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Using pea as our model crop, we sought to understand the regulatory control over the import of sugars and amino acids into the developing seeds and its importance for seed yield and quality. Transgenic peas simultaneously overexpressing a sucrose transporter and an amino acid transporter were developed. Pod walls, seed coats, and cotyledons were analysed separately, as well as leaves subtending developing pods. Sucrose, starch, protein, free amino acids, and endogenous cytokinins were measured during development. Temporal gene expression analyses (RT-qPCR) of amino acid (AAP), sucrose (SUT), and SWEET transporter family members, and those from cell wall invertase, cytokinin biosynthetic (IPT) and degradation (CKX) gene families indicated a strong effect of the transgenes on gene expression. In seed coats of the double transgenics, increased content and prolonged presence of cytokinin was particularly noticeable. The transgenes effectively promoted transition of young sink leaves into source leaves. We suggest the increased flux of sucrose and amino acids from source to sink, along with increased interaction between cytokinin and cell wall invertase in developing seed coats led to enhanced sink activity, resulting in higher cotyledon sucrose at process pea harvest, and increased seed number and protein content at maturity. [ABSTRACT FROM AUTHOR]

Grain legumes such as pea (Pisum sativum L.) are highly valued as a staple source of protein for human and animal nutrition. However, their seeds often contain limited amounts of high-quality, sulfur (S) rich proteins, caused by a shortage of the S-amino acids cysteine and methionine. It was hypothesized that legume seed quality is directly linked to the amount of organic S transported from leaves to seeds, and imported into the growing embryo. We expressed a high-affinity yeast (Saccharomyces cerevisiae) methionine/cysteine transporter (Methionine UPtake 1) in both the pea leaf phloem and seed cotyledons and found source-to-sink transport of methionine but not cysteine increased. Changes in methionine phloem loading triggered improvements in S uptake and assimilation and long-distance transport of the S compounds, S-methylmethionine and glutathione. In addition, nitrogen and carbon assimilation and source-to-sink allocation were upregulated, together resulting in increased plant biomass and seed yield. Further, methionine and amino acid delivery to individual seeds and uptake by the cotyledons improved, leading to increased accumulation of storage proteins by up to 23%, due to both higher levels of S-poor and, most importantly, S-rich proteins. Sulfate delivery to the embryo and S assimilation in the cotyledons were also upregulated, further contributing to the improved S-rich storage protein pools and seed quality. Overall, this work demonstrates that methionine transporter function in source and sink tissues presents a bottleneck in S allocation to seeds and that its targeted manipulation is essential for overcoming limitations in the accumulation of high-quality seed storage proteins. [ABSTRACT FROM AUTHOR]