Biochemical and anatomical investigation of Sesbania herbacea (Mill.) McVaugh nodules grown under flooded and non-flooded conditions

Authors: Krishnan, Hari; Oehrle, Nathan; ALASWAD, ALAA - University Of Missouri; STEVENS, WILLIAM - University Of Missouri; JOHN, MARIA - Forestry Services; Luthria, Devanand - Dave; Natarajan, Savithiry - Savi

Submitted to: International Journal of Molecular Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2019
Publication Date: 4/12/2019
Citation: Krishnan, H.B., Oehrle, N.W., Alaswad, A.A., Stevens, W., John, M.K., Luthria, D.L., Natarajan, S.S. 2019. Biochemical and anatomical investigation of Sesbania herbacea (Mill.) McVaugh nodules grown under flooded and non-flooded conditions. International Journal of Molecular Sciences. 20(8):1824. https://doi.org/10.3390/ijms20081824.
DOI: https://doi.org/10.3390/ijms20081824

Interpretive Summary: Rhizobia present in the soil are responsible for formation of nodules on the roots of legumes such as soybean. Nodules are specialized structures where atmospheric nitrogen is fixed by the rhizobia to benefit the host plant. This process is termed biological nitrogen fixation and it enables legumes to grow in nitrogen-poor soils. Nodulation is significantly reduced in waterlogged soils. However, some legumes occurring in wetlands have evolved adaptations that enable them the overcome the deleterious effect of flooding. In this study we have examined how Sesbania, an annual legume is able to thrive and efficiently fix nitrogen in flooded conditions. Our study demonstrates the important role of biochemical and anatomical modifications that enables this legume to form nodules even under adverse conditions. The results of our study reveal key adaptations that enables this legume to form nodules even under adverse conditions. 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: Sesbania herbacea, a native North American fast-growing legume, thrives in wet and waterlogged conditions. This legume enters into symbiotic association with rhizobia resulting in the formation of nitrogen-fixing nodules on the roots. Flooding-induced anaerobic environment imposes a challenge for the survival of rhizobia and negatively impacts nodulation. Very little information is available on how S. herbacea is able to thrive and efficiently fix N2 in flooded conditions. In this study, we found that Sesbania plants grown under flooded conditions were significantly taller, produced more biomass, and formed more number nodules when compared to plants grown on dry land. Transmission electron microscopy of Sesbania nodules revealed bacteroids from flooded nodules contained prominent polyhydroxybutyrate crystals, which were absent in non-flooded nodules. Gas and ion chromatography mass spectrometry analysis of nodule metabolites revealed a marked decrease in asparagine and an increase in the levels of gamma aminobutyric acid in flooded nodules. 2-D gel electrophoresis of nodule bacteroid proteins revealed flooding-induced changes in their protein profiles. Several of the bacteroid proteins that were prominent in flooded nodules were identified by MALDI-TOFF analysis to be members of the ABC transporter family. The activities of several key enzymes involved in nitrogen metabolism was altered in Sesbania flooded nodules. Aspartate aminotransferase (AspAT), an enzyme with a vital role in the assimilation of reduced nitrogen, was dramatically elevated in flooded nodules. The results of our study highlight the potential of S. herbacea as a green manure and sheds light on the morphological, structural and biochemical adaptations that enable S. herbacea to thrive and efficiently fix N2 in flooded conditions.