Mode of interaction of Stenotrophomonas rhizophila

Salinated soils become a more and more ecological and economical relevant problem world-wide. In former studies we showed that Stenotrophomonas rhizophila P69 (DSM 14405) is able to promote plant growth and to protect plants against soil-borne pathogens in salinated soils. The mode of interaction is only partly understood, especially under salt stress. Preliminary results show a strong influence of salt on the antagonistic and plant growth promoting activity. For survival in salted habitats, the synthesis of osmoprotective substances is important. For S. rhizophila synthesis of the osmolytes trehalose and glucosylglycerol was demonstrated. In the proposed project we aim to investigate the interactions between S. rhizophila with plants and the influence of salt stress and osmolyte synthesis. Further studies focus on the interactions between S. rhizophila and the phytopathogenic fungus Verticillium. For these experiments molecular techniques in combination with confocal laser scanning microscopy will be applied. The knowledge gained in this project will result in novel and environmental friendly and efficient strategies to control pathogens in salinated soils.

The bacterial genus Stenotrophomonas includes species with entirely different habitats and host interactions. While clinical strains of S. maltophilia are opportunistic human pathogens, S. rhizophila includes significant biocontrol and plant growth promoting agents. One objective of our project was to characterize the interaction of S. rhizophila DSM14405, a biocontrol strain that protects and promotes plants in salinated soil, with its plant hosts, soil-borne pathogenic fungi, and the a-biotic environment. We discovered that the beneficial plant interaction is controlled by the DSF (Diffusible Signal Factor) quorum sensing system which senses the amount of DSF molecules corresponding with the abundance of cells.The DSFcontrolled effects on the host plant relied upon the regulation of a significant number of physiologically crucial genes and were evident in seed germination, plant growth promotion, and plant tissue colonization.In addition, the genome of the plant growth promoting and biocontrol strain S. rhizophila DSM14405T was characterized in regards to both general and specific characteristics as well as biological functions. Moreover, entire genome comparisons between the plant-associated beneficial S. maltophilia R551-3 model strain and the clinical S. maltophilia K279a were performed and revealed a high degree of similarity among all three strains. The substantial genome-wide similarity found between S. rhizophila and the clinical S. maltophilia K279a is intriguing, as these species have entirely different habitats, lifestyles and economic significance. Nevertheless, a great number of physiologically essential genes were specific to S. rhizophila DSM14405T, and play a crucial role in enabling S. rhizophila to function as a distinguished plant growth promoting and biocontrol agent as revealed by transcriptomic studies. Furthermore, the impact of salt stress on S. rhizophila, which is known to be highly resistant towards salinity, was also studied using transcriptomics. The production and excretion of glucosylglycerol (GG) was found as the main salient substance responsible for salt stress protection. In addition, for S. rhizophila treated with root exudates, the shift from a planktonic to sessile lifestyle was measured through the expression of downregulatedflagellar-driven motility. Spermidine, a plant growth regulator and general stress protector, protector, was also newly identified in plant-microbe interaction. Overall, this work significantly contributed to achieving a better understanding of the mechanisms behind biological control, the plant growth promoting effect, as well as the stress tolerance of Stenotrophomonas strains including the regulatory role of the DSF quorum sensing system. In addition, our study can be used as a scientific basis for the safe and efficient use of S. rhizophila DSM14405T as a biological stress protecting agent for plants.