Abstract
Plants respond to severe temperature changes by inducing the expression of numerous genes whose products enhance stress tolerance and responses. Dehydration-responsive element (DRE)-binding protein 1/C-repeat binding factor (DREB1/CBF) transcription factors act as master switches in cold-inducible gene expression. Since DREB1 genes are rapidly and strongly induced by cold stress, the elucidation of the molecular mechanisms of DREB1 expression is vital for the recognition of the initial responses to cold stress in plants. A previous study indicated that the circadian clock-related MYB-like transcription factors REVEILLE4/LHY-CCA1-Like1 (RVE4/LCL1) and RVE8/LCL5 directly activate DREB1 expression under cold stress conditions. These RVEs function in the regulation of circadian clock-related gene expression under normal temperature conditions. They also activate the expression of HSF-independent heat-inducible genes under high-temperature conditions. Thus, there are thought to be specific regulatory mechanisms whereby the target genes of these transcription factors are switched when temperature changes are sensed. We revealed that NIGHT LIGHT-INDUCIBLE AND CLOCK-REGULATED (LNK) proteins act as coactivators of RVEs in cold and heat stress responses in addition to regulating circadian-regulated genes at normal temperatures. We found that among the four Arabidopsis LNKs, LNK1 and LNK2 function under normal and high-temperature conditions, and LNK3 and LNK4 function under cold conditions. Thus, these LNK proteins play important roles in inducing specific genes under different temperature conditions. Furthermore, LNK3 and LNK4 are specifically phosphorylated under cold conditions, suggesting that phosphorylation is involved in their activation.