Biodiversity of endophytic bacteria which colonize red clover nodules, roots, stems and foliage and their influence on host growth

Abstract

The aim of this study was to identifiy the endophytic bacteria recovered from the foliage, tap roots and nodules of red clover plants (Trifolium pratense L.); and to assess the effects of the nodule bacteria, alone and in combination with Rhizobium spp., on the growth and development of red clover seedlings. Thirty-one bacteria species from 14 different genera were recovered from within the foliage, roots and nodules of red clover plants cv. AC Charlie. Genera diversity and species number were greatest in foliage tissues. Pantoea agglomerans (59.6%) was the most frequent species recovered in foliage tissues, Agrobacterium rhizogenes A in the tap root (49.2%) and Rhizobium leguminosarum BV phaseoli and R. loti B in the nodules (27.2% each). Recovery of Rhizobium species was not restricted to the nodules, and species of this genus were systemic throughout the plant. Clover root nodules were host to 12 bacteria species other than rhizobia, of which 8 were specific to this tissue. Using non-selective media, R. leguminosarum BV trifolii constituted only 8.8% of all the root nodule bacteria recovered. In root bacterization experiments, species of nodule bacteria promoted growth of red clover more often when applied in combination with R. leguminosarum BV trifolii than when applied singly. However, Bacillus megaterium, Bordetella avium and Curtobacterium luteum consistently promoted growth either individually or in combination with R. leguminosarum BV trifolii. Nodulation was promoted when R. leguminosarum BV trifolii was coinoculated with Bacillus insolitus, B. brevis or A. rhizogenes A. Single isolate applications of Rhizobium species to roots always led to the depression of clover growth, but mixtures of R. leguminosarum BV trifolii and R. leguminosarum BV phaseoli resulted in growth promotion. The latter is considered further evidence of the beneficial allelopathic side effect of strain competition for the same ecological niche.