Department of Plant Pathology, Iowa State University, Ames, IA 50011, U.S.A.
The physiological mechanisms by which plants limit the growth of bacterial pathogens during gene-for-gene resistance are poorly understood. We characterized early events in the Arabidopsis thaliana--Pseudomonas syringae pathosystem to identify physiological changes for which the kinetics are consistent with bacterial growth restriction. Using a safranine-O dye solution to detect vascular activity, we demonstrated that A. thaliana Col-0 resistance to P. syringae pv. tomato DC3000 cells expressing avrRpm1 involved virtually complete cessation of vascular water movement into the infection site within only 3 h postinoculation (hpi), under the conditions tested. This vascular restriction preceded or was simultaneous with precipitous decreases in photosynthesis, stomatal conductance, and leaf transpiration, with the latter two remaining at detectable levels. Microscopic plant cell death was detected as early as 2 hpi. Interestingly, suppression of bacterial growth during AvrRpm1-mediated resistance was eliminated by physically blocking leaf water loss through the stomata without altering plant cell death and was nearly eliminated by incubating plants at high relative humidity. The majority of the population growth benefit from blocking leaf water loss occurred early after inoculation, i.e., between 4 and 8 hpi. Collectively, these results support a model in which A. thaliana suppresses P. syringae growth during gene-for-gene resistance, at least in part, by coupling restricted vascular flow to the infection site with water loss through partially open stomata; that is, the plants effectively starve the invading bacteria for water.
