Influence of root exudates on the extracellular proteome of the plant growth-promoting bacterium Bacillus amyloliquefaciens FZB42.

Proteins secreted by Bacillus amyloliquefaciens FZB42, a root-associated plant growth-promoting rhizobacterium, are thought to play an important role in the establishment of beneficial interactions with plants. To investigate the possible role of proteins in this process, extracellular proteome maps of B. amyloliquefaciens FZB42 during the late exponential and stationary growth phases were generated using 2D gel electrophoresis. Out of the 121 proteins identified by MALDI-TOF MS, 61 were predicted to contain secretion signals. A few of the others, bearing no signal peptide, have been described as elicitors of plant innate immunity, including flagellin proteins, cold-shock proteins and the elongation factor Tu, suggesting that B. amyloliquefaciens FZB42 protects plants against disease by eliciting innate immunity. Our reference maps were used to monitor bacterial responses to maize root exudates. Approximately 34 proteins were differentially secreted in response to root exudates during either the late exponential or stationary phase. These were mainly involved in nutrient utilization and transport. The protein with the highest fold change in the presence of maize root exudates during the late exponential growth phase was acetolactate synthase (AlsS), an enzyme involved in the synthesis of the volatile acetoin, known as an inducer of systemic resistance against plant pathogens and as a trigger of plant growth.