Soy J, Leivar P, González-Schain N, Sentandreu M, Prat S, Quail PH, Monte E.
Arabidopsis seedlings display rhythmic growth when grown under diurnal
conditions, with maximal elongation rates occurring at the end of the
night in short day photoperiods. Current evidence indicates that this
behavior involves the action of the growth-promotive
PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) and PIF5 bHLH factors at the end
of the night, through a coincidence mechanism that combines their
transcriptional regulation by the circadian clock with the control of
protein accumulation by light. Here, to assess the possible role of PIF3
in this process, we have analyzed the hypocotyl response and
marker-gene expression of pif single and higher order mutants.
The data show that PIF3 plays a prominent role as 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 capacity to regulate phytochrome B (phyB) levels.
Furthermore, in sharp contrast to PIF4 and PIF5, our data show that the PIF3
gene is not subjected to transcriptional regulation by the clock, but
that PIF3-protein abundance oscillates under diurnal conditions as a
result of a progressive decline in photoactivated-phyB-mediated
PIF3-protein degradation, 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, where it accelerates hypocotyl growth, together
with PIF4 and PIF5, by directly regulating the induction of
growth-related genes.