Our objectives were to identify substances produced by plant roots that might act as nutritional mediators of specific plant-bacterium relationships and to delineate the bacterial genes responsible for catabolizing these substances. We discovered new compounds, which we call calystegins, that have the characteristics of nutritional mediators. They were detected in only 3 of 105 species of higher plants examined: Calystegia sepium, Convolvulus arvensis (both of the Convolvulaceae family), and Atropa belladonna. Calystegins are abundant in organs in contact with the rhizosphere and are not found, or are observed only in small quantities, in aerial plant parts. Just as the synthesis of calystegins is infrequent in the plant kingdom, their catabolism is rare among rhizosphere bacteria that associate with plants and influence their growth. Of 42 such bacteria tested, only ohe (Rhizobium meliloti 41) was able to catabolize calystegins and use them as a sole source of carbon and nitrogen. The calystegin catabolism gene(s) (cac) in this strain is located on a self-transmissible plasmid (pRme4la), which is not essential to nitrogen-fixing symbiosis with legumes. We suggest that under natural conditions calystegins provide an exclusive carbon and nitrogen source to rhizosphere bacteria which are able to catabolize these compounds. Calystegins (and the corresponding microbial catabolic genes) might be used to analyze and possibly modify rhizosphere ecology.The factors regulating microbial growth in the rhizosphere are not completely understood but are thought to include substances released by roots (6, 33, 34) that could be species specific and act on microbial populations through negative or positive selection. In the latter case, these secondary metabolites could be responsible for nutritional selection, if they were produced in sufficient quantities and if they were refractory to catabolism by most soil microorganisms. Such substances would then provide an exclusive nutrient source to microorganisms possessing the genetic information necessary for their catabolism (26). The opines encoded by the Ti (tumor-inducing) and Ri (root-inducing) T-DNAs of Agrobacterium tumefaciens and Agrobacterium rhizogenes (see reference 40 for a review) are selective nutrients for a variety of soil bacteria under laboratory conditions (5, 32, 41), but their ability to influence bacterial growth under natural conditions has not been reported. The use of these opines as selective agents in the rhizosphere, by inserting opine synthesis genes into the plant host, would run the risk of encouraging the growth of wild opine-utilizing, pathogenic agrobacteria.We describe here the bacterial catabolism of substances produced by two species of the Convolvulaceae family and by Atropa belladonna. The synthesis of these compounds, which we call calystegins, is uncommon in the plant kingdom, as is their utilization in a sample of 42 rhizosphere bacteria, most of which were chosen for their ability to interact with plants. Under laboratory conditions these...

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