Ascorbate Metabolism and Nitrogen Fixation in Legumes

32 Pags.- 6 Figs.- 1 Tabl. The definitive version is available at: https://link.springer.com/book/10.1007/978-3-319-74057-7Ascorbic acid (AsA) is a major antioxidant and redox buffer in plants that acts as a direct scavenger of reactive oxygen species (ROS), substrate of ascorbate peroxidase (APX), and cofactor of key enzymes involved in cell wall expansion and hormone biosynthesis. N2-fixing root nodules of legumes contain high levels of AsA (1–2 mM), APX (0.9% of the total soluble protein), and the other enzymes of the AsA-glutathione pathway. The AsA content and their associated enzyme activities are strongly correlated with N2 fixation, providing adequate protection of nodules against ROS generated by, among other processes, the oxidation of nitrogenase and leghemoglobin. These antioxidant defenses are concentrated in the infected zone and in the endodermis/nodule parenchyma, where they may be part of the O2 diffusion barrier that restricts entry of O2 into the nodule interior to avoid nitrogenase inactivation. AsA can enhance N2 fixation fourfold to fivefold when supplied by stem infusion to soybean plants or when added, along with APX, to an in vitro reconstitution system. GDP-d-mannose 3,5-epimerase (GME) and GDP-l-galactose phosphorylase (GGP) catalyze critical limiting steps in AsA biosynthesis. Consequently, studies are underway to produce transgenic N2-fixing plants that overexpress GME and GGP. Preliminary results have indicated that these plants have higher rates of N2 fixation but with a metabolic cost, resulting in slightly smaller plants. Additionally, since AsA oxidation and decline are early symptoms of nodule aging, a high AsA content might delay nodule senescence and extend the period of N2 fixation.Work from our laboratories described in this review article was funded by grants AGL2011-24524 and AGL2014-53717-R from the Ministry of Economy and Competitiveness-Fondo Europeo de Desarrollo Regional to M.B. and by grants DCB-8903254, IBN-9206453, IBN-9507491, and IOS-0517688 from the National Science Foundation to D.A.D.Peer reviewe