The Basic Helix-loop-helix Transcription Factor PIF5 Acts on Ethylene Biosynthesis and Phytochrome Signaling by Distinct Mechanisms

PHYTOCHROME-INTERACTING FACTOR5 (PIF5), a basic helix-loop-helix transcription factor, interacts specifically with the photoactivated form of phytochrome B (phyB). Here, we report that dark-grown Arabidopsis thaliana seedlings overexpressing PIF5 (PIF5-OX) exhibit exaggerated apical hooks and short hypocotyls, reminiscent of the triple response induced by elevated ethylene levels, whereas pif5 mutants fail to maintain tight hooks like those of wild-type seedlings. Silver ions, an ethylene receptor blocker, rescued the triple-response phenotype, and we show that PIF5-OX seedlings express enhanced levels of key ethylene biosynthesis enzymes and produce elevated ethylene levels. Exposure of PIF5-OX seedlings to prolonged continuous red light (Rc) promotes hypocotyl elongation relative to dark controls, the reciprocal of the Rc-imposed hypocotyl inhibition displayed by wild-type seedlings. In contrast with this PIF5-OX hyposensitivity to Rc, pif5 mutant seedlings are hypersensitive relative to wild-type seedlings. We show that this contrast is due to reciprocal changes in phyB protein levels in prolonged Rc. Compared with wild-type seedlings, PIF5-OX seedlings have reduced, whereas pif5 mutants have increased, phyB (and phyC) levels in Rc. The phyB degradation in the overexpressors depends on a functional phyB binding motif in PIF5 and involves the 26S proteasome pathway. Our data thus indicate that overexpressed PIF5 causes altered ethylene levels, which promote the triple response in darkness, whereas in the light, the interaction of photoactivated phyB with PIF5 causes degradation of the photoreceptor protein. The evidence suggests that endogenous PIF5 negatively regulates phyB-imposed hypocotyl inhibition in prolonged Rc by reducing photoreceptor abundance, and thereby photosensory capacity, rather than functioning as a signaling intermediate.