Calcium regulates growth of Rhizobium fredii and its ability to nodulate soybean cv. Peking

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Abstract
We investigated the role of calcium in growth of Rhizobium fredii USDA208 and its influence on nodulation of cv. Peking soybean. The placement and numbers of nodules on soybean roots in growth pouches is relatively insensitive to levels of Ca2+ in the root zone. In contrast, adequate Ca2+ in the Rhizobium culture media is essential for induction of a normal distribution of nodules on the taproot and lateral roots. Ca2+-deprived rhizobia attach in reduced numbers to soybean roots, do not initiate nodules in the most infectible region of the taproot, and induce nodules almost exclusively on lateral roots. Ca2+ starvation also greatly reduces growth of USDA208 in culture, but it has little effect on exopolysaccharide production. Strain USDA208 produces two detectable 45Ca2+-binding proteins in culture. A major 38-kDa bacterial protein is regulated by the availability of Ca2+, but this protein does not itself bind 45Ca2+. Key words: autoregulation, nodule regulation, nodule initiation.

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