Daylength measurements by rice plants in photoperiodic short-day flowering.

Plants set seed at appropriate seasons. One major mechanism responsible for this adaptation involves photoperiodic flowering. Most plants are classified as either long-day plants, which flower under a longer photoperiod, or short-day plants, which flower under a shorter photoperiod. A third group, day-neutral plants, is not responsive to changes in photoperiod. During the past decade, molecular analysis has revealed at the molecular level how the long-day plant Arabidopsis thaliana measures daylength in photoperiodic flowering. In contrast, the molecular mechanisms underlying the responses of short-day plants are still under investigation. Progress in understanding photoperiodic flowering in rice (Oryza sativa), a short-day plant, revealed unique, evolutionarily conserved pathways involved in photoperiodic flowering at the molecular level. Furthermore, the conserved pathways promote flowering under short-day conditions and suppress flowering under long-day conditions in rice, but promote flowering under long-day conditions in Arabidopsis. In this chapter, we discuss the molecular mechanisms responsible for short-day flowering in rice in comparison with long-day flowering in Arabidopsis.