PHYTOCHROME-INTERACTING FACTORS (PIFs) are a group of basic helix-loop-helix transcription factors that can physically interact with photoreceptors, including phytochromes and cryptochromes. It was previously demonstrated that PIFs accumulated in darkness and repressed seedling photomorphogenesis, and that PIFs linked different photosensory and hormonal pathways to control plant growth and development. In this study, we show that PIFs positively regulate the ABA signaling pathway during the seedling stage specifically in darkness. We found that PIFs positively regulate ABI5 transcript and protein levels in darkness in response to exogenous ABA treatment by binding directly to the G-box motifs in the ABI5 promoter. Consistently, PIFs and the G-box motifs in the ABI5 promoter determine ABI5 expression in darkness, and overexpression of ABI5 could rescue the ABA-insensitive phenotypes of pifq mutants in the dark. Moreover, we discovered that PIFs can physically interact with the ABA receptors PYL8 and PYL9, and that this interaction is not regulated by ABA. Further analyses showed that PYL8 and PYL9 promote PIF4 protein accumulation in the dark and enhance PIF4 binding to the ABI5 promoter, but negatively regulate PIF4mediated ABI5 activation. Taken together, our data demonstrate that PIFs interact with ABA receptors to orchestrate ABA signaling in darkness by controlling ABI5 expression, providing new insights into the pivotal roles of PIFs as signal integrators in regulating plant growth and development.

Light and temperature are two core environmental factors that coordinately regulate plant growth and survival throughout their entire life cycle. However, the mechanisms integrating light and temperature signaling pathways in plants remain poorly understood. Here, we report that CBF1, an AP2/ERF‐family transcription factor essential for plant cold acclimation, promotes hypocotyl growth under ambient temperatures in Arabidopsis. We show that CBF1 increases the protein abundance of PIF4 and PIF5, two phytochrome‐interacting bHLH‐family transcription factors that play pivotal roles in modulating plant growth and development, by directly binding to their promoters to induce their gene expression, and by inhibiting their interaction with phyB in the light. Moreover, our data demonstrate that CBF1 promotes PIF4/PIF5 protein accumulation and hypocotyl growth at both 22°C and 17°C, but not at 4°C, with a more prominent role at 17°C than at 22°C. Together, our study reveals that CBF1 integrates light and temperature control of hypocotyl growth by promoting PIF4 and PIF5 protein abundance in the light, thus providing insights into the integration mechanisms of light and temperature signaling pathways in plants.

Phytochrome A (phyA) is the only plant photoreceptor that perceives far-red light and then mediates various responses to this signal. Phosphorylation and dephosphorylation of oat phyA have been extensively studied, and it was shown that phosphorylation of a serine residue in the hinge region of oat phyA could regulate the interaction of phyA with its signal transducers. However, little is known about the role of the hinge region of Arabidopsis phyA. Here, we report that three sites in the hinge region of Arabidopsis phyA (i.e., S590, T593, and S602) are essential in regulating phyA function. Mutating all three of these sites to either alanines or aspartic acids impaired phyA function, changed the interactions of mutant phyA with FHY1 and FHL, and delayed the degradation of mutant phyA upon light exposure. Moreover, the in vivo formation of a phosphorylated phyA form was greatly affected by these mutations, while our data indicated that the abundance of this phosphorylated phyA form correlated well with the extent of phyA function, thus suggesting a pivotal role of the phosphorylated phyA in inducing the far-red light response. Taking these data together, our study reveals the important role of the hinge region of Arabidopsis phyA in regulating phyA phosphorylation and function, thus linking specific residues in the hinge region to the regulatory mechanisms of phyA phosphorylation.Arabidopsis | far-red light | phyA | hinge region | phosphorylation

Phytochrome A (phyA) is the far-red (FR) light photoreceptor in plants that is essential for seedling de-etiolation under FR-rich environments, such as canopy shade. TANDEM ZINC-FINGER/PLUS3 (TZP) was recently identified as a key component of phyA signal transduction in Arabidopsis thaliana; however, how TZP is integrated into the phyA signaling networks remains largely obscure. Here, we demonstrate that ELONGATED HYPOCOTYL5 (HY5), a well-characterized transcription factor promoting photomorphogenesis, mediates FR light induction of TZP expression by directly binding to a G-box motif in the TZP promoter. Furthermore, TZP physically interacts with CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), an E3 ubiquitin ligase targeting HY5 for 26S proteasome-mediated degradation, and this interaction inhibits COP1 interaction with HY5. Consistent with those results, TZP post-translationally promotes HY5 protein stability in FR light, and in turn, TZP protein itself is destabilized by COP1 in both dark and FR light conditions. Moreover, tzp hy5 double mutants display an additive phenotype relative to their respective single mutants under high FR light intensities, indicating that TZP and HY5 also function in largely independent pathways. Together, our data demonstrate that HY5 and TZP mutually upregulate each other in transmitting the FR light signal, thus providing insights into the complicated but delicate control of phyA signaling networks.

Soil salinity is one of the most detrimental abiotic stresses affecting plant survival, and light is a core environmental signal regulating plant growth and responses to abiotic stress. However, how light modulates the plant’s response to salt stress remains largely obscure. Here, we show that Arabidopsis (Arabidopsis thaliana) seedlings are more tolerant to salt stress in the light than in the dark, and that the photoreceptors phytochrome A (phyA) and phyB are involved in this tolerance mechanism. We further show that phyA and phyB physically interact with the salt tolerance regulator SALT OVERLY SENSITIVE2 (SOS2) in the cytosol and nucleus, and enhance salt-activated SOS2 kinase activity in the light. Moreover, SOS2 directly interacts with and phosphorylates PHYTOCHROME-INTERACTING FACTORS PIF1 and PIF3 in the nucleus. Accordingly, PIFs act as negative regulators of plant salt tolerance, and SOS2 phosphorylation of PIF1 and PIF3 decreases their stability and relieves their repressive effect on plant salt tolerance in both light and dark conditions. Together, our study demonstrates that photo-activated phyA and phyB promote plant salt tolerance by increasing SOS2-mediated phosphorylation and degradation of PIF1 and PIF3, thus broadening our understanding of how plants adapt to salt stress according to their dynamic light environment.