Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/14/2016
Publication Date: 11/14/2016
Publication URL: http://handle.nal.usda.gov/10113/5600610
Citation: Krishnan, H.B., Alaswad, A.A., Oehrle, N.W., Gillman, J.D. 2016. Deletion of the SACPD-C locus alters the symbiotic relationship between Bradyrhizobium japonicum USDA110 and soybean, resulting in elicitation of plant defense response and nodulation defects. Molecular Plant-Microbe Interactions. 29(11):862-877.
Interpretive Summary: Soybean, an economically important legume, interacts with some soil-dwelling bacteria (collectively called as rhizobia) resulting in formation of nitrogen-fixing nodules on the roots and in few cases on stems. Nodules are a specialized plant organ where atmospheric nitrogen is reduced to ammonia by the bacterial encoded enzyme nitrogenase. This process is termed biological nitrogen fixation and contributes significantly to the overall nitrogen requirements of growing plants for the synthesis of seed storage reserves. Fatty acids (FA) play an important role in nodule formation. Stearoyl-Acyl Carrier Protein Desaturase (SACPD) enzymes play an important role in fatty acid biosynthesis. In this study, we have carried out in-depth cytological and biochemical investigation of nodule development using two sacpd-c mutant lines, M25 and MM106, in comparison with its parent cultivar ‘Bay’. The results of our study demonstrate a critical role for SACPD-C in nodule maintenance and in control of symbiosis and plant defense response. Information obtained from this study will help scientists to better understand the factors that limit the formation of nitrogen-fixing nodules on legumes. Such an understanding should enable scientists to manipulate biological nitrogen fixation so that farmers can increase yields with minimal use of nitrogen fertilizers.
Technical Abstract: Legumes form symbiotic association with soil-dwelling bacteria collectively called rhizobia. This association results in the formation of nodules, unique plant-derived organs, within which the rhizobia are housed. Rhizobia encoded-nitrogenase facilitates the conversation of atmospheric nitrogen into ammonia, which is utilized by the plants for its growth and development. Fatty acids have been shown to play an important role in root nodule symbiosis. In this study, we have investigated the role of Stearoyl-Acyl Carrier Protein Desaturase (SACPD), an enzyme that catalyzes the conversion of stearic acid into oleic acid, in soybean nodulation. In-depth cytological investigation of nodule development in sacpd-c mutant lines M25 and MM106 revealed gross anatomical alteration in the sacpd-c mutants. Transmission electron microscopy observations exhibited ultrastructural alterations in the sacpd-c mutants that are typically associated with plant defense response to pathogens. In contrast to salicylic acid (SA), whose concentration in the nodules of the three genotypes were not significantly different, combined jasmonic acid (JA) species (JA, JA-ile, and OPDA) levels were significantly higher in the nodules of sacpd-c mutant lines. The activities of antioxidant enzymes, ascorbate peroxidase (APX) and superoxide dismutase (SOD) were also found to be higher in the sacpd-c mutants. PR-1 gene expression was extremely elevated in M25 and MM106, while the expression of nitrogenase was significantly reduced in these sacpd-c mutants compared with the parent cultivar ‘Bay’. Two-dimensional gel electrophoresis and MALDI-TOF mass spectrometry analyses confirmed sacpd-c mutants also accumulated higher amounts of pathogenesis protein (PR-1) in the nodules. Our study establishes a major role for SACPD-C as essential for proper maintenance of soybean nodule morphology and physiology and involved in attenuation of nodule biotic defense responses.