Role of the Hrp type III protein secretion system in growth of Pseudomonassyringae pv. syringae B728a on host plants in the field

hrp genes are reportedly required for pathogenicity in Pseudomonas syringae pv, syringae (Pss) and other phytopathogenic bacterial species, A subset o f these genes encodes a type III secretion system through which virulence f actors are thought to be delivered to plant cells, In this study, we sought to better understand the role that hrp genes play in interactions of Pss w ith its host as they occur naturally under field conditions. Population siz es of hrp mutants with defects in genes that encode components of the Hrp s ecretion system (Delta hrcC::nptII and hrpJ:: Omega Spc) and a protein secr eted via the system (Delta hrpZ::nptII) were similar to B728a on germinatin g seeds. However, phyllosphere (i,e,, leaf) population sizes of the hrcC an d hrpJ secretion mutants, but not the hrpZ mutant, were significantly reduc ed relative to B728a, Thus, the Hrp type III secretion system, but not HrpZ , plays an important role in enabling Pss to flourish in the phyllosphere, but not the spermosphere, The hrcC and hrpJ mutants caused brown spot lesio ns on primary leaves at a low frequency when they were inoculated onto seed s at the time of planting. Pathogenic reactions also were found when the hr p secretion mutants were co-infiltrated into bean leaves with a non-lesion- forming gacS mutant of B728a, In both cases, the occurrence of disease was associated with elevated population sizes of the hrp secretion mutants. The role of the Hrp type III secretion system in pathogenicity appears to be l argely mediated by its requirement for growth of Pss in the phyllosphere, W ithout growth, disease does not occur.