Roles Of Xanthomonas Oryzae PV. Oryzae Secreted Proteins XopP/Q And Hpa1in Plant Pathogenesis And Defense Responses

Xanthomonas outer proteins (Xop) were primarily defined as extracellular proteins secreted by type III secretion system (T3SS) in Xanthomonas oryzae pathovars oryzae (Xoo) and oryzicola (Xoc). Subsequently, Xop proteins were found to be conserved in several genera of plant pathogenic bacteria, including Erwinia, Ralstonia, Rhizomonas, Pseudomonas in addition to Xanthomonas. Of about30Xop proteins, XopP and XopQ have not been studied in regard to their contributions to pathogenicity on rice and the induction of hypersensitive response (HR) in the no-host plant tobacco. To infer their functional basis, bioinformatics analysis reveals that XopQxoo is homologous to HopQ1-1in P. syringae pv. tomato and XopQ1Xoc in Xoc. The Pseudomonas effector HopQ1promotes bacterial virulence and interacts with tomato14-3-3proteins in a phosphorylation dependent manner. XopQ1Xoc may contribute to enhanced virulence to the pathogen in rice by suppressing an unknown target in plant immune system, while XopP has been reported to be translocated into plant cells through T3SS with AvrBs2as reporting assay. Herein, we mutated XopPxoo and XopQxoo, trying to unravel their virulence functions and recognition mechanisms on rice. And we found that mutations of these two genes cause no remarkable difference in virulence in IR24, but each of them can lead to significant virulence changes in certain rice species. Besides, both mutants are compromised in the ability to induce HR symptoms on Nicotiana benthamiana, suggesting that when injected into N. benthamiana cells, they are recognized by the immune system and prevents infection.Haipin proteins secreted from T3SS have been proven to activate the plant defense pathway, which includes hydrogen peroxide (H2O2) transduction signal generated in the apoplast. Neverthless, how harpins are recognized in that pathway and what kind of roles the apoplast H2O2play is still there to uncover. Xanthomonas oryzae pv. oryzae can produce a harpin protein called Hpal which is related to plant defense. In this study, we investigated whether the cellular localization of Hpa1Xoo can affect the H2O2production and pathogen resistance in Arabidopsis thaliana. We generated hpa1Xoo-and shpa1Xoo-expressing transgenic lines in A. thaliana (HETAt and SHETAt) by transforming hpa1Xoo and hpa1Xoo with apoplastic localization signal(shpa1Xoo).Hpa1xoo localized mainly in apoplast in SHETAt plant while in cytoplasm in HETAt plant. What’s more, plant treated with Hpa1Xoo cause H2O2accumulation in apoplast and cytoplasm in SHETAt and only cytoplasm in HETAt. Apoplastic H2O2generation relies on NADPH oxidase (NOX). In SHETAt plants, H2O2was produced in a NOX-dependent manner and accumulated more in cytoplasm than apoplast. In wild type and SHETAt plants, inhibiting apoplast generation reduced both cytoplasmic accumulation and plant resistance to pathogens.