Os03g0302900

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The rice gene Os03g0302900 was reported as ssd1 in 2010[1].

Annotated Information

Function

  • SSD1 plays an important role in the regulation of cell division[1].
  • Using the WoLF PSORT programs (http:// wolfpsort.org/), it was predicted that the SSD1 protein contained a putative nuclear localization signal (NLS, RKKLFSNSPEGAKLVKR, amino acid residues 37-53; Fig. 6). This prediction suggests that SSD1 functions in the nucleus. This NLS is highly conserved among various plant species (data not shown), and thus this domain may have an essential role in SSD1 function[1].
Fig. 6. Deduced amino acid sequence of SSD1. The insertion site of Tos17 is indicated by the arrowhead. Bold letters and underline denote the putative nuclear localization signal predicted by WoLF PSORT. mutant. [1].

Mutation

  • Characterization of the ssd1 mutant. The ssd1 mutant was screened from the Tos17 mutant library, which is a mutant population induced by the Tos17 retrotransposon.23) In the progeny of heterozygous plants, the segregation ratio of the normal phenotype to the dwarf was 90:31, which corresponded to the expected 3:1 segregation ratio of a single recessive gene (χ2 ¼ 0.003). The dwarf phenotype in rice is generally caused by a reduction in culm length. Based on the elongation pattern of internodes, rice dwarf mutants are classified into six types: N-, dn-, dm-, d6-, nl-, and sh-type.24) Of these, the dn-type is defined by reduction in internodes length in the same proportion to the WT. ssd1 exhibited a reduction in the length of all internodes in the same proportion as in the WT, which is characteristic of the dn-type dwarf (Fig. 2A), with severe dwarf and wide, dark green leaves (Fig. 1A, B). Culm length of the mutant at harvest was about 17 cm, whereas the WT grew to about 90 cm (Figs. 1A, B and 2A). Elongation of the seminal and crown roots was also inhibited in the mutant (Figs. 1E and 2B), and the development of floral organs was also impaired in ssd1. Rice flowers are composed of four kinds of glumes, two rudimentary glumes, two empty glumes, lemma and palea, and three kinds of floral organs, two lodicules, six stamens, and one pistil.25) The ssd1 glumes were shorter than that of the WT (Fig. 1C, D). The ssd1 flowers also developed short anthers and filaments (Fig. 1F) and short, shrunken stigma (Fig. 1G). Some pistils developed three stigmas (Fig. 1G). These observations suggest that SSD1 has a fundamental role in cell division and/or elongation in various organs[1].
Fig. 1. Morphological characterization of the ssd1 mutant. [1].
Fig. 2. Lengths of panicle, internodes, and seminal roots. [1].
  • Cell morphology of ssd1 leaves. Usually, dwarf is caused by a defect in cell division and/or elongation. To clarify which defect causes dwarf phenotype in ssd1 mutant, we observed the microscopic structure of ssd1 leaves. In the WT, cell files that were well organized in a longitudinal manner were observed (Fig. 3A). In contrast, in ssd1, longitudinally longitudinally arranged cells were not well organized and the cells were enlarged and distorted, leading to a disorganized cell files (Fig. 3B). Moreover, abnormality in the shape and size of cells were observed in ssd1. In the WT, only rectangular cells were observed (Fig. 3A), but trapezoidal, triangular, circular, and diamond-shaped cells were found in ssd1 (Fig. 3B, arrowheads). These abnormal cell shapes and disorganized cell arrangements are probably caused by a defect in synchronous division in these cells. In fact, the transverse division of cells was often slanted in the mutant (Fig. 3B), whereas this abnormal division pattern was not observed in the WT (Fig. 3A)[1].
Fig. 3. Structure of cells in the leaf sheath of ssd1. (A) and (B) Epidermal cell morphology of Nipponbare and ssd1, respectively. Arrowheads in B indicate cells with abnormal shape and size. Bar ¼ 50 mm. [1].

Expression Pattern

  • The researchers performed an RNA gel blot analysis to examine expression profiles, but signals were not detected, probably due to the low expression levels of SSD1 (data not shown). Thus, the researchers compared the expression pattern by using semiquantitative reverse transcription (RT)-PCR. As expected, the expression of SSD1 was observed in each of the organs we tested (Fig. 4). Expression levels in the shoot apex and elongating roots were relatively higher than in other organs, whereas expression levels in the leaf blades, leaf sheaths, and flowers were relatively lower. The preferential expression of SSD1 in the shoot apex and root may correspond to the ability of each organ to carry out cell division[1].
Fig. 4 Expression analysis of SSD1 in various organs. Total RNAs were isolated from the leaf blade (LB), leaf sheath (LS), elongating root (RT), shoot apex (SA), and flower (FL), and quantitative RT-PCR was performed. OsActin1 was used as a control [1].

Evolution

  • Basic Local Alignment Search Tool (BLAST) analyses identified SSD1-like genes from diverse other plant species, including monocots and dicots, but not from moss (Physcomitrella patens) and fern (Selaginella moellendorffii). Three putative homologous genes, At5g26910, At3g58650, and At3g05750, which shared around 30% amino acid sequence identity with the SSD1 protein, were identified in the Arabidopsis genome. Putative SSD1 homologous genes were also identified from the genomes of Sorghum bicolor, castor bean (Ricinus communis), grape (Vitis vinifera), and western balsam poplar (Populus trichocarpa) (Fig. 5). Database searches could not identify animal or yeast proteins that have significant similarity to SSD1. Although high similarity among diverse plant species implies that the SSD1 protein family has a fundamental function in plants, searches failed to identify any protein with known biological function in the public database[1].
Fig. 5. Phylogenetic relationship between SSD1 and SSD1-like proteins in plants. The structural relationship was calculated using CLUSTALW followed by manual alignment and illustrated using SplitsTree. [1].

Subcellular localization

  • The pleiotropic effect of the ssd1 mutation in various organs indicates that the SSD1 gene functions in all of these organs[1].

Labs working on this gene

  • Bioscience and Biotechnology Center, Nagoya University, Aichi, Japan.
  • Division of Genome and Biodiversity Research, National Institute of Agrobiological Sciences, Ibaraki, Japan.

References

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 Asano K, Miyao A, Hirochika H, Kitano H, Matsuoka M, Ashikari M. SSD1, which encodes a plant-specific novel protein, controls plant elongation by regulating cell division in rice. Proc Jpn Acad Ser B Phys Biol Sci. 2010;86(3):265-73. PubMed PMID: 20228626; PubMed Central PMCID: PMC3417851.

Structured Information