The CRISPR/Cas revolution reaches the RNA world: Cas13, a new Swiss Army knife for plant biologists.

Application of the bacterial CRISPR/Cas systems to eukaryotes is revolutionizing biology. Cas9 and Cas12 (previously called Cpf1) are widely used as DNA nucleases for inducing site-specific DNA breaks for different kinds of genome engineering applications, and in their mutated forms as DNA-binding proteins to modify gene expression. Moreover, histone modifications, as well as cytosine methylation or base editing, were achieved with these systems in plants. Recently, with the discovery of the nuclease Cas13a (previously called C2c2), molecular biologists have obtained a system that enables sequence-specific cleavage of single-stranded RNA molecules. The latest experiments with this and also the alternative Cas13b system demonstrate that these proteins can be used in a similar manner in eukaryotes for RNA manipulation as Cas9 and Cas12 for DNA manipulations. The first application of Cas13a for post-transcriptional regulation of gene expression in plants has been reported. Recent results show that the system is also applicable for combating viral infection in plants. As single-stranded RNA viruses are by far the most abundant class of viruses in plants, the application of this system is of special promise for crops. More interesting applications are imminent for plant biologists, with nuclease dead versions of Cas13 enabling the ability to visualize RNA molecules in vivo, as well as to edit different kinds of RNA molecules at specific bases by deamination or to modify them by conjugation. Moreover, by combining DNA- and RNA-directed systems, the most complex of changes in plant metabolism might be achievable.