Composition of the phyllospheric microbial populations on vegetable plants with different glucosinolate and carotenoid compositions.

The plant phyllosphere is intensely colonized by a complex and highly diverse microbial population and shows pronounced plant-species-specific differences. The mechanisms and influencing factors determining whether and in which density microorganisms colonize plant phyllosphere tissues are not yet fully understood. One of the key influencing factors is thought to be phytochemical concentration and composition. Therefore, correlations between various concentrations of individual glucosinolates and carotenoids in four different plant species-Brassica juncea, Brassica campestris, Cichorium endivia, and Spinacea oleracea-and the phyllospheric bacterial population size associated with the aerial parts of the same plants were analyzed. The concentration of various individual glucosinolates and carotenoids were measured using high-performance liquid chromatography. The phyllospheric bacterial population size including both nonculturable and culturable organisms was assessed using quantitative real-time polymerase chain reaction, and the physiological profile of the culturable microbial community was analyzed using the Biolog system. Results show significant differences between plant species in both concentration and composition of secondary metabolites, bacterial population size, and microbial community composition in three consecutively performed experiments. An interesting and underlying trend was that bacterial density was positively correlated to concentrations of beta-carotene in the plant phyllosphere of the four plant species examined. Likewise, the alkenyl glucosinolates, 2-propenyl, 3-butenyl, and 4-pentenyl, concentrations were positively correlated to the bacterial population density, whereas the aromatic glucosinolate 2-phenylethyl showed a negative correlation to the phyllospheric bacterial population size. Thus, we report for the first time the relationship between individual glucosinolate and carotenoid concentrations and the phyllospheric bacterial population size of nonculturable and culturable organisms and the phyllospheric microbial physiological profiles.