Comparative Genomics and Functional Characterisation of the GIGANTEA Gene from the Temperate Forage Perennial Ryegrass Lolium perenne

Lolium perenne (ryegrass) is a perennial forage grass of worldwide importance. It is a temperate climate grass and flowering is induced by long day photoperiods. The agronomic productivity of ryegrass is strongly influenced by flowering time, but less is known about ryegrass flowering time regulators in other temperate Poaceae like wheat, barley and the model grass Brachypodium. The GIGANTEA (GI) gene was first identified in Arabidopsis and is an important regulator of photoperiodic flowering in angiosperms. GI usually exists as a conserved, single-copy gene. However, recently, genome sequencing has revealed that some plants, including the tropical grass maize, carry more than one full-length copy of GI. Here we describe the isolation and characterisation of the ryegrass L. perenne GIGANTEA gene (LpGI) gene. Comparative genomic analysis indicates that LpGI gene structure is very well conserved overall with GI genes from monocots and eudicots. LpGI protein clusters with GI proteins from other temperate grasses and is most closely related to the GI protein from meadow fescue. Genetic mapping locates LpGI to a chromosomal region that is syntenic with rice and Brachypodium GI. Our functional characterisation shows that LpGI shows a diurnal pattern of expression, continues to oscillate under constant light and adjusts its phase in response to changes in day length as seen in other plants. Constitutive expression of LpGI completely rescues the Arabidopsis gigantea-3 allele (gi-3) mutation, confirming that the isolated ryegrass gene is fully functional. Taken together, it is highly likely that LpGI is orthologous to Arabidopsis GI and involved in photoperiodic flowering time control in ryegrass.