Accumulation of transcripts of defense enzymes involved during rhizobacteria mediated induced resistance against pearl millet downy mildew disease

Abstract

Plant growth promoting rhizobacteria strain Bacillus pumilus INR 7 induced systemic and durable resistance against downy mildew disease of pearl millet caused by the oomycete Sclerospora graminicola. The underlying mechanism of this induced resistance was studied by analyzing the pattern of transcript accumulation of some of the important defense enzymes in the B. pumilus INR 7 treated seedlings in comparison to the downy mildew resistant and susceptible seedlings. Accumulation of mRNA for defense enzymes like glucanase, chitinase, phenylalanine ammonia lyase, peroxidase and polyphenol oxidase, gene was measured in response to B. pumilus INR 7 treatments and the same was compared with the resistant and susceptible seedlings. The time course of enzyme expression correlated with the earlier report that, in an incompatible interaction, it is the speed and intensity of the defense reaction that determine the resistance to the pathogen. For most of the enzymes examined, the highest levels of transcripts were observed in resistant seedlings followed by induced resistant seedlings in comparison with the susceptible seedlings. For each enzyme, very low levels of mRNA were present in susceptible seedlings.  Inducer treatment caused an increase in transcript accumulation, but generally not to the same degree as resistant seedlings. PAL and PPO transcripts were expressed in highest levels in the induced resistant seedlings implicating a major role in B. pumilus induced resistance against pearl millet downy mildew disease.