Characterization of PHYTOCHROME B and CONSTANS: Key Regulators of Flowering Time in Sorghum

Abstract

Sorghum genotypes used for grain production in temperate regions are photoperiod insensitive, while energy sorghum is highly photoperiod sensitive and flowers late in long days, resulting in enhanced biomass accumulation. The sorghum floral repressors PSEUDORESPONSE REGULATOR PROTEIN (SbPRR37, Ma1) and GRAIN NUMBER, PLANT HEIGHT AND HEADING DATE 7 (SbGHD7, Ma6) contribute to photoperiod sensitivity by delaying flowering in long days with minimal influence in short days. Phytochrome B (PhyB) was found to mediate light signaling required for expression of the floral repressors SbPRR37 and SbGHD7 in the evening of long days and for photoperiod regulated flowering in sorghum. In genotypes lacking PHYB, SbPRR37 and SbGHD7 did not delay flowering in long days. The floral activators EARLY HEADING DATE 1 (SbEHD1) and SbCN8 (ZCN8) were highly expressed in long and short days in 58M (phyB-1) but repressed in 100M (PHYB) in long days. Sorghum alleles of CONSTANS (SbCO) were identified through QTL analysis. SbCO is an activator of flowering that is repressed post-transcriptionally in long days by the floral inhibitor SbPRR37, contributing to photoperiod sensitive flowering in Sorghum. Analysis of the flowering time QTL on SBI10 in Recombinant Inbred Lines (RILs) derived from BTx642 and Tx7000 revealed that BTx642 encodes a recessive CONSTANS allele containing a His106Tyr substitution in B-box 2. Genetic analysis characterized SbCO as a floral activator that promotes flowering by inducing the expression of SbEHD1, SbCN8 and SbCN12 (ZCN12). The floral repressor SbPRR37 inhibits SbCO activity and flowering in long days.

These studies further characterize the genetic regulatory pathway that modulates photoperiodic flowering-time in sorghum. The balance between repressors and activators provides the basis for a wide range of flowering times in response to diverse environmental factors contributing to sorghum’s wide adaptation.