Mechanism of low-temperature tolerance in wucai induced by exogenous thermospermine

Wucai is an important specialty vegetable, and its low-temperature tolerance is associated with a polyamine, thermospermine (T-Spm) according to our previous study. However, the mechanism underlying the effect of T-Spm has not been elucidated yet. In this study, we investigated the relationship between T-Spm and cold tolerance of wucai by exogenously applying T-Spm to wucai seedlings exposed to low-temperature stress with water application as control. Under low-temperature condition, seedlings sprayed with water (LT) displayed a significant decrease in the photosynthesis parameters and stunted growth; however, foliar application of T-Spm to the seedlings (LT + T) led to marked increase in the net photosynthetic and transpiration rates, thereby alleviating growth inhibition and wilting phenotype. Compared with LT treatment, the LT + T group experienced simultaneous decrease in the malondialdehyde content, electrolyte leakage and O₂^•- generation rate by 40.3 %, 51.3 % and 44.0 %, respectively. LT treatment also led to increases in the superoxide dismutase activity and osmotic regulation substance contents, which were increased further after LT + T treatment. The ascorbate peroxidase activity of LT group was obviously declined on the seventh day, whereas that of LT + T treatment was maintained at a higher level, which underscored the role of T-Spm in enhancing antioxidant ability and reducing oxidative damage, eventually contributing to plant photosynthesis and growth. Furthermore, spermidine and T-Spm levels were markedly elevated after three days of LT treatment, and exogenous T-Spm application led to further increase in the content of polyamines, except for spermine. T-Spm treatment also promoted polyamine oxidase activity in response to low temperature, while its effect on the activities of ornithine and arginine decarboxylase was not pronounced; and genes encoding these enzymes showed similar variation trends in their expression, indicating that T-Spm treatment facilitated the homeostasis between PA and proline metabolism. Finally, transcriptome sequencing data revealed that exogenous T-Spm plays a role in regulating the expression of many genes involved in diverse processes, including MAPK signaling, suggesting the mediating role of these signaling pathways in T-Spm-induced cold tolerance.