RICE FLOWERING LOCUS T 1 Research Articles

Research Progress on Photoperiod Gene Regulation of Heading Date in Rice.

Heading date is a critical physiological process in rice that is influenced by both genetic and environmental factors. The photoperiodic pathway is a primary regulatory mechanism for rice heading, with key florigen genes Hd3a (Heading date 3a) and RFT1 (RICE FLOWERING LOCUS T1) playing central roles. Upstream regulatory pathways, including Hd1 and Ehd1, also significantly impact this process. This review aims to provide a comprehensive examination of the localization, cloning, and functional roles of photoperiodic pathway-related genes in rice, and to explore the interactions among these genes as well as their pleiotropic effects on heading date. We systematically review recent advancements in the identification and functional analysis of genes involved in the photoperiodic pathway. We also discuss the molecular mechanisms underlying rice heading date variation and highlight the intricate interactions between key regulatory genes. Significant progress has been made in understanding the molecular mechanisms of heading date regulation through the cloning and functional analysis of photoperiod-regulating genes. However, the regulation of heading date remains complex, and many underlying mechanisms are not yet fully elucidated. This review consolidates current knowledge on the photoperiodic regulation of heading date in rice, emphasizing novel findings and gaps in the research. It highlights the need for further exploration of the interactions among flowering-related genes and their response to environmental signals. Despite advances, the full regulatory network of heading date remains unclear. Further research is needed to elucidate the intricate gene interactions, transcriptional and post-transcriptional regulatory mechanisms, and the role of epigenetic factors such as histone methylation in flowering time regulation. This review provides a detailed overview of the current understanding of photoperiodic pathway genes in rice, setting the stage for future research to address existing gaps and improve our knowledge of rice flowering regulation.

Knockout of OsWOX13 moderately delays flowering in rice under natural long-day conditions.

Plants are sensitive to photoperiods and are also equipped with systems to adjust their flowering time in response to various changes in the environment and developmental hormones. In the present study, previously generated rice OsWOX13 overexpression and newly generated OsWOX13 knockout lines constructed via CRISPR/Cas9 technology flowered 10 days earlier and 4-6 days later than the wild type, respectively. Quantitative real-time polymerase chain reaction analyses revealed that OsWOX13 might be involved in drought escape responses through the b-ZIP TRANSCRIPTION FACTOR 23 signaling pathway during rice flowering via photoperiod signaling genes such as Grain number, plant height and heading date 7, Early heading date 1, RICE FLOWERING LOCUS T1, Heading date 3a, and MADS14. Future investigations of OsWOX13 may provide insight into how plants adjust their flowering under stress conditions and how OsWOX13 could be precisely controlled to achieve maximum productivity in rice breeding.

Life-Cycle Multiomics ofRice Shoots Reveals Growth Stage-Specific Effects ofDrought Stress andTime-Lag Drought Responses.

Field-grown rice plants are exposed to various stresses at different stages of their life cycle, but little is known about the effects of stage-specific stresses on phenomes and transcriptomes. In this study, we performed integrated time-course multiomics on rice at 3-d intervals from seedling to heading stage under six drought conditions in a well-controlled growth chamber. Drought stress at seedling and reproductive stages reduced yield performance by reducing seed number and setting rate, respectively. High temporal resolution analysis revealed that drought response occurred in two steps: a rapid response via the abscisic acid (ABA) signaling pathway and a slightly delayed DEHYDRATION-RESPONSIVE ELEMENT-BINDING PROTEIN (DREB) pathway, allowing plants to respond flexibly to deteriorating soil water conditions. Our long-term time-course multiomics showed that temporary drought stress delayed flowering due to prolonged expression of the flowering repressor gene GRAIN NUMBER, PLANT HEIGHT AND HEADING DATE 7 (Ghd7) and delayed expression of the florigen genes HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1). Our life-cycle multiomics dataset on rice shoots under drought conditions provides a valuable resource for further functional genomic studies to improve crop resilience to drought stress.

The WD40 domain-containing protein Ehd5 positively regulates flowering in rice (Oryza sativa).

Heading date (flowering time), which greatly influences regional and seasonal adaptability in rice (Oryza sativa), is regulated by many genes in different photoperiod pathways. Here, we characterized a heading date gene, Early heading date 5 (Ehd5), using a modified bulked segregant analysis method. The ehd5 mutant showed late flowering under both short-day and long-day conditions, as well as reduced yield, compared to the wild type. Ehd5, which encodes a WD40 domain-containing protein, is induced by light and follows a circadian rhythm expression pattern. Transcriptome analysis revealed that Ehd5 acts upstream of the flowering genes Early heading date 1 (Ehd1), RICE FLOWERING LOCUS T 1 (RFT1), and Heading date 3a (Hd3a). Functional analysis showed that Ehd5 directly interacts with Rice outermost cell-specific gene 4 (Roc4) and Grain number, plant height, and heading date 8 (Ghd8), which might affect the formation of Ghd7-Ghd8 complexes, resulting in increased expression of Ehd1, Hd3a and RFT1. Taken together, these results demonstrate that Ehd5 functions as a positive regulator of rice flowering and provide insight into the molecular mechanisms underlying heading date.

The transcription factor HBF1 directly activates expression of multiple flowering time repressors to delay rice flowering.

The online version contains supplementary material available at 10.1007/s42994-023-00107-7.

Rice florigens control a common set of genes at the shoot apical meristem including the F-BOX BROADER TILLER ANGLE 1 that regulates tiller angle and spikelet development.

Rice flowering is triggered by transcriptional reprogramming at the shoot apical meristem (SAM) mediated by florigenic proteins produced in leaves in response to changes in photoperiod. Florigens are more rapidly expressed under short days (SDs) compared to long days (LDs) and include the HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T1 (RFT1) phosphatidylethanolamine binding proteins. Hd3a and RFT1 are largely redundant at converting the SAM into an inflorescence, but whether they activate the same target genes and convey all photoperiodic information that modifies gene expression at the SAM is currently unclear. We uncoupled the contribution of Hd3a and RFT1 to transcriptome reprogramming at the SAM by RNA sequencing of dexamethasone-inducible over-expressors of single florigens and wild-type plants exposed to photoperiodic induction. Fifteen highly differentially expressed genes common to Hd3a, RFT1, and SDs were retrieved, 10 of which still uncharacterized. Detailed functional studies on some candidates revealed a role for LOC_Os04g13150 in determining tiller angle and spikelet development and the gene was renamed BROADER TILLER ANGLE 1 (BRT1). We identified a core set of genes controlled by florigen-mediated photoperiodic induction and defined the function of a novel florigen target controlling tiller angle and spikelet development.

Two florigens and a florigen-like protein form a triple regulatory module at the shoot apical meristem to promote reproductive transitions in rice.

Many plant species monitor and respond to changes in day length (photoperiod) for aligning reproduction with a favourable season. Day length is measured in leaves and, when appropriate, leads to the production of floral stimuli called florigens that are transmitted to the shoot apical meristem to initiate inflorescence development1. Rice possesses two florigens encoded by HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1)2. Here we show that the arrival of Hd3a and RFT1 at the shoot apical meristem activates FLOWERING LOCUS T-LIKE 1 (FT-L1), encoding a florigen-like protein that shows features partially differentiating it from typical florigens. FT-L1 potentiates the effects of Hd3a and RFT1 during the conversion of the vegetative meristem into an inflorescence meristem and organizes panicle branching by imposing increasing determinacy to distal meristems. A module comprising Hd3a, RFT1 and FT-L1 thus enables the initiation and balanced progression of panicle development towards determinacy.

Two interacting basic helix-loop-helix transcription factors control flowering time in rice.

Flowering time is one of the most important agronomic traits affecting the adaptation and yield of rice (Oryza sativa). Heading date 1 (Hd1) is a key factor in the photoperiodic control of flowering time. In this study, two basic helix-loop-helix (bHLH) transcription factors, Hd1 Binding Protein 1 (HBP1) and Partner of HBP1 (POH1) were identified as transcriptional regulators of Hd1. We generated knockout mutants of HBP1 and ectopically expressed transgenic lines of the two bHLH transcription factors and used these lines to investigate the roles of these two factors in regulating flowering time. HBP1 physically associated with POH1 forming homo- or heterodimers to perform their functions. Both HBP1 and POH1 bound directly to the cis-acting elements located in the promoter of Hd1 to activate its expression. CRISPR/Cas9-generated knockout mutations of HBP1, but not POH1 mutations, promoted earlier flowering time; conversely, HBP1 and POH1 overexpression delayed flowering time in rice under long-day and short-day conditions by activating the expression of Hd1 and suppressing the expression of Early heading date 1 (Ehd1), Heading date 3a (Hd3a), and Rice Flowering locus T 1 (RFT1), thus controlling flowering time in rice. Our findings revealed a mechanism for flowering time control through transcriptional regulation of Hd1 and laid theoretical and practical foundations for improving the growth period, adaptation, and yield of rice.

The tetratricopeptide repeat protein OsTPR075 promotes heading by regulating florigen transport in rice.

Rice (Oryza sativa) is one of the most important crops worldwide. Heading date is a vital agronomic trait that influences rice yield and adaption to local conditions. Hd3a, a proposed florigen that primarily functions under short-day (SD) conditions, is a mobile flowering signal that promotes the floral transition in rice. Nonetheless, how Hd3a is transported from leaves to the shoot apical meristem (SAM) under SDs remains elusive. Here, we report that FT-INTERACTING PROTEIN9 (OsFTIP9) specifically regulates rice flowering time under SDs by facilitating Hd3a transport from companion cells (CCs) to sieve elements (SEs). Furthermore, we show that the tetratricopeptide repeat (TPR) protein OsTPR075 interacts with both OsFTIP9 and OsFTIP1 and strengthens their respective interactions with Hd3a and the florigen RICE FLOWERING LOCUS T1 (RFT1). This in turn affects the trafficking of Hd3a and RFT1 to the SAM, thus regulating flowering time under SDs and long-day conditions, respectively. Our findings suggest that florigen transport in rice is mediated by different OsFTIPs under different photoperiods and those interactions between OsTPR075 and OsFTIPs are essential for mediating florigen movement from leaves to the SAM.

Natural alleles of CIRCADIAN CLOCK ASSOCIATED1 contribute to rice cultivation by fine-tuning flowering time.

The timing of flowering is a crucial factor for successful grain production at a wide range of latitudes. Domestication of rice (Oryza sativa) included selection for natural alleles of flowering-time genes that allow rice plants to adapt to broad geographic areas. Here, we describe the role of natural alleles of CIRCADIAN CLOCK ASSOCIATED1 (OsCCA1) in cultivated rice based on analysis of single-nucleotide polymorphisms deposited in the International Rice Genebank Collection Information System database. Rice varieties harboring japonica-type OsCCA1 alleles (OsCCA1a haplotype) flowered earlier than those harboring indica-type OsCCA1 alleles (OsCCA1d haplotype). In the japonica cultivar “Dongjin”, a T-DNA insertion in OsCCA1a resulted in late flowering under long-day and short-day conditions, indicating that OsCCA1 is a floral inducer. Reverse transcription quantitative PCR analysis showed that the loss of OsCCA1a function induces the expression of the floral repressors PSEUDO-RESPONSE REGULATOR 37 (OsPRR37) and Days to Heading 8 (DTH8), followed by repression of the Early heading date 1 (Ehd1)–Heading date 3a (Hd3a)–RICE FLOWERING LOCUS T 1 (RFT1) pathway. Binding affinity assays indicated that OsCCA1 binds to the promoter regions of OsPRR37 and DTH8. Naturally occurring OsCCA1 alleles are evolutionarily conserved in cultivated rice (O. sativa). Oryza rufipogon-I (Or-I) and Or-III type accessions, representing the ancestors of O. sativa indica and japonica, harbored indica- and japonica-type OsCCA1 alleles, respectively. Taken together, our results demonstrate that OsCCA1 is a likely domestication locus that has contributed to the geographic adaptation and expansion of cultivated rice.

Cytokinin increases vegetative growth period by suppressing florigen expression in rice and maize.

Increasing the vegetative growth period of crops can increase biomass and grain yield. In rice (Oryza sativa), the concentration of trans -zeatin, an active cytokinin, was high in the leaves during vegetative growth and decreased rapidly upon induction of florigen expression, suggesting that this hormone is involved in the regulation of the vegetative phase. To elucidate whether exogenous cytokinin application influences the length of the vegetative phase, we applied 6-benzylaminopurine (BAP) to rice plants at various developmental stages. Our treatment delayed flowering time by 8-9 days when compared with mock-treated rice plants, but only at the transition stage when the flowering signals were produced. Our observations also showed that flowering in the paddy field is delayed by thidiazuron, a stable chemical that mimics the effects of cytokinin. The transcript levels of florigen genes Heading date 3a (Hd3a) and Rice Flowering locus T1 (RFT1) were significantly reduced by the treatment, but the expression of Early heading date 1 (Ehd1), a gene found directly upstream of the florigen genes, was not altered. In maize (Zea mays), similarly, BAP treatment increased the vegetative phage by inhibiting the expression of ZCN8, an ortholog of Hd3a. We showed that cytokinin treatment induced the expression of two type-A response regulators (OsRR1 and OsRR2) which interacted with Ehd1, a type-B response regulator. We also observed that cytokinin did not affect flowering time in ehd1 knockout mutants. Our study indicates that cytokinin application increases the duration of the vegetative phase by delaying the expression of florigen genes in rice and maize by inhibiting Ehd1.

RID1 sets rice heading date by balancing its binding with SLR1 and SDG722.

Rice (Oryza sativa) is a major crop that feeds billions of people, and its yield is strongly influenced by flowering time (heading date). Loss of RICE INDETERMINATE1 (RID1) function causes plants not to flower; thus, RID1 is considered a master switch among flowering-related genes. However, it remains unclear whether other proteins function together with RID1 to regulate rice floral transition. Here, we revealed that the chromatin accessibility and H3K9ac, H3K4me3, and H3K36me3 levels at Heading date 3a (Hd3a) and RICE FLOWERING LOCUS T1 (RFT1) loci were significantly reduced in rid1 mutants. Notably, RID1 interacted with SET DOMAIN GROUP PROTEIN 722 (SDG722), a methyltransferase. We determined that SDG722 affects the global level of H3K4me2/3 and H3K36me2/3, and promotes flowering primarily through the Early heading date1-Hd3a/RFT1 pathway. We further established that rice DELLA protein SLENDER RICE1 (SLR1) interacted with RID1 to inhibit its transactivation activity, that SLR1 suppresses rice flowering, and that messenger RNA and protein levels of SLR1 gradually decrease with plant growth. Furthermore, SLR1 competed with SDG722 for interaction with RID1. Overall, our results establish that interplay between RID1, SLR1, and SDG722 feeds into rice flowering-time control.

Cloning and functional analysis of LH2, a gene controlling late heading in rice

AbstractHeading date is an important agronomic trait in rice (Oryza sativa L.). We previously fine‐mapped two complementary genes, Late Heading Date 1 and Late Heading Date 2 (LH2), that control the late heading trait in rice. Here, we cloned and analyzed the function of LH2. LH2 was fine‐mapped to a 53‐kb genomic region. Sequencing analysis revealed that there were differences in the coding sequence of LOC_Os08g07740 between the varieties ‘Bo B’ and ‘Yuefeng B’. Therefore, LOC_Os08g07740 was identified as a candidate gene for LH2 and subsequently determined that LH2 is a Type 3 allele of Days To Heading 8. Transgenic plants of Yuefeng B carrying LH2 from Bo B significantly delayed the heading date under short‐day and long‐day conditions. A yeast two‐hybrid analysis revealed that LH2 physically interacted with Heme Activator Protein (HAP) 5A, ‐H, ‐I, ‐L, ‐K, and HAP5‐LIKE. The HAP5 domain of HAP5 family members has various functions. Further analysis revealed that LH2 represses photoperiodic flowering by controlling the expression of Early Heading Date 1, Heading date 3a, and Rice Flowering Locus T1 in rice.

The Rice CHD3/Mi-2 Chromatin Remodeling Factor Rolled Fine Striped Promotes Flowering Independent of Photoperiod.

Genetic studies have revealed that chromatin modifications affect flowering time, but the underlying mechanisms by which chromatin remodeling factors alter flowering remain largely unknown in rice (Oryza sativa). Here, we show that Rolled Fine Striped (RFS), a chromodomain helicase DNA-binding 3 (CHD3)/Mi-2 subfamily ATP-dependent chromatin remodeling factor, promotes flowering in rice. Diurnal expression of RFS peaked at night under short-day (SD) conditions and at dawn under long-day (LD) conditions. The rfs-1 and rfs-2 mutants (derived from different genetic backgrounds) displayed a late-flowering phenotype under SD and LD conditions. Reverse transcription-quantitative PCR analysis revealed that among the flowering time-related genes, the expression of the major floral repressor Grain number and heading date 7 (Ghd7) was mainly upregulated in rfs mutants, resulting in downregulation of its downstream floral inducers, including Early heading date 1 (Ehd1), Heading date 3a (Hd3a), and Rice FLOWERING LOCUS T 1 (RFT1). The rfs mutation had pleiotropic negative effects on rice grain yield and yield components, such as plant height and fertility. Taking these observations together, we propose that RFS participates in multiple aspects of rice development, including the promotion of flowering independent of photoperiod.

OsFD4 promotes the rice floral transition via florigen activation complex formation in the shoot apical meristem.

In rice, the florigens Heading Date 3a (Hd3a) and Rice Flowering Locus T 1 (RFT1), OsFD-like basic leucine zipper (bZIP) transcription factors, and Gf14 proteins assemble into florigen activation/repressor complexes (FACs/FRCs), which regulate transition to flowering in leaves and apical meristem. Only OsFD1 has been described as part of complexes promoting flowering at the meristem, and little is known about the role of other bZIP transcription factors, the combinatorial complexity of FAC formation, and their DNA-binding properties. Here, we used mutant analysis, protein-protein interaction assays and DNA affinity purification (DAP) sequencing coupled to in silico prediction of binding syntaxes to study several bZIP proteins that assemble into FACs or FRCs. We identified OsFD4 as a component of a FAC promoting flowering at the shoot apical meristem, downstream of OsFD1. The osfd4 mutants are late flowering and delay expression of genes promoting inflorescence development. Protein-protein interactions indicate an extensive network of contacts between several bZIPs and Gf14 proteins. Finally, we identified genomic regions bound by bZIPs with promotive and repressive effects on flowering. We conclude that distinct bZIPs orchestrate floral induction at the meristem and that FAC formation is largely combinatorial. While binding to the same consensus motif, their DNA-binding syntax is different, suggesting discriminatory functions.

Isolation and molecular phylogeny of RFT1 gene from upland rice leaves cultivar

Rice is a modern short-day plant but equally flower during non-inductive long-day condition due to the possession of second florigen, Rice Flowering Locus T1 (RFT1). RFT1 is an ortholog of Arabidopsis FT gene which triggers flowering induction under long-day (LD) condition. The gene isolation from Malaysian upland rice cv. Wai that induces flowering were reported. Nucleic acids isolation and amplification of the RFT1 were observed, then phylogenetic relationship among other 5 indica and 1 japonica rice were inferred by comparing the gene amino acids sequence data sets using multiple sequence alignment databases. The amplicon quality result indicated that the gene was fully amplified at above 0.5kb, whereas the sequence alignment revealed that the RFT1 gene was partially amplified based on the gaps identified. Among the 6 indica cultivars sequence aligned, they were almost all conserved except on only 12 amino acids. While the cladogram tree result classified the cultivars into three major groups and include Wai under clade that contains both indica and japonica cultivars. This finding concluded that the cultivars are of high evolutionary relationship and well established their molecular relationship. The cultivars relationship provides necessary information for better understanding of molecular evolution and designing scientific breeding system for generation of new rice cultivar.

Regulatory Genes and Enzymatic Complex of Flowering Time in Rice

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 CK2α, 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. MRG702, a reader protein, promotes flowering. Casein kinases, CKI and CK2α directly influence the function of the early heading regulator PRR37. Hd16/CKI hinders flowering time in the Ehd1-concerned pathway through phosphorylation of Ghd7 and PRR37. Natural variants of Hd1, PRR37, Ghd7, DTH8, Hd6, and Hd16 were found in the rice varieties that are cultivated presently in Asia and Europe, and their variants play significant roles in the down streaming of Ehd1 expression to delay flowering time in natural long day (LD) conditions.

Overexpression of RICE FLOWERING LOCUS T 1 (RFT1) Induces Extremely Early Flowering in Rice.

RICE FLOWERING LOCUS T 1 (RFT1) is a major florigen that functions to induce reproductive development in the shoot apical meristem (SAM). To further our study of RFT1, we overexpressed the gene and examined the expression patterns of major regulatory genes during floral transition and inflorescence development. Overexpression induced extremely early flowering in the transgenics, and a majority of those calli directly formed spikelets with a few spikelets, thus bypassing normal vegetative development. FRUITFULL (FUL)-clade genes OsMADS14, OsMADS15, and OsMADS18 were highly induced in the RFT1-expressing meristems. OsMADS34 was also induced in the meristems. This indicated that RFT1 promotes the expression of major regulatory genes that are important for inflorescence development. RFT1 overexpression also induced SEPALLATA (SEP)-clade genes OsMADS1, OsMADS5, and OsMADS7 in the greening calli before floral transition occurred. This suggested their possible roles at the early reproductive stages. We found it interesting that expression of OsFD1 as well as OsFD2 and OsFD3 was strongly increased in the RFT1-expressing calli and spikelets. At a low frequency, those calli produced plants with a few leaves that generated a panicle with a small number of spikelets. In the transgenic leaves, the FUL-clade genes and OsMADS34 were induced, but SEP-clade gene expression was not increased. This indicated that OsMADS14, OsMADS15, OsMADS18, and OsMADS34 act immediately downstream of RFT1.

Rice second florigen is crucial for expansion of its’ cultivation areas: A Bioinformatics

Flowering state is an important agronomic trait in determining the plant cropping season and best ecological examples between plant and its existing ecosystem. Rice is a model plant for molecular studies and pertinent for flowering development analysis. Rice Flowering Locus T1 (RFT1) is the second rice florigen which promotes flowering under long-day (LD) condition. This study investigates the correlation between RFT1 gene established accessions towards an expansion of rice cultivation areas in Asia using bioinformatics tools. Fourteen (14) different RFT1 accessions were collected including 9 indica and 5 japonica, and analyzed for molecular evolution. Also, the protein 3-dimensional (3D) structure were predicted and validated. The genes appeared to be highly conserved and functional across the diverse accessions. Cultivars from the two sub-species form a separate sub-group except for Koshihikari, Akamai and Nara aswari which form a major group, while Basmati370 was in the same clade with japonica cultivars. The 3D model indicated that the protein accessions has slight differences and VERIFY 3D model refinement has the best stereochemistry score at 99.38. The study clarified on the cultivars’ evolutionary relationships and suggested that diverse functional RFT1 variants is crucial for expansion of rice cultivation areas.Keyword: Rice, RFT1 gene, Accessions, Phylogenetic tree and 3D model

Histone Deacetylase 701 (HDT701) Induces Flowering in Rice by Modulating Expression of OsIDS1.

Rice is a facultative short-day (SD) plant in which flowering is induced under SD conditions or by other environmental factors and internal genetic programs. Overexpression of Histone Deacetylase 701 (HDT701) accelerates flowering in hybrid rice. In this study, mutants defective in HDT701 flowered late under both SD and long-day conditions. Expression levels of florigens Heading date 3a (Hd3a) and Rice Flowering Locus T1 (RFT1), and their immediate upstream floral activator Early heading date 1 (Ehd1), were significantly decreased in the hdt701 mutants, indicating that HDT701 functions upstream of Ehd1 in controlling flowering time. Transcript levels of OsINDETERMINATE SPIKELET 1 (OsIDS1), an upstream repressor of Ehd1, were significantly increased in the mutants while those of OsGI and Hd1 were reduced. Chromatin-immunoprecipitation assays revealed that HDT701 directly binds to the promoter region of OsIDS1. These results suggest that HDT701 induces flowering by suppressing OsIDS1.