Physiological and proteomic analysis reveals the impact of boron deficiency and surplus on alfalfa (Medicago sativa L.) reproductive organs

Boron (B), an essential element for increasing seed yield and germinability in alfalfa (Medicago sativa L.), plays a vital role in its reproductive processes. However, effects of B stress on physiological and proteomic changes in reproductive organs related to alfalfa seed yield and germinability are poorly understood. In order to gain a better insight into B response or tolerance mechanisms, field trials were designed for B deficiency (0 mg B L^-1), B sufficiency (800 mg B L^-1), and B surplus (1600 mg B L^-1) application during alfalfa flowering to analyze the proteomics and physiological responses of alfalfa 'Aohan' reproductive organs. Results showed that B deficiency weakened the stress-responsive ability in these organs, while B surplus reduced the sugar utilization of 'Aohan' flowers and caused lipid membrane peroxidation in 'Aohan' seeds. In addition, four upregulated stress responsive proteins (ADF-like protein, IMFP, NAD(P)-binding Rossmann-fold protein and NAD-dependent ALDHs) might play pivotal roles in the response of 'Aohan' reproductive organs to conditions of B deficiency and B surplus. All of the above results would be helpful to understand the tolerance mechanisms of alfalfa reproductive organs to both B deficiency and B surplus conditions, and also to give insight into the regulatory role of B in improving seed yield and germinability in alfalfa seed production. In summary, B likely plays a structural and regulatory role in relation to lipid metabolism, carbohydrate metabolism, amino acid metabolism, and signal transduction, thus regulates alfalfa reproductive processes eventually affecting the seed yield and germinability of alfalfa seeds.