Signaling by the hormones brassinosteroid (BR) and gibberellin (GA) is critical to normal plant growth and development and is required for hypocotyl elongation in response to dark and elevated temperatures. Active BR signaling is essential for GA promotion of hypocotyl growth and suppresses the dwarf phenotype of GA mutants. Cross-talk between these hormones occurs downstream from the DELLAs, as GA-induced destabilization of these GA signaling repressors is not affected by BRs. Here we show that the light-regulated PIF4 (phytochromeinteracting factor 4) factor is a phosphorylation target of the BR signaling kinase BRASSINOSTEROID-INSENSITIVE 2 (BIN2), which marks this transcriptional regulator for proteasome degradation. Expression of a mutated PIF41A protein lacking a conserved BIN2 phosphorylation consensus causes a severe elongated phenotype and strongly up-regulated expression of the gene targets. However, PIF41A is not able to suppress the dwarf phenotype of the bin2-1 mutant with constitutive activation of this kinase. PIFs were shown to be required for the constitutive BR response of bes1-D and bzr1-1D mutants, these factors acting in an interdependent manner to promote cell elongation. Here, we show that bes1-D seedlings are still repressed by the inhibitor BRZ in the light and that expression of the nonphosphorylatable PIF41A protein makes this mutant fully insensitive to brassinazole (BRZ). PIF41A is preferentially stabilized at dawn, coinciding with the diurnal time of maximal growth. These results uncover a main role of BRs in antagonizing light signaling by inhibiting BIN2-mediated destabilization of the PIF4 factor. This regulation plays a prevalent role in timing hypocotyl elongation to late night, before light activation of phytochrome B (PHYB) and accumulation of DELLAs restricts PIF4 transcriptional activity.

The Arabidopsis PIF4 and BES1/BZR1 transcription factors antagonize light signaling by facilitating co‐activated expression of a large number of cell wall‐loosening and auxin‐related genes. While PIF4 directly activates expression of these targets, BES1 and BZR1 activity switch from a repressive to an activator function, depending on interaction with TOPLESS and other families of regulators including PIFs. However, the complexity of this regulation and its role in diurnal control of plant growth and brassinosteroid (BR) levels is little understood. We show by using a protein array that BES1, PIF4, and the BES1‐PIF4 complex recognize different DNA elements, thus revealing a distinctive cis‐regulatory code beneath BES1‐repressive and PIF4 co‐activation function. BES1 homodimers bind to conserved BRRE‐ and G‐box elements in the BR biosynthetic promoters and inhibit their expression during the day, while elevated PIF4 competes for BES1 homodimer formation, resulting in de‐repressed BR biosynthesis at dawn and in response to warmth. Our findings demonstrate a central role of PIF4 in BR synthesis activation, increased BR levels being essential to thermomorphogenic hypocotyl growth.

z These authors contributed equally to this work. SummaryThe cambium of woody plants cycles between active and dormant states. Dormancy can be subdivided into eco-and endodormant stages. Ecodormant trees resume growth upon exposure to growth-promotive signals, while the establishment of endodormant state results in loss of the ability to respond to these signals. In this paper, we analysed the regulation of cyclin-dependent kinases (CDKs) to understand the differential response of cell division machinery to growth-promotive signals during the distinct stages of dormancy in hybrid aspen. We show that 4 weeks of short-day (SD) treatment causes termination of the cambial cell division and establishment of the ecodormant state. This coincides with a steady decline in the histone H1 kinase activity of the PSTAIRE-type poplar CDKA (PttCDKA) and the PPTTLRE-type PttCDKB kinase complexes. However, neither the transcript nor the polypeptide levels of PttCDKA and PttCDKB are reduced during ecodormancy. In contrast, 6 weeks of SD treatment establishes endodormancy, which is marked by the reduction and disappearance of the PttCDKA and PttCDKB protein levels and the PttCDKB transcript levels. The transition to endodormancy is preceded by an elevated E2F (adenosine E2 promoter binding factor) phosphorylation activity of the PttCDKA kinase that reduces the DNA-binding activity of E2F in vitro. The transition to endodormancy is followed by a reduction of retinoblastoma (Rb) phosphorylation activity of PttCDKA protein complexes. Both phosphorylation events could contribute to block the G1 to S phase transition upon the establishment of endodormancy. Our results indicate that eco-and endodormant stages of cambial dormancy involve a stage-speci®c regulation of the cell cycle effectors at multiple levels.