GIGANTEA promotes sorghum flowering by stimulating floral activator gene expression

The C4 grass Sorghum bicolor is an important grain and subsistence crop, animal forage, and cellulosic biofuel feedstock that is tolerant of abiotic stresses and marginal soils. Sorghum is short-day flowering, an obstacle for adaptation as a grain crop but a benefit as a biofuel feedstock. To identify genes underlying sorghum photoperiodic flowering behavior this study characterized the Sbgi-ems1 nonsense mutation in the sorghum GIGANTEA ( SbGI ) gene from a sequenced M4 EMS-mutagenized BTx623 population. Sbgi-ems1 plants had reduced stature and leaf blades exhibiting increased lateral growth combined with reduced proximal-distal growth. Mutant plants flowered later than normal siblings under long-day conditions provided by greenhouse or field. Delayed flowering in Sbgi-ems1 plants accompanied by an increase in internode number, indicating an extended vegetative growth phase prior to flowering. Sbgi-ems1 plants had reduced expression of floral activator genes SbCO and SbEhd1 and downstream FT-like florigen genes SbFT , SbCN8 , and SbCN12 . Therefore, SbGI accelerates flowering by promotion of SbCO and SbEhd1 expression. Circadian clock-associated genes SbTOC1 and SbLHY had disrupted expression in Sbgi-ems1 plants. This work demonstrates SbGI is a key upstream activator in the regulatory networks dictating sorghum flowering time and growth, as well as gene expression regulation within the circadian clock.