Transcriptional activator that acts as floral inducer to promote short-day (SD) flowering pathway. Activates HD3A and other FT-like genes independently from HD1. May also activate MADS-box transcription factors involved in flowering regulation (PubMed:15078816). Functions as a response regulator involved in His-to-Asp phosphorelay signal transduction system. Phosphorylation of the Asp residue in the receiver domain activates the ability of the protein to promote the transcription of target genes. May directly activate some type-A response regulators in response to cytokinins (By similarity). Daily oscillation and diurnal expression in plants grown in short day (SD) but not in long day (LD) conditions. Two-component system major event consists of a His-to-Asp phosphorelay between a sensor histidine kinase (HK) and a response regulator (RR). In plants, the His-to-Asp phosphorelay involves an additional intermediate named Histidine-containing phosphotransfer protein (HPt). This multistep phosphorelay consists of a His-Asp-His-Asp sequential transfer of a phosphate group between first an His and an Asp of the HK protein, followed by the transfer to a conserved His of the HPt protein and finally the transfer to an Asp in the receiver domain of the RR protein. The cultivar Taichung 65 exhibits a long basic vegetative growth and reduced response to photoperiod due to loss-of-function alleles of HD1 and EHD1. This confers almost constant and sufficient vegetative growth periods even in low latitudes where short photoperiod conditions continue almost throughout the year. Belongs to the ARR family. Type-B subfamily.