Submitted to: Molecular Plant Microbe Interactions
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/15/1997
Publication Date: N/A
Citation: Interpretive Summary: Soybeans obtain a large proportion of their requirement for nitrogen (N) through symbiotic N2 fixation with the soil bacterium Bradyrhizobium. Small wart like structures on roots, called nodules, are the site of this symbiosis. Both plant and bacterial genes control the formation of effective N2 fixing nodules. While a great deal is known about bacterial genes controlling this process, much less is known about the plant genes. In this report we identify in soybean a single gene which controls ineffective non-N2-fixation with selected strains of Bradyrhizobium. Moreover, we describe the physiological, biochemical, and structural characteristics of both effective N2 fixing and ineffective non-N2-fixing nodules controlled by this single plant gene. Soybean plants containing this gene respond to the Bradyrhizobium not as though it is a symbiosis but as though it is a pathogen. Our studies show that subtle changes in the genetic makeup of the plant controls whether microorganisms are recognized as a pathogen or symbiont. Isolation and characterization of the plant genes controlling symbiotic N2 fixing root nodules will help in the development of plant germplasm with improved N2 fixation, which in turn can reduce the dependence of U.S. agriculture on N fertilizer additions.
Technical Abstract: We have previously reported a soybean plant introduction, PI 437153A, which was ineffectively nodulated by Bradyrhizobium japonicum strain UMR 161. In this study, we further characterize this nodulation system by: I) examining the nodulation of 15 serogroups, with PI 437153A, II) ascertaining the inheritance of the ineffective nodulation, and III) characterizing the symbiosis both morphologically and physiologically. Only USDA 126 (serogroup 125) and UMR 161 failed to produce effective nodules with PI 437153A. Results from a segregation analysis of F2 and F3 plants from a cv. Lambert x PI 437153A population are consistent with control of this trait by a single, dominant gene. In reciprocal grafting studies between cv. Lambert and PI 437153A, ineffective nodulation was determined by the root genotype, and was independent of plant growth temperature and inoculant concentration. In PI 437153A plants inoculated with UMR 161, nodule dry weights were 10-fold less and nodule soluble protein levels wer reduced four-fold compared to those obtained with PI 437153A and USDA 110. Nitrogenase activity and leghemoglobin could not be detected in ineffective nodules. Phenylalanine ammonia-lyase RNA level was also significantly higher one to six days after inoculation (DAI) in the ineffectively nodulated phenotype than in the effectively nodulated one. Microscopic analysis of the ineffective nodules, done 35 DAI, revealed the presence of large deteriorating regions near apparently infected plant cells. This suggests a plant defense response instead of a symbiotic association. However, the lack of chalcone synthase expression and the appearance of infected cells within the ineffective nodules suggests that some steps in the infection process do occur.