HYPERSENSITIVE TO RED AND BLUE 1 and its C-terminal regulatory
function
define a signaling step in photoperiodic control of FLOWER LOCUS T
expression
Summerary:
The red and far-red light-absorbing phytochromes and UV-A/blue
light-absorbing crytochromes regulate seedling de-etiolation and
photoperiodic flowering. The signaling steps mediating
photoreceptor
regulation on key genes that affect flowering time are largely
unknown. We
report here that a photomorphogenic mutant, hypersensitive to red
and blue
1 or hrb1, flowered late and attenuated the expression of FT under
both
long-day and short-day. In consistence, transgenic plants
overexpressing
full-length or C-terminal HRB1 flowered early and accumulated more
FT mRNA
under short-day. By contrast, HRB1 knockout or overexpression did
not
affect CO expression under either long-day or short-day, and HRB1
appears
to target the expression of FT through a CO
transcription-independent
pathway. The transgenic plants also displayed de-etiolation
phenotypes
opposite to that of hrb1 and the expression of hypocotyl and
cotyledon
phenotypes is PIF4-depedent. Moreover, hrb1 is epistatic to cry2-1
in
long-day flowering and FT expression, whereas phyB-9 is epistatic
to hrb1
in both long-day and short-day flowering and FT expression. HRB1
therefore
acts downstream of crys but modulates phyB signaling in the
photoperiodic
flowering pathway. HRB1 expression oscillates with different
patterns under
long-day and short-day, which is likely caused by a synchronized
light and
circadian regulation. Under constant light, HRB1 expression showed
a
free-running circadian rhythm and HRB1 behaved as a typical
morning gene.
The amplitude of its circadian expression was compromised in
phyB-9, gi-3,
and lhy-20. Overexpression of full-length HRB1 did not alter the
circadian
expression patterns of two central oscillators, TOC1 and CCA1.
HRB1
therefore represents a signaling step downstream of photoreceptors
and
circadian clock in photoperiodic control of FT expression.