Extracellular Polysaccharide Is Not Responsible for Aluminum Tolerance of Rhizobium leguminosarum bv. Phaseoli CIAT899.

Strain UHM-5, a pSym Exo derivative of the aluminum-tolerant Rhizobium leguminosarum bv. phaseoli strain CIAT899, was equally tolerant of aluminum (Al) as the parental culture. Dialyzed culture supernatants of the wild-type cells grown in YEM broth (10 cells ml) contained 185 mug of glucose equivalents ml whereas UHM-5 culture supernatants yielded 2 mug of glucose ml. The Exo derivative and the parental strain gave essentially similar growth in medium containing from 0 to 300 muM Al, indicating that the pSym of CIAT899, and extracellular polysaccharide, were not involved in the aluminum tolerance of this strain. However, increasing the level of Al from 80 to 150 muM increased the lag phase, induced a slight killing of the inoculum, and depressed the final populations by about fivefold. Doubling the aluminum concentration from 150 to 300 muM presented a severe aluminum stress to CIAT899 and UHM-5: the inoculum level dropped 10-fold, indicating killing of the inoculum, and remained depressed for ca. 4 days before continuing to grow slowly; the final population was decreased 15-fold relative to that of cultures grown in medium containing 80 muM Al. The production by CIAT899 of other extracellular or intracellular aluminum tolerance factors was investigated in culture by using aluminum-sensitive rhizobia as stress indicators. These experiments, conducted at 80 muM Al, demonstrated that CIAT899 produced neither extracellular nor intracellular products that could alleviate toxicity for the Al-sensitive indicator rhizobia.