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Classical Soybean (Glycine max (L.) Merr) Symbionts, Sinorhizobium fredii USDA191 and Bradyrhizobium diazoefficiens USDA110, Reveal Contrasting Symbiotic Phenotype on Pigeon Pea (Cajanus cajan (L.) Millsp)

A. A. Alaswad, N. W. Oehrle, H. B. Krishnan
International journal of molecular sciences 2019 v.20 no.5 pp. 1091
Bradyrhizobium diazoefficiens, Bradyrhizobium japonicum, Cajanus cajan, Glycine max, Sinorhizobium fredii, leghemoglobin, mutants, nitrogen-fixing bacteria, nodulation, phenotype, pigeon peas, rhizosphere bacteria, root inoculation, root nodules, semiarid zones, soybeans, symbionts, symbiosis, type III secretion system
Pigeon pea [Cajanus cajan (L.) Millspaugh] is cultivated widely in semi-arid agricultural regions in over 90 countries around the world. This important legume can enter into symbiotic associations with a wide range of rhizobia including Bradyrhizobium and fast-growing rhizobia. In comparison with other major legumes such as soybean and common bean, only limited information is available on the symbiotic interaction of pigeon pea with rhizobia. In this study, we investigated the ability of two classical soybean symbionts, S. fredii USDA191 and B. japonicum USDA110 and their type 3 secretion system (T3SS) mutants, to nodulate pigeon pea. Both S. fredii USDA191 and a T3SS mutant S. fredii RCB26 formed nitrogen-fixing nodules on pigeon pea. Inoculation of pigeon pea roots with B. japonicum USDA110 and B. japonicum '136 (a T3SS mutant) resulted in the formation of Fix- and Fix+ nodules, respectively. Light and transmission electron microscopy of Fix- nodules initiated by B. japonicum USDA110 revealed the complete absence of rhizobia within these nodules. In contrast, Fix+ nodules formed by B. japonicum '136 revealed a central region that was completely filled with rhizobia. Ultrastructural investigation revealed the presence of numerous bacteroids surrounded by peribacteroid membranes in the infected cells. Analysis of nodule proteins by one and two-dimensional gel electrophoresis revealed that leghemoglobin was absent in B. japonicum USDA110 nodules, while it was abundantly present in B. japonicum '136 nodules. Results of competitive nodulation assays indicated that B. japonicum '136 had greater competitiveness for nodulation on pigeon pea than did the wild-type strain. Our results suggest that this T3SS mutant of B. japonicum, due to its greater competitiveness and ability to form Fix+ nodules, could be exploited as an inoculant to boost pigeon pea productivity.