COMPETITIVENESS AND COMMUNICATION FOR EFFECTIVE INOCULATION BY RHIZOBIUM, BRADYRHIZOBIUM AND VESICULAR-ARBUSCULAR MYCORRHIZA FUNGI

After a short summary on the ecology and rhizosphere biology of symbio tic bacteria and vesicular-arbuscular (VA) mycorrhiza fungi and their application as microbial inocula, results on competitiveness and commu nication are summarized. Stress factors such as high temperature, low soil pH, aluminium concentrations and phytoalexins produced by the hos t plants were studied with Rhizobium leguminosarum bv. phaseoli and Rh izobium tropici on Phaseolus beans. Quantitative data for competitiven ess were obtained by using gus(+) (glucoronidase) labelled strains, wh ich produce blue-coloured nodules. For Phaseolus-nodulating rhizobia, a group specific DNA probe was also developed, which did not hybridize with more than 20 other common soil and rhizosphere bacteria. Results from several laboratories contributing to knowledge of signal exchang e and communication in the Rhizobium/Bradyrhizobium legume system are summarized in a new scheme, including also defense reactions at the ea rly stages of legume nodule initiation. Stimulating effects of flavono ids on germination and growth of VA mycorrhiza fungi were also found. A constitutive antifungal compound in pea roots, beta-isoxazolinonyl-a lanine, was characterized.