The reversibly red (R)/far-red (FR)-Iight-responsive phytoch rome (phy) photosensory system initiates both the deetiolation process in dark-germinated seedlings upon first exposure to light,and the shade-avoidance process in fully deetiolated seedlings upon exposure to vegetational shade.The intracellular signaling pathway from the light-activated photoreceptor conformer (Pfr) to the transcriptional network that drives these responses involves direct,physical interaction of Pfr with a small subfamily of bHLH transcription factors,termed Phy-lnteracting Factors (PIFs),which induces rapid PIF proteolytic degradation.In addition,there is evidence of further complexity in light-grown seedlings,whereby phyB-PIF interaction reciprocally induces phyB degradation,in a mutually-negative,feedback-loop configuration.Here,to assess the relative contributions of these antagonistic activities to the net phenotypic readout in light-grown seedlings,we have examined the magnitude of the light- and simulated-shade-induced responses of a pentuple phyBpif1pif3pif4pif5 (phyBpifq) mutant and various multiple pif-mutant combinations.The data (1) reaffirm that phyB is the predominant,if not exclusive,photoreceptor imposing the inhibition of hypocotyl elongation in deetiolating seedlings in response to prolonged continuous R irradiation and (2) show that the PIF quartet (PIF1,PIF3,PIF4,and PIF5) retain and exert a dual capacity to modulate hypocotyl elongation under these conditions,by concomitantly promoting cell elongation through intrinsic transcriptional-regulatory activity,and reducing phyB-inhibitory capacity through feedback-loop-induced phyB degradation.In shade-exposed seedlings,immunoblot analysis shows that the shade-imposed reduction in Pfr levels induces increases in the abundance of PIF3,and mutant analysis indicates that PIF3 acts,in conjunction with PIF4 and PIF5,to promote the known shade-induced acceleration of hypocotyl elongation.Conversely,although the quadruple pifq mutant displays clearly reduced hypocotyl elongation compared to wild-type in response to prolonged shade,immunoblot analysis detects no elevation in phyB levels in the mutant seedlings compared to the wild-type during the majority of the shade-induced growth period,and phyB levels are not robustly correlated with the growth phenotype across the pif-mutant combinations compared.These results suggest that PIF feedback modulation of phyB abundance does not play a dominant role in modulating the magnitude of the PIF-promoted,shade-responsive phenotype under these conditions.In seedlings grown under diual light-dark cycles,the data show that FR-pulse-induced removal of Pfr at the beginning of the dark period (End-of-Day-FR (EOD-FR) treatment) results in longer hypocotyls relative to no EOD-FR treatment and that this effect is attenuated in the pif-mutant combinations tested.This result similarly indicates that the PIF quartet members are capable of intrinsically promoting hypocotyl cell elongation in light-grown plants,independently of the effects of PIF feedback modulation of photoactivated-phyB abundance.