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

Abstract

We show that in the temperate grass, Brachypodium distachyon, PHYTOCHROME C (PHYC), is necessary for photoperiodic flowering. In loss-of-function phyC mutants, flowering is extremely delayed in inductive photoperiods. PHYC was identified as the causative locus by utilizing a mapping by sequencing pipeline (Cloudmap) optimized for identification of induced mutations in Brachypodium. In phyC mutants the expression of Brachypodium homologs of key flowering time genes in the photoperiod pathway such as GIGANTEA (GI), PHOTOPERIOD 1 (PPD1/PRR37), CONSTANS (CO), and florigen/FT are greatly attenuated. PHYC also controls the day-length dependence of leaf size as the effect of day length on leaf size is abolished in phyC mutants. The control of genes upstream of florigen production by PHYC was likely to have been a key feature of the evolution of a long-day flowering response in temperate pooid grasses.

Night-Break Experiments Shed Light on the Photoperiod1-Mediated Flowering

“…The different roles of PHYC on NB parallel the different roles played by this phytochrome in the photoperiodic response in wheat and rice. PHYC is a positive regulator of flowering time in some temperate grasses such as wheat, barley, and Brachypodium distachyon (Nishida et al, 2013;Chen et al, 2014;Woods et al, 2014) but has limited or no effect on flowering time in rice and Arabidopsis (Monte et al, 2003;Takano et al, 2005;Hu et al, 2013). These results suggest PHYC plays a more critical role in the photoperiod and NB response in the LD temperate grasses than in other plant species.…”

Genetic Architecture of Flowering-Time Variation in Brachypodium distachyon

“…The two major QTLs coincide with the location of the genes VRN1 and PHYC on chromosome 1 (QTL1) and VRN2 on chromosome 3 (QTL2). These genes have previously been shown to play important roles in flowering-time regulation in B. distachyon, and contribute to variation in flowering-time responses in wheat and barley (Woods et al, 2014b(Woods et al, , 2016Woods and Amasino 2015;Distelfeld et al, 2009). Thus, it appears that allelic variation in VRN1 and VRN2 likely contributes to flowering differences in an undomesticated pooid grass.…”

“…PHYC is an essential light receptor for photoperiodic flowering in pooids (Woods et al, 2014b;Chen et al, 2014) and allelic variation of PHYC in barley and pearl millet (Pennisetum glaucum) has been implicated in influencing flowering in these species (Nishida et variation at PHYC included a single nonsynonymous variant as well as several SNPs within the promoter region between Bd21 and Bd1-1, which might influence PHYC function. Mutations in PHYC result in a nonflowering phenotype in B. distachyon even after prolonged exposure to cold (Woods et al, 2014b); thus, the variants are unlikely to cause a loss of PHYC function given that both Bd21 and Bd1-1 respond to vernalization.…”

“…Depending upon the condition, QTL1 (peak marker Bd1_6006000) explains between 1.8% and 20.5% of the phenotypic variance observed in this mapping population (Supplemental Table S2). Although several genes are within the QTL interval (46 cM to 61cM in 20 h nonvernalized conditions), the tightly linked flowering-time genes VRN1 and PHYC are likely candidates because previous reverse and forward genetic studies have shown that both genes play important roles in flowering-time regulation in B. distachyon as well as in wheat (Woods et al, 2014b(Woods et al, , 2016Chen and Dubcovsky 2012;Chen et al, 2014). The major QTL on chromosome 3 (QTL2) was detected only under 16-h nonvernalized and 20-h nonvernalized conditions with peak LOD scores under 16-h (LOD 5.42) and 20-h (LOD 5.23; Fig.…”

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.…”