Evidence for dose-dependent effects on plant growth by Stenotrophomonas strains from different origins.

AIMS To assess the influence of Stenotrophomonas on plants, the interaction of 16 Stenotrophomonas strains from clinical and environmental sources with strawberry plant seedlings was analysed. METHODS AND RESULTS In vitro, all Stenotrophomonas strains influenced plant growth when applied to seedlings. Whereas most of the Stenotrophomonas strains promoted root growth and hair development, a statistically significantly negative influence on the length of stem was found. Although strains from a clinical origin also showed statistically significant effects on plants, this was generally lower when compared with environmental strains. For three selected strains, a strong dose-dependent effect was observed for all parameters. In vitro, a correlation was found between plant growth promotion and production of a plant growth hormone, indole-3-acetic acid (IAA). Xanthomonas campestris, a phylogenetically very closely related species to Stenotrophomonas, was used as a phytopathogenic control. It too confirmed the reduction of plant growth in this in vitro system. CONCLUSIONS Independent of their origin, Stenotrophomonas strains can produce IAA in vitro and subsequently, influence plant growth. The effect of Stenotrophomonas presence on plants was dose-dependent. SIGNIFICANCE AND IMPACT OF THE STUDY The dose-dependent effect of Stenotrophomonas, a bacterium of both biotechnological and medical interest, is of great interest for biocontrol applications of plant-associated strains. This paper is the first report that clearly demonstrates the phytopathogenic capacity of Stenotrophomonas.