Endogenous small RNAs (miRNAs and siRNAs) regulate gene expression in diverse biological processes.Research with the Arabidopsis-Pseudomonas syringae system has shown that small RNAs contribute to plant immunity by regulating the expression of their target genes.Plant immunity can be triggered by pathogen-associated molecular pattes (PAMPs) or effector proteins that are delivered into the host cell by the pathogen.Experimental evidence indicates that the miRNA pathway play a major role in PAMP-triggered immunity while some of the siRNA pathways appear to be more important in effector-triggered immunity.In addition,some P.syringae effector proteins appear to inhibit miRNA biogenesis or function to enhance bacterial virulence.These exciting findings illustrate a new battle ground for plant-pathogen interactions.