Genomes of higher eukaryotes encode small regulatory RNAs that direct transcriptional and post-transcriptional gene silencing.Two major classes of small RNA occur in plants, microRNAs (miRNAs) and small interfering RNAs (siRNAs), with the major difference being their biogenesis pathways. Most small RNAs are generated through a series of maturation steps and incorporated into RNA-induced silencing complexes (RISC), which invariably contains a member of the Argonaute (AGO) protein family. The AGO protein forms the core effector component, while the small RNA functions and serves as a guide to direct RISC to target RNAs through base complementarity.The roles of small RNA during plant pathogenesis have recently come to light. In plants, inducible defence responses are triggered by the recognition of conserved microbial molecules referred to pathogen-associated molecular patterns (PAMPs), via pattern recognition receptors (PRRs). This layer of defence has been termed PAMP-triggered immunity (PTI), which in some cases can be suppressed by successful pathogens that secret effectors that suppress PTI, leading to susceptibility. However, some plant cultivars have evolved resistance proteins to recognise particular effectors, leading to an effector-triggered immunity (ETI).The mechanisms by which pathogens suppress the PTI are of special interest.Bacterial pathogens overcome PTI by delivering effector proteins through the Type III secretion system. In biotrophic and hemi-biotrophic pathogens, secretion of fungal effector proteins involves the formation of haustoria within living plant cells. This proposal will test the new hypothesis that small RNAs (or their precursors), as do effector proteins, can move and function across the plant-pathogen interface. While it is known that transfer of silencing signals occurs between interacting species in different kingdoms, this project will shed light on the functional roles of small RNA transfer during host-pathogen interactions.