A perspective on cross-kingdom RNA interference in mutualistic symbioses

RNA interference (RNAi) is arguably one of the more versatile mechanisms in cell biology, facilitating the fine regulation of gene expression and protection against mobile genomic elements, whilst also constituting a key aspect of induced plant immunity. More recently, the use of this mechanism to regulate gene expression in heterospecific partners - cross-kingdom RNAi (ckRNAi) - has been shown to form a critical part of bidirectional interactions between hosts and endosymbionts, regulating the interplay between microbial infection mechanisms and host immunity. Here, we review the current understanding of ckRNAi as it relates to interactions between plants and their pathogenic and mutualistic endosymbionts, with particular emphasis on evidence in support of ckRNAi in the arbuscular mycorrhizal symbiosis.