Metabolic regulation of polyamines and γ-aminobutyric acid in relation to spermidine-induced heat tolerance in white clover.

Heat stress decreases crop growth and yield worldwide. Spermidine (Spd) is small aliphatic amine and acts as ubiquitous plant growth regulator to regulate plant growth, development, and stress tolerance. Objectives of this study was to determine effects of exogenous Spd on changes of endogenous polyamines and γ-aminobutyric acid metabolism, oxidative damage, senescence response, and heat shock proteins (HSPs) expression in white clover under heat stress. Physiological and molecular methods including colorimetric assay, high performance liquid chromatography, and qRT-PCR were applied. Results showed that exogenous Spd significantly alleviated heat-induced stress damage. Spd application not only increased endogenous putrescine (Put), Spd, spermine (Spm), and total polyamine (PAs) accumulation, but also accelerated PAs oxidation and improved glutamic acid decarboxylase activity leading to GABA accumulation in leaves under heat stress. Spd-pretreated white clover maintained significantly higher chlorophyll (Chl) content than untreated plants under heat stress, which could be related to roles of Spd in up-regulating genes encoding Chl synthesis (PBGD and Mg-CHT) and maintaining lower Chl degradation (PaO and CHLASE) during heat stress. In addition, Spd up-regulated HSP70, HSP70B, and HSP70-5 expression that could have function of stabilizing denatured protein and helping proteins to folding correctly in white clover under high temperature stress. In summary, exogenous Spd treatment improves the heat tolerance of white clover by altering endogenous PAs and GABA content and metabolism, enhancing the antioxidant system and HSPs expression, and slowing down leaf senescence related to an increase in Chl biosynthesis and a decrease in Chl degradation during heat stress.