PHYTOCHROME C plays a major role in the acceleration of wheat flowering under long-day photoperiod

Abstract

Phytochromes are dimeric proteins that function as red and far-red light sensors influencing nearly every phase of the plant life cycle. Of the three major phytochrome families found in flowering plants, PHYTOCHROME C (PHYC) is the least understood. In Arabidopsis and rice, PHYC is unstable and functionally inactive unless it heterodimerizes with another phytochrome. However, when expressed in an Arabidopsis phy-null mutant, wheat PHYC forms signaling active homodimers that translocate into the nucleus in red light to mediate photomorphogenic responses. Tetraploid wheat plants homozygous for loss-of-function mutations in all PHYC copies (phyC AB ) flower on average 108 d later than wild-type plants under long days but only 19 d later under short days, indicating a strong interaction between PHYC and photoperiod. This interaction is further supported by the drastic down-regulation in the phyC AB mutant of the central photoperiod gene PHOTOPERIOD 1 (PPD1) and its downstream target FLOWERING LOCUS T1, which are required for the promotion of flowering under long days. These results implicate light-dependent, PHYC-mediated activation of PPD1 expression in the acceleration of wheat flowering under inductive long days. Plants homozygous for the phyC AB mutations also show altered profiles of circadian clock and clock-output genes, which may also contribute to the observed differences in heading time. Our results highlight important differences in the photoperiod pathways of the temperate grasses with those of well-studied model plant species.

PHYTOCHROME C Is an Essential Light Receptor for Photoperiodic Flowering in the Temperate Grass, Brachypodium distachyon

“…Our studies in Brachypodium demonstrate an essential role for PHYC in photoperiodic flowering (Figure 1 and Figure 3). While our article on the role of PHYC in Brachypodium was undergoing revision, it was reported that phyC mutations also result in delayed flowering in wheat, supporting PHYC's role regulating photoperiodic flowering broadly within Pooideae (Chen et al 2014). Thus, the acquisition by PHYC of a major role in flowering is likely to have been part of the evolutionary pathway to a LD requirement for flowering in the Poodieae.…”

“…In tetraploid wheat (cultivar Kronos), loss of PHYC activity results in a lack of FT expression and a substantial delay of flowering, but the delay of flowering is not as strong as that exhibited by the mvp mutant (Chen et al 2014). As noted by Chen et al (2014), the difference between the mvp phenotype in diploid wheat and the loss of PHYC phenotype in tetraploid wheat may be due to the additional genes deleted in the mvp mutant. That three independent Brachypodium phyC mutants all have a severe delayed flowering phenotype indicates that in a close relative of the crown pooids, loss of PHYC alone can create an mvp-like phenotype.…”

A Constitutively Active Allele of Phytochrome B Maintains Circadian Robustness in the Absence of Light

“…PhyD single mutants have a wild-type circadian phenotype but have an additive effect when introgressed into a phyB background (Devlin and Kay, 2000). Although recent studies on temperate grasses have established a direct role for phyC in photoperiod sensing (Chen et al, 2014;Woods et al, 2014), phyC has not been formally described as part of the circadian system in Arabidopsis. However, phyC is presumed to act similarly to phyE as a modulator of phyB activity since neither phyC nor phyE is capable of forming the homodimers necessary for signaling activity in Arabidopsis (Clack et al, 2009) or in rice (Oryza sativa; Xie et al, 2014), and instead, both function as heterodimers with other phys.…”

Molecular Regulation of Flowering Time in Grasses

“…In addition, studies have shown that PHYC plays an essential role in the acceleration of wheat flowering under LD photoperiods. Moreover, it is stable and functionally active even in the absences of other phytochromes, compared with rice and Arabidopsis [67]. Blue and far-red lights promote flowering in Arabidopsis and rice, acting through the action of PHYA, CRY1 and CRY2 photoreceptors in Arabidopsis, while red light delays flowering [12,33,43,67].…”

“…Moreover, it is stable and functionally active even in the absences of other phytochromes, compared with rice and Arabidopsis [67]. Blue and far-red lights promote flowering in Arabidopsis and rice, acting through the action of PHYA, CRY1 and CRY2 photoreceptors in Arabidopsis, while red light delays flowering [12,33,43,67]. On the other hand, PHYB modulates the expression of genes in response to red light and is the main component of the shade-avoidance mechanism in Arabidopsis.…”