Chitinase mRNA and enzyme activity in Phaseolus vulgaris (L.) increase more rapidly in response to avirulent than to virulent cells of Pseudomonas syringae pv. phaseolicola

Abstract

Changes in chitinase mRNA concentration in leaves of French bean cultivar Red Mexican, after inoculation with virulent or avirulent races of Pseudomonas syringae pv. phaseolicola, were estimated by probing Northern and dot blots with radiolabelled cDNA for bean chitinase. In leaves inoculated with the avirulent race 1 isolate of the pathogen, chitinase mRNA levels had begun to increase by 6 h after inoculation. In contrast, chitinase mRNA levels first began to increase between 20 to 24 h after inoculation of leaves with a virulent race 3 isolate of the pathogen. Chitinase mRNA activity was estimated by immunoprecipitation of in vitro translation products using antiserum raised against bean chitinase. Changes in chitinase mRNA activity reflected closely, changes in mRNA concentration. Increases in extractable chitinase enzyme activity occurred 3–6 h after increases in mRNA concentration and activity. As far as we are aware this is the first report of the differential induction of chitinase mRNA and enzyme activity in a host plant by virulent and avirulent races of a homologous bacterial plant pathogen. Heat-killed and UV-killed cells of both avirulent and virulent races of P.s. pv. phaseolicola induced chitinase enzyme activity. This shows that chitinase is induced separately from the hypersensitive reaction and phytoalexin synthesis; both of which are induced only by metabolically active, living bacteria.