Light signalling mediated by phytochrome plays an important role in cold‐induced gene expression through the C‐repeat/dehydration responsive element (C/DRE) in Arabidopsis thaliana

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Abstract
Summary: Low temperature induces a number of genes that encode the proteins promoting tolerance to freezing, mediated by ABA‐dependent and ABA–independent pathways in plants. The cis‐acting element called C/DRE is known to respond to low temperature independently of ABA action. To investigate the signalling and network of ABA‐independent pathways, the transgenic Arabidopsis plants were generated containing several copies of the C/DRE derived from cor15a gene with a minimal promoter fused to a GUS reporter gene. The transgenic plants containing four copies of the C/DRE (4C/DRE‐GUS) showed responsiveness to cold and drought treatments and were used for characterization of cold signalling and cross‐talk. Cold‐induced GUS expression was inhibited by okadaic acid at 1 nm, indicating that protein phosphatase 2A might act as a positive regulator. Light was shown to activate cold‐ and drought‐induced GUS expression. Photo‐reversibility of the GUS mRNA by red and far‐red light with concomitant cold treatment suggests a role of phytochrome as a photoreceptor in mediating light signalling to activate the cold‐induced gene expression through the C/DRE. Furthermore, GUS expression analysis in phyA or phyB or phyAphyB mutant backgrounds showed that phytochrome B is a primary photoreceptor responsible for the activation of cold‐stress signalling in response to light. Light enhanced the induction kinetics of CBF1, 2, and 3 encoding the cognate transcription factors, and cor15a, in a consecutive manner compared to the dark condition in the cold, suggesting that the connection point between cold and light signalling mediated by phytochrome is at a higher step than the expression of CBF genes.