Targeted gene deletion with SpCas9 and multiple guide RNAs in Arabidopsis thaliana: four are better than two

Engineering a robust Cas12i3 variant-mediated wheat genome editing system

Wheat (Triticum aestivum L., 2n = 6x = 42, AABBDD) is one of the most important food crops in the world. CRISPR/Cas12i3, which belongs to the type V-I Cas system, has attracted extensive attention recently due to its smaller protein size and its less-restricted canonical 'TTN' protospacer adjacent motif (PAM). However, due to its relatively lower editing efficacy in plants and the hexaploidy complex nature of wheat, Cas12i3/Cas12i3-5M-mediated genome editing in wheat has not been documented yet. Here, we report the engineering of a robust Cas12i3-5M-mediated genome editing system in wheat through the fusion of T5 exonuclease (T5E) in combination with an optimised crRNA expression strategy (Opt). We first showed that fusion of T5E, rather than ExoI, to Cas12i3-5M increased the gene editing efficiencies by up to 1.34-fold and 3.87-fold, compared to Cas12i3-5M and Cas12i3 in HEK293T cells, respectively. However, its editing efficiency remains low in wheat. We then optimised the crRNA expression strategy and demonstrated that Opt-T5E-Cas12i3-5M could enhance the editing efficiency by 1.20- to 1.33-fold and 4.05- to 7.95-fold in wheat stable lines compared to Opt-Cas12i3-5M and Opt-Cas12i3, respectively, due to progressive 5'-end resection of the DNA strand at the cleavage site with increased deletion size. The Opt-T5E-Cas12i3-5M enabled an editing efficiency ranging from 60.71% to 90.00% across four endogenous target genes in stable lines of three elite Chinese wheat varieties. Together, the developed robust Opt-T5E-Cas12i3-5M system enriches wheat genome editing toolkits for either biological research or genetic improvement and may be extended to other important polyploidy crop species.

A nucleoside signal generated by a fungal endophyte regulates host cell death and promotes root colonization

The intracellular colonization of plant roots by the beneficial fungal endophyte Serendipita indica follows a biphasic strategy, including a host cell death phase that enables successful colonization of Arabidopsis thaliana roots. How host cell death is initiated and controlled is largely unknown. Here, we show that two fungal enzymes, the ecto-5'-nucleotidase SiE5NT and the nuclease SiNucA, act synergistically in the apoplast at the onset of cell death to produce deoxyadenosine (dAdo). The uptake of extracellular dAdo but not the structurally related adenosine activates cell death via the equilibrative nucleoside transporter ENT3. We identified a previously uncharacterized Toll-like interleukin 1 receptor (TIR)-nucleotide-binding leucine-rich repeat receptor (NLR) protein, ISI (induced by S. indica), as an intracellular factor that affects host cell death, fungal colonization, and growth promotion. Our data show that the combined activity of two fungal apoplastic enzymes promotes the production of a metabolite that engages TIR-NLR-modulated pathways to induce plant cell death, providing a link to immunometabolism in plants.

Versatile Cloning Strategy for Efficient Multigene Editing in Arabidopsis

CRISPR-Cas9 technology has become an essential tool for plant genome editing. Recent advancements have significantly improved the ability to target multiple genes simultaneously within the same genetic background through various strategies. Additionally, there has been significant progress in developing methods for inducible or tissue-specific editing. These advancements offer numerous possibilities for tailored genome modifications. Building upon existing research, we have developed an optimized and modular strategy allowing the targeting of several genes simultaneously in combination with the synchronized expression of the Cas9 endonuclease in the egg cell. This system allows significant editing efficiency while avoiding mosaicism. In addition, the versatile system we propose allows adaptation to inducible and/or tissue-specific edition according to the promoter chosen to drive the expression of the Cas9 gene. Here, we describe a step-by-step protocol for generating the binary vector necessary for establishing Arabidopsis edited lines using a versatile cloning strategy that combines Gateway® and Golden Gate technologies. We describe a versatile system that allows the cloning of as many guides as needed to target DNA, which can be multiplexed into a polycistronic gene and combined in the same construct with sequences for the expression of the Cas9 endonuclease. The expression of Cas9 is controlled by selecting from among a collection of promoters, including constitutive, inducible, ubiquitous, or tissue-specific promoters. Only one vector containing the polycistronic gene (tRNA-sgRNA) needs to be constructed. For that, sgRNA (composed of protospacers chosen to target the gene of interest and sgRNA scaffold) is cloned in tandem with the pre-tRNA sequence. Then, a single recombination reaction is required to assemble the promoter, the zCas9 coding sequence, and the tRNA-gRNA polycistronic gene. Each element is cloned in an entry vector and finally assembled according to the Multisite Gateway® Technology. Here, we detail the process to express zCas9 under the control of egg cell promoter fused to enhancer sequence (EC1.2en-EC1.1p) and to simultaneously target two multiple C2 domains and transmembrane region protein genes (MCTP3 and MCTP4, respectively at3g57880 and at1g51570), using one or two sgRNA per gene. Key features • A simple method for Arabidopsis edited lines establishment using CRISPR-Cas9 technology • Versatile cloning strategy combining various technologies for convenient cloning (Gateway®, Golden Gate) • Multigene targeting with high efficiency.

Efficient scar-free knock-ins of several kilobases in plants by engineered CRISPR-Cas endonucleases

In plants and mammals, non-homologous end-joining is the dominant pathway to repair DNA double-strand breaks, making it challenging to generate knock-in events. In this study, we identified two groups of exonucleases from the herpes virus and the bacteriophage T7 families that conferred an up to 38-fold increase in homology-directed repair frequencies when fused to Cas9/Cas12a in a tobacco mosaic virus-based transient assay in Nicotiana benthamiana. We achieved precise and scar-free insertion of several kilobases of DNA both in transient and stable transformation systems. In Arabidopsis thaliana, fusion of Cas9 to a herpes virus family exonuclease led to 10-fold higher frequencies of knock-ins in the first generation of transformants. In addition, we demonstrated stable and heritable knock-ins in wheat in 1% of the primary transformants. Taken together, our results open perspectives for the routine production of heritable knock-in and gene replacement events in plants.