GIGANTEA regulates photoperiodic flowering and photoperiod-independent flowering in Brachypodium distachyon

Sensing daylength as a seasonal cue to determine when to flower is a critical adaptation for many plants. Here, we identify a photoperiod-insensitive mutant in Brachypodium distachyon caused by a null mutation in a circadian clock regulated gene named GIGANTEA (GI). We show that the gi mutant flowers around 100 days after germination regardless of daylength, unlike wildtype which flowers rapidly in long days (LD) and never flowers in short days (SD). Thus, this single-gene mutation converts Brachypodium distachyon from a long-day plant into a day-neutral plant. Gene expression and genetic interaction analyses show that the delayed flowering of the gi mutant in LD is associated with altered expression of flowering-promoting genes FLOWERING LOCUS T1 (FT1), VERNALIZATION1 (VRN1), PHOTOPERIOD1 (PPD1), and CONSTANS1 (CO1) as well as the flowering-inhibiting gene VERNALIZATION2 (VRN2), whereas the ability to flower in SD is associated with the accumulation of flowering promoters FLOWERING LOCUS T2 (FT2) and FRUITFUL2 (FUL2). Furthermore, gi is epistatic to a delayed-flowering mutation in the light receptor PHYTOCHROME C and a rapid-flowering mutation in the circadian clock gene EARLY FLOWERING3, indicating the important role of GI in the photoperiod response and circadian clock regulation in relation to the timing of flowering.