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Regulatory Genes and Enzymatic Complex of Flowering Time in Rice

Satyen Mondal
Bangladesh Rice Research Institute, Gazipur-1701, Bangladesh

Najeebul Rehman Sofi
Mountain Research Centre for Field Crops, Khudwani SKUAST-Kashmir 192102, India

M M Emam Ahmed
Bangladesh Rice Research Institute, Gazipur-1701, Bangladesh

Tuhin Halder
Bangladesh Rice Research Institute, Gazipur-1701, Bangladesh

Partha S Biswas *
Bangladesh Rice Research Institute, Gazipur-1701, Bangladesh

Abstract/ Executive Summary:

Flowering time (heading date) of the rice plant is considered an important agronomic trait for environmental adaptation and grain yield. It is controlled by multiple genes and is regulated by different environmental factors, such as day length, temperature, soil moisture, etc. So far, approximately 125 genes regulating flowering process and floral organ identity or development directly or indirectly have been reported in rice. Among these genes, Heading date 3a (Hd3a), RICE FLOWERING LOCUS T1 (RFT1), Heading date 5 (Hd5), MORF-RELATED GENE702 (MRG702), Casein kinases, CKI and CK2?, Pseudo-Response Regulator 37 (PRR37), Hd gene family have been reported as the key genes regulating flowering time in rice; however, their functions are mostly inter-related. Hd3a and RFT1 that encode florigens, are known as the floral transition genes in rice. In rice, florigen immediately induces downstream genes in the shoot apical meristem (SAM) to start the transition from vegetative to reproductive phase. RFT1 gene regulates flowering time with pivotal action while Hd3a, regulates under long day conditions. The Hd5 gene regulates flowering time in variation of early heading for adaptation depending on environmental signals. .

Keywords: Enzymes, Florigen, Flowering time, Genes, Rice

Location:

Start Date:

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Program Area: Variety and Species

Commodity: Rice

Objectives:

In this paper, we reviewed the functional role of the most recently reported genes and their alleles controlling heading date in rice plants.

Methodology:

Overview of responses of genes to heading date/ flowering time Rice heading date is one of the most important determinants of rice adaptability to various seasons and geographical regions. It is a complex trait controlled by multiple genes and is influenced by different environmental factors, particularly day length, temperature, and soil moisture. Several quantitative trait loci (QTLs) controlling flowering time have been mapped using different diverse rice germplasms and several genes have been identified by positional cloning strategies (Miura et al. 2011; Tsuji et al. 2011; Fujino et al. 2013). So far, approximately 125 genes were reported to be involved in the process of flowering in rice (Table 1). However, the majority of these genes are inter-related and epistatically controlled for their functions. In a QTL mapping work using 12 diverse rice cultivars from Asia, Ebana et al. (2011) showed that combinations of different alleles of QTLs for heading date could generate a large portion of the phenotypic variations in heading date. QTL analysis for very late heading from an indica variety, Nona Bokra, suggested that strong photoperiod sensitivity of this variety was mainly due to additive effects of particular alleles (Uga et al. 2007). It was found that genetic variations could be attributed to the coalition of alleles in loci involved in heading date (Fujino et al. 2013). We describe the detailed functions of few key genes involved in the regulation of flowering time in the next few sections. Role of Heading date5 (Hd5) controlling heading date Hd5 gene was identified on the short arm of chromosome 8 from the F2 population of a cross between Hayamasari and Hoshinoyume (Nonoue et al. 2008). A previous study revealed that days to heading (DTH) difference between Kitaibuki (Japonica) and Hoshinoyume was controlled by a single gene (Fujino 2011). The sequence analysis showed that Kitaibuki carries a loss function of Hd5 with a 19 bp deletion (Gen Bank accession number AB693201), the same as for Hayamasari, while Hoshinoyume carries the functional Hd5 (Fujino et al. 2013); and simultaneously, Fujino et al. (2013) found differences in heading date and photoperiod sensitivity among different japonica varieties (Fig. 1). The transgenic lines overexpressed with functional Hd5 gene showed delayed DTH than that of Hayamasari under long day (LD) conditions. These results clearly revealed that allelic variations in Hd5 such as functional and loss-of-function are major factors influencing heading date in Hoshinoyume with Kitaibuki and Hayamasari. In a separate study in Hokkaido, Japan (2003-2004) with Hd5 gene carrying 19 bp deletion and non-deletion showed a significant (P = 0.0039) difference in flowering time between the two groups (Fujino et al. 2013). These results revealed that the loss-of-function Hd5 has a vital role in generating early heading on variations. Loss of-function of Hd5 was also reported as a major determinant of early heading among the tested populations by Kim et al. (2013) and could contribute to extremely early heading behavior in presence of (strong floral repressor) Ghd7, while Shibaya et al. (2011) reported loss-of -function of qDTH7/ghd7 for extremely early heading varieties. In some cases, Hd5 expressed epistatic interactions with Hd1 and Ghd7, thus causing these various effects on dates of heading (Nonoue et al. 2008; Xue et al. 2008; Yan et al. 2011). Nevertheless, heading date genes such as Hd5 control agronomic features like plant height and yield potential as well (Wei et al. 2010; Yan et al. 2011). Flowering time controlling Heading date gene family and Early heading date 1 (Ehd1) Heading-date 1 (Hd1) is the first gene cloned through natural variation incur for rice flowering time (Yano et al. 2000; Hori et al. 2016). It is one of the principal flowering time controlling genes, which is an ortholog of Arabidopsis CONSTANS (CO), promoting transition to flower in shortday (SD) and suppresses it strongly in LD conditions (Endo-Higashi and Izawa 2011). Hd1 is homologous to SE1 (Photoperiodic Sensitivity 1), one of the major QTLs for photoperiodic sensitivity. Los of-function of Hd1 causes delayed flowering in SD but early flowering under long day conditions (Inoue et al. 1992; Lin et al. 2000; Gomez-Ariza et al. 2015), demonstrating that Hd1 may act either as a flowering activator or repressor, but depends on light periods. However, how Hd1 functions differently between SD and LD is still unknown. Ishikawa et al. (2005) observed that the over expression of Hd1 gene induced late-flowering but they did not notice any phenotypic expression in the OsphyB mutant background suggesting OsPhyB is necessary for the repressive function of Hd1. Hd1 is a member of the CO-like proteins family, which contains conserved B-box zinc fingers as well as CCT domains (Robson et al. 2001; Griffiths et al. 2003). A strong flowering time regulating gene, Early heading date 1 (Ehd1), induces floral transition under short day (SD) rather than in LD conditions (Endo-Higashi and Izawa 2011; Zhao et al. 2015). Ehd1 is successively repressed by the upregulation of Ghd7 under LD conditions; this partially describes the critical day length identification of Heading date 3a (Hd3a), a rice florigen transcriptional control (Itoh et al. 2010). In the case of crop production, controlling of flowering time is often encompassed with yield-related features, as it is speculated that the longer vegetative periods result in larger biomass.

Source of Information: Plant Breed. Biotech. 2019 (September) 7(3):161~174 Online ISSN: 2287-9366 Print ISSN: 2287-9358

Article Link (Online Journal): https://doi.org/10.9787/PBB.2019.7.3.161

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Knowledge Management: Journal