Integration of low temperature and circadian signals by a higher plant sigma factor. Plant sigma factors and cold

Low temperatures cause widespread alterations in plant physiology and development. This has selected for the evolution of mechanisms that allow plants to sense, respond and acclimate to cold temperatures. Chloroplasts are a defining feature of plant cells and their metabolism is influenced by temperature. However, it is not known how information about low temperature conditions is communicated to chloroplasts. Here, we show that in Arabidopsis, a nuclear-encoded sigma factor (SIGMA FACTOR5, SIG5) regulates the expression of a chloroplast gene in response to cold, and this requires the bZIP transcription factors HY5 and HYH. The response of this pathway to cold is gated by the circadian clock. The pathway also underlies changes in the nuclear-encoded transcriptome that occur in response to low temperature. The mechanism enhances the tolerance of photosynthesis to long-term cold and impacts lipid remodelling, but does not confer freezing tolerance. We identify a mechanism that integrates low temperature and circadian signals, and communicates this information to chloroplasts to modulate the response of plants to low temperatures.