Sorghum Phytochrome B Inhibits Flowering in Long Days by Activating Expression of SbPRR37 and SbGHD7, Repressors of SbEHD1, SbCN8 and SbCN12

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.

Transcriptome Profiling of Tiller Buds Provides New Insights into PhyB Regulation of Tillering and Indeterminate Growth in Sorghum

“…Sorghum and maize florigens are produced in leaves by ZCN8/ SbCN8 and ZCN12/SbCN12 (Danilevskaya et al, 2010;Murphy et al, 2011). Flowering in sorghum is inhibited in long day photoperiods by repressing the expression of SbCN8 and SbCN12 through a pathway that requires phyB-signaling (Murphy et al, 2011;Yang et al, 2014). Therefore, phyB-1 or shaded plants have greater propensity to produce FT, and increased expression of TFL1 (SbCN2) in axillary buds of these plants may be needed to prevent precocious floral transition.…”

Molecular Regulation of Flowering Time in Grasses

“…In SD conditions, PHYB may have a limited effect on the expression of these genes as peak SbPRR37 and SbGHD7 expression is highest in the morning and lowest during the evening compared with expression in LD. The inactivation of PHYB results in early flowering in LD [43,52].…”

“…This result emphasizes the importance of adaptation in plants to the broad range of agro-environmental conditions in which they grow. In sorghum, three significant QTL associated with flowering time, PHYB (Ma3), PHYC (Ma5) [52] and SbGHD7 (Ma6) [14,52] were identified, through analysis of flowering variation in LD using an F 2 population, which explained~50% of the phenotypic variance for flowering time [52]. Recessive ma3R alleles from 58 M populations associated with Ma3 QTL produced early flowering time phenotypes; however, dominant alleles of SbGhd7 (Ma6) and SbPRR37 act in an additive manner to delay floral initiation for~175 days until day-lengths decrease below 12.3 h [14,52].…”

“…The photoperiod response on flowering time varies among grasses. Barley (Hordeum vulgare L.) and wheat are LD plants, while rice and sorghum are SD plants [52]. Flowering is regulated through the CO and FT genes [66].…”

“…For example, over-expression of the photoreceptor PHYA under SD conditions promotes flowering, but phyA mutants delay flowering in LD conditions. In contrast to Arabidopsis, rice and sorghum encodes three phytochromes (PHYA, PHYB and PHYC) [12,52], where phyA mutants of rice do not produce significant alterations in flowering time [52,63]. This is despite the high similarity between the PHYA locus in Arabidopsis rice, sorghum and maize [62] suggesting that a similar response would be expected.…”