Os06g0110000

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Annotated Information

Function

In the case of the rice plant, more tillering equates to more grain-bearing branches, hence a higher grain yield.The D3 gene plays an important role in the control of tiller bud dormancy to suppress bud activity, encoding an F-box leucine-trich-repeat protein orthologous to Arabidopsis MAX2/ORE9. Besides, the d3 protein is also involved in darkness-induced senescence or H2O2-induced cell death in the plant leaves.

Mutation

Figure 1．Phenotype of tillering dwarf mutants 6 and 10 weeks after the germination.(from reference ).

To identify genes involved in the control of rice tillering, Ishikawa et al. analyzed d3, d7, d10, d14, d27 tillering dwarf mutants exhibit reduction of plant stature and an increase in tiller numbers, in the mutants, axillary meristems are normally established but the suppression of tiller bud activity is weakened. All mutants exhibited a similar abnormal appearance from the early stage of development, we can the this phenomena from figure 1.Yan et al found that darkness-induced senescence or H2O2-induced cell death in the thisd leaf [as measured by chlorophyll degradation, membrane ion leakage and expression of senescence-associated genes(SAGs)] in a d3 rice mutant was dalayed compared to that in its reference line Shiokari, we can see the leaf senescence phenomenon from the figure 2.

Figure 2．Representative third leaves of 1-d-old light-grown Shiokari and Id3 seeding transferred to darkness for the indicated times.(from reference ).

In addition,Yasuno et al.compare the phenotype of the d3rcn1 double mutant with each single mutant and parental rice cultivar‘‘Shiokari’’, The reduction in tillering by the rcn1 mutation was independent of the d3 genotype, and tillering number of d3rcn1 double mutant was between those of the d3 and rcn1 mutants. The phenotypes of the 4 genotypes at heading time are shown in Figure 3.These results demonstrated that the Rcn1 gene was not involved in the D3-associated pathway in tillering control.

Figure 3.phenotype of Shiokari, a new rcn1 mutant S-97-61, d3 mutant of near-isogenic line ID3, and d3rcn1 double mutant(DM) at heading time. S-97-61 was recovered fromM2 progeny derived from gamma-ray–irradiated Shiokari in the present study. DM was developed from progeny of a cross betweenID3 and S-97-61 in the present study. Vertical bar represents20 cm.(from reference ).

Expression

Tissue specificity of D3 expression was examined by RT–PCR analysis (Fig. 4A, B),the expression of D3 mRNA is regulated posttranscriptionaly or that transcripts with different lengths of 3′-untranslated regions are generated.The effect of d3 mutation on D3 mRNA expression was also analyzed by RT–PCR (Fig. 4C),it is strongly suggested that the function of the D3 gene is hampered in the Id3 mutant.The mRNA levels of D3 is increased during cell death by qRT-PCR analyzed.These results suggest that D3 protein is involved in leaf senescence or cell death.

Figure 4．Analysis of D3 expression. (A) Exon/intron structure of the D3 gene predicted on the basis of the cDNA sequence (AK065478) and cDNA regions amplified in RT–PCR analysis. D3 comprises four exons and three introns, and the predicted protein-coding region,shown as an open square, is present in the first exon. a–h represent the cDNA regions amplified in the RT–PCR analysis. A closed triangle represents the putative transposon insertion region in Id3. (B) Tissue specificity of D3 expression was examined by RT–PCR analysis using two sets of primers that amplify the b and d regions. L, leaves; R, roots; VM, vegetative meristems; RM, reproductive meristems. (C) Expression of D3 in the Id3 mutant. RNA samples isolated from young leaves were used for the RT–PCR analysis. S, wild-type Shiokari; Id3, Id3 mutant .

Evolution

The D3 gene sequenced and cloned by Ishikawa et al.A database search with the predicted D3 amino acid sequence indicated the presence of an N-terminal F-box and an LRR in the middle, suggesting that D3 is a member of the Fbox LRR family of proteins that function as a component of the ubiquitin E3 ligase complex(Figure5).The D3 gene showed the highest sequence similarity to MAX2/ORE9 of Arabidopsis.The insertion of a putative transposon at the 154th amino acid of the predicted D3 protein caused an alteration of the amino acid sequence and generated a stop codon.

Figure 5

Knowledge Extension

A proposed model of Strigolactone(SL) signalling patyway ).

Strigolactones (SLs) are a group of newly identified plant hormones that control plant shoot branching. SL signaling requires the hormone-dependent interaction of DWARF14 (D14) which is regulated by the interaction of OsMADS57 with OsTB1. In this study, they have identified theD53gene that encodes a substrate of the SCF(D3) ubiquitination complex, and revealed that D53 functions as a repressor ofSL signalling. These results allow to establish a model of SL signalling that is centred around a D14–D3–D53 signalling axis . In the presence of SLs, perception of SL by D14 and the SCF(D3) complex leads to ubiquitination of D53 and its subsequent degradation by the ubiquitin proteasome system, which in turn releases the repression of downstream target genes . In the d53 plant, the mutated D53 protein is resistant to ubiquitination and degradation, leading to the accumulation of d53, which blocks SL signalling and results in dwarf and high tillering phenotypes. The signalling paradigm of SLs is still emerging as SLs are a relatively new class of plant hormone for which many knowledge gaps still exist. Identification of D53 as a repressor of SL signalling adds a critical piece of information that helps to paint the whole picture of the SL signalling pathways. Moreover, the work has also provided an important paradigm for understanding signalling pathways of other plant hormones, for example, karrikins, a class of plant growth regulators found in the smoke of burning plants. Karrikin signalling involves MAX2 and KAI2, a D14-like α / β-hydrolase. It is probable that a similar protein to D53 could serve as the repressor of karrikin signalling. Indeed, multiple D53-like proteins are found in rice and inArabidopsis. We propose that these proteins could serve as repressors of signalling by karrikin and other plant hormones, in a similar way to D53 in SL signalling.

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Labs working on this gene

(1) Graduate School of Agriculture and Life Sciences, University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo, 113-8657 Japan

(2)CREST, Japan Science and Technology Corporation, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012 Japan

(3) Research Institute for Bioresources, Okayama University,2-20-1Chuo, Kurashiki, Okayama, 710-0046 Japan

(4) Faculty of Agriculture, Hokkaido University, Sapporo, 060 Japan

(5) Department of Crop Science Obihiro University of Agriculture and Veterinary Medicine, Nishi 2-5 Inada-cho,Obihiro, Hokkaido, 080-8555 Japan

(6) Graduate School of Agriculture, Hokkaido University, Kita 9 Nishi 9, Kita-ku, Sapporo, Hokkaido, 060-8589 Japan