BR-dependent phosphorylation modulates PIF4 transcriptional activity and shapes diurnal hypocotyl growth

Abstract

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.

Phytochromes and Phytochrome Interacting Factors

“…CK2 phosphorylates PIF1 (Bu et al, 2011b), while BIN2 phosphorylates PIF3 and PIF4 (Ling et al, 2017;Bernardo-García et al, 2014). However, these kinases are not necessary for the light-induced phosphorylation of PIFs in vivo.…”

“…The GSK3-like kinase BRASSINOSTEROID-INSENSITIVE (BIN2) has been shown to phosphorylate both PIF3 and PIF4 in vitro (Bernardo-García et al, 2014;Ling et al, 2017). BIN2-mediated phosphorylation promotes the degradation of PIF3 and PIF4 in the dark.…”

Reply: Interaction between Brassinosteroids and Gibberellins: Synthesis or Signaling? In Arabidopsis, Both!

“…In Arabidopsis and other species, BR mutants phenotypically resemble GA-deficient plants in impaired germination (Unterholzner et al, 2015), hypocotyl elongation, dwarfism, darker green leaves, late flowering, and reduced fertility (Clouse, 2011). Numerous studies have shown that BRs and GAs interact at the signaling level Gallego-Bartolomé et al, 2012;Li et al, 2012;Bernardo-García et al, 2014). GAs signal through the degradation of the DELLA transcriptional repressors (Schwechheimer, 2012;Daviè re and Achard, 2013), which is initiated following the perception of bioactive GA by the GA receptor GID1 (Griffiths et al, 2006;Willige et al, 2007).…”

CUL4 forms an E3 ligase with COP1 and SPA to promote light-induced degradation of PIF1

“…A mutant form of PIF1 deficient in CK2 phosphorylation sites displayed reduced degradation in response to light in vivo, suggesting CK2 promotes PIF1 degradation. BIN2 phosphorylates PIF4 in vitro and BIN2-mediated phosphorylation affects degradation of PIF4 under light 21 . However, both CK2-and BIN2-mediated phosphorylation of PIFs are light independent, suggesting that additional kinases are necessary for light-induced phosphorylation of PIFs 22 .…”