Salt stress improves the low-temperature tolerance in sugar beet in which carbohydrate metabolism and signal transduction are involved

Low temperatures can inhibit crop growth and yield. To investigate the physiological mechanisms by which salt stress affects low temperature tolerance, sugar beet seedlings subjected to low temperature stress after treated with 0, 200 and 400 mM NaCl were analyzed for physiome, transcriptome and metabolome profiles. Salt stress significantly alleviated the maximum quantum efficiency of PS II (Fv/Fm) and the reduction of relative leaf water content (RWC) caused by low temperature. Salt stress enhanced reactive oxygen species (ROS) metabolism and significantly increased the activities of superoxide dismutase, peroxidase (POX) and dehydroascorbate reductase, and it also increased the gene expression of glutathione S-transferase, POX and L-ascorbate peroxidase. Meanwhile, salt stress also contributed to the increase in carbohydrate enzyme activities (e.g. vacuolar invertase, hexokinase and fructokinase) and gene expression. Salt stress resulted in a decrease in sucrose concentration and an increase in proline concentration. Sucrose concentration was significantly and negatively correlated with the gene expression of the signal transduction pathway. Additionally, the expression of genes related to abscisic acid (protein phosphatase 2 C and receptor), gibberellin signaling (DELLA and F-box protein) and 3-ketoacyl-CoA synthase was significantly increased by salt stress under low temperature. These results suggest that salt stress regulated ROS metabolism, carbohydrate metabolism and hormone signaling, thereby enhancing low temperature tolerance in sugar beet.