Suppression of Plant Defence in the Medicago Sativa (Alfalfa)-Sinorhizobium meliloti Symbiosis
The gene cluster of Sinorhizobium meliloti directing the biosynthesis of the exopolysaccharide succinoglycan (EPS I) was analyzed in detail. S. meliloti mutants deficient in EPS I production induced the formation of non-infected pseudonodules displaying plant defence reactions. The hypothesis that the plant defence in alfalfa is suppressed by the S. meliloti exopolysaccharide EPS I was verified using elicitor responsive alfalfa suspension cultures. It was shown that yeast elicitors, chitin oligosaccharides and cell wall fragments of alfalfa cells induced an alkalinization of the cell culture medium. The alkalinization of alfalfa cell cultures could be reduced drastically by adding low molecular weight EPS I indicating that this compound acts as a signal substance in the suppression of alfalfa defence reactions.
KeywordsNitrogen Fixation Plant Defence Nodulation Factor Alfalfa Root Cell Wall Fragment
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