Bauer, Dietz#, Max Teplitski# and Jayne Robinson*

Horticulture & Crop Science, Ohio State Univ#. and Biology Department, U. Dayton.* contact bauer.7@osu.edu

N-acyl homoserine lactones (AHLs) are important signal molecules in Gram negative bacteria, regulating the cell-density dependent expression of diverse genetic pathways, particularly those involved in association with eukaryotic hosts. Our studies show that plant seedling exudates contain substances that mimic AHLs in stimulating some AHL regulated behaviors while inhibiting others. Such substances could have major effects on bacterial colonization of plants in general as well as specific pathogenic and symbiotic plant-bacteria interactions. Giskov et al (J. Bacteriol. 178:6618-6622) isolated a mutant of Serratia liquefaciens, MG44, that is defective in synthesis of AHLs. They used this mutant, and a Tn5 swrAluxAB derivative of it, PL10, to demonstrate that halogenated furanones produced by a seaweed inhibited the ability of added AHLs to stimulate bacterial surface swarming and synthesis of the surfactant, serrawettin. We found that methanol extracts of lyophilized pea root exudates stimulated both surface swarming of MG44 and bioluminescence of the luxAB serawettin reporter PL10. Similarly, McClean et al. (1997) isolated a mutant of Chromobacterium violaceum, CV206, that is defective in AHL synthesis. We used this reporter strain to show that methanol extracts of lyophilized pea root exudate inhbited AHL-induced synthesis of the purple pigment, violacein, and synthesis of extracellular protease and chitinases. The nature of the active compounds in pea root exudates is unknown. Their insolubility in ethyl acetate indicates that the plant compounds are not typical AHLs. Reverse phase HPLC resulted in the separation of two retained fractions active in the Serratia bioassays. The void peak contained activity in both theSerratia and Chromobacterium bioassays. Young seedlings of various plant species were directly assayed for activity on agar plates. When roots of pea, corn, soybean and Medicago truncatula,were inoculated with MG44 or PL10, then rinsed and placed on an agar surface, the bacteria were induced to swarm out over the nutrient agar or luminesce along the length of the root, whereas toothpicks inoculated in the same way induced no swarming or luminescence. No plants tested were able to induce violacein synthesis in Chromobacterium, but roots of pea and crown vetch strongly inhbited violacein production. These results suggest that various plant species may have complex, co evolved abilities to interfere with normal AHL signaling in associated bacteria.