A double-stranded RNA binding protein, HYL1, regulates plant immunity via the jasmonic acid pathway

MicroRNA (miRNA) and small interference RNA (siRNA) regulate not only plant growth and development, but also various responses to stress, including biotic stress, through post-transcriptional gene regulation. Hyponastic leaves 1 (HYL1), a double-stranded RNA binding protein, plays an important role in miRNA and siRNA processing, along with Dicer-like 1 (DCL1) and Hua Enhancer 1 (HEN1). Of five double-stranded RNA binding protein (DRB) deficient mutants in Arabidopsis, only the hyl1 mutant compromised innate immune responses to Pseudomonas syringae pv. tomato (Pst) DC3000 hrcC (-) infection when compared to wild-type plants. This was similar to the response of the hen1 mutant. Furthermore, the hyl1 mutant was susceptible to Pst (avrRpt2) infections, and showed a delay in the hypersensitive response that should result from gene-for-gene interactions between RPS2 and AvrRpt2. These results suggest that HYL1 is a major contributor in the DRB family, influencing miRNA and siRNA biogenesis and therefore playing essential roles in PAMP- and effect-triggered immune responses in plants. DNA microarray analysis revealed that the expression of genes involved in jasmonic acid (JA) biosynthesis, signal transduction, and downstream responses was significantly upregulated in the hyl1 mutant compared with wild-type plants. Consistent with this, transgenic plants overexpressing HYL1 are hypersensitive to the necrotrophic pathogen Botrytis cinerea, of which resistance is mainly regulated by JA. These results elucidate the importance of small RNA-mediated JA regulation in plant immunity.