PHYSIOLOGICAL RELEVANCE OF SULFUR TRANSPORT PROCESSES IN NITROGEN FIXING LEGUMES

It can generally be assumed that in nitrogen (N)-fixing legumes S uptake from the soil and long-distance transport of S from root-to-leaf-to-nodule or from source-to-seed rely on membrane proteins functioning in phloem loading and import into the nodule and embryo, respectively. However, up to date, relatively little is known about the physiological function of S transporters in nodulated legumes and their importance for legume performance. This project will analyze nodulated, N-fixing pea and soybean lines with altered S transport processes in source and sink and aims to resolve how these impact nodule activity and development, as well as seed yield, and seed protein amount and quality.Objective 1: Determine the role of S partitioning processes in biological N fixation in nodules (Years 1-3). S nutrition is essential for N fixation by affecting both nodule N-fixation rates and nodule establishment and growth. Soybean and pea plants, in which S transporters are overexpressed, will be analyzed to resolve the importance of transport processes for S delivery to nodules and nodule function. It will be determined (i) if S allocation from roots or leaves to nodules and S uptake by nodules can be increased, (ii) if and how changes in S partitioning affect nodule number, size and/or N fixation, (iii) which components of the N-fixation apparatus are affected, (iv) the effects on nodule S/N/carbon (C) metabolisms, (v) the consequences for shoot and root growth, and (vi) if or how the outcomes for nodule function and plant growth differ dependent on the respective overexpressed S transporter.Objective 2: Determine the importance of S partitioning processes for seed nutritional quality in N-fixing legumes (Years 1-3). Using transgenic pea and soybean plants with altered S transport processes, the role of source-to-seed S partitioning in seed protein yield and quality will be analyzed. We aim to uncover (i) to what extend supply of S to seeds is changed, (ii) if seed (or pod) numbers, size and/or total protein levels are altered, (iii) if and how seed protein composition and nutritional quality are affected, and (iv) if alteration in S partitioning to seeds co-regulates N and C allocation to seeds and downstream metabolic processes.Objective 3: Determine the role of S partitioning processes in N-fixing legumes under S deficiency (Year 4). S deficiency severely limits N-fixation rates, reduces nodule development, and decreases S-rich seed proteins, thus seed nutritional quality. We aim to resolve if overexpression of S transporters provides our legume plants with an advantage under S limitation conditions, and how they balance S availability, assimilate partitioning and metabolism in support of N fixation in nodules and storage protein synthesis in seeds.