CRISPR-Cas nucleases and base editors for plant genome editing

Clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein (Cas) and base editors are fundamental tools in plant genome editing. Cas9 from Streptococcus pyogenes (SpCas9), recognizing an NGG protospacer adjacent motif (PAM), is a widely used nuclease for genome editing in living cells. Cas12a nucleases, targeting T-rich PAMs, have also been recently demonstrated in several plant species. Furthermore, multiple Cas9 and Cas12a engineered variants and orthologs, with different PAM recognition sites, editing efficiencies and fidelity, have been explored in plants. These RNA-guided sequence-specific nucleases (SSN) generate double-stranded breaks (DSBs) in DNA, which trigger non-homologous end-joining (NHEJ) repair or homology-directed repair (HDR), resulting in insertion and deletion (indel) mutations or precise gene replacement, respectively. Alternatively, genome editing can be achieved by base editors without introducing DSBs. So far, several base editors have been applied in plants to introduce C-to-T or A-to-G transitions, but they are still undergoing improvement in editing window size, targeting scope, off-target effects in DNA and RNA, product purity and overall activity. Here, we summarize recent progress on the application of Cas nucleases, engineered Cas variants and base editors in plants.