Effect of environmental factors on the interaction plant-pathogen-Bacillus amyloliquefaciens S499

As living organisms, growth and antagonistic activity of biological control agents depend on environmental conditions. Unfortunately, unsatisfactory or inconsistent disease biocontrol has frequently been reported under field conditions. Although relevant, the effect of environment factors on some biocontrol mechanisms is far from being full understood. Temperatures and water stress are generally considered the most detrimental factors for survival and activity of microbial biocontrol agents. Bacillus amyloliquefaciens S499 (S499) can readily colonize the rhizosphere of many plants and induce systemic resistance (ISR) against several pathogens. While the role of surfactin production by S499 in ISR is clearly demonstrated, the influence of environmental factors on this biocontrol mechanism is still unclear. The aims of our research were to understand the effect of temperature and water stress on systemic resistance induced by S499. We evaluated the effect of temperature on S499 growth and lipopeptide production and exposure of plants to temperature and water stress prior and after S499 inoculation on the level of ISR. Under laboratory conditions, a marked influence of temperature on growth and production of lipopeptides by strain S499 was observed. Final cell densities were globally similar in cultures performed at 15, 20, 30, 40 and 48°C, but growth at low temperatures (15 and 20°C) was characterized by a much longer lag-phase, probably necessary for cell adaptation. Higher concentrations of surfactin were also measured after growth at 15 and 20°C compared to 30°C and the lipopeptide production is clearly impaired at higher temperatures. The effect of temperature (15, 25 and 35°C) and water stress on ISR, was evaluated on zucchini, bean and tomato inoculated with the pathogens (Podosphaera xanthii, Botrytis cinerea, and Phytophthora infestans on zucchini, bean and tomato, respectively). A short exposure of plants to temperature and water stress reduced the level of ISR on plants and influenced at different extent the root colonization on the three crops. Even if exposure to temperature and water stress generally reduced the level of ISR on the three crops, the highest reduction was found on zucchini. This is the first study of an interaction between plants, bacteria and ISR under temperature and water stress conditions.