W. Dietz Bauer*, Max Teplitski*, Vaidehi Deshmukh* and Jayne B. Robinson#

*Horticulture & Crop Science, Ohio State University and

# Biology Department, University of Dayton

Bacterial signaling. In bacteria, many genes and behaviors are turned on only when there are enough bacterial cells present in a given location to efficiently attack or colonize a plant or animal host. This population density-dependent regulation of genes, called "quorum sensing", works by the synthesis and perception of small, diffusible signal molecules that move in and out through the bacterial cell membrane and between bacterial cells. When there are lots of bacterial cells present, the concentration of these small signal compounds is high enough to activate specific receptor proteins inside the bacteria and initiate the expression of a variety of genes in the bacteria. In Gram negative bacteria, the most important of the small, diffusible signals that mediate quorum sensing are compounds known as N-acyl homoserine lactones (AHLs). Many plant-associated bacterial species have been found to make AHLs, and a recent study has shown that over 250 different genes are regulated by AHL quorum sensing Pseudomonas aeruginosa. Thus AHL signaling is both common among relevant bacteria and affects many aspects of their growth, survival and virulence.

AHL signal-mimic compounds from plants. We recently discovered that various higher plant species have the ability to affect the behavior of associated bacteria by secreting compounds that mimic the bacterium's own AHL signal substances (Teplitski et al. 2000). The signal-mimic compounds from plants therefore have the potential to manipulate bacterial pathogens and symbionts in useful ways - to reduce disease or enhance symbiotic benefits. They may also prove to be of medical value.

The AHL signal-mimic compounds were detected by exposing seedlings or seedling rinsates to special reporter bacteria constructed by other labs to respond to exogenously added AHLs. Soybean, pea, crown vetch, barrel medic, tomato, and rice each activated several different AHL reporter strains. Lettuce and Arabidopsis seedlings, however, showed no AHL-like activity. Tests indicated that the plant compounds did not behave chemically like bacterial AHLs, even though the reporter bacteria responded to them as though they were AHLs. Thus, the plant compounds appear to be AHL signal-mimics.

Attempts to purify and identify the plant AHL signal-mimic compounds are in progress. Pea appears to synthesize and secrete at least 10 separate mimic compounds. We have preliminary evidence to indicate that exposure of seedlings to bacteria induces the synthesis and/or secretion of specific mimic compounds.

Teplitski, M., J.B. Robinson and W.D. Bauer. 2000. Plants secrete substances that mimic bacterial N-acyl homoserine lactone signal activities and disrupt cell density dependent behaviors in associated bacteria. Mol. Plant-Microbe Interact. 13:637-648.