Although flowering regulatory mechanisms have been extensively studied in Arabidopsis (Arabidopsis thaliana), those in other species have not been well elucidated. Here, we investigated the role of OsMADS51, a type I MADS-box gene in the short-day (SD) promotion pathway in rice (Oryza sativa). In SDs OsMADS51 null mutants flowered 2 weeks later than normal, whereas in long days loss of OsMADS51 had little effect on flowering. Transcript levels of three flowering regulators-Ehd1, OsMADS14, and Hd3a-were decreased in these mutants, whereas those of OsGI and Hd1 were unchanged. Ectopic expression of OsMADS51 caused flowering to occur about 7 d earlier only in SDs. In ectopic expression lines, transcript levels of Ehd1, OsMADS14, and Hd3a were increased, but those of OsGI and Hd1 remained the same. These results indicate that OsMADS51 is a flowering promoter, particularly in SDs, and that this gene functions upstream of Ehd1, OsMADS14, and Hd3a. To further investigate the relationship with other flowering promoters, we generated transgenic plants in which expression of Ehd1 or OsGI was suppressed. In Ehd1 RNA interference plants, OsMADS51 expression was not affected, supporting our conclusion that the MADS-box gene functions upstream of Ehd1. However, in OsGI antisense plants, the OsMADS51 transcript level was reduced. In addition, the circadian expression pattern for this MADS-box gene was similar to that for OsGI. These results demonstrate that OsMADS51 functions downstream of OsGI. In summary, OsMADS51 is a novel flowering promoter that transmits a SD promotion signal from OsGI to Ehd1.

In much of the tropics and subtropics, rice (Oryza sativa L.) is grown under long days (LDs). Therefore, LD must play a major role in inducing flowering signal in rice. However, little is known on LD-dependent flowering signal in the species. We previously reported that OsMADS50, which is highly homologous to Arabidopsis SOC1, functions as a positive regulator for flowering. However, its detailed photoperiodic mechanism was not yet elucidated. Here, we report the functional analysis of OsMADS50 and its closely related gene OsMADS56. Knock-out of OsMADS50 caused a late-flowering phenotype only under LD conditions. Overexpression of OsMADS56 (56OX) also resulted in delayed flowering under LD. In the osmads50 mutants and 56OX transgenic plants, transcripts of Ehd1, Hd3a and RFT1 were reduced, although that of OsLFL1 increased. On the other hand, mRNA levels of OsGI, Hd1, OsId1, OsDof12, Ghd7, Hd6 and SE5 were unchanged. These observations imply that OsMADS50 and OsMADS56 function antagonistically through OsLFL1-Ehd1 in regulating LD-dependent flowering. Yeast two-hybrid and co-immunoprecipitation analyses indicated an interaction between those two proteins as well as their formation of homodimers. These results suggest that OsMADS50 and OsMADS56 may form a complex that regulates downstream target genes.

SUMMARYSeed shattering is an important trait that influences grain yield. A major controlling quantitative trait locus in rice is qSH1. Although the degree of shattering is correlated with the level of expression of qSH1, some qSH1-defective cultivars display moderate shattering while others show a non-shattering phenotype. Os05 g38120 (SH5) on chromosome 5 is highly homologous to qSH1. Although we detected SH5 transcripts in various organs, this gene was highly expressed at the abscission zone (AZ) in the pedicels. When expression of this gene was suppressed in easy-shattering 'Kasalath', development of the AZ was reduced and thereby so was seed loss. By contrast, the extent of shattering, as well as AZ development, was greatly enhanced in moderate-shattering 'Dongjin' rice when SH5 was overexpressed. Likewise, overexpression of SH5 in the non-shattering 'Ilpum' led to an increase in seed shattering because lignin levels were decreased in the basal region of spikelets in the absence of development of an AZ. We also determined that two shattering-related genes, SHAT1 and Sh4, which are necessary for proper formation of an AZ, were induced by SH5. Based on these observations, we propose that SH5 modulates seed shattering by enhancing AZ development and inhibiting lignin biosynthesis.

SummaryIndeterminate 1 (Id1), a classical flowering gene first reported in 1946, is one of the earliest genes whose expression in leaf tissues affects the floral transition in the shoot meristem. How Id1 is integrated into the flowering process is largely unknown. In this study, we examined the genetic action of the rice (Oryza sativa) ortholog OsId1. In rice, OsId1 is preferentially expressed in young leaves, but the overall expression pattern is broader than that in maize (Zea mays). OsId1 is able to activate transcription in yeast. RNAi mutants show a delay in flowering under both short-day (SD) and long-day (LD) conditions. OsId1 regulates the expression of Ehd1 (Early heading date 1) and its downstream genes, including Hd3a (a rice ortholog of FT) and RFT1 (Rice Flowering Locus T1), under both SD and LD conditions. In rice, the expression of Ehd1 is also controlled by the photoperiodic flowering genes OsGI (a rice ortholog of GI) and OsMADS51. However, the expression of OsId1 is independent of OsGI, OsMADS51, and OsMADS50 (a rice SOC1 ortholog). This study demonstrates that the activation of Ehd1 by OsId1 is required for the promotion of flowering.

The area between the upper part of the leaf sheath and the basal portion of the leaf blade contains several specialized organs, such as the laminar joint, auricle and ligule. Here we report the identification of T-DNA insertional mutant lines that lack all of these organs. The gene knocked out in the mutant lines encodes a protein that contains a SBP (SQUAMOSA promoter Binding Protein)-domain and is highly homologous to the maize LIGULELESS1 (LG1) gene. At the amino acid sequence level, the OsLG1 protein is 69% identical to maize LG1 and 78% identical to barley LG1. We named the rice gene OsLIGULELESS1 (OsLG1). Transient expression of an OsLG1:RFP (Red Fluorescent Protein) fusion protein indicated that the protein is localized to the nucleus. Transgenic plants harboring the OsLG1 promoter:GUS (beta-glucuronidase) reporter gene construct display preferential expression in developing laminar joint regions and meristemic regions. The gene is also weakly expressed in the ligule, auricles, and leaf sheaths at the basal region. These results indicate that OsLG1 is a transcriptional factor that plays an important role in building the laminar joint between leaf blade and leaf sheath boundary, thereby controlling ligule and auricle development.

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Plant growth and development are coordinately controlled by environmental signals and internal factors. Light signals, mediated by phytochromes, regulate photomorphogenesis by interacting with endogenous programmes that involve multiple phytohormones. Brassinosteroids (BRs) are a group of growth-promoting phytohormones with a crucial role in the light-dependent development of plants. However, the interaction between light-signalling pathways and BR signalling is not well understood. Here, we examined the responses of lamina joint inclination and coleoptile elongation to exogenous brassinolide (BL) under light or dark conditions. Both responses were more pronounced under darkness, implying that BR signalling is inhibited by light. To elucidate which phytochrome is involved in this interaction, we isolated rice phytochrome-deficient mutants (osphyA, osphyB and osphyC) from a T-DNA insertional population. Whereas the osphyA and osphyC knockout mutants did not differ from the wild-type plants in their BL responses, osphyB mutants were more sensitive. In addition, RT-PCR analysis revealed enhanced expression of BR-inducible genes and decreased transcript levels of BR-biosynthetic genes in osphyB plants. These results suggest that Phytochrome B acts as a negative regulator of BL-regulated growth and development processes in rice.

As an extremely early flowering cultivar, rice cultivar Kitaake is a suitable model system for molecular studies. Expression analyses revealed that transcript levels of the flowering repressor Ghd7 were decreased while those of its downstream genes, Ehd1, Hd3a, and RFT1, were increased. Sequencing the known flowering-regulator genes revealed mutations in Ghd7 and OsPRR37 that cause early translation termination and amino acid substitutions, respectively. Genetic analysis of F2 progeny from a cross between cv. Kitaake and cv. Dongjin indicated that those mutations additively contribute to the early-flowering phenotype in cv. Kitaake. Because the short life cycle facilitates genetics research, this study generated 10 000 T-DNA tagging lines and deduced 6758 flanking sequence tags (FSTs), in which 3122 were genic and 3636 were intergenic. Among the genic lines, 367 (11.8%) were inserted into new genes that were not previously tagged. Because the lines were generated by T-DNA that contained the promoterless GUS reporter gene, which had an intron with triple splicing donors/acceptors in the right border region, a high efficiency of GUS expression was shown in various organs. Sequencing of the GUS-positive lines demonstrated that the third splicing donor and the first splicing acceptor of the vector were extensively used. The FST data have now been released into the public domain for seed distribution and facilitation of rice research.