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(9) cells ml-1) contained 185-mu-g of glucose equivalents ml-1 whereas UHM-5 culture supernatants yielded 2-mu-g of glucose ml-1. The Exo- derivative and the parental strain gave essentially similar growth in medium containing from 0 to 300-mu-M 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-mu-M 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-mu-M 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-mu-M 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-mu-M Al, demonstrated that CIAT899 produced neither extracellular nor intracellular products that could alleviate toxicity for the Al-sensitive indicator rhizobia.