A HOST-CONTROLLED, SEROGROUP-SPECIFIC, INEFFECTIVE NODULATION SYSTEM IN THE BRADYRHIZOBIUM-SOYBEAN (GLYCINE-MAX) SYMBIOSIS

We have previously reported a soybean plant introduction, PI 437153A, which was ineffectively nodulated (definition of G. Vest, D.F. Weber, and C. Sloger, Agronomy 16:353-390, 1973) by Bradyrhizobium japonicum strain UMR (University of Minnesota Rhizobium) 161. In this study, we further characterize this nodulation system by (i) examining the nodul ation of 21 B. japonicum strains, representing 15 serogroups, with PI 437153A, (ii) ascertaining the inheritance of the ineffective nodulati on, and (iii) characterizing the symbiosis both morphologically and ph ysiologically. Only USDA 126 (serogroup 125) and UMR 161 failed to pro duce effective nodules with PI 437153A. Segregation among F-2 and F-3 plants from a cross between cv. Lambert and PI 437153A was consistent with control of this trait by a single, dominant gene. In reciprocal g rafts between cv. Lambert and PI 437153A, ineffective nodulation was d etermined by the root genotype, and was independent of plant growth te mperature and inoculant concentration. The nodule dry weight of UMR 16 1-inoculated plants of PI 437153A plants was approximately 10-fold les s than that of comparable plants inoculated with USDA 110; nodule solu ble protein levels were also significantly reduced. Nitrogenase activi ty and leghemoglobin could not be detected in the ineffectively nodula ted phenotype, but 1 to 6 days after inoculation (DAI) phenylalanine a mmonialyase RNA level was higher in the ineffectively nodulated phenot ype than in the effectively nodulated one. Microscopic examination of effective and ineffective nodules 20 and 35 DAI revealed major morphol ogical differences. Ineffective nodules contained large, deteriorating regions near apparently infected plant cells at 35 DAI, and sclerench yma cells that were stained red, suggestive of lignin deposition. Thes e changes could be the result of incompatibility between host and Brad yrhizobium, though the lack of chalcone synthase expression and the ap pearance of infected cells within the ineffective nodules suggest that some later steps in the infection process do occur.