Bacterial Immobilization and Toxicity Induced by a Bean Plant Immune System

Pseudomonas savastanoi pv. phaseolicola causes halo blight disease in the common
bean Phaseolus vulgaris. The bacterium
invades the leaf apoplast and uses a type III secretion system to
inject effector proteins into a bean cell to interfere with the bean
immune system. Beans counter with resistance proteins that can detect
effectors and coordinate effector-triggered immunity responses transduced
by salicylic acid, the primary defense hormone. Effector-triggered
immunity halts bacterial spread, but its direct effect on the bacterium
is not known. In this study, mass spectrometry of bacterial infections
from immune and susceptible beans revealed that immune beans inhibited
the accumulation of bacterial proteins required for virulence, secretion,
motility, chemotaxis, quorum sensing, and alginate production. Sets
of genes encoding these proteins appeared to function in operons,
which implies that immunity altered the coregulated genes in the bacterium.
Immunity also reduced amounts of bacterial methylglyoxal detoxification
enzymes and their transcripts. Treatment of bacteria with salicylic
acid, the plant hormone produced during immunity, reduced bacterial
growth, decreased gene expression for methylglyoxal detoxification
enzymes, and increased bacterial methylglyoxal concentrations in vitro. Increased methylglyoxal concentrations reduced
bacterial reproduction. These findings support the hypothesis that
plant immunity involves the chemical induction of adverse changes
to the bacterial proteome to reduce pathogenicity and to cause bacterial
self-toxicity.