ROLE OF AMINO ACID AND SUCROSE PARTITIONING PROCESSES IN SOURCE AND SINK PHYSIOLOGY OF PEA (PISUM SATIVUM L.) PLANTS

Rachel E Snyder Garneau

Carbon (C), nitrogen (N), and sulfur (S) are essential macronutrients required for plant growth and development. Inorganic C, N, and S are acquired from the atmosphere or soil and assimilated into sucrose and amino acids within source organs. These assimilates are used in source metabolism or loaded into the plant vasculature for partitioning to sinks such as roots, nodules, developing leaves, and seeds. In legumes, source-to-sink transport of sucrose and organic S compounds via the phloem is especially critical for the process of symbiotic N fixation that occurs within root nodules, while translocation of amino acids, including the S-containing amino acids methionine and S-methylmethionine (SMM), to seed sinks is vital for the biosynthesis of nutritious S-rich storage proteins. This research examines the importance of plasma membrane-localized transport proteins in source-to-sink partitioning of sucrose and S-amino acids in Pisum sativum L. plants and their role in source-sink physiology and sink development. First, transporter function in source-to-sink partitioning of sucrose was examined and its impact on source and sink physiology was elucidated using transgenic pea plants overexpressing the endogenous SUcrose Transporter 1 (PsSUT1) in the leaf phloem and seed embryo. The SUT1-overexpressors displayed increased sucrose phloem loading and uptake into seeds. This change in C partitioning positively affected leaf C fixation and co-regulated N assimilation and amino acid translocation from leaf to seed sinks. Consequently, this resulted in improved seed yield and seed protein and starch concentrations in the transgenic pea plants. Second, it was investigated if altered SMM phloem loading affects leaf-to-sink S partitioning, and nodule organogenesis and function in N-fixing pea plants expressing a yeast S-Methyl Methionine Permease (MMP1) transporter in the leaf phloem. The transgenic MMP1-expressing plants showed improvements in source-to-sink transport of SMM and other organic S compounds as well as amino acids and sucrose, resulting in adjustments in leaf and nodule metabolisms, and enhanced nodule development. The overall outcome was a significant increase in nodule-to-shoot N and S partitioning that resulted in improved increases in growth, seed yield, and seed protein accumulation in the MMP1 plants. Finally, the role of methionine phloem loading in source-to-sink S transport was examined and its impact on nodule development and function was evaluated. N-fixing pea plants expressing the yeast Methionine UPtake 1 (MUP1) transporter in the phloem showed increased source-to-nodule partitioning of methionine and other S compounds, resulting in a higher number of nodules as well as increased N fixation per nodule. These changes were accompanied by improvements in C fixation and whole plant adjustments in S, N, and C partitioning resulting in enhanced vegetative growth. This further led to increases in seed yield and seed protein concentration in the MUP1 plants.

ROLE OF AMINO ACID AND SUCROSE PARTITIONING PROCESSES IN SOURCE AND SINK PHYSIOLOGY OF PEA (PISUM SATIVUM L.) PLANTS

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