Usually, moderately warm constant ambient temperatures tend to oppose light signals in the control of plant growth. However, in this study, Karayekov and co-workers showed that brief heat shocks enhanced the inhibition of hypocotyl growth induced by light in deetiolation Arabidopsis thaliana seedlings, and that the light was perceived by phytochrome B, a circadian clock input component of Arabidopsis. In the experiments with just the wild-type seedlings, the authors discovered a synergism between high temperature and light signals in hypocotyl responsiveness. In order to further study this synergistic mechanism, the authors tested heat shock and light combination conditions in many PhyA and PhyB single or double mutant because PhyA and PhyB are red light perceiving components in the clock network. They have found that any mutants of PhyB showed absence of the synergy between heat shock and light. They then test this synergy condition with other major clock mutants that were either the main clock genes or genes in PhyB singling pathway. As for the subsequent experiments, the authors used red light instead. The experimental transgenic plants were either single or double mutants of cca1 and/or lhy, single or triple mutants of prr3/5/7/9, hy5, cop1, Pif3/4/5, elf3/4, gi and toc1. Among all the mutants and wild-type plants, they have observed that the hypocotyl length was synergistically inhibited by the combination of heat shock and red light in wild-type, but this synergy was absent in phyB and phyAphyB mutants and reduced in some mutants like pif3/4/5, elf3/4 , cop1, hy5, gi, toc1, prr7/9 and cca1 lhy. They then used luciferase reporter assay and found out that heat shock could generate a rhythm of hypocotyl growth sensitive to red light. They then started to test the affect of heat shock and red light on all the mutants that had either absence or reduced synergy between heat shock and light. They found out through the experiments that heat shock reduced the PhyB nuclear body formation and nuclear abundance of COP1 under red light, but enhanced the stability of HY5. They also found out that heat shock treatments also generated circadian rhythms of CCA1 and LHY gene expression in darkness and that temperature gating of the hypocotyl growth response to red light required CCA1, LHY, PRR7 and PRR9, and that the plants had impaired PIF4 and PIF5 expression in cca1 lhy double mutant. Since all these circadian clock genes were connected in light perceiving signaling pathway, the authors have proposed a mechanism in which how plants response to heat shock and light signals according to the figure shown above. With the presence of heat shock, there are two signaling pathways that converge to enhance the sensitivity of hypocotyl growth to light perceived by PhyB. First, the heat decreases abundance of COP1 and causes an increase in HY5, which is a positive regulator of photomorphogenesis. The second pathway is that heat shock induces transient oscillation in the PRR7 and PRR9 expression, which leads to the oscillation of CCA1 and LHY expressions, and this causes an oscillation of PIF4 and PIF5 expressions, which are the negative regulator of photomorphogenesis. The authors from this paper also indicated that the convergence between heat shock and light signaling could affect a wide range of processes as revealed by RNA-seq analysis. As a result, the high temperature signals enhance the sensitivity to light through PhyB mediated signaling pathway in preparing the seedlings for deetiolation upon light exposure.
Reference: Karayekov, E., Sellaro, R., Legris, M., Yanovsky, M., & Casal, J. (2013). Heat Shock-Induced Fluctuations in Clock and Light Signaling Enhance Phytochrome B-Mediated Arabidopsis Deetiolation. The Plant Cell, 2892-2906.