Regulation of Plant Responses to High Temperatures by Phytochrome-Interacting Factors

Light and temperature are critical environmental signals for the regulation of plant growth and development. Thus, plants have to properly sense and respond to changing light and temperature conditions in surrounding environments. In particular, as global warming intensifies, adaptation to changes in ambient temperatures is considered as a crucial issue that has direct impacts on plant survival and reproduction. The suite of morphological and architectural changes induced by elevated ambient temperatures are collectively referred as thermomorphogenesis, in which representative phenotypes include elongated hypocotyls and petioles in seedlings, stem elongation, hyponastic leaf growth, decrease in leaf thickness, and early flowering. Plant red and far-red light-sensing phytochrome B (phyB) acts as a temperature sensor and phytochrome-interacting factor (PIF)-auxin signaling module is known to play critical roles for plant responses to high temperatures or thermomorphogenesis. Especially, PIF4 is well-known to be a key transcription factor for regulating responses to high temperatures in plants. In general, phyB acts as a negative regulator of PIF4 at optimum temperature but phyB is inactivated at higher temperatures, which increases the protein stability of PIF4. Consequently, the accumulated PIF4 induces thermomorphogenic responses by inducing the expression of genes involved in auxin biosynthesis and responsiveness. PIF4 is regulated not only by phyB but also by a key component of the circadian clock, evening complex (EC), constitutive photomorphogenic 1 (COP1), and elongated hypocotyl 5 (HY5). Recently, PIF7 has also been reported as another essential transcription factor in regulating high temperature responses in plants. In particular, as PIF7 is known to play important roles in shade avoidance responses of plants, it is involved in the regulation of thermomorphogenic responses under shade combined with high temperature conditions. In these conditions, PIF7 is dephosphorylated (i.e., active form) and induces the expression of auxin biosynthesis genes. In this review, we intend to summarize the function of phyB and PIFs for the regulation of plant responses to high temperatures.