Os01g0182600

The rice gene of Gigantea(GI) is solo ortholog of GI in the Arabidopsis circadian system and related to photoperiodic control of flowering.

Annotated Information

Function

• OsGI,a GI ortholog, regulates flowering time in response to photoperiodic conditions. Mutations in this gene delay flowering in inductive SDs (short-days),whereas flowering is marginally induced under LDs(long-days).In comparison, over expression of OsGI in transgenic rice causes late flowering in both SDs and LDs.In addition, OsGI has function to regulate Hd1, a gene that also shows circadian rhythm. Os-GI affects gene expression of approximately 75% of genes among 27,201 genes and is required for strong amplitudes and proper phase-setting of global gene expression under natural field conditions.
• The cDNA of OsGI firstly found in se5 mutant that is completely photoperiod-insensitive, the expression patterns of rice and Arabidopsis GI gene are both photoperiodically regulated and are very similar under LD and SD conditions(Figure 1).
  

  Figure 1 Possible roles of isolated genes in the photoperiodic regulation of flowering in rice, including OsGI, may function to inhibit flowering under LD conditions.(from reference ).
  GI is required for phase-dependent COtranscription through the interaction with FKF1, which leads to cycling DNA binding with one finger (CDF) degradation and consequent derepression of CO. In addition, it has been known that GI protein interacts with ZEITLUPE (ZTL) F-box protein to control the diurnal rhythm of TOC1 protein in regulation of the Arabidopsis circadian clock.
• Os-GI also has function to maintain bioactive GA level through the regulation of the GA-deactivating enzyme genes in rice, Os-GI represses major OsGA2ox genes in order to maintain a proper bioactive GA level, osgi-1 plants showed a semi-dwarf phenotype with elevated OsGA2ox gene expression(Figure 2) .
  

  Figure 2 WT rice and osgi-1 plants grown under natural field conditions and osgi-1 plant exhibits Semi-dwarf phenotype(from reference ).

Expression

• GI encodes a nuclear protein that defines the proper amplitude and period length of circadian rhythms. GI expression is regulated by the circadian clock, with a peak in transcript levels at 8 to 10 h after dawn. The GI protein acts between the circadian oscillator and CO to promote flowering by increasing CO and FT mRNA abundance. The expression of the OsGI messenger RNA was circadian-controlled and that its temporal expression pattern was very similar to that of GI under both SD and LD conditions. Decreased OsGI expression led to early flowering under LD conditions and late flowering under SD conditions relative to the wild type, and that this early flowering phenotype under LD conditions was the opposite of that of the Arabidopsis gi mutants, confirming the opposite role of the GI gene in flowering under LD conditions in rice and Arabidopsis.

Mutation

• The single osgi-1 mutation extended the time to flowering only under SD conditions. Under LD conditions the single hd1 mutation induced early flowering, but osgi-1 plants exhibited no phenotypic change

Evolution

• There is some evolutionary conservation between rice circadian clock–related genes and those associated with Arabidopsis circadian clocks, where the circadian clock consists of interlocked subloops, it is also possible that Os-GI is a member of a subloop in rice circadian clocks.But there is a divergence in the photoperiodic pathways of these two species arises from the molecular mechanisms controlling the expression of FT/Hd3a by CO/Hd1(Se1) (Figure 3)
  

  Figure 3 The molecular mechanisms for the regulation of GI/OsGI expression, and the regulation of CO/Hd1(Se1) expression by GI/OsGI, might be conserved between Arabidopsis and rice. Both the GI–CO and OsGI–Hd1(Se1) pathways are activated under LD conditions; the former promotes the flowering of Arabidopsis, whereas the latter inhibits the flowering of rice. The divergence in the photoperiodic pathways of these two species arises from the molecular mechanisms controlling the expression of FT/Hd3a by CO/Hd1(Se1)(from reference ).

Labs working on this gene

• Photosynthesis and Photobiology Research Unit, National Institute of Agrobiological Sciences, 2-1-2 Kannondai, Tsukuba 305-8602, Japan
• Laboratory of Plant Molecular Genetics, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma 630-0101, Japan
• Plant Genomics Research Unit, National Institute of Agrobiological Sciences, 2-1-2 Kannondai, Tsukuba 305-8602, Japan

Knowledge Extension

• The rice orthologues of the two key regulators of flowering time in Arabidopsis, CO and FT, were recently shown to be important in the photoperiodic control of flowering in rice. The rice Hd1(Se1)gene, which is an orthologue of the Arabidopsis CO gene, was required for the suppression of flowering under LD conditions and for the promotion of flowering under SD conditions. Furthermore, the rice Hd3a gene, an orthologue of the Arabidopsis FT gene, was shown to be an activator of flowering in rice. It was previously demonstrated that CO controls the flowering time by integrating signals from the circadian clock and light to regulate FT expression in Arabidopsis.Unlike Arabidopsis FT, expression of Hd3a mRNA is more or less opposite to that of Hd1 under inductive short-day conditions.