Environmental Signal-Dependent Regulation of Flowering Time in Rice

Abstract

The transition from the vegetative to the reproductive stage of growth is a critical event in the lifecycle of a plant and is required for the plant's reproductive success. Flowering time is tightly regulated by an internal time-keeping system and external light conditions, including photoperiod, light quality, and light quantity. Other environmental factors, such as drought and temperature, also participate in the regulation of flowering time. Thus, flexibility in flowering time in response to environmental factors is required for the successful adaptation of plants to the environment. In this review, we summarize our current understanding of the molecular mechanisms by which internal and environmental signals are integrated to regulate flowering time in Arabidopsis thaliana and rice (Oryza sativa).

Keywords: drought; environmental signals; flowering time; photoperiod; regional adaptation; rice; temperature.

Figure 1

Photoperiodic regulation of FT expression is mediated by CO in Arabidopsis. Photoperiodic flowering in Arabidopsis is primarily mediated by transcriptional and posttranslational regulation of the transcription factor gene CONSTANS (CO). Under inductive long-day conditions, the CYCLING DOF FACTOR (CDF)-dependent repression of CO expression is diminished by the blue light-dependent FKF1-GI complex. FBHs and TCPs then induce CO expression. CO protein stability is further regulated by PHYB, SK12, PRRs, FKF1, CRYs, COP1, and SPAs. PHYB mediates red light dependent CO destabilization. SK12 interacts with CO and phosphorylate it for destabilization. During the night, CO is degraded by the COP1-SPAs complex. This COP1-SPA-dependent CO degradation is inhibited by FKF1 and COP1 in the presence of blue light. In addition, the circadian clock component PRRs interact with and stabilize CO. Once CO has been stabilized in the afternoon, it activates the transcription of FT, which in turn induces flowering. Under non-inductive short-day conditions, CDFs continuously repress the transcription of CO during the day. In addition, the PHYB-HOS1 and COP1-SPAs complexes inhibit CO accumulation, thereby preventing flowering under short-day conditions.

Figure 2

The regulatory network controlling Hd3a and RFT1 expression in rice. Flowering is regulated by two distinct pathways in rice, Hd1-Hd3a and Ghd7-Ehd1-Hd3a/RFT1. Under short-day conditions, Hd1 positively regulates Hd3a expression. DTH8 interacts with Hd1 to help upregulate Hd3a expression. The expression of Ehd1, which encodes another activator of Hd3a, is induced by Ehd2, Ehd3, and Ehd4 regardless of photoperiod. Under long-day conditions, Hd1 is converted to a negative regulator of Hd3a expression. Hd6 enhances the negative function of Hd1 on the Hd3a expression under long day conditions. In addition, Ghd7 acts as a repressor of Ehd1 expression, leading to the suppression of RFT1 expression. The Ghd7-dependent suppression of Ehd1 expression is enhanced by Hd16. Hd1 physically interacts with Ghd7 to repress Ehd1 expression under long-day conditions. OsPRR37 affects flowering by suppressing the transcription of Ehd1 and Hd3a under long day conditions, but activating expression of Ehd1 and Hd3a under short day conditions.