Phytochromes as low temperature sensors in Arabidopsis.

The ability to efficiently synchronise internal processes with the external environment is essential to plant survival. The circadian clock is a timing mechanism to allow plants to maintain synchronicity between internal processes and changes in external environmental conditions. Both light and temperature are important external cues, indicating both day-length and season. The largest group of photoreceptors present in Arabidopsis are phytochromes. Most Arabidopsis accessions have five phytochromes (A-E). These different phytochrome isoforms have overlapping and distinct functions; we are elucidating their specific roles in detecting temperature. As many phytochromes play a role in detecting day length through duration of light, we are particularly interested in the integrated transitions of temperature and light at dusk and dawn. Extensive time series experiments with differing light and temperature regimes were carried out to compare the expression of both circadian clock and cold-regulated genes in Arabidopsis wild type and phytochrome mutant lines. We found that in ambient growth conditions of warm days and cool nights specific phytochromes help ensure the correct phasing of CBF transcription factor expression and amplitude of circadian clock gene expression. When entrained in non-physiological conditions of warm nights and cold days, phytochrome mutants were unable to express CBF and cold-regulated genes to the same level as the wild type plants but phasing of expression was similar. Furthermore, mutant lines showed significant morphological changes compared to wildtype under these stress conditions.