Method for making site-directed modification to plant genomes by using non-inheritable materials

Abstract

The invention disclosed a method for conducting site-directed modification to a plant genome using non-inheritable materials. The method provided in the present invention specifically comprises the following steps: introducing a non-inheritable material into a cell or a tissue or a part of the plant of interest; wherein said non-inheritable material is a nuclease specific to said target fragment or an mRNA expressing said nuclease, thereby the target fragment is cleaved by said nuclease and site-directed modification to the target fragment is achieved through DNA repairing in the plant. By introducing a non-inheritable material of sequence-specific nuclease, site-directed mutation in a plant gene can be achieved, and no exogenous gene or nucleic acid fragments will be integrated into the plant as obtained. Therefore, the present invention can lead to more precise genome function study and higher biosafety in breeding.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase of International Patent Application No. PCT/CN2016/076244, filed on Mar. 14, 2016, which published as WO 2016/155482 A1 on Oct. 6, 2016, and claims priority to Chinese Patent Application No. 201510114017.4, filed on Mar. 16, 2015, all of which are herein incorporated by reference in their entirety.

SEQUENCE LISTING

The instant application contains a Sequence Listing which has been submitted in ASCII format via EFS-Web and is hereby incorporated by reference in its entirety. Said ASCII copy, created on Feb. 24, 2019, is named seqlistKWSO244PCTUS_ST25.txt, and is 87,241 bytes in size.

TECHNICAL FIELD

The present invention belongs to the field of plant genetic engineering, and relates to method for making site-directed modification to plant genomes by using non-inheritable materials, specifically to a non-transgenic method for making site-directed modification to plant genome by using protein or mRNA.

TECHNICAL BACKGROUND

Genome editing technology is the most promising means for investigating gene function and improving crops genetically. Currently available genome editing technologies include Zinc finger nucleases (ZFN), Transcription activator-like effector nucleases (TALEN), and Clustered regularly interspaced short palindromic repeats/CRISPR associated systems (CRISPR/Cas9), which are called sequence specific nucleases (SSN). Their common feature is that they can act as an endonuclease to cleave specific DNA sequences, producing DNA double-strand break (DSB). DSB can activate intrinsic repair mechanism of the cell, Non-homologous end joining (NHEJ) and Homologous recombination (HR), so as to repair the DNA damages. Thereby site-directed substitution or insertion mutant can be generated. Currently, genome editing technologies have been efficiently used in some plants (e.g., rice, Arabidopsis, maize, wheat) to modify the plant genome, and show significant potential in improving agricultural traits of important crops.

However, although genome editing brings about a promising chance for crop improvement, there is still a great challenge. To conduct genome editing, the sequence-specific nuclease should be expressed in the cell. Currently, the method for expressing the sequence-specific nuclease in plant cells is to deliver an expression vector or DNA fragment expressing the nuclease into the cells via convention transformation approaches (Agrobacterium-mediated transformation, particle bombardment, injection and the like). Those inheritable materials randomly integrate into the plant chromosome and transcribe to perform editing. These convention transformation approaches involve the integration of exogenous genes into the plant genome and require selection markers (selection pressure) during the transformation, which may lead to undesirable phenotypes. The application of the plants as obtained would be controlled under GMO regulations. Therefore, it is necessary to establish a method for conducting genome editing in plants without the need of introducing inheritable material DNA.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method for conducting site-directed modification to a target fragment of a target gene in a plant.

The method provided in the present invention for conducting site-directed modification to a target fragment of a target gene in a plant, specifically comprises the following steps: introducing a non-inheritable material into a cell or a tissue or a part of the plant of interest; wherein said non-inheritable material is a nuclease specific to said target fragment or an mRNA expressing said nuclease, thereby the target fragment is cleaved by said nuclease and site-directed modification to the target fragment is achieved through DNA repairing in the plant.

In the present method, a non-inheritable material is introduced in a cell or a tissue or a part of the plant of interest. The non-inheritable material can express a nuclease for conducting site-directed modification to the target fragment, or the non-inheritable material can direct act on the target fragment and achieve the site-directed modification. Along with or after the site-directed modification, said non-inheritable material can be degraded by the metabolic mechanism in the cell. The modified cell or tissue can be regenerated into an intact plant by conventional tissue culture. Consequently, a transgene-free mutant plant is obtained, in which only the target fragment is modified and no exogenous inheritable material has been introduced.

In the present method, said nuclease is a TALEN nuclease, a Zinc finger nuclease, a CRISPR/Cas9 nuclease, or any other nuclease that can achieve genome editing.

Correspondingly, the non-inheritable material can be selected from any one of following (a)-(c):

(a) the non-inheritable material is a TALEN nuclease, or a mRNA capable of expressing paired TALEN proteins; wherein the TALEN protein is composed of a DNA binding domain capable of recognizing and binding to the target fragment, and a Fok I domain.

In one embodiment of the invention (Example 1), said non-inheritable material is composed of mRNAs of SEQ ID NO: 3 and 4. In another embodiment of the invention (Example 2), said non-inheritable material is composed of proteins of SEQ ID NOS: 7 and 8.

(b) the non-inheritable material is a Zinc finger nuclease or a mRNA capable of expressing paired ZFN proteins; wherein the ZFN protein is composed of a DNA binding domain capable of recognizing and binding to the target fragment, and a Fok I domain.

(c) the non-inheritable material is composed of a Cas9 protein or a mRNA capable of expressing a Cas9 protein, and a guide RNA; wherein said guide RNA is an RNA with a palindromic structure which is formed by partial base-pairing between a crRNA and a tracrRNA; said crRNA contains an RNA fragment capable of complementarily binding to the target fragment.

In one embodiment of the invention (Example 3), said non-inheritable material is composed of a protein as shown in SEQ ID NO: 10 and a sgRNA as shown in SEQ ID NO: 11. In another embodiment of the invention (Example 4), said non-inheritable material is composed of a protein as shown in SEQ ID NO: 10 and a sgRNA as shown in SEQ ID NO: 12.

In the present method, said cell may be any cell into which the non-inheritable material can be introduced and which can regenerate into an intact plant through tissue culture. Said tissue may be any tissue into which the non-inheritable material can be introduced and which can regenerate into an intact plant through tissue culture. Said part of the plant is a part of an intact plant (not an ex vivo part) into which the non-inheritable material can be introduced.

Specifically, said cell can be a protoplast cell or a suspension cell. Said tissue can be a callus, an immature embryo, or a mature embryo. Said part of the plant can be a leaf, a shoot apex, a hypocotyl, a young spike or a pollen tube.

In said method, the approach for introducing the non-inheritable material into a cell or a tissue or a part of the plant of interest can be particle bombardment, PEG-mediated protoplast transformation, pollen tube approach, or any other approach that can be used for introducing the non-inheritable material.

In said method, the site-specific modification is nucleotide insertion, deletion, and/or replacement in the target fragment.

Another object of the invention is to provide a method for making a transgene-free mutant plant.

The method of the invention for making a transgene-free mutant plant specifically can comprises the following steps: conducting a site-directed modification to a target fragment of a target gene in a plant of interest, thereby a plant is obtained in which the functions of the target gene are lost or changed and the genome thereof is free of integrated exogenous gene.

In the present invention, the plant can be a monocotyledon or a dicotyledon. In some embodiments, the plant is rice, maize, wheat or tobacco.

Compared with the inheritable material DNA, protein and mRNA are two types of non-inheritable materials which can be easily degraded in the cell by the defense mechanism. Through the transient introduction of an mRNA or a protein of sequence-specific nuclease, mutants with site-directed knocked out genes can be obtained without the integration of the sequence-specific nuclease gene or vector fragment in the plane genome, namely, transgene-free. The method of the invention achieves higher biosafety, and the crop varieties produced by the method would not be regulated as GMO. The present invention has significant values in basic study and crop breeding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows that TaGW2 gene mutations were generated by transforming wheat immature embryo with Cas9 mRNA and sgRNA. A: a gel electrophoretogram of Cas9-mRNA in vitro transcribed with a mRNA transcription kit (AM1344, Ambion). B: PCR/RE results showing the mutations in target site of TaGW2 in TO plants generated by Cas9 mRNA and sgRNA-GW2-C14. C: the sequencing results indicate in vitro transcribed Cas9 mRNA and sgRNA-GW2-C14 induced mutations at the target site. WT represents wild-type gene sequence, “−” represents a sequence with deletion, “+” represents a sequence with insertion, the number after “−/+” represents the number of the deleted or inserted nucleotides.

FIG. 2 shows that OsBADH2 gene mutations were generated by transiently transforming rice protoplasts with mRNA-TALEN. A: a gel electrophoretogram showing in vitro transcription of T-BADH2b-L and T-BADH2b-R with a mRNA transcription kit (AM1344, Ambion), and a PolyA tail was added to the 3′end of the mRNA. B: PCR/RE results showing the mutations in target site generated by in vitro transcribed mRNA in the protoplasts. C: the sequencing results indicate in vitro transcribed mRNA induced mutations at the target site. WT represents wild-type gene sequence, “−” represents a sequence with deletion, “+” represents a sequence with insertion, the number after “−/+” represents the number of the deleted or inserted nucleotides.

FIG. 3 shows mutagenesis of wheat MLO gene by transformation of wheat protoplasts with MLO-TALEN proteins. A: SDS-PAGE results showing prokaryotic expression and purification of T-MLO-L and T-MLO-R for the MLO target site. B: PCR/RE results showing the mutations in target site generated by the TALEN proteins in the protoplasts. C: the sequencing results indicate in vitro generated TALEN proteins induced mutations at the target site. WT represents wild-type gene sequence, “−” represents a sequence with deletion, “+” represents a sequence with insertion, the number after “−/+” represents the number of the deleted or inserted nucleotides.

FIG. 4 shows mutagenesis of wheat TaGASR7 gene by transformation of wheat protoplasts with Cas9 protein and in vitro transcribed sgRNA. A: SDS-PAGE results showing prokaryotic expression and purification of Cas9 protein. B: PCR/RE results showing the mutations in target site generated by Cas9 protein and in vitro transcribed sgRNA. C: the sequencing results indicate in vitro generated Cas9 protein and in vitro transcribed sgRNA induced mutations at the target site. WT represents wild-type gene sequence, “−” represents a sequence with deletion, “+” represents a sequence with insertion, the number after “−/+” represents the number of the deleted or inserted nucleotides.

FIG. 5 shows that NtPVY gene mutations were generated by co-transformation of Cas9 protein and in vitro transcribed sgRNA into tobacco protoplasts, and mutant plants were obtained by regeneration. A: PCR/RE results of the protoplasts showing the mutations in target site generated by Cas9 protein and in vitro transcribed sgRNA. B: the sequencing results indicate co-transformation of in vitro generated Cas9 protein and in vitro transcribed sgRNA into tobacco protoplasts induced mutations at the target site. C: Detection of mutant plants regenerated from the protoplasts, and sequencing results of the target sites. WT represents wild-type gene sequence, “−” represents a sequence with deletion, “+” represents a sequence with insertion, the number after “−/+” represents the number of the deleted or inserted nucleotides.

DETAILED EMBODIMENTS

The experimental methods used in the following Examples are all conventional methods, unless otherwise indicated.

The materials, reagents used in the following Examples are all commercially available, unless otherwise indicated.

The wheat variety Bobwhite is disclosed in “Weeks, J. T. et al. Rapid production of multiple independent lines of fertile transgenic wheat. Plant Physiol. 102: 1077-1084, (1993)”, and can be obtained from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences.

Wheat TaMLO gene-targeting TALENs vector T-MLO is disclosed in “Wang, Y, Cheng, X., Shan, Q., Zhang, Y, Liu, J., Gao, C., and Qiu, J. L. (2014). Simultaneous editing of three homoeoalleles in hexaploid bread wheat confers heritable resistance to powdery mildew. Nature Biotechnology. 32, 947-951”, and can be obtained from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences.

Prokaryotic expression vector pGEX-4T was obtained from Shanghai BeiNuo Biotechnology Co. Ltd., Cat. No. 1110024.

Cas9-mRNA in vitro transcription vector pXT7-Cas9 was disclosed in “Chang N, Sun C, Gao L, Zhu D, Xu X, et al. 2013. Genome editing with RNA-guided Cas9 nuclease in zebrafish embryos. Cell research 23: 465-72”, and can be obtained from the authors.

pT7-gRNA vector was disclosed in “A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity. Science 337(6096): 816-821”, and can be obtained from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences.

Maize variety Hill was disclosed in “Armstrong, C. L., Green, C. E.& Phillips, R. L. Development and availability of germplasm with high type II culture formation response. Maize Genet. Coop. News Lett. 65, 92-93 (1991)”, and can be obtained from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences.

Solutions used in the preparation and transformation of rice protoplast are shown in Tables 1-5.

TABLE 1
50 ml enzymolysis solution
	The amount	Final
	added	Concentration
Cellulase R10	0.75 g	1.5%
Macerozyme R10	0.375 g	0.75%
mannitol	5.4651 g	0.6M
2-(N-Morpholino)ethanesulfonic	0.1066 g	10 mM
acid
made up to 50 ml with double distilled water, pH adjusted to 5.7 with
KOH; incubated in 55° C. water bath for 10 min, and cooled at room
temperature before adding
CaCl2	0.0735 g	10 mM
BSA	0.05 g	0.1%
filtered with a 0.45 μm filter

TABLE 2
500 ml W5
	The amount added	Final Concentration
NaCl	4.5 g	154 mM
CaCl2	9.189 g	125 mM
KCl	0.1864 g	5 mM
2-(N-	0.2132 g	2 mM
Morpholino)ethanesulfonic
acid
made up to 500 ml with double distilled water, pH adjusted to 5.7
with NaOH

TABLE 3
10 ml MMG solution
	The amount added	Final Concentration
mannitol (0.8M)	5 ml	0.4M
MgCl2 (1M)	0.15 ml	15 mM
2-(N-	0.2 ml	4 mM
Morpholino)ethanesulfonic
acid (200 mM)
double distilled water	Made up to 10 ml

TABLE 4
4 ml PEG solution
		The amount added	Final Concentration
	PEG4000	1.6 g	40%
	mannitol (0.8M)	1 ml	0.2M
	CaCl2 (1M)	0.4 ml	0.1M
	double distilled water	Made up to 4 ml

TABLE 5
250 ml WI solution
	The amount added	Final Concentration
mannitol	27.324 g	0.6M
KCl	0.07456 g	4 mM
2-(N-	0.2135 g	4 mM
Morpholino)ethanesulfonic
acid (200 mM)
made up to 250 ml with double distilled water, pH adjusted to 5.7
with KOH

% in above Tables 1-5 indicates weight-volume percentage, g/100 ml.

The medium used for wheat tissue culture include:

Hypertonic medium: MS minimal medium, 90 g/L mannitol, 5 mg/L 2,4-D, 30 g/L sucrose, and 3 g/L, phytogel, pH 5.8.

Induction medium: MS minimal medium, 2 mg/L 2,4-D, 0.6 mg/L cupric sulfate, 0.5 mg/L casein hydrolysates, 30 g/L, sucrose, and 3 g/L phytogel, pH 5.8.

Differentiation medium: MS minimal medium, 0.2 mg/L kinetin, 30 g/L sucrose, and 3 g/L phytogel, pH 5.8.

Rooting medium: ½ of MS minimal medium, 0.5 mg/L ethanesulfonic acid, 0.5 mg/L α-naphthylacetic acid, 30 g/L, sucrose, and 3 g/L phytogel, pH 5.8.

Example 1. Site-Directed Editing of TaGW2 by Transforming Wheat Immature Embryo with In Vitro Transcribed Cas9 mRNA and sgRNA

I. Design of the Target Fragment: Target-C14

Target-C14:
SEQ ID NO: 18
5′- CCAGGATGGGGTATTTCTAGAGG-3′ (in the conserved
region of exon 8 of wheat TaGW2, Groups A, B and
D;).

II. In Vitro Transcription and Purification of Cas9-mRNA

1. pXT7-Cas9 vector was digested with XbaI. The digested product was purified with a purification kit (Axygen) to a concentration of higher than 100 ng/μl, and designated as pXT7-Cas9-XbaI.

2. The purified product pXT7-Cas9-XbaI was transcribed with an in vitro transcription kit (AM1344, Ambion). The product was purified with a mRNA purification kit (AM1908, Ambion) to a concentration of higher than 500 ng/μl. The Agarose gel electrophoretogram of the in vitro transcribed Cas9-mRNA was shown in FIG. 1A.

III. In Vitro Transcription of sgRNA Against the Target Site

1. The Target Site of TaGW2 was Constructed in the pTaU6-gRNA Vector

The following single-stranded oligonucleotides with sticky ends (underlined) were synthesized:

C14F:
(SEQ ID NO: 19)
5′-CTTGCAGGATGGGGTATTTCTAG-3′;
C14R:
(SEQ ID NO: 20)
5′-AAACCTAGAAATACCCCATCCTG-3′.

Double-stranded DNA with sticky ends was formed through annealing between C14F/C14R, and inserted between the two BbsI restriction sites in pTaU6-gRNA plasmid, resulting in a pTaU6-gRNA plasmid containing C14 site. The positive plasmid was verified by sequencing. A recombinant plasmid, which was obtained by inserting the DNA fragment as shown in 5′-CTTGCAGGATGGGGTATTTCTAG-3′ (SEQ ID NO: 19) in forward direction at the BbsI restriction site of pTaU6-gRNA plasmid, was positive, and designated as pTaU6-gRNA-C14.

2. In Vitro Amplification and Purification of the DNA Fragment of T7-TaGW2-gRNA Primer Design

T7-TaGW2-F:
(SEQ ID NO: 21)
TAATACGACTCACTATAGGCAGGATGGGGTATTTCTAG;
gRNA-PCR-R:
(SEQ ID NO: 22)
AGCACCGACTCGGTGCCACTT.

PCR amplification was performed with pTaU6-gRNA-C14 as the template. PCR product was purified with a PCR purification kit (AP-GX-250G, Axygen) to a concentration of higher than 100 ng/μl. The resulted PCR product is a sgRNA containing T7 promoter and the TaGW2 target site, and designated as T7-TaGW2-gRNA.

3. In Vitro Transcription of the sgRNA Containing the TaGW2 Target Site

sgRNA-GW2-C14 (as shown in SEQ ID NO:17) was in vitro transcribed with a T7 in vitro transcription kit (E2040S, NEB).

IV. Site Directed Editing of Wheat TaGW2 Gene by Particle Bombardment Transformation of In Vitro Transcribed Cas9-mRNA and In Vitro Transcribed sgRNA

1. Loading In Vitro Transcribed Cas9-mRNA and In Vitro Transcribed sgRNA to 0.6 nm Gold Powder

5 μl 0.6 nm gold powder, 3 μl Cas9-mRNA, 1 μl sgRNA-GW2-C14, 1 μl 5M ammonium acetate, 20 μl isopropanol were mixed and precipitated at −20° C. for 1 h, so as to allow the Cas9-mRNA and sgRNA-GW2-C14 to attach to the gold powder. The mixture was centrifuged at 1000 rpm for 5 sec and washed in 100 μl dehydrated alcohol after discarding the supernate, then centrifuged at 1000 rpm for 5 sec again and resuspended in 20 μl dehydrated alcohol after discarding the supernate.

2. Transformation of Wheat Recipient Materials Using Particle Bombardment

• • 1) Immature embryo of the wheat variety KN199 was taken and treated for 4 hours using hypertonic medium.
  • 2) A particle bombardment device was used to bombard the wheat immature embryo that was hypertonically cultured in step 1). 20 μl of the sgRNA-Cas9-mRNA mixture was loaded on the membrane and bombarded; the bombarding distance for each bombardment was 6 cm, the bombarding pressure was 1100 psi, the bombarding diameter was 2 cm.
  • 3) The wheat immature embryo bombarded in step 2) was hypertonically cultured for 16 hours;
  • 4) The wheat immature embryo hypertonically cultured in step 3) were then sequentially subjected to 14 days of callus tissue induction culture, 28 days of differentiation culture, and 14-28 days of rooting culture, so as to obtain wheat plants.
  • 5) DNA was extracted from the wheat seedlings generated in step 4) and mutants with gene knocked-out (site-directed) were detected through PCR/RE tests (for specific test method, please refer to step IV). Wild-type wheat variety Kn199 was used as control.

Since there is a sequence recognized by the restriction endonuclease XbaI in the target fragment of wheat endogenous gene TaGW2, XbaI was used to perform the PCR/RE tests. The primers used in PCR amplification are primers specific to Groups A, B and D, having the following sequences:

TaGW2-AF:
(SEQ ID NO: 23)
5′- CTGCCATTACTTTGTATTTTGGTAATA-3′;
TaGW2-BF:
(SEQ ID NO: 24)
5′- GTTCAGATGGCAATCTAAAAGTT-3′;
TaGW2-DF:
(SEQ ID NO: 25)
5′- GCATGTACTTTGATTGTTTGCGTGA-3′;
TaGW2-R:
(SEQ ID NO: 26)
5′- TCCTTCCTCTCTTACCACTTCCC-3′.

The results of some detection tests indicate that mutations occurred in the target site of wheat TaGW2 gene. Bands were recovered for sequencing. The sequencing results indicate that insertion/deletion (indel) occurred in the target site of wheat TaGW2 gene (FIGS. 1 B and C).

Example 2. Site-Directed Editing of OsBADH2 Gene by Transforming Rice Protoplasts with In Vitro Transcribed TALEN mRNA

I. TALEN Target Fragment

The sequence of rice BADH2 gene is shown in SEQ ID NO:1.

TALEN target fragment is located in the fourth exon of rice BADH2 gene, and has the following sequence:

5′-GCTGGATGCTTTGAGTActttgcagatcttgcagaATCCTTGGACAAAAGGC-3′ (SEQ ID NO: 27; positions 1589-1640 of SEQ ID NO: 1); the lower case letters in the middle represent a spacer sequence; and the flanking uppercase letters represent the sequences recognized by the TALEN modules (designated as L-b and R-b). Underlined is the sequence recognized by BglII.

II. Design and Synthesis of Talen Encoding Genes

The TALEN protein that recognizes L-b in the target sequence was designated as T-BADH2b-L, while the encoding sequence is shown in positions 7-2952 of SEQ ID NO: 2. Positions 7-27 of SEQ ID NO: 2 encodes foe a nucleic localization signal (NLS); positions 463-2154 encodes for the L-b sequence recognizing module protein; positions 2350-2953 (603 bp) encodes for an endonuclease Fok I.

The TALEN protein that recognizes R-b in the target sequence was designated as T-BADH2b-R, while the encoding sequence is shown in positions 3085-6018 of SEQ ID NO:2. Positions 3085-3105 of SEQ ID NO: 2 encodes foe a nucleic localization signal (NLS); positions 3541-5232 encodes for the L-b sequence recognizing module protein; positions 5428-6018 (591 bp) encodes for an endonuclease Fok I.

Positions 2953-3006 of SEQ ID NO: 2 encodes for T2A which is composed of 18 amino acids and allows T-BADH2b-L and T-BADH2b-R expressed in a same expression cassette to break into two individual proteins.

III. In Vitro Synthesis of mRNA of TALEN Gene

The two components of TALEN for rice BADH2 gene, T-BADH2b-L and T-BADH2b-R, were in vitro transcribed with an mRNA transcription kit (Ambion) by using the T7 promoter to initiate the transcription. mRNA-L-T-OsBADH2b and mRNA-R-T-OsBADH2b were obtained, and PolyA tails were added to the 3′ end thereof for increasing the stability of the mRNA.

The sequence of mRNA-L-T-OsBADH2b is shown in SEQ ID NO: 3, and the sequence of mRNA-R-T-OsBADH2b is shown in SEQ ID NO: 4.

IV. Introduction of the Mixture of two mRNAs of TALEN Obtained by In Vitro Transcription into Rice Protoplasts

1. Preparation of the Materials

The rice variety as used is Nipponbare. Seeds were rinsed in 75% ethanol, then treated with 2.5% sodium hypochlorite for 20 min, washed with sterile water for more than 5 times, and cultured on ½ MS medium for 7-10 days under 26° C., 12 h light (150 μmol·m⁻¹·s⁻¹). 15 seeds may be cultured in a big glass culture bottle. For one experiment, 40-60 seedlings are required and the amount of isolated protoplasts is sufficient for transformation of 6 plasmids.

2. Isolation of Protoplasts

• • 1) Shoots and leaf sheathes were used for isolation of protoplasts. They were cut into 0.5 mm threads;
  • 2) The threads were transferred to 0.6M mannitol solution immediately, placed in dark for 10 min;
  • 3) The mannitol solution was removed by filtration, and the threads were transferred into enzymolysis solution, treated in a vacuum pump for 30 min at −15˜−20 (mmHg) in dark;
  • 4) the samples were digested for additional 4-5 hours with gentle shaking (on a shaker at a speed of 10 rpm);
  • 5) equal volume of W5 solution was added after digestion and the solution should be shaken for 10 sec so as to release the protoplasts;
  • 6) the protoplasts were filtrated into a 50 ml round bottom centrifuge tube using a 40 μm Nylon filter membrane, and W5 solution was added for washing;
  • 7) 250 g centrifugation for 3 min for precipitating the protoplasts, and the supernatant was discarded;
  • 8) the protoplasts were resuspended in 10 ml W5, centrifuged at 250 g for 5 min, and the supernatant was discarded;
  • 9) the protoplasts were resuspended by adding a proper amount of MMG solution. The concentration of the protoplasts is 2×10⁶/ml, as determined by counting with a haemocytometer.

Note: all the above steps were performed under room temperature.

3. Transformation of Protoplasts

• • 1) 10 μg mRNA-L-T-OsBADH2b and 10 μg mRNA-R-T-OsBADH2b were added into a 2 ml centrifuge tube. 200 μl of the protoplasts (about 4×10⁵ cells) were added. Then 220 μl of fresh PEG solution was added and mixed. Transformation was performed in dark for 10-20 min under room temperature;
  • 2) after transformation, 880 μl W5 was added slowly and mixed by reversing, 250 g centrifugation for 3 min, and the supernatant was discarded;
  • 3) the protoplasts was resuspended by adding 1 ml WI, and transferred to a 6-well plate (with pre-added 1 ml WI), and then cultured at RT or 28° C. in the dark for 6-16 hours (for 48 hours if the protoplasts are used for genomic DNA extraction);
  • 4. Using PCR/RE experiments to analyze the mutagenesis of rice endogenous gene BADH2 resulted from in vitro transcribed TALEN

48 hours after the transformation of the protoplasts, genomic DNA was extracted, which was used as template for PCR/RE (Polymerase Chain Reaction/Restriction digestion) experiment analysis. At the same time, the protoplasts of wild-type rice variety Nipponbare were used as a control. PCR/RE analysis method is based on Shan, Q. et al. Rapid and efficient gene modification in rice and Brachypodium using TALENs. Molecular Plant (2013). Since the target site of rice endogenous gene BADH2 contains the recognition sequence of restriction endonuclease BglII, the restriction endonuclease BglII was used in the experiment for conducting the PCR/RE test. Primers used in the PCR amplification are:

OsBADH-F:
(SEQ ID NO: 28)
5′-GATCCCGCAGCGGCAGCTCTTCGTCG-3′;
OsBADH2-R:
(SEQ ID NO: 29)
5′-GAGGAATAAAATCTCAAATGTCTTCAACTT-3′.

The results of PCR/RE experiments can be seen in FIG. 2B, and the results showed that: mutations occurred at the target site of BADH2 gene, and the mutagenesis efficiency is about 5%. The bands in the figure were recovered and sequenced, and the sequencing results showed that insertion/deletion (indel) occurred at the target site of BADH2 gene (FIG. 2C).

Example 3. Expression and Purification of TALEN Proteins in a Prokaryotic Expression System, and Transformation of the Same into Wheat Protoplasts or Immature Embryos for Site-Directed Modification of MLO Gene

I. Selection of Target Sequences and Design of the TALENs

A conserved region in exon 2 of wheat MLO gene was used as the target sequence to design a pair of TALENs (consisting of TAL-MLO-L protein and TAL-MLO-R protein; TAL-MLO-L protein is composed of two functional fragments, namely a fragment specifically binds to upstream nucleotides of the target sequence and a Fok I endonuclease with EL mutation; TAL-MLO-R protein is composed of two functional fragments, namely a fragment specifically binds to downstream nucleotides of the target sequence and a Fok I endonuclease with KK mutation). The target sequences of said TALENs in TaMLO-A, TaMLO-B and TaMLO-D genes are listed as follows:

TaMLO-A gene:
(SEQ ID NO: 30)
5′-TCGCTGCTGCTCGCCGTcacgcaggacccaatctcCGGGATATGCAT
CTCCCA-3′;
TaMLO-B gene:
(SEQ ID NO: 31)
5′-TCGCTGCTGCTCGCCGTgacgcaggaccccatctcCGGGATATGCAT
CTCCGA-3′;
TaMLO-D gene:
(SEQ ID NO: 32)
5′-TCGCTGCTGCTCGCCGTgacgcaggacccaatctcCGGGATATGCAT
CTCCGA-3′.

In the wheat cell, when the TAL-L fragment and TAL-R fragment bind to respective binding region, the two different monomer Fok I endonucleases (Fok I endonuclease with EL mutation and Fok I endonuclease with KK mutation) will form an active Fok I dimmer endonuclease which cleaves in the target sequence region (including the target sequence and the flanking sequences) to generate a double-strand break. During the repair of said break by the cell, a number of mutations will be introduced. Here, “mutation” has a broad meaning, including insertion, deletion, replacement and the like, most of which result in loss of gene function.

In the above target sequences, the underlined portion is the recognition sequence of restriction nuclease AvaII which can be cut by AvaII. After the generation of break, if a mutation occurs and interrupts the AvaII recognition sequence, the target sequence cannot be cut by AvaII; if no mutation occurs, the target sequence can be cut by AvaII.

II. Expression and Purification of TALEN Proteins for MLO Gene Target in a Prokaryotic Expression System

1. Construction of Prokaryotic Expression Vectors for Expressing TALEN Proteins

• • 1) Encoding regions of TAL-L (SEQ ID NO: 5) and TAL-R (SEQ ID NO: 6) of the TALEN gene were constructed into a prokaryotic expression vector pGEX-4T, so that a recombinant vector was obtained with the TAL-L encoding region (SEQ ID NO: 5) inserted between the BamHI and XbaI sites of pGEX-4T in a forward direction, while the TAL-R encoding region (SEQ ID NO: 6) inserted between the XbaI and BamHI sites of pGEX-4T in a forward direction. The recombinant vector was transformed into E. coli BL21. A positive colony was inoculated into LB medium supplemented with ampicillin and chloramphenicol and cultured under 37° C. over night. The culture was then inoculated to 5 ml fresh LB medium at a ratio of 1:100, cultured under 37° C. at 225 rpm to OD600≈0.5. 1 ml of the culture was taken as the negative control (no induction). Controls of empty pGEX-4T vector were also set up, with or without induction. For the remaining culture, IPTG was added (final concentration of 1 mM) to induce expression under 37° C. at 225 rpm for 8 h.
  • 2) 1 ml of each of the control or induced culture was taken and centrifuged at 12000 rpm for 10 min to collect the bacteria cells, discarding the supernatant. The cells were resuspended by adding 50 μL protein loading buffer, boiled for 7 min. The supernatant was analyzed by 10% SDS-PAGE. The molecular weight of each TALEN protein is about 100 Kda. The amino acid sequence of the TAL-MLO-L protein is shown in SEQ ID NO: 7. The amino acid sequence of the TAL-MLO-R protein is shown in SEQ ID NO: 8.

2. Purification of TALEN Proteins

The bacteria culture was centrifuged under 4° C. for 10 min to collect the bacteria cells. 10 ml lysis buffer (50 mM Tris-HCl, 2 mM EDTA, 100 mM NaCl, 1 mg/ml lysozyme, pH 8.5) was added to the pellet, mixed on ice for 45 min. After ultrasonication, pellet was collected by centrifugation, washed with 4M Imidazole. The pellet obtained after a further centrifugation was dissolved in 50 mM phosphate buffer (containing 8M Urea) of pH 7.4. (FIG. 3A)

III. Introduction of the Purified TALEN Proteins into Wheat Protoplasts for Site-Directed Editing of the MLO Gene

The purified TALEN proteins against the target site of MLO gene were introduced into protoplasts of wheat variety Bobwhite via PEG-mediated approach as follows:

1. Growth of Wheat Seedling

Wheat seeds were grown in a culturing room, under 25±2° C., illuminance 1000 Lx, 14-16 h light/d, for about 1-2 weeks.

2. Isolation of Protoplast

• • 1) Tender leaves of wheat were taken, and the middle part thereof was cut into 0.5-1 mm threads using a cutter blade, placed into 0.6M of mannitol solution (using water as solvent) for 10 min in dark. The mixture was then filtrated using a filter, then placed in 50 ml enzymolysis solution for 5 h of digestion (0.5 h enzymolysis in vacuum, then 4.5 h slow shaking at 10 rpm).

Note: The temperature during enzymolysis should be kept between 20-25° C., the reaction should be carried out in the dark; and the solution should be gently shaken after the reaction so as to release the protoplasts.

• • 2) the enzymolysis product was diluted by adding 10 ml of W5, and filtrated into a 50 ml round bottom centrifuge tube using a 75 μm Nylon filter membrane.

Note: The Nylon filter membrane should be submerged in 75% (volume percentage) ethanol, washed with water and then soaked in W5 for 2 min before use.

• • 3) 23° C., 100 g centrifugation for 3 min, and the supernatant was discarded.
  • 4) the pellet was suspended with 10 ml W5, placed on ice for 30 min; the protoplasts eventually formed sedimentation, and the supernatant was discarded.
  • 5) the protoplasts were suspended by adding a proper amount of MMG solution, placed on ice until transformation.

Note: The concentration of the protoplasts needs to be determined by microscopy (×100). The amount of protoplasts was 2×10⁵/ml to 1×10⁶/ml.

3. Transformation of Wheat Protoplast

• • 1) 15 μg TALEN proteins (TAL-MLO-L protein and TAL-MLO-R protein mixed in equal amount) or 20 μg T-MLO vector (control) were added into a 2 ml centrifuge tube. 200 μl of the isolated protoplasts was added using a pipette and then mixed by gentle patting, kept still for 3-5 min. Then 250 μl of PEG4000 was added and mixed by gentle patting. Transformation was performed in dark for 30 min;
  • 2) 900 μl W5 (room temperature) was added and mixed by reversing, 100 g centrifugation for 3 min, and the supernatant was discarded;
  • 3) 1 ml W5 was added and mixed by reversing, the content was gently transferred to a 6-well plate (with pre-added 1 ml W5), and then cultured at 23° C. overnight.

4. Using PCR/RE Experiments to Analyze the Mutagenesis of Wheat Endogenous Gene MLO Resulted from Purified TALEN Proteins

48 hours after the transformation of wheat protoplasts, genomic DNA was extracted, which was used as template for PCR/RE (Polymerase Chain Reaction/Restriction digestion) experiment analysis. At the same time, the protoplasts transformed with T-MLO plasmid or protoplasts of wild-type wheat variety Bobwhite were used as control. PCR/RE analysis method is based on Shan, Q. et al. Rapid and efficient gene modification in rice and Brachypodium using TALENs. Molecular Plant (2013). Since the target fragment of wheat endogenous gene MLO contains the recognition sequence of restriction endonuclease AvaII, AvaII was used in the experiment for conducting the PCR/RE test. Primers used in the PCR amplification were:

TaMLO-F:
(SEQ ID NO: 33)
5′-TCATCGTCTCCGTCCTCCTGGAGCA-3′;
TaMLO-R:
(SEQ ID NO: 34)
5′-TGGTATTCCAAGGAGGCGGTCTCTGTCT-3′.

The results of PCR/RE experiments showed that: mutations occurred at the target site of MLO gene. The bands were recovered and sequenced, and the sequencing results showed that insertion/deletion (indel) occurred at the target site of MLO gene. (FIGS. 3B and 3C)

IV. Site-Directed Editing of the MLO Gene by Introduction of the TALEN Proteins Using Particle Bombardment

Generally, transformation of an expression plasmid into cells by particle bombardment is using gold powder as the carrier to carry the DNA plasmid into the cells. However, for proteins, gold powder is not suitable as the carrier as it is difficult to bind a protein to the gold powder. In the present invention, silica is used as the carrier for transforming proteins with particle bombardment.

1. Loading Proteins to Silica

Silica Au-MSN with aperture of 10 nm was used as the carrier. 20 mg of Au-MSN was added to 5 ml phosphate buffer (PBS) of pH 7.4 for sonication, and then 7 mg of purified TAL-MLO-L protein and TAL-MLO-R protein were added. The mixture was stirred under 22° C. for 24 hours, centrifuged at 12000 rpm. The supernatant was discarded. Pellet was suspended with PBS buffer.

2. Transformation of Wheat Recipient Materials Using Particle Bombardment

• • 1) Immature embryo of the wheat variety Bobwhite was taken and treated for 4 hours using hypertonic medium.
  • 2) A particle bombardment device was used to bombard the wheat immature embryo that was hypertonically cultured in step 1). Au-MSN loaded with TALEN proteins (5 μl, 20 μg/μl) was loaded on the membrane and bombarded; the bombarding distance for each bombardment was 6 cm, the bombarding pressure was 1100 psi, the bombarding diameter was 2 cm.
  • 3) The wheat immature embryo bombarded in step 2) was hypertonically cultured for 16 hours;
  • 4) The wheat immature embryo hypertonically cultured in step 3) were then sequentially subjected to 14 days of callus tissue induction culture, 28 days of differentiation culture, and 14-28 days of rooting culture, so as to obtain wheat plants.
  • 5) DNA was extracted from the wheat seedlings generated in step 4) and mutants with gene knocked-out (site-directed) were detected through PCR/RE tests (for specific test method, please refer to step III). Wild-type wheat variety Bobwhite was used as control.

The detection results of some mutants indicate that mutations occurred in the target site of wheat MLO gene. Bands were recovered for sequencing. The sequencing results indicate that insertion/deletion (indel) occurred in the target site of wheat MLO gene.

The above results demonstrated that site-directed editing of a target site can be achieved by introducing nuclease protein into wheat. The mutants obtained by this method are free of exogenous DNA, and the protein as introduced will be degraded by the plant cell. Therefore, the mutants obtained by this method are transgene-free plants, having high biosafety.

Example 4. Site-Directed Editing of TaGASR7 Gene by Co-Transformation of a Cas9 Protein Expressed and Purified in Prokaryotic Expression System and an In Vitro Transcribed sgRNA

I. Design of the Target Fragment: Target-C5

Target-C5:
(SEQ ID NO: 35)
5′-CCGCCGGGCACCTACGGCAAC-3′; (in the
TaGASR7 gene as shown in Genbank No. EU095332,
positions 248-268).

II. Prokaryotic Expression and Purification of Cas9 Protein

1. Cas9 gene (optimized for plant codon usage and added with NLS at both ends) was constructed into a prokaryotic expression vector pGEX-4T, so that a recombinant vector was obtained with a Cas9 gene of SEQ ID NO: 9 (optimized for plant codon usage and added with NLS at both ends) inserted between BamHI and SpeI of the pGEX-4T vector. The recombinant vector was transformed into E. coli BL21. A positive colony was inoculated into LB medium supplemented with ampicillin and chloramphenicol and cultured under 37° C. over night. The culture was then inoculated to 5 ml fresh LB medium at a ratio of 1:100, cultured under 37° C. at 225 rpm to OD600≈0.5. 1 ml of the culture was taken as the negative control (no induction). Controls of empty pGEX-4T vector were also set up, with or without induction. For the remaining culture, IPTG was added (final concentration of 1 mM) to induce expression under 37° C. at 225 rpm for 8 h.

2. 1 ml of each of the control or induced culture was taken and centrifuged at 12000 rpm for 10 min to collect the bacteria cells, discarding the supernatant. The cells were resuspended by adding 50 μL protein loading buffer, boiled for 7 min. The supernatant was analyzed by 10% SDS-PAGE. The molecular weight of the Cas9 protein is about 200 KDa. The amino acid sequence of the Cas9 protein is shown in SEQ ID NO: 10.

2. Purification of the Cas9 Protein

The bacteria culture was centrifuged under 4° C. for 10 min to collect the bacteria cells. 10 ml lysis buffer (50 mM Tris-HCl, 2 mM EDTA, 100 mM NaCl, 1 mg/ml lysozyme, pH 8.5) was added to the pellet, mixed on ice for 45 min. After ultrasonication, pellet was collected by centrifugation, washed with 4M Imidazole. The pellet obtained after a further centrifugation was dissolved in 50 mM phosphate buffer (containing 8M Urea) of pH 7.4. (FIG. 4A)

III. In Vitro Transcription of the sgRNA of the Target Site

1. The Target Site of TaGASR7 was Constructed into the pT7-gRNA Vector

C5 is the DNA sequence coding for the RNA that can complementarily bind to target-C5.

The following single-stranded oligonucleotides with sticky ends (underlined) were synthesized:

C5F:
(SEQ ID NO: 36)
5′-CTTGTTGCCGTAGGTGCCCGG-3′;
C5R:
(SEQ ID NO: 37)
5′-AAACCCGGGCACCTACGGCAA-3′.

Double-stranded DNA with sticky ends was formed through oligonucleotides annealing process, and inserted between the two BbsI restriction sites in pT7-gRNA plasmid, resulting in a pT7-gRNA plasmid containing the C5 site. The positive plasmid was verified by sequencing. A recombinant plasmid, which was obtained by inserting the DNA fragment as shown in 5′-CTTGTTGCCGTAGGTGCCCGG-3′ (SEQ ID NO: 36) in forward direction at the BbsI restriction site of pT7-gRNA plasmid, was positive and designated as pT7-gRNA-C5.

2. In Vitro Transcription of the sgRNA Containing Target Site of TaGASR7

With the T7 promoter for initiate the transcription, sgRNA for the TaGASR7 gene was in vitro transcribed using an mRNA transcription kit (Ambion) into sgRNA-GASR7-C5 (SEQ ID NO: 11), and a PolyA tail was added to the 3′end thereof for increasing the stability of the mRNA.

IV. Editing the TaGASR7 Gene by Co-Transformation of the Cas9 Protein and the In Vitro Transcribed sgRNA into Wheat Protoplasts

1. The Preparation of Protoplasts is Identical to Example 3.

2 Transformation of the Protoplasts

• • 1) 15 μg purified Cas9 protein and 20 μg sgRNA-GASR7-C5 were added into a 2 ml centrifuge tube. 200 μl of the protoplasts (about 4×10⁵ cells) was added and then 250 μl of fresh PEG solution was added and mixed. Transformation was performed in dark for 30 min;
  • 2) 900 μl W5 (room temperature) was added and mixed by reversing, 100 g centrifugation for 3 min, and the supernatant was discarded;
  • 3) 1 ml W5 was added and mixed by reversing, the content was gently transferred to a 6-well plate (with pre-added 1 ml W5), and then cultured at 23° C. overnight.

3. Using PCR/RE Experiments to Analyze the Mutagenesis of Wheat Endogenous Gene TaGASR7 Resulted from Purified Cas9 Protein and the In Vitro Transcribed sgRNA.

48 hours after the transformation of wheat protoplasts, genomic DNA was extracted, which was used as template for PCR/RE (Polymerase Chain Reaction/Restriction digestion) experiment analysis. At the same time, the protoplasts of wild-type wheat variety Bobwhite were used as control. PCR/RE analysis method is based on Shan, Q. et al. Rapid and efficient gene modification in rice and Brachypodium using TALENs. Molecular Plant (2013). Since the target site (positions 248-268 of Genbank No. EU095332) of wheat endogenous gene TaGASR7 (Genbank No. EU095332) contains the recognition sequence (5′-CCSGG-3′) of restriction endonuclease NciI, NciI was used in the experiment for conducting the PCR/RE test. Primers used in the PCR amplification were:

TaGASR7-F:
(SEQ ID NO: 38)
5′-GGAGGTGATGGGAGGTGGGGG-3′;
TaGASR7-R:
(SEQ ID NO: 39)
5′-CTGGGAGGGCAATTCACATGCCA-3′.

The results of PCR/RE experiments showed that mutations occurred at the target site of TaGASR7 gene. The bands in the figure were recovered and sequenced, and the sequencing results showed that insertion/deletion (indel) occurred at the target site of TaGASR7 gene. (FIGS. 4B and C).

V. Site-Directed Editing of Wheat TaGASR7 Gene Through Particle Bombardment Transformation of Purified Cas9 Protein and In Vitro Transcribed sgRNA

1. Loading Purified Cas9 Protein and In Vitro Transcribed sgRNA to Silica

Silica Au-MSN with aperture of 10 nm was used as the carrier. 20 mg of Au-MSN was added to 5 ml phosphate buffer (PBS) of PH 7.4 for sonication. Then 7 mg of purified Cas9 protein was added. The mixture was stirred under 22° C. for 24 hours, centrifuged at 12000 rpm. The supernatant was discarded. Pellet was suspended with PBS buffer. 4 μl of the in vitro transcribed sgRNA (250 ng/μl) was added into 10 μl Cas9 protein-Au-MSN (10 μg/μl) carrier. Then 12.5 μl 2.5M CaCl₂) and 5 μl 0.1M spermidine were added, centrifuged at 5000 rpm for 15 s, discarding the supernatant. The Au-MSN carrying Cas9 protein and coated with mRNA was washed with 100% ethanol twice, and resuspended in 5 μl 100% ethanol, designated sgRNA-Cas9-Au-MSN.

2. Transformation of Wheat Recipient Materials Using Particle Bombardment

• • 1) Immature embryo of the wheat variety Bobwhite was taken and treated for 4 hours using hypertonic medium.
  • 2) A particle bombardment device was used to bombard the wheat immature embryo that was hypertonically cultured in step 1). 5 μl of sgRNA-Cas9-Au-MSN was loaded on the membrane and bombarded; the bombarding distance for each bombardment was 6 cm, the bombarding pressure was 1100 psi, the bombarding diameter was 2 cm.
  • 3) The wheat immature embryo bombarded in step 2) was hypertonically cultured for 16 hours;
  • 4) The wheat immature embryo hypertonically cultured in step 3) were then sequentially subjected to 14 days of callus tissue induction culture, 28 days of differentiation culture, and 14-28 days of rooting culture, so as to obtain wheat plants.
  • 5) DNA was extracted from the wheat seedlings generated in step 4) and mutants with gene knocked-out (site-directed) were detected through PCR/RE tests (for specific test method, please refer to step IV). Wild-type wheat variety Bobwhite was used as control.

The detection results of some mutants indicate that mutations occurred in the target site of wheat TaGASR7 gene. Bands were recovered for sequencing. The sequencing results indicate that insertion/deletion (indel) occurred in the target site of wheat TaGASR7 gene.

Example 5. Site-Directed Editing of Maize Endogenous ZmIPK Gene by Introducing Purified Cas9 Protein and sgRNA into Plant Via Pollen Tube Approach

I. Design of the Target Fragment: Target-C2

Target-C2:
SEQ ID NO: 40
5′-CCGAGCTCGACCACGCCGCCGAC-3′; (position
393-415 of the gene ZmIPK as shown in Genbank No.
AY172635).

II. Prokaryotic Expression and Purification of Cas9 Protein

Identical to Example 3, step II.

III. In Vitro Transcription of the sgRNA of the Target Site

1. The Target Site of ZmIPK was Constructed into the pT7-gRNA Vector

C2 is the DNA sequence coding for the RNA that can complementarily bind to target-C2.

The following single-stranded oligonucleotides with sticky ends (underlined) were synthesized:

C2-1F:
(SEQ ID NO: 41)
5′-AGCAGTCGGCGGCGTGGTCGAGCT-3′;
C2-1R:
(SEQ ID NO: 42)
5′-AAACAGCTCGACCACGCCGCCGAC-3′.

Double-stranded DNA with sticky ends was formed through oligonucleotides annealing process, and inserted between the two BbsI restriction sites in pT7-gRNA plasmid, resulting in a pT7-gRNA plasmid containing the C2 site. The positive plasmid was verified by sequencing. A recombinant plasmid, which was obtained by inserting the DNA fragment as shown in 5′-AGCAGTCGGCGGCGTGGTCGAGCT-3′ (SEQ ID NO: 41) in forward direction at the BbsI restriction site of pT7-gRNA plasmid, was positive and designated as pT7-gRNA-C2.

2. In Vitro Transcription of the sgRNA Containing Target Site of ZmIPK

With the T7 promoter for initiate the transcription, sgRNA for the ZmIPK gene was in vitro transcribed using an mRNA transcription kit (Ambion) into sgRNA-IPK-C2 (SEQ ID NO: 12), and a PolyA tail was added to the 3′end thereof for increasing the stability of the mRNA.

IV. Site-Directed Editing of Maize Endogenous ZmIPK Gene by Introducing Purified Cas9 Protein and In Vitro Transcribed sgRNA Via Pollen Tube Approach

Strong plants of maize inbred Hill in the field were selected as the recipient materials. The plants were self-fertilized at 14: 00-16:00 of a sunny day. 16-20 hr post pollination, namely 10:00-12:00 of the next day, the styles of the recipients were cut. A mixture of 10 μg/μl Cas9 protein and 250 ng/μl sgRNA was dripped to the incision. The stigmas were bagged until fructifcation. The obtained maize seeds were grown, and genomic DNA was extracted for use in the PCR/RE experiment as a template. Wild type maize variety Hill was set as control in parallel. PCR/RE analysis method is based on Shan, Q. et al. Rapid and efficient gene modification in rice and Brachypodium using TALENs. Molecular Plant (2013). Since the target fragment (positions 393-415 of Genbank No. AY172635) of maize endogenous gene ZmIPK (Genbank No. AY172635) contains the recognition sequence (5′-GAGCTC-3′) of restriction endonuclease SacI, the restriction endonuclease SacI was used in the experiment for conducting the PCR/RE test. Primers used in the PCR amplification were:

ZmIPK-1F:
(SEQ ID NO: 43)
5′- TCGCAGCCCCTGGCAGAGCAA-3′;
ZmIPK-1R:
(SEQ ID NO: 44)
5′- GAGACCTGGGAGAAGGAGACGGATCC-3′.

The results of PCR/RE experiments showed that: mutations occurred at the target site of ZmIPK gene. The uncut bands was recovered and sequenced, and the sequencing results showed that insertion/deletion (indel) occurred at the target site of ZmIPK gene.

Example 6. Site-Directed Editing of NtPVY Gene by Co-Transformation of a Cas9 Protein Expressed and Purified in Prokaryotic Expression System and an In Vitro Transcribed sgRNA into Tobacco Protoplasts, and the Regeneration into Plants

I. Design of the Target Fragment: Target-P4

Target-P4:
(SEQ ID NO: 45)
5′-TGATACCAGCTGGCTATACACGG-3′

II. Prokaryotic Expression and Purification of Cas9 Protein is Identical to Example 3.

III. In Vitro Transcription of the sgRNA of the Target Site

1. The Target Site of NtPVY was Constructed into the pHSN401 Vector

P4 is the DNA sequence coding for the RNA that can complementarily bind to target-P4.

The following single-stranded oligonucleotides with sticky ends (underlined) were synthesized:

P4-F:
(SEQ ID NO: 46)
5′-ATTGTGATACCAGCTGGCTATACA-3′;
P4-R:
(SEQ ID NO: 47)
5′-AAACTGTATAGCCAGCTGGTATCA-3 ′ .

Double-stranded DNA with sticky ends was formed through oligonucleotides annealing process, and inserted between the two BsaI restriction sites in the pHSN401 plasmid, resulting in a pHSN401 plasmid containing P4. The positive plasmid was verified by sequencing. A recombinant plasmid, which was obtained by inserting the DNA fragment as shown in 5′-ATTGTGATACCAGCTGGCTATACA-3′ (SEQ ID NO: 46) in forward direction at the BsaI restriction site of pHSN401 plasmid, was positive and designated as p pHSN401-P4.

2. In Vitro Transcription of the sgRNA Containing Target Site of NTPVY

With the T7 promoter for initiate the transcription, sgRNA for the NTPVY gene (SEQ ID NO:13, 14, 15) was in vitro transcribed using an mRNA transcription kit (Ambion) into sgRNA-PVY-P4 (SEQ ID NO:16).

IV. Editing of NtPVY Gene by Co-Transformation of a Cas9 Protein and In Vitro Transcribed sgRNA into Tobacco Protoplasts.

1. Preparation of the Materials

The tobacco variety as used is Honghua Dajinyuan. Seeds were treated with 20% sodium hypochlorite for 20 min, and washed with sterile water for 5 times. Then the seeds were cultured on ½ MS medium under 25° C., 16 h light.

2. Isolation of Protoplasts

• • 1) 6 leaves of 30 day old tobacco plants were selected and cut into sections of about 1 cm under sterile conditions. The sections were placed in a culture plate containing 15 ml enzymolysis solution. The plate was sealed and kept in the dark under 25° C. overnight (most preferable 12 h).
  • 2) After the enzymolysis reaction, a suitable amount of W5 solution was added. The plate was gently shaken to release the protoplasts. Then the protoplast suspension was filtered with 100 μm and 40 μm sterile filter, centrifuged at 70 g for 5 min, discarding the supernatant.
  • 3) The protoplasts were resuspended by adding 5 ml 22% sucrose solution. Then, 2 ml W5 solution was added and centrifuged at 70 g for 5 min. Protoplasts now suspended at the interface.
  • 4) The protoplasts were taken from the interface. 5 ml W5 solution was added and mixed following by 70 g centrifugation for 5 min.
  • 5) The supernatant was discarded. 1 ml MMG transformation solution was added to resuspended the protoplasts. The yield of the protoplasts was determined by microscopy.

3. Transformation and Regeneration of the Protoplasts

• • 1) 20 μg purified Cas9 protein and 20 μg mRNA-PVY-P4 were added into a 14 ml centrifuge tube. 300 μl of the protoplast (about 5×10⁵ cells) was added following by 300 μl of fresh PEG solution, mixed and kept in dark for 20 min.
  • 2) 10 ml W5 was added and mixed, 70 g centrifugation for 3 min, and the supernatant was discarded; this step was repeated.
  • 3) 1 ml of K3:H medium containing 0.6% Sea Plaque agarose (incubated in 40-45° C. water bath before use) was added and mixed. The mixture was transformed into a sterile 30 mm culture plate.
  • 4) After solidification of the medium, the plate was placed in the dark under 24° C. for 24 h, the cultured in dark for another 6 d until the first cell division occurred.
  • 5) The agarose gel was transferred into a 90 mm culture plate, and a suitable amount of liquid A medium was added. Cultivation was continued under 24° C. in dark.
  • 6) 3-4 weeks later, visible callus emerged in the plate. And the callus reached diameters of 8-10 mm after cultivation of 5-6 weeks.
  • 7) The calli were transferred to differentiation medium and cultured for 1-2 weeks until adventitious buds were formed on the surface.
  • 8) Adventitious buds of 3-4 cm were cut and transferred to rooting medium to induce the generation of roots, until the formation of intact plants.
  • 9) The seedlings were transplanted in soil when the roots reach a certain length.

DNA of the transgenic tobacco was extracted and used as the template for PCR/RE (Polymerase Chain Reaction/Restriction digestion) analysis. Wild type tobacco DNA was used as control in parallel. PCR/RE analysis method is based on Shan, Q. et al. Rapid and efficient gene modification in rice and Brachypodium using TALENs. Molecular Plant (2013). Since the target fragment of tobacco endogenous gene NtPVY contains the recognition sequence (5′-CAGCTG-3′) of restriction endonuclease PvuII, the restriction endonuclease PvuII was used in the PCR/RE test. Primers used in the PCR amplification were:

NtPVY-F:
(SEQ ID NO: 48)
5′-TGGATTAGATGTTTTCAAATGC-3′;
NtPVY-R:
(SEQ ID NO: 49)
5′-CATTCTTTTGGGGACGGACAAA-3′.

The results of PCR/RE experiments showed that co-transformation of Cas9 protein and in vitro transcribed sgRNA into tobacco protoplasts resulted in mutations in the target site of NtPVY gene. The uncut bands was recovered and sequenced, and the sequencing results showed that insertion/deletion (indel) occurred at the target site of NtPVY gene (FIGS. 5 A and B). In addition, the regenerated transgenic tobacco plants also showed mutation in the target site of NtPVY gene. The sequencing results showed that insertion/deletion (indel) occurred at the target site of NtPVY gene (FIG. 5 C).

Solution for Tobacco protoplast isolation and culture are listed in following Table 6-10.

TABLE 6
50 ml enzymolysis solution
	The amount added	Final Concentration
Cellulase R10	0.6	1.2%
Macerozyme R10	0.3	0.6%
made up to 50 ml with K4 medium, pH adjusted to 5.6 with KOH;
centrifugation at 7000 g for 10 min; filtered with a 0.22 μm filter.

TABLE 7
500 ml W5
		The amount added	Final Concentration
	NaCl	4.5 g	154 mM
	CaCl2	9.189 g	125 mM
	KCl	0.1864 g	5 mM
	Glucose	0.45 g	5 mM
	made up to 500 ml with double distilled water, pH adjusted to
	5.8 with KOH, autoclaved.

TABLE 8
10 ml transformation solution
		The amount added	Final Concentration
	mannitol (0.8M)	6.33 ml	0.5M
	MgCl2 (1M)	0.15 ml	15 mM
	MES	0.01 g	0.1%
	made up to 10 ml with double distilled water, pH adjusted to
	5.8 with KOH, autoclaved.

TABLE 9
4 ml PEG solution
	The amount added	Final Concentration
PEG4000	1.6 g	40%
mannitol (0.8M)	2 ml	0.4M
Ca(NO3)2		0.1M
made up to 4 ml with double distilled water, pH adjusted to
8-9 with KOH, autoclaved.

TABLE 10
Stock solution for Tobacco protoplast isolation and culture
					MS
Medium (ml/L)	A	H	K3	MS	mopho
1000 mg/50 ml Stock
KNO3	50.5	95	125	95	95
NH4NO3		40	30	12.5	82.5
CaCl2•2H2O	22	30	45	22	36.5
MgSO4•7H2O	37	15	12.5	18.5	18.5
1000 mg/100 ml
(NH4)2SO4	0	0	25	0	0
KH2PO	0	13.6	17	0	17
NaH2PO4	0	0	15	0	0
(NH4)succinate	5	0	0	0	0
CaHPO4	0	0	0	5	0
Microelements (MS microelements 10× from Sigma, 100 ml/l)
	100	100	100	100	100
Carbohydrates (g/l) final concentration
Sucrose (+)	30	30	30	20	30
D-sorbitol	0	0	45.5	20	0
D-Mannital	0	0	45.5	20	0
Hormones (mg/l final concentration)
2,4-D	0	1.5	5	1.5	0
Kinetin	0	0	0	0	0.2
Vitamins (mg/l final concentration)
PyridoxineHCl	0.5	0.5	0.5	1.5	0.5
Thiamine HCI	0.1	0.1	0.1	10	0.1
Nicotinic acid	0	0.5	0.5	0.5	0.5
Inositol	100	100	100	100	100
Other organics (mg/l final concentration)
Glycine	2	2	2	7.5	2
L-Glutamine	0	0	0	877	0
L-Asparagine	0	0	0	266	0
Caseinhydrolysate	400	400	400	0	0

Claims

1. A method for conducting site-directed modification to a target fragment of a target gene in a plant of interest comprising introducing a non-inheritable material into a tissue of the plant of interest by particle bombardment using particles comprising silica and/or gold; wherein said non-inheritable material is a nuclease of a CRISPR associated system specific to said target fragment, wherein the target fragment is cleaved by said nuclease, and wherein a site-directed modification to the target fragment is achieved through a DNA repairing event in the plant; wherein said tissue is a hypertonically cultured immature or mature embryo; andwherein the non-inheritable material is composed of a mixture of an in vitro transcribed Cas9 mRNA comprising a nuclear localization sequence (NLS) and an in vitro transcribed guide RNA; wherein said guide RNA is an RNA with a palindromic structure which is formed by partial base-pairing between a crRNA and a tracrRNA; said crRNA contains an RNA fragment capable of complementarily binding to the target fragment,wherein the mixture of the Cas9 mRNA and the guide RNA is precipitated on the particles, by combining the Cas9 mRNA, the guide RNA, and the particles with ammonium acetate and isopropanol, incubating the Cas9 mRNA, the guide RNA, and the particles, collecting the particles with bound Cas9 mRNA and guide RNA by centrifugation, resuspending the collected particles with dehydrated ethanol, collecting the washed particles with bound Cas9 mRNA and guide RNA by centrifugation, and resuspending the collected particles in absolute ethanol before adding the collected particles to a membrane of a particle bombardment device,wherein the DNA template is prepared to a concentration of greater than 100 ng/μl and the DNA template is in vitro transcribed to produce the guide RNA, andwherein the guide RNA is present in a concentration of 250 ng/μl.

2. The method of claim 1, wherein the site-directed modification is nucleotide insertion, deletion, and/or replacement in the target fragment.

3. A method for making a transgene-free mutant plant, specifically comprising the following steps: conducting a site-directed modification to a target fragment of a target gene in a plant of interest according to the method of claim 1,subjecting the modified immature or mature embryo to callus induction culture;subjecting the induced callus to differentiation culture; andsubjecting the differentiated callus to rooting culture, resulting in a plant in which the functions of the target gene are lost or changed and the genome of the plant is free of an integrated exogenous gene.

4. The method of claim 1, wherein the guide RNA is an sgRNA.

5. The method of claim 1, wherein the non-inheritable material is introduced into the tissue, and wherein the tissue is the immature embryo.

6. The method of claim 1, wherein the non-inheritable material is introduced into the tissue, and wherein the tissue is the mature embryo.

7. The method of claim 1, wherein the particle comprises Silica Au-MSN.