Reversal of photoperiodic flowering in temperate grasses rewired light inputs and floral outputs of the circadian clock

The transition from long-day (LD) to short-day (SD) flowering enabled temperate grasses of the tribe Stipeae (Poaceae: Pooideae) to colonise neotropical montane habitats, yet the transcriptomic basis of this shift remains poorly understood. Here, we used comparative transcriptomics and functional principal component analysis to examine diel gene expression in SD-flowering Nassella pubiflora and LD-flowering Oloptum miliaceum. Although photoperiod substantially rewired the diel transcriptome in both species, interspecific expression differences were limited to a small subset of orthologous genes. Divergence was most pronounced under SDs and involved floral integrators, light-signalling genes, and circadian clock targets, while core circadian clock gene expression remained largely conserved. These results suggest that the reversal from LD to SD flowering in Stipeae involved modifications to relatively few but central flowering pathway genes that convey photoperiod signals, rather than widespread changes to the endogenous timekeeping system. Our findings provide insight into the genetic architecture underlying niche transitions and photoperiod responsiveness in grasses and identify candidate genes that could potentially be targeted to optimise flowering time in cereal crops.