Microscopy of the interaction of hrp mutants of Pseudomonas syringae pv. phaseolicola with a nonhost plant

Abstract

Pseudomonas syringae pv. phaseolicola is a pathogen of green bean and induces localized, rapid plant cell death (the hypersensitive response or HR) when placed into leaves of the nonhost plant tobacco. The hrp genes control both pathogenicity and the ability to cause the HR. In this study we examined the interaction of the wild-type parent strain (NPS3121) and three independent, prototrophic hrp mutants (NPS4000, NPS4003, NPS4005) with tobacco by use of light and transmission electron microscopy. Even though the hrp mutants did not induce macroscopic symptoms on leaves by 48 h postinoculation, localized disruption of parenchyma cells did occur. The rapidity and severity of microscopic symptoms was related to the relative abilities of the mutants to cause macroscopic symptoms on green bean. Cells of the hrp mutants exhibited extensive surface blebbing. The increased surface blebbing of the mutants may reflect either outer membrane disorganization and/or a reduced ability to cope with an hypoosmotic apoplastic environment by production of periplasmic glucans. Unlike induction of a macroscopic HR, induction of more subtle responses by nonhost plant cells is not dependent on the presence of an intact hrp cluster.