The Molecular Basis of Diversity in the Photoperiodic Flowering Responses of Arabidopsis and Rice

Fluctuations in the length of the day affect developmental processes and behaviors of many organisms. Mammals and birds reproduce in spring in response to lengthening days and insects pupate in autumn when daylength shortens. These phenomena, called photoperiodism, allow detection of seasonal changes and anticipation of environmental conditions such as low temperatures and desiccation. Photoperiodism was first described in detail by Garner and Allard in 1920 through the demonstration that many plants flower in response to changes in daylength Subsequently, they showed that some plant species promote flowering when daylength falls below a critical daylength, whereas other plants accelerate flowering in response to daylengths longer than a critical daylength. These plants are called short-day (SD) and long-day (LD) plants, respectively. During the last decade, molecular-genetic approaches were applied to understanding the control of flowering time, mainly in the LD plant Arabidopsis, and notable progress has been made in identifying the molecular mechanisms by which Arabidopsis recognizes daylength and promotes flowering specifically under LDs. Also, recent genetic studies in rice enabled the mechanisms of the daylength response in this SD plant to be compared with those of Arabidopsis. Here we review the recent advances in understanding the regulatory mechanisms for daylength response of flowering in Arabidopsis and compare them with those of rice.