Bacterial Light - Pseudomonas

Collaborator: Jacyn Baker
USDA-ARS
Molecular and Plant Pathogen Interactions Laboratory

Bacterial blight caused by Pseudomonas savastanoi pv. glycinea occurs worldwide and is the most common bacterial disease of soybean.  Losses in the United States have been reported as high as 40 % in areas where susceptible varieties are used; losses are generally lower where resistant or tolerant cultivars are used.  The pathogen over-winters in crop residue and seeds and during the season exists epiphytically on leaf surfaces and buds.  When cool conditions with wind blown rain exist, the bacteria enter tissue through wounds and natural openings.  Within 5 – 7 days yellow and brown water-soaked lesions occur on leaves as well as stems and pods.  Young seedlings are generally stunted or die, while older plants will defoliate and often produce infected seeds. 

In the pathogen, at least three genes  - avrA, avrB, and avrC – determine race phenotype and account for resistance in soybean cultivars carrying the complementary resistance genes.  Four resistance genes have been identified in soybean, Rpg1, Rpg2, Rpg3, and Rpg4. Strains of the pathogen carrying avrB gene elicit a hypersensitive response (HR) in cultivars carrying the resistance gene Rpg1, while strains carrying avrA elicit HR in cultivars carrying Rpg2.  Thus, this gene-for-gene specificity can provide an excellent model for studying and differentiating both the unique mechanisms of specific race and cultivar recognition as well as the recognition mechanisms common to general resistance.

Biochemical and genetic evidence suggests that the mechanics of this gene-for- gene specificity entails the recognition of a pathogen elicitor (avirulence gene product) being recognized by a plant receptor (resistance gene product).  Following this initial recognition, multiple pathways are likely involved in the cascade of events triggering resistance.  There is biochemical and molecular evidence that an oxidative burst occurs upon detection of stress in plants and that it in part resembles that in mammalian neutrophils.  One of the central components of the oxidative burst is the NADPH oxidase that produces superoxide.  This enzyme and associated components have been reported in plants.  Cloning of mRNA induced in soybean suspension cells (carrying Rpg2) during HR after inoculation with P. savastanoi pv. glycinea (expressing avrA) indicated that several related pathways appeared to be turned on including the phenolic pathway and the pentose phosphate shunt.  Products of these pathways are critical to phytoalexin production, lignification, cell wall fortification, and NADPH production.