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Changes in root hair morphogenesis and nodulation of Phaseolus vulgaris L. in the presence of bacillus and rhizobium Srinivasan, Murali


Twenty two Bacillus spp. isolates from the Phaseolus vulgaris rhizosphere were identified using Biolog™, GC-FAME and 23S rDNA analysis. Some Bacillus isolates produced copious amounts of indoleacetic acid in vitro, in culture medium supplemented with L-tryptophan. Spontaneous cc-methyl tryptophan resistant mutants of Bacillus megaterium S49 exhibited altered IAA production and excreted tryptophan into the culture medium. Coinoculation of Rhizobium etli TAL 182 with some Bacillus spp. under gnotobiotic conditions promoted nodulation of P. vulgaris. In contrast, decreased nodulation was observed in response to low and high lAA-producing Bacillus mutants, suggesting that the nodulation response in bean was not dependent on the in vitro level of IAA production by the Bacillus coinoculant. Coinoculation of Bacillus spp. and Rhizobium spp. resulted in a unique and significant synergistic enhancement of lateral root formation and root hair initiation which was independent of plant host. These observations provided direct evidence for a beneficial interaction on root morphology which may account for enhanced nodulation by creating additional infection sites for the Rhizobium. The presence of live Rhizobium and Bacillus was necessary for the root hair proliferation response. Coinoculation of Rhizobium TAL 182 with cell extracts of Bacillus S49 failed to induce root hair proliferation or enhance nodulation of P. vulgaris. A biologically active Rhizobium TAL 182 was essential for the observed response, indicating that root hair proliferation and enhanced nodulation were direct consequences of the bacterial species interaction. Nodulation and symbiotic specificity of Rhizobium TAL 182 were altered by the presence of Bacillus S49. Split-root experiments showed that coinoculation suppressed host-controlled regulation of nodulation. Bacillus S49 facilitated heterologous nodulation of Rhizobium TAL 182 on P. acutifolius, indicating that a host plant response was also involved in the bacterial interaction. Exogenous aminoethoxy vinyl glycine, a negative regulator of ethylene biosynthesis, inhibited root hair formation in P. vulgaris. These inhibitory effects were reversed by the presence of both Bacillus S49 and Rhizobium TAL 182. The interaction between bacterial species on root hair induction and nodulation patterns in P. vulgaris is discussed.

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