Small RNAs regulate plant responses to filamentous pathogens

• Small RNAs regulate plant immunity against filamentous pathogens.
• Pathogen-responsive small RNAs enhance resistance by facilitating the transition from growth to defense.
• PhasiRNAs directly target defense-related genes including NB-LRRs.
• Filamentous pathogens have evolved effectors to disrupt small RNA-based plant defense.

Small RNAs are central players of RNA silencing in eukaryotes. These short RNA molecules (20–25 nucleotides in length) repress target gene expression based on sequence complementarity. While small RNAs are well-known for their essential function in regulating growth and development, recent research has revealed that they also influence plant immunity. Extensive changes in small RNA accumulation have been observed during infection. This review focuses on specific small RNA changes that are involved in plant responses to filamentous eukaryotic pathogens including fungi and oomycetes. We describe how changes in small RNA accumulation influence plant immunity and summarize the cellular processes affected by these small RNAs. In particular, we discuss secondary small interfering RNAs that directly modulate the expression of defense-related genes.