Os01g0972900
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Annotated Information
Function
During the whole plant life cycle, cell division and expansion work alternately to build and control the precise size of specific organs. In addition, during cell differentiation of post-embryonic tissue, either cell expansion or endoreduplication control the final size. CCS52 protein has been reported to be involved in cell expansion and endoreduplication, the functional characterization of B-type CCS52 (CCS52B) in plants is relatively limited. Previous studies of model plants (Medicago and Arabidopsis) have proposed that MtCCS52B and AtCCS52B play a crucial role in plant cell division. Yeast overexpressing AtCCS52B had smaller cells compared to the wild type, indicating an early start of the mitotic cell division cycle. Moreover, yeast cells harboring OsCCS52B grow slower than the wild type during the cycle, as determined by the proliferation assay (Fig. 1). Another distinct phenotype is yeast cell elongation prior to division. These results indicate that OsCCS52B may function in cell expansion rather than cell division. Although the yeast cell is elongated, the nuclei size remains similar to the wild type, indicating that no additional endoreduplication cycle occurs in OsCCS52B overexpressing cells (Fig. 2). In Arabidopsis, when AtCCS52B is overexpressed in fission yeast, the cells loose polarity control. Therefore, the nuclei location is not always in the center of the cells. In contrast to this phenomenon, and presented here show that OsCCS52B overexpressing cells maintain the nuclei within the center of the cell. To further confirm the functional role of OsCCS52B in plants, a T-DNA insertion mutant line 1B-10423 was characterized. These results indicate that OsCCS52B plays an essential role in the growth and development of rice, not only during the early developmental stage, but also during the grain filling stage. Grain filling is a critical stage, during which cereal plants modulate endosperm (seed) size due to the accumulation of photosynthates, such as starch and protein. Two main processes, endoreduplication and cell expansion, are reported to occur concomitantly with grain filling. Furthermore, endoreduplication and cell expansion might work concurrently or independently. In most cereals, successive rounds of endoreduplication start at 10 DAP and peak at 15–18 DAP. To determine whether the mutation in line 1B-10423 interferes with cellular development during endosperm formation, 15 DAP endosperm was sectioned and stained with DAPI to compare with the corresponding wild-type endosperm. Interestingly, microscopic analysis of the endosperm cells in line 1B-10423 revealed a smaller cell size without altering the nuclei size. Hence, cell size regulation in mutant seed could be separate and independent from the cell cycle progression associated with DNA endoreduplication. Therefore, OsCCS52B is more likely to be involved in the regulation of cell growth during endosperm development. Taken together with the yeast overexpression data, this result supports the hypothesis for the involvement of OsCCS52B in cell expansion rather than endoreduplication. Previous studies have reported that the level of endoreduplication in maize endosperm does not always affect the cell size, thus providing evidence of another mechanism for seed size regulation. Thus, cell expansion and endoreduplication may be independent events and not necessarily related under certain conditions. Cell expansion during endosperm development is also regulated by the surrounding phenomena, such as embryo development. In studies using a RNAi approach, the down-regulation of Orysa;CycB1;1 increases embryo size and inhibits endosperm development. This demonstrates the effect of a physical interaction in bordering cells between the embryo and endosperm. In fact, the cell numbers in mutant endosperm are relatively higher than those in the wild-type. This may be due to a decrease in cell size, which can stimulate a compensatory increase in cell proliferation. In conclusion, these results provide a genetic basis for the involvement of OsCCS52B in maintaining growth and development. In addition, OsCCS52B is responsible for the normal shape and size of rice seeds via cell expansion regulation. Further study will be required to confirm whether transgenic rice overexpressing OsCCS52B alter plant morphology, especially in seed size.
Expression
It was previously reported that Medicago CCS52B was highly expressed in shoot apices. In addition, Arabidopsis CCS52B is mainly expressed in young leaves and bolting plants. To investigate the expression pattern of OsCCS52B, RNA from vegetative and generative organs of wild-type plants was isolated and the transcription levels were assessed by semi-quantitative RT-PCR. The results showed that OsCCS52B was highly expressed in young organs, both in leaves and roots. However, the expression was very weak in mature roots and barely detectable in mature leaves. In contrast to mature vegetative organs, OsCCS52B was expressed strongly in generative organs such as flowers and kernels (Fig. 3).
Evolution
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Labs working on this gene
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References
1. Mukhamad Su’udi, Joon-Yung Cha, Il-Pyung Ahn, Youn-Sig Kwak, Young-Min Woo, Daeyoung Son. Functional characterization of a B-type cell cycle switch 52 in rice. Plant Cell Tiss Organ Cult, 2012, 111:101–111.
