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Title: Phytochrome signaling in green Arabidopsis seedlings: impact assessment of a mutually-negative phyB-PIF feedback loop

Author
item LEVIAR, PABLO - University Of California
item MONTE, ELANA - Consejo Superior De Investigaciones Cientificas (CSIC)
item COHN, MEGAN - University Of California
item QUAIL, PETER - US Department Of Agriculture (USDA)

Submitted to: Molecular Plant
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2012
Publication Date: 5/3/2012
Citation: Leviar, P., Monte, E., Cohn, M.M., Quail, P.H. 2012. Phytochrome signaling in green Arabidopsis seedlings: impact assessment of a mutually-negative phyB-PIF feedback loop. Molecular Plant. 5:734-749.

Interpretive Summary: This study shows that a previously identified negative feedback loop that reduces the sensitivity of young seedlings to light by reducing the levels of the phyB photoreceptor, does not play a major role in modulating light-sensitivity in established green seedlings exposed to vegetational shade from neighboring plants. This mechanism, thus, appears not to play a major role in controlling the vegetative architecture of the plant, an important determinant of crop yield.

Technical Abstract: The reversibly red (R)-far-red (FR)-light-responsive phytochrome (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-Interacting 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 (a) reaffirm that phyB is the predominant, if not exclusive, photoreceptor imposing inhibition of hypocotyl elongation in deetiolating seedlings in response to prolonged continuous R irradiation, and (b) 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 diurnal 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.