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Identification of Novel Type III Secretion Effectors in Xanthomonas oryzae pv. oryzae

¹National Institute of Agrobiological Sciences, Tsukuba, Japan; ²Laboratory of Molecular Plant Pathology, Faculty of Life and Environmental Sciences, Prefectural University of Hiroshima, Shobara, Japan; ³Laboratory of Plant Pathology, Graduate School of Agriculture, Kyoto Prefectural University, Kyoto, Japan; ⁴Faculty of Agriculture, Miyazaki University, Miyazaki, Japan

Many gram-negative bacteria secrete so-called effector proteins via a type III secretion (T3S) system. Through genome screening for genes encoding potential T3S effectors, 60 candidates were selected from rice pathogen Xanthomonas oryzae pv. oryzae MAFF311018 using these criteria: i) homologs of known T3S effectors in plant-pathogenic bacteria, ii) genes with expression regulated by hrp regulatory protein HrpX, or iii) proteins with N-terminal amino acid patterns associated with T3S substrates of Pseudomonas syringae. Of effector candidates tested with the Bordetella pertussis calmodulin-dependent adenylate cyclase reporter for translocation into plant cells, 16 proteins were translocated in a T3S system-dependent manner. Of these 16 proteins, nine were homologs of known effectors in other plant-pathogenic bacteria and seven were not. Most of the effectors were widely conserved in Xanthomonas spp.; however, some were specific to X. oryzae. Interestingly, all these effectors were expressed in an HrpX-dependent manner, suggesting coregulation of effectors and the T3S system. In X. campestris pv. vesicatoria, HpaB and HpaC (HpaP in X. oryzae pv. oryzae) have a central role in recruiting T3S substrates to the secretion apparatus. Secretion of all but one effector was reduced in both HpaB^-- and HpaP^-- mutant strains, indicating that HpaB and HpaP are widely involved in efficient secretion of the effectors.