Plant volatiles can minimize the growth suppression of epiphytic bacteria by the phytopathogenic fungus Botrytis cinerea in co-culture experiments

Leaf surfaces of plants are colonized by microbes, although the ecological roles of most of these epiphytes are unknown. Eleven non-pathogenic bacteria were isolated from strawberry (Fragaria x ananassa) plants and tested for their ability to interact with plant volatiles and the phytopathogenic fungus Botrytis cinerea. None of the bacterial epiphytes produced antimicrobial compounds. Light microscopic and SEM analysis of F. x ananassa leaf surfaces showed that capitate glands are densely colonized by microorganisms. Benzyl alcohol, 2-phenylethanol, R,S-linalool and nonanal were identified as major volatiles emitted by intact strawberry leaves, while R,S-linalool and nonanal were released by the capitate glands. The isolated epiphytes cannot utilize these leaf volatiles as sole carbon source, but some of the bacteria metabolize them, e.g. to the corresponding acids. However, the leaf volatiles have a stronger inhibitory effect on different strains of the plant pathogenic fungus B. cinerea than on the isolated epiphytic bacteria. In co-culture experiments, B. cinerea strains suppress the proliferation of epiphytes but low concentrations of 1-5 ppm of R,S-linalool, 2-phenylethanol and in particular nonanal significantly enhance the progeny of a number of epiphytic bacteria, while the growth of B. cinerea strains is retarded. Thus, native volatile compounds can affect population dynamics of epiphytes and the phytopathogenic fungus. Our findings have significant implications for pest management notably on the use of antagonistic bacteria as biocontrol agents.