Plant Immunity eXtinction by bacterial pathogen-InducEd host Small RNAs – PIXIES

Eukaryotic Small RNAs (sRNAs) have emerged as fundamental regulatory players in gene expression of many biological functions including development, growth, responses to abiotic and biotic stresses. In the cell, sRNAs typically range from 20-24 nucleotides in length, originate from double stranded RNA precursors and are processed into sRNA duplexes by Dicer-like enzymes. sRNAs associate with RNA-induced gene silencing complexes and target complementary nucleotides to reduce gene expression. The nature of sRNA biogenesis and activity remains heavily underexplored in model systems of important crops such as rice during biotic attack. Rice (Oryza sativa) is a major crop in global agriculture and stands as an economic and nutritional cornerstone in the developing world. Global production of rice is limited by plant diseases caused by pathogens such as viruses, fungi, nematodes and bacteria. In particular, the bacterium Xanthomonas oryzae causes foliar diseases of rice and is one of the three most important pathogens of rice worldwide. The PIXIES proposal aims at investigating an unsuspected bacterial pathogen virulence strategy that presumably relies on RNA silencing of host immunity. High-throughput sRNA sequencing data mining has identified several sRNAs that strongly accumulate in rice leaves infected with virulent strains of X. oryzea. These Xanthomonas-induced small RNAs (xisRNAs) have features of regulatory sRNAs and are predicted to target related genes involved in plant immune signaling. Several experimental approaches will be pursued in order to elucidate the bacterial factors and the plant sRNA biogenesis pathways responsible for xisRNA synthesis. The nature of the target genes and the mode of action of xisRNA at the molecular level will also be examined. Finally, the biological relevance of xisRNA targets with respect to establishing plant susceptibility will be addressed. Understanding these fundamental biological processes will undoubtedly lead to the discovery of unconventional molecular mechanisms underlying the production of host genome-derived sRNA and will be important to design ways to control disease in a strategic crop.