Real-Time Monitoring of Key Gene Products Involved in Rice Photoperiodic Flowering.

Hayato Yoshioka,Yuko Ogo,Takeshi Izawa,Seiichi Toki,Keiko Kimura,Namie Ohtsuki,Ayako Nishizawa-Yokoi,Hironori Itoh

doi:10.3389/fpls.2021.766450

Abstract

Flowering is an important biological process through which plants determine the timing of reproduction. In rice, florigen mRNA is induced more strongly when the day length is shorter than the critical day length through recognition of 30-min differences in the photoperiod. Grain number, plant height, and heading date 7 (Ghd7), which encodes a CCT-domain protein unique to monocots, has been identified as a key floral repressor in rice, and Heading date 1 (Hd1), a rice ortholog of the Arabidopsis floral activator CONSTANS (CO), is another key floral regulator gene. The Hd1 gene product has been shown to interact with the Ghd7 gene product to form a strong floral repressor complex under long-day conditions. However, the mRNA dynamics of these genes cannot explain the day-length responses of their downstream genes. Thus, a real-time monitoring system of these key gene products is needed to elucidate the molecular mechanisms underlying accurate photoperiod recognition in rice. Here, we developed a monitoring system using luciferase (LUC) fusion protein lines derived from the Ghd7-LUC and Hd1-LUC genes. We successfully obtained a functionally complemented gene-targeted line for Ghd7-LUC. Using this system, we found that the Ghd7-LUC protein begins to accumulate rapidly after dawn and reaches its peak more rapidly under a short-day condition than under a long-day condition. Our system provides a powerful tool for revealing the accurate time-keeping regulation system incorporating these key gene products involved in rice photoperiodic flowering.

Highlights

The floral transition is an important biological event in which a plant switches from the vegetative phase to the reproductive phase
Heading date 1 (Hd1) alone promotes Early heading date1 (Ehd1) at night under short-day conditions, whereas Hd1 and Ghd7 cooperate to suppress Ehd1 transcription during daytime under long-day conditions through the formation of a floral suppressor complex including both Ghd7 and Hd1 gene products, which can bind to the promoter region of Ehd1 (Nemoto et al, 2016). This finding partially explains the molecular mechanism through which Hd1 promotes flowering under short-day conditions and suppresses it under long-day conditions, Ghd7 exhibits significant repressor activity even in a background with hd1 deficiency. These findings demonstrate that the diurnal dynamics of Hd1 and Ghd7 mRNA cannot explain how rice precisely recognizes day length and controls downstream genes such as Heading date 3a (Hd3a)/RICE FLOWERING LOCUS T 1 (RFT1) in leaves (Nemoto et al, 2016)
Two diphtheria toxin A fragment (DT-A) genes under the control of the maize polyubiquitin 1 promoter (Pubi) or rice elongation factor-1α promoter (Pef) (Terada et al, 2002) within the right and left border sequences of the T-DNA vector were used as negative selection marker genes in this study (NishizawaYokoi et al, 2015), whereas the hygromycin phosphotransferase (HPT) gene containing piggyBac (Cary et al, 1989) inverted repeat (IR) sequences as flanking sequences was used as a positive selection marker

Summary

Introduction

The floral transition is an important biological event in which a plant switches from the vegetative phase to the reproductive phase. Many plants monitor changes in external environmental factors such as temperature and the photoperiod to release offspring during the most favorable season. Based on extensive molecular genetic analysis of floral formation, florigen has been identified as evolutionarily conserved small proteins encoded by genes such as FLOWERING LOCUS T (FT) (Kardailsky et al, 1999; Kobayashi et al, 1999) in Arabidopsis thaliana and two orthologous genes. In A. thaliana, a long-day plant, the zinc-finger transcription factor CONSTANS (CO) directly induces FT gene expression in a photoperiod-dependent manner (Samach et al, 2000). The GI-CO-FT pathway is currently recognized as an evolutionarily conserved genetic pathway that controls flowering timing in many plants

Methods

Results

Conclusion