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.