Phytochrome‐imposed oscillations in PIF3 protein abundance regulate hypocotyl growth under diurnal light/dark conditions in Arabidopsis

Abstract

SUMMARY Arabidopsis seedlings display rhythmic growth when grown under diurnal conditions, with maximal elongation rates occurring at the end of the night under short-day photoperiods. Current evidence indicates that this behavior involves the action of the growth-promoting bHLH factors PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) and PHYTOCHROME-INTERACTING FACTOR 5 (PIF5) at the end of the night, through a coincidence mechanism that combines their transcriptional regulation by the circadian clock with control of protein accumulation by light. To assess the possible role of PIF3 in this process, we have analyzed hypocotyl responses and marker gene expression in pif single- and higher-order mutants. The data show that PIF3 plays a prominent role as a promoter of seedling growth under diurnal light/dark conditions, in conjunction with PIF4 and PIF5. In addition, we provide evidence that PIF3 functions in this process through its intrinsic transcriptional regulatory activity, at least in part by directly targeting growth-related genes, and independently of its ability to regulate phytochrome B (phyB) levels. Furthermore, in sharp contrast to PIF4 and PIF5, our data show that the PIF3 gene is not subject to transcriptional regulation by the clock, but that PIF3 protein abundance oscillates under diurnal conditions as a result of a progressive decline in PIF3 protein degradation mediated by photoactivated phyB, and consequent accumulation of the bHLH factor during the dark period. Collectively, the data suggest that phyB-mediated, post-translational regulation allows PIF3 accumulation to peak just before dawn, at which time it accelerates hypocotyl growth, together with PIF4 and PIF5, by directly regulating the induction of growth-related genes.

Photobody Localization of Phytochrome B Is Tightly Correlated with Prolonged and Light-Dependent Inhibition of Hypocotyl Elongation in the Dark

“…Among the PIFs, PIF3 plays an import role in promoting hypocotyl growth in the dark under diurnal conditions; in particular, PIF3 is not regulated at the transcriptional level by the circadian clock but mainly at the step of protein degradation by photoactivated phytochromes (Soy et al, 2012). Therefore, we decided to use PIF3 as a model to examine whether phytochromemediated PIF3 degradation is differentially regulated in the PBG and NGB lines during the R-to-D transition.…”

“…a dramatic decrease in hypocotyl growth inhibition under diurnal conditions (Elich and Chory, 1997). Recently, it has been shown that the Pfr of phyB is responsible for repressing PIF1 and PIF3 accumulation and, consequently, hypocotyl growth in the dark (Soy et al, 2012(Soy et al, , 2014. Interestingly, the in vivo dark reversion rate of phyB is much slower than the rate measured in vitro, and photobodies have been proposed through mathematical modeling to be involved in stabilizing the Pfr of phyB in the dark (Rausenberger et al, 2010).…”

“…The circadian clock mediates an increase in PIF4 and PIF5 gene expression at the end of the night (Nozue et al, 2007;Leivar et al, 2012a;Soy et al, 2012). In parallel, because of the slow dark reversion rate of phyB (Hennig et al, 1999;Rausenberger et al, 2010), photoactivated phyB persists into the night to inhibit the accumulation of PIFs (Nozue et al, 2007;Soy et al, 2012Soy et al, , 2014. Based on these recent results, it would be interesting to investigate whether photobodies are required for phyB-mediated hypocotyl growth inhibition and PIF stability in the dark.…”

“…In particular, recent studies show that, although plants perceive light during the day, under short days the maximum hypocotyl growth rate occurs at the end of the night (Nozue et al, 2007). Hypocotyl growth during the dark period is mediated by PIF3 as well as by PIF1, PIF4, and PIF5; the levels of PIFs are coincidentally regulated by the circadian clock and light (Nozue et al, 2007;Leivar et al, 2012a;Soy et al, 2012Soy et al, , 2014. The circadian clock mediates an increase in PIF4 and PIF5 gene expression at the end of the night (Nozue et al, 2007;Leivar et al, 2012a;Soy et al, 2012).…”

“…Hypocotyl growth during the dark period is mediated by PIF3 as well as by PIF1, PIF4, and PIF5; the levels of PIFs are coincidentally regulated by the circadian clock and light (Nozue et al, 2007;Leivar et al, 2012a;Soy et al, 2012Soy et al, , 2014. The circadian clock mediates an increase in PIF4 and PIF5 gene expression at the end of the night (Nozue et al, 2007;Leivar et al, 2012a;Soy et al, 2012). In parallel, because of the slow dark reversion rate of phyB (Hennig et al, 1999;Rausenberger et al, 2010), photoactivated phyB persists into the night to inhibit the accumulation of PIFs (Nozue et al, 2007;Soy et al, 2012Soy et al, , 2014.…”