S-Methylmethionine Phloem Loading Affects Source-Sink Physiology and Assimilate Partitioning in Nitrogen-Fixing Pea Plants, Resulting in Improved Growth and Seed Protein Yields

Legumes like pea (Pisum sativum L.) fix atmospheric nitrogen (N) in a symbiosis with bacteria, thus reducing the need for N fertilizer and its negative impact on the environment and human health. In addition, their seeds are a vital source of protein for human diets and animal feed. Legume productivity depends on efficient sulfur (S) partitioning within the plant to support N fixation in nodules and seed protein synthesis. S movement from source leaves to sinks occurs in the phloem, and we tested the hypothesis that phloem loading of the organic S compound S-methylmethionine (SMM) is a key regulatory step controlling sink S supply. In pea plants relying solely on N fixation for N nutrition, expression of an SMM transporter in the leaf phloem enhanced source-to-sink movement of SMM and other S assimilates, resulting in increased nodule number, N fixation, and total plant N. These changes triggered coordinated increases in S, N, and carbon acquisition, metabolism, and partitioning, leading to greater vegetative growth, seed yield, and improved seed protein quantity and quality. Overall, this study identifies source-to-sink transport of SMM as a promising target for improving legume productivity.