Designing Flowering Times for Climate Optimized Grain and Energy Sorghum Crops

Sorghum is widely used for production of grain, forage, sugar and biomass.  The crop is especially important in the semi-arid tropics where adverse growing conditions occur frequently and yield stability is of high value.  Flowering time has a large impact on yield potential and yield stability.  For grain crops, early flowering often maximizes yield stability by avoiding adverse environments during the reproductive phase.  In contrast, late flowering and long vegetative growth duration increases biomass yield potential of energy sorghum.  Our understanding of the genetics of flowering time regulation has advanced recently to the point where inbreds and hybrids with predictable flowering times can be constructed through selection of alleles for key genes in this regulatory pathway.  Sorghum is a short day plant, and flowering is delayed in long days by repressors encoded by SbPRR37 and SbGHD7.   Grain sorghum inbreds and hybrids used at higher latitudes are largely photoperiod insensitive because they encode null or weak alleles of SbPRR37 and SbGHD7.  Flowering time variation in photoperiod insensitive grain sorghum varies in part due to alleles of EHD1 and CONSTANS, activators of SbCN8/SbCN12, the source of florigen that induces floral initiation.  Therefore, the flowering time of grain sorghum can be optimized in short day environments using alleles of these floral activators.  Delayed flowering in energy and forage sorghums in long day environments is mediated by light and clock regulated expression of SbPRR37 and SbGHD7.  Light signals that modulate SbPRR37 and SbGHD7 expression are mediated by PhyB and PhyC therefore alleles of these genes can be used to optimize flowering time in long days.  This system has also been used to facilitate production of photoperiod sensitive hybrids using specially constructed genotypes of early flowering inbred lines that can be propagated in regions optimal for hybrid seed production.   Current knowledge of the sorghum flowering time pathway and alleles of key genes allows breeders to optimize flowering times for specific sorghum crop types (grain, forage, energy) within their regions of production.   Additional yield gains may be realized once we learn how sorghum modifies flowering time in response to variation in temperature.