THE ROLE OF NITROGEN TRANSPORT PROCESSES IN SOYBEAN [GLYCINE MAX (L.) MERR.] PHYSIOLOGY AND STRESS RESPONSE

Sandi Win Thu

doi:10.7273/000007482

Legumes develop a symbiotic relationship with rhizobial bacteria that reside in root nodules and fix atmospheric di-nitrogen (N2 ). In soybean [Glycine max (L.) Merr.] nodules, the fixed nitrogen (N) is used to produce the ureides allantoin and allantoic acid that serve as the primary forms of N transported from nodules to shoot tissues. Cellular import and long-distance transport of ureides is facilitated by plasma membrane-localized ureide permeases 1 (UPS1). Recent research has shown that the overexpression of UPS1 (UPS1-OE) in cortex and endodermal cells of soybean nodules led to increased nodule export of ureides and their enhanced partitioning in the xylem to the shoot, finally resulting in improved seed development. The current work demonstrates that ureide partitioning processes are equally important for the performance of non-fixing, non-nodulated soybean plants that primarily use amino acids as main N transport forms. Non-fixing soybean plants overexpressing UPS1 in the leaf phloem displayed enhanced phloem loading of ureides and sink N supply. The changes in ureide partitioning positively affected amino acid homeostasis and transport, photosynthesis, and sucrose phloem transport, leading to improved vegetative growth and higher seed yields in UPS1-OE plants compared to the wild-type control. Overall, these findings support that ureide transport processes in non-fixing soybean plants exert regulatory control over N and carbon acquisition, assimilation, and source-to-sink translocation affecting soybean productivity. Soybeans are extremely sensitive to drought, which inhibits N fixation, likely due to a decrease in the export of ureides from nodules and their subsequent accumulation in these organs. It was hypothesized that UPS1 overexpression in the nodule inner cortex and endodermal cells could mitigate N-fixation inhibition under drought by promoting continuous ureide export and partitioning from nodules to shoot tissues. N-fixing UPS1-OE plants were exposed to moderate and severe drought stress and results showed increased ureide synthesis in nodules and nodule-to-shoot ureide partitioning under drought. These enhancements positively affected N and carbon fixation, metabolism, and partitioning, ultimately improving soybean growth during water-deficit conditions. The data also suggest that changes in ureide partitioning or leaf ureide pools can lead to improved protection against oxidative and osmotic stress, contributing to a better performance of the drought-stressed UPS1-OE soybean plants compared to the wild type. Thus, ureides not only function as key N transport compounds, but also trigger physiological processes that improve soybean’s response to drought stress. In legume nodules, a significant amount of reactive oxygen species (ROS) are produced during N fixation. The host cells deploy antioxidant systems to counteract oxidative stress and protect themselves from ROS-induced cellular damage. An important antioxidant delivered from leaves to, or produced in, nodule cells is glutathione (GSH). GSH also acts as a transport form of N and sulfur, with the latter being a key component of the bacterial nitrogenase and other enzymes essential for N fixation and nodule function. To understand the role of GSH partitioning processes in soybean N fixation, transgenic plants were produced that express the GSH transporter OPT4 (oligopeptide transporter 4) from Arabidopsis thaliana in the phloem. The aim of the current study was to confirm the presence of the transgene in the soybean genome, screen the transgenic lines for homozygosity, and determine the genome copy number of the transgene. Five homozygous OPT4 lines were identified, each carrying a single copy of the transgene. These soybean lines are an invaluable source for future studies on the importance of GSH transport processes for nodule function and whole plant physiology.

THE ROLE OF NITROGEN TRANSPORT PROCESSES IN SOYBEAN [GLYCINE MAX (L.) MERR.] PHYSIOLOGY AND STRESS RESPONSE

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