Daylength Measurements by Rice Plants in Photoperiodic Short‐Day Flowering

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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.

Section snippets

Introduction: History of Studies on Photoperiodic Flowering Before Molecular Cloning

Photoperiodic flowering was first reported by Garner and Allard in the 1920s (Garner 1920, Garner 1923). Their research revealed that many plants flowered during appropriate seasons regardless of the timing of sowing. They subsequently demonstrated that flowering of these plants was controlled by photoperiod and categorized the plants into two groups: short‐day plants and long‐day plants (Table I). Subsequent physiological analysis of a range of species revealed that many short‐day plants

Photoperiodic Flowering in Arabidopsis

To compare the molecular mechanisms responsible for photoperiodic flowering in rice, a short‐day plant, with those in long‐day plants, it is helpful to summarize progress toward understanding the molecular mechanisms responsible for photoperiodic flowering in a well‐studied model plant. In this section, I discuss our knowledge of these mechanisms in Arabidopsis thaliana, a long‐day plant (Fig. 2).

Molecular genetics revealed four distinct flowering pathways in Arabidopsis: long‐day promotion,

External Coincidence Model in Rice

E. Bünning first proposed the involvement of circadian clocks in photoperiodic flowering in the 1930s (Bünning, 1960). Later, Pittendrigh and his colleagues refined Bünning's hypothesis and proposed the external coincidence model (Fig. 1) to explain how plants measure daylength (Pittendrigh and Minis, 1964). In this refined model, light signals play two distinct roles—one in the entrainment of circadian clocks and another as acute light signals that transmit the light conditions to downstream

Concluding Remarks

The survival strategy of plant species, including their adaptation to different areas, varies seasonally. There are three main types of adaptation in terms of the regulation of flowering time (Table III). Typical short‐day plants, including rice, represent one class. In this class, plants germinate in the spring and grow during the summer; they initiate floral transition under appropriate daylength conditions (i.e., time points in decreasing daylength) and flower in the middle of the summer so

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