微生物および生化学技術ジャーナル

概要

Effect of Cyanophycin Metabolism in Recombinant Sinorhizobium (Ensifer) meliloti 1021 on the Symbiosis with Alfalfa (Medicago sativa)

Yasser Abd-El-Karem, Rudolf Reichelt, Martin Krehenbrink and Alexander Steinbüchel1

The aim of this study was to investigate the effect of the synthesis and degradation of the nitrogen-rich polymer cyanophycin in rhizobia on symbiotic nitrogen fixation and crop yield in legumes. For this, cyanophycin synthetase from Anabaena sp. PCC7120 was expressed in the bacteroids of the symbiont S. meliloti 1021 either alone or together with an intracellular cyanophycinase from the same bacterium, either in the wild type or in a polyhydroxybutyratenegative (PHB-) mutant, and the effect on the growth of alfalfa host plants was studied. All strains induced the formation of nitrogen-fixing nodules in the host, but clear differences in various parameters were noticeable. Alfalfa plants infected with the wild type expressing only cyanophycin synthetase showed significantly lower shoot nitrogen contents and higher nitrogen fixation rates than plants inoculated with wild type, but the wild type phenotype was exceeded in S. meliloti 1021 expressing cyanophycinase in addition to cyanophycin synthetase. Growth of plants infected with the PHB- mutant expressing only cyanophycin synthetase was severely impaired in comparison to growth of plants infected with the PHB- mutant expressing both or neither of the two proteins. Transmission electron micrographs of sections of nodules induced by wild type S. meliloti 1021 producing cyanophycin synthetase and cyanophycinase showed that the rough endoplasmic reticulum and Golgi membranes were extended further in comparison to the nodules induced by the wild type, the PHB- mutant, or all other recombinant S. meliloti strains, indicating higher metabolic activities in these nodules.