Abstract
Light signaling by phytochrome B in long days inhibits flowering in sorghum by increasing expression of the long day floral repressors PSEUDORESPONSE REGULATOR PROTEIN (SbPRR37, Ma1) and GRAIN NUMBER, PLANT HEIGHT AND HEADING DATE 7 (SbGHD7, Ma6). SbPRR37 and SbGHD7 RNA abundance peaks in the morning and in the evening of long days through coordinate regulation by light and output from the circadian clock. 58 M, a phytochrome B deficient (phyB-1, ma3R) genotype, flowered ∼60 days earlier than 100 M (PHYB, Ma3) in long days and ∼11 days earlier in short days. Populations derived from 58 M (Ma1, ma3R, Ma5, ma6) and R.07007 (Ma1, Ma3, ma5, Ma6) varied in flowering time due to QTL aligned to PHYB/phyB-1 (Ma3), Ma5, and GHD7/ghd7-1 (Ma6). PHYC was proposed as a candidate gene for Ma5 based on alignment and allelic variation. PHYB and Ma5 (PHYC) were epistatic to Ma1 and Ma6 and progeny recessive for either gene flowered early in long days. Light signaling mediated by PhyB was required for high expression of the floral repressors SbPRR37 and SbGHD7 during the evening of long days. In 100 M (PHYB) the floral activators SbEHD1, SbCN8 and SbCN12 were repressed in long days and de-repressed in short days. In 58 M (phyB-1) these genes were highly expressed in long and short days. Furthermore, SbCN15, the ortholog of rice Hd3a (FT), is expressed at low levels in 100 M but at high levels in 58 M (phyB-1) regardless of day length, indicating that PhyB regulation of SbCN15 expression may modify flowering time in a photoperiod-insensitive manner.
Highlights
Flowering time has a significant impact on plant adaptation to agro-ecological environments, biomass accumulation and grain yield [1]
We report that PHYB is required for light activation of SbPRR37 and SbGHD7 expression in the evening of long days, resulting in repression of SbEHD1, SbCN12, SbCN8 and floral initiation
The peak of SbCO expression at dawn was reduced and of similar amplitude in plants entrained and sampled in short days (SD) (Figure 4A, lower). These results show that the peak of SbCO expression at dawn is dependent on PhyB, most likely because expression of SbPRR37 in the evening of Long day (LD) is dependent on PhyB (Figure 3A)
Summary
Flowering time has a significant impact on plant adaptation to agro-ecological environments, biomass accumulation and grain yield [1]. Floral initiation is regulated by plant development, photoperiod, shading, temperature, nutrient status, and many other factors [2,3,4,5]. Signals from many input pathways are integrated in the shoot apical meristem (SAM) through regulation of the meristem identity genes LEAFY (LFY) and APETALA1 (AP1), which are activated during transition of the SAM from a vegetative meristem to a floral meristem. Long day (LD) plants, such as Arabidopsis, flower earlier in LD compared to short days (SD). SD plants, such as rice and sorghum, show delayed floral initiation under LD conditions. Photoperiod regulated flowering is mediated by light signaling from photoreceptors and output from the endogenous circadian clock consistent with external coincidence models of flowering time regulation [6]
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