A method of genetic transformation and gene editing of succulents without tissue culture.

Succulents, valued for their drought tolerance and ornamental appeal, are important in the floriculture market. However, only a handful of succulent species can be genetically transformed, making it difficult to improve these plants through genetic engineering. In this study, we adapted the recently developed cut-dip-budding (CDB) gene delivery system to transform three previously recalcitrant succulent varieties - the dicotyledonous Kalanchoe blossfeldiana and Crassula arborescens and the monocotyledonous Sansevieria trifasciata. Capitalizing on the robust ability of cut leaves to regenerate shoots, these plants were successfully transformed by directly infecting cut leaf segments with the Agrobacterium rhizogenes strain K599. The transformation efficiencies were approximately 74%, 5% and 3.9%-7.8%, respectively, for K. blossfeldiana and C. arborescens and S. trifasciata. Using this modified CDB method to deliver the CRISPR/Cas9 construct, gene editing efficiency in K. blossfeldiana at the PDS locus was approximately 70%. Our findings suggest that succulents with shoot regeneration ability from cut leaves can be genetically transformed using the CDB method, thus opening up an avenue for genetic engineering of these plants.

Efficient and heritable A-to-K base editing in rice and tomato.

Cytosine and adenosine base editors (CBE and ABE) have been widely used in plants, greatly accelerating gene function research and crop breeding. Current base editors can achieve efficient A-to-G and C-to-T/G/A editing. However, efficient and heritable A-to-Y (A-to-T/C) editing remains to be developed in plants. In this study, a series of A-to-K base editor (AKBE) systems were constructed for monocot and dicot plants. Furthermore, nSpCas9 was replaced with the PAM-less Cas9 variant (nSpRY) to expand the target range of the AKBEs. Analysis of 228 T0 rice plants and 121 T0 tomato plants edited using AKBEs at 18 endogenous loci revealed that, in addition to highly efficient A-to-G substitution (41.0% on average), the plant AKBEs can achieve A-to-T conversion with efficiencies of up to 25.9 and 10.5% in rice and tomato, respectively. Moreover, the rice-optimized AKBE generates A-to-C conversion in rice, with an average efficiency of 1.8%, revealing the significant value of plant-optimized AKBE in creating genetic diversity. Although most of the A-to-T and A-to-C edits were chimeric, desired editing types could be transmitted to the T1 offspring, similar to the edits generated by the traditional ABE8e. Besides, using AKBEs to target tyrosine (Y, TAT) or cysteine (C, TGT) achieved the introduction of an early stop codon (TAG/TAA/TGA) of target genes, demonstrating its potential use in gene disruption.

Genome editing in rice using CRISPR/Cas12i3.

The CRISPR/Cas type V-I is a family of programmable nuclease systems that prefers a T-rich protospacer adjacent motif (PAM) and is guided by a short crRNA. In this study, the genome-editing application of Cas12i3, a type V-I family endonuclease, was characterized in rice. We developed a CRIPSR/Cas12i3-based Multiplex direct repeats (DR)-spacer Array Genome Editing (iMAGE) system that was efficient in editing various genes in rice. Interestingly, iMAGE produced chromosomal structural variations with a higher frequency than CRISPR/Cas9. In addition, we developed base editors using deactivated Cas12i3 and generated herbicide-resistant rice plants using the base editors. These CRIPSR/Cas12i3-based genome editing systems will facilitate precision molecular breeding in plants.

Cut-dip-budding delivery system enables genetic modifications in plants without tissue culture.

Of the more than 370 000 species of higher plants in nature, fewer than 0.1% can be genetically modified due to limitations of the current gene delivery systems. Even for those that can be genetically modified, the modification involves a tedious and costly tissue culture process. Here, we describe an extremely simple cut-dip-budding (CDB) delivery system, which uses Agrobacterium rhizogene to inoculate explants, generating transformed roots that produce transformed buds due to root suckering. We have successfully used CDB to achieve the heritable transformation of plant species in multiple plant families, including two herbaceous plants (Taraxacum kok-saghyz and Coronilla varia), a tuberous root plant (sweet potato), and three woody plant species (Ailanthus altissima, Aralia elata, and Clerodendrum chinense). These plants have previously been difficult or impossible to transform, but the CDB method enabled efficient transformation or gene editing in them using a very simple explant dipping protocol, under non-sterile conditions and without the need for tissue culture. Our work suggests that large numbers of plants could be amenable to genetic modifications using the CDB method.

Base editing-mediated targeted evolution of ACCase for herbicide-resistant rice mutants.

Improved cytosine and adenine base editors and an efficient dual editor were applied in targeted evolution of ACETYL COA CARBOXYLASE in rice, resulting in the generation of dozens of herbicide-resistant mutations, at least three of which, W2125L, W2125Q and C2186H, have not been reported previously.

Efficient generation of homozygous substitutions in rice in one generation utilizing an rABE8e base editor.

A new deaminase, TadA8e, was recently evolved in the laboratory. TadA8e catalyzes DNA deamination over 1,000 times faster than ABE7.10. We developed a high-efficiency adenine base editor, rABE8e (rice ABE8e), combining monomeric TadA8e, bis-bpNLS and codon optimization. rABE8e had substantially increased editing efficiencies at NG-protospacer adjacent motif (PAM) and NGG-PAM target sequences compared with ABEmax. For most targets, rABE8e exhibited nearly 100% editing efficiency and high homozygous substitution rates in the specific editing window, especially at Positions A5 and A6. The ability to rapidly generate plant materials with homozygous base substitutions will benefit gene function research and precision molecular breeding.

SWO1 modulates cell wall integrity under salt stress by interacting with importin ɑ in Arabidopsis.

Maintenance of cell wall integrity is of great importance not only for plant growth and development, but also for the adaptation of plants to adverse environments. However, how the cell wall integrity is modulated under salt stress is still poorly understood. Here, we report that a nuclear-localized Agenet domain-containing protein SWO1 (SWOLLEN 1) is required for the maintenance of cell wall integrity in Arabidopsis under salt stress. Mutation in SWO1 gene results in swollen root tips, disordered root cell morphology, and root elongation inhibition under salt stress. The swo1 mutant accumulates less cellulose and pectin but more lignin under high salinity. RNA-seq and ChIP-seq assays reveal that SWO1 binds to the promoter of several cell wall-related genes and regulates their expression under saline conditions. Further study indicates that SWO1 interacts with importin ɑ IMPA1 and IMPA2, which are required for the import of nuclear-localized proteins. The impa1 impa2 double mutant also exhibits root growth inhibition under salt stress and mutations of these two genes aggravate the salt-hypersensitive phenotype of the swo1 mutant. Taken together, our data suggest that SWO1 functions together with importin ɑ to regulate the expression of cell wall-related genes, which enables plants to maintain cell wall integrity under high salinity.

Targeted, efficient sequence insertion and replacement in rice.

CRISPR-Cas9 methods have been applied to generate random insertions and deletions, large deletions, targeted insertions or replacements of short sequences, and precise base changes in plants1-7. However, versatile methods for targeted insertion or replacement of long sequences and genes, which are needed for functional genomics studies and trait improvement in crops, are few and largely depend on the use of selection markers8-11. Building on methods developed in mammalian cells12, we used chemically modified donor DNA and CRISPR-Cas9 to insert sequences of up to 2,049 base pairs (bp), including enhancers and promoters, into the rice genome at an efficiency of 25%. We also report a method for gene replacement that relies on homology-directed repair, chemically modified donor DNA and the presence of tandem repeats at target sites, achieving replacement with up to 130-bp sequences at 6.1% efficiency.

Multiplex gene editing in rice with simplified CRISPR-Cpf1 and CRISPR-Cas9 systems.

We developed simplified single transcriptional unit (SSTU) CRISPR systems for multiplex gene editing in rice using FnCpf1, LbCpf1 or Cas9, in which the nuclease and its crRNA array are co-expressed from a single Pol II promoter, without any additional processing machinery. Our SSTU systems are easy to construct and effective in mediating multiplex genome editing.

Application of Cre-lox gene switch to limit the Cry expression in rice green tissues.

The presence of genetically modified (GM) protein in the endosperm is important information for the public when considering the biological safety of transgenic rice. To limit the expression of GM proteins to rice green tissues, we developed a modified Cre-lox gene switch using two cassettes named KEY and LOCK. KEY contains a nuclear-localized Cre recombinase driven by the green-tissue-specific promoter rbcS. LOCK contains a Nos terminator (NosT), which is used to block the expression of the gene of interest (GOI), bounded by two loxP sites. When KEY and LOCK are pyramided into hybrid rice, a complete gene switch system is formed. The Cre recombinase from KEY excises loxP-NosT in LOCK and unlocks the GOI in green tissues but keeps it locked in the endosperm. This regulatory effect was demonstrated by eYFP and Bt expression assays. The presence of eYFP and Cre were confirmed in the leaf, sheath, stem, and glume but not in the root, anther or seed of the gene-switch-controlled eYFP hybrids. Meanwhile, gene switch-controlled Bt hybrid rice not only confined the expression of Bt protein to the green tissues but also showed high resistance to striped stem borers and leaffolders.

The RING Finger Ubiquitin E3 Ligase OsHTAS Enhances Heat Tolerance by Promoting H2O2-Induced Stomatal Closure in Rice.

Heat stress often results in the generation of reactive oxygen species, such as hydrogen peroxide, which plays a vital role as a secondary messenger in the process of abscisic acid (ABA)-mediated stomatal closure. Here, we characterized the rice (Oryza sativa) HEAT TOLERANCE AT SEEDLING STAGE (OsHTAS) gene, which plays a positive role in heat tolerance at the seedling stage. OsHTAS encodes a ubiquitin ligase localized to the nucleus and cytoplasm. OsHTAS expression was detected in all tissues surveyed and peaked in leaf blade, in which the expression was concentrated in mesophyll cells. OsHTAS was responsive to multiple stresses and was strongly induced by exogenous ABA. In yeast two-hybrid assays, OsHTAS interacted with components of the ubiquitin/26S proteasome system and an isoform of rice ascorbate peroxidase. OsHTAS modulated hydrogen peroxide accumulation in shoots, altered the stomatal aperture status of rice leaves, and promoted ABA biosynthesis. The results suggested that the RING finger ubiquitin E3 ligase OsHTAS functions in leaf blade to enhance heat tolerance through modulation of hydrogen peroxide-induced stomatal closure and is involved in both ABA-dependent and DROUGHT AND SALT TOLERANCE-mediated pathways.

Gene editing by co-transformation of TALEN and chimeric RNA/DNA oligonucleotides on the rice OsEPSPS gene and the inheritance of mutations.

Although several site-specific nucleases (SSNs), such as zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and the clustered regularly interspaced short palindromic repeat (CRISPR)/Cas, have emerged as powerful tools for targeted gene editing in many organisms, to date, gene targeting (GT) in plants remains a formidable challenge. In the present study, we attempted to substitute a single base in situ on the rice OsEPSPS gene by co-transformation of TALEN with chimeric RNA/DNA oligonucleotides (COs), including different strand composition such as RNA/DNA (C1) or DNA/RNA (C2) but contained the same target base to be substituted. In contrast to zero GT event obtained by the co-transformation of TALEN with homologous recombination plasmid (HRP), we obtained one mutant showing target base substitution although accompanied by undesired deletion of 12 bases downstream the target site from the co-transformation of TALEN and C1. In addition to this typical event, we also obtained 16 mutants with different length of base deletions around the target site among 105 calli lines derived from transformation of TALEN alone (4/19) as well as co-transformation of TELAN with either HRP (5/30) or C1 (2/25) or C2 (5/31). Further analysis demonstrated that the homozygous gene-edited mutants without foreign gene insertion could be obtained in one generation. The induced mutations in transgenic generation were also capable to pass to the next generation stably. However, the genotypes of mutants did not segregate normally in T1 population, probably due to lethal mutations. Phenotypic assessments in T1 generation showed that the heterozygous plants with either one or three bases deletion on target sequence, called d1 and d3, were more sensitive to glyphosate and the heterozygous d1 plants had significantly lower seed-setting rate than wild-type.

Assembling long heteroduplexes by asymmetric polymerase chain reaction and annealing the resulting single-stranded DNAs.

We developed an effective protocol for generating high-purity heteroduplexes via annealing single-stranded DNAs (ssDNAs) derived from plasmid DNA by asymmetric polymerase chain reaction (A-PCR). With the addition of dimethyl sulfoxide, a one-step A-PCR procedure can generate ssDNAs stably at a range of reaction temperatures. Several annealing buffers can anneal two ssDNAs into heteroduplexes effectively. We further developed a simple strategy to create d(GATC) hemimethylated heteroduplexes by annealing fully methylated homoduplexes in the presence of excessive unmethylated ssDNAs. The constructed heteroduplexes have been well tested as substrates for mismatch repair in Escherichia coli and, thus, can be used in various biotechnology applications.