A Negative Feedback Loop between PHYTOCHROME INTERACTING FACTORs and HECATE Proteins Fine-Tunes Photomorphogenesis in Arabidopsis

Abstract

The phytochrome interacting factors (PIFs), a small group of basic helix-loop-helix transcription factors, repress photomorphogenesis both in the dark and light. Light signals perceived by the phytochrome family of photoreceptors induce rapid degradation of PIFs to promote photomorphogenesis. Here, we show that HECATE (HEC) proteins, another small group of HLH proteins, antagonistically regulate PIFs to promote photomorphogenesis. HEC1 and HEC2 heterodimerize with PIF family members. PIF1, HEC1, and HEC2 genes are spatially and temporally coexpressed, and HEC2 is localized in the nucleus. hec1, hec2, and hec3 single mutants and the hec1 hec2 double mutant showed hyposensitivity to light-induced seed germination and accumulation of chlorophyll and carotenoids, hallmark processes oppositely regulated by PIF1. HEC2 inhibits PIF1 target gene expression by directly heterodimerizing with PIF1 and preventing DNA binding and transcriptional activation activity of PIF1. Conversely, PIFs directly activate the expression of HEC1 and HEC2 in the dark, and light reduces the expression of these HECs possibly by degrading PIFs. HEC2 is partially degraded in the dark through the ubiquitin/26S-proteasome pathway and is stabilized by light. HEC2 overexpression also reduces the light-induced degradation of PIF1. Taken together, these data suggest that PIFs and HECs constitute a negative feedback loop to fine-tune photomorphogenesis in Arabidopsis thaliana.

Reciprocal proteasome-mediated degradation of PIFs and HFR1 underlies photomorphogenic development in Arabidopsis

“…PIFs), and positively acting HLH factors (e.g. HFR1, PRE6/KIDARI, PAR1, PAR2, HECs and possibly others) (Fairchild et al, 2000;Fankhauser and Chory, 2000;Hyun and Lee, 2006;Kim et al, 2002;Lorrain et al, 2009;RoigVillanova et al, 2006;Zhou et al, 2014;Zhu et al, 2016). Similar to the established antagonistic relationship between bHLH and HLH proteins in eukaryotic systems (Littlewood and Evans, 1998;Toledo-Ortiz et al, 2003), HFR1 sequesters PIF1/PIF4/PIF5/PIF7 to regulate red light-induced seed germination and shade avoidance responses (Hersch et al, 2014;Hornitschek et al, 2009;Shi et al, 2013).…”

“…These dual mechanisms ensure inhibition of PIF activity to optimize plant development in response to light. This is in contrast with another small family of HLH proteins named HECATE, which stabilizes PIF1 (Zhu et al, 2016). Thus, bHLH-HLH interactions not only result in sequestration, but also post-translational regulation of protein levels.…”

“…A recent study showed that overexpression of HECATE2 (HEC2), a HLH transcription factor, stabilizes PIF1 in the dark and reduces the light-induced degradation of PIF1 (Zhu et al, 2016). HFR1, another HLH factor, was originally identified as an important positive regulator of phyA-mediated far-red light signaling and shade avoidance pathways (Fairchild et al, 2000;Fankhauser and Chory, 2000;Hersch et al, 2014;Lorrain et al, 2009).…”

“…Quantitative RT-PCR (RT-qPCR) for seedlings and seeds was performed as previously described Zhu et al, 2015Zhu et al, , 2016. For seedlings, total RNA of 3-or 4-day-old dark-grown seedlings were extracted with the Spectrum plant total RNA kit (Sigma-Aldrich).…”

Phytochromes and Phytochrome Interacting Factors

“…As HLH proteins, they do not bind to DNA directly by themselves; however, they sequester PIFs by heterodimerizing with them (Hornitschek et al, 2009;Hao et al, 2012;Shi et al, 2013;Zhu et al, 2016b). HFR1 and HEC1/HEC2 interact with PIF1 and inhibit seed germination in response to red and far-red light (Shi et al, 2013;Zhu et al, 2016b). HFR1, PAR1, and PAR2 interact with PIF4/PIF5 and inhibit shade avoidance responses Hao et al, 2012).…”