Cold acclimation induces freezing tolerance via antioxidative enzymes, proline metabolism and gene expression changes in two chrysanthemum species

Cold acclimation is necessary for chrysanthemum to achieve its genetically determined maximum freezing tolerance, but the underlying physiological and molecular mechanisms are unclear. The aim of this study was to discover whether changes in antioxidative enzymes, proline metabolism and frost-related gene expression induced by cold acclimation are related to freezing tolerance. Our results showed that the semi-lethal temperature (LT50) decreased from −7.3 to −23.5 °C in Chrysanthemum dichrum and −2.1 to −7.1 °C in Chrysanthemum makinoi, respectively, after cold acclimation for 21 days. The activities of SOD, CAT and APX showed a rapid and transient increase in the two chrysanthemum species after 1 day of cold acclimation, followed by a gradual increase during the subsequent days and then stabilization. qRT-PCR analysis showed that the expression levels of some isozyme genes (Mn SOD, CAT and APX) were upregulated, which was consistent with the SOD, CAT and APX activities, while others remained relatively constant (Fe SOD and Cu/Zn SOD). P5CS and PDH expression were increased under cold acclimation and the level of P5CS presented similar trends as proline content, indicating proline accumulation was via P5CS and PDH cooperation. Cold acclimation also promoted DREB, COR413 and CSD gene expression. The activities of three enzymes and gene expression were higher in C. dichrum than in C. makinoi after cold acclimation. Our data suggested that cold-inducible freezing-tolerance could be attributed to higher activity of antioxidant enzymes, and increased proline content and frost-related gene expression during different periods.