Abstract
Crotalaria zanzibarica is an exotic and widely distributed leguminous plant in Taiwan. The relationship between C. zanzibarica and its rhizobial symbionts has been suggested to contribute to its successful invasion. A rhizobial strain (designed as CzR2) isolated from the root nodules of C. zanzibarica and cultivated in standard YEM medium displayed pleomorphism, with cells ranging between 2 and 10 μm in length and some branching. In the present study, we identified this rhizobial strain, investigated the causes of pleomorphism, and examined the nodules formed. The results of a multilocus sequence analysis of the atpD, dnaK, glnII, gyrB, recA, and rpoB genes revealed that CzR2 belongs to Bradyrhizobium arachidis, a peanut symbiont recently isolated from China. Cells of the strain were uniformly rod-shaped in basal HM medium, but displayed pleomorphism in the presence of yeast extract, mannitol, or fructose. These results indicate that the morphology of CzR2 in its free-living state is affected by nutrient conditions. Several highly pleomorphic bacteroids enclosed in symbiosomes were frequently detected in FM and TEM observations of sections of the indeterminate nodules induced by CzR2; however, no infection thread was identified. Flow cytometric analyses showed that CzR2 cells in YEM medium and in the nodules of C. zanzibarica had two or more than two peaks in relative DNA contents, respectively, suggesting that the elongated cells of CzR2 in its free-living state occur due to a cell cycle-delayed process, while those in its symbiotic state are from genomic endo-reduplication.