Exogenous spermidine regulates starch synthesis and the antioxidant system to promote wheat grain filling under drought stress

Polyamines (PAs) markedly affect the cereal's grain filling characteristics under drought stress. But little is known that the regulatory mechanism that governs this process. So, in the present study, drought was imposed, and exogenous spermidine (Spd) was applied at anthesis. The grain filling characteristics of and gene expression in the grains were measured. The research purpose of this study was to investigate whether and how the PAs regulated the grain filling in wheat under drought stress. In the present study, external Spd markedly relieved the inhibitory action of drought stress on grain filling. Moreover, RNA-seq analysis revealed that the effects of drought and exogenous Spd on grain filling in wheat were strongly related to protein processing in the endoplasmic reticulum, phenylalanine metabolism, hormone signaling, and starch and sucrose metabolism. In addition, under drought stress, the exogenous Spd significantly promoted the synthesis of cytokinin (CTK) and starch in wheat grains of plants. In contrast, exogenous Spd significantly decreased ethylene (ETH) synthesis in the grains of plants under drought stress. Beside this, exogenous Spd application notably promoted the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) and decreased the malondialdehyde (MDA) content in the grains of plants under drought stress. In summary, the promoting action of Spd on the grain filling characteristics in wheat under drought stress was notably related to an increase in endogenous zeatin (Z) + zeatin riboside (ZR) contents, the synthesis of starch from sucrose, the activities of antioxidant enzymes and a decrease in ETH synthesis in the grains.