With an autotrophic, N-flee medium, Xanthobacter populations were isolated from the roots of wetland rice grown under field conditions. Xanthobacter populations ranged from 3.2 x 10 4 to 5.1 × 10 5 colony-forming units (cfu) g-~ of root and averaged 47-fold higher on the root or rhizoplane than in the neighbouring nonrhizosphere. Characterization studies indicated dissimilarities in carbon utilization and motility among the isolated Xanthobacter strains and other recognized Xanthobacter species. Under gnotobiotic conditions, the population of one isolate, Xanthobacter sp. JW-KR1, increased from 105 to 107 cfu plant-l 1 d after inoculation when a rice plant was present, but declined to numbers below the limit of detection (<104cfu assembly -l) after 3d in the absence of a plant. Scanning electron microscopy revealed Xanthobacter as pleomorphic forms on the rhizoplane. To assess the effect of Xanthobacter on plant growth, rice plants were grown under greenhouse conditions in plant assemblies containing sand and half-strength Hoagland's nutrient solution with and without nitrogen. Plants were either inoculated with 105 cfu Xanthobacter g-t of sand or left uninoculated. After 40 d, plants without nitrogen showed no significant differences in top or root dry weight, plant height, root length, or number of tillers or leaves, whether the plants were inoculated or uninoculated. However, when nitrogen was added, inoculated plants had a significantly larger top dry weight (15%) and number of leaves (19%) than uninoculated plants. Under conditions of added and no added nitrogen, acetylene reduction assays showed Xanthobacter sp. JW-KR1 produced <0.1 (below detection limit) and 7 nmol C2H 4 plant-~ h-1, respectively. Under the conditions studied, the results suggest that both Xanthobacter and wetland rice derive some benefits from their association.

Xiznthobucterfluvus 301T (T = type strain) and other strains, including H4-14, both of which were previously described as nonmotile, were reproducibly motile and peritrichously flagellated during the log phase when they were cultured in medium lacking tricarboxylic acid cycle intermediates. Therefore, the species description is emended to include motility and flagellation. Similarly, Xunthobucter uutotrophicus was found to be flagellated and motile, but this finding was not consistently reproducible and was mainly found with the mutant strain Fe-".Currently, the genus Xanthobacter (10) consists of three species, Xanthobacter autotrophicus (the type species), Xanthobacter flavus, and Xanthobacter agilis. Strains of X . auiotrophicus (11) and X. flavus (3, 5 ) have been described as being nonmotile, whereas strains of X. agilis have been described as being motile (2). X. flavus is the only biotin-tequiring species. Typically, new Xanthobacter isolhtes are classified tentatively into one of the three Xanthobacter species on the basis of motility and biotin requirement (12). Recently, Reding et al. reported that Xanthobacter strains isolated from the roots of rice were motile but differed morphologically and physiologically from strains of X. agilis (7, 8). Meijer et al. have described a Xanthobacter strain, strain 25a, that is motile and requires biotin (6).During routine growth experiments, we observed that X. flavus was motile under certain culture conditions. Motility depended on the growth phase and the carbon source (Table 1). In general, growth in the presence of tricarboxylic acid cycle intermediates produced nonmotile cells, whereas growth on alcohols or transfer of washed cells to medium lacking tricarboxylic acid cycle intermediates resulted in motile cells. Growth on H,-CO,, glutamate, or glutamine also produced nonmotile cells. However, we emphasize the effect of tricarboxylic acid cycle intermediates because agilis. This characteristic has been used to differentiate Xanthobacter species (10). Cells of X. flavus produce large amounts of poly-P-hydroxybutyrate as a carbon reserve (lo), and we assume that this compound serves as a carbon and energy source for flagellum synthesis and motility when cells are incubated in a medium lacking usable carbon. The nitrogen source used (NH,, NO3, or N2) had no effect on motility.When cultures were grown with 0.1% n-propanol as the

When Bradyrhizobiumjaponicum 1-110 was transferred into medium containing 40 mM succinate or 40 mM fumarate, over 90% of the bacteria acquired a swollen, pleomorphic form similar to that of bacteroids. The induction of pleomorphism was dependent on the carbon substrate and concentration but was independent of the hydrogen ion and sodium ion concentration. Cell extracts of rod-shaped and pleomorphic cells contained enzymes required for sugar catabolism and gluconeogenesis. Variations in these enzyme profiles were correlated with the carbon source used and not with the conversion to the bacteroid-like morphology. Rod-shaped cells cultured on glucose or 10 mM succinate transported glucose and succinate; however, the pleomorphic cells behaved similarly to symbiotic bacteroids in that they lacked the ability to transport glucose and transported succinate at lower rates than did rod-shaped cells.

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