We have identified a drought-inducible gene, designated as SoDip22, in sugarcane leaves. The cDNA encoded a hydrophilic protein with a calculated molecular mass of 15.9 kDa and the amino acid sequence was similar to that of ABA, stress and ripening-inducible protein from various plant species. ABA or mannitol-treatment of the detached leaves also induced SoDip22 expression. Stepwise homogenization of the stressed leaves showed that SoDip22 is localized in bundle sheath cells. These results suggest that SoDip22 functions to adapt to drought stress in the bundle sheath cell, and that the signaling pathway for the induction is, at least in a part, mediated by ABA.

In eukaryotes, the cell cycle consists of four distinct phases: G1, S, G2 and M. In certain condition, the cells skip M-phase and undergo endoreduplication. Endoreduplication, occurring during a modified cell cycle, duplicates the entire genome without being followed by M-phase. A cycle of endoreduplication is common in most of the differentiated cells of plant vegetative tissues and it occurs extensively in cereal endosperm cells. Endoreduplication occurs when CDK/Cyclin complex low or inactive caused by ubiquitin-mediated degradation by APC and their activators. In this study, rice cell cycle switch 52 A (OsCCS52A), an APC activator, is functionally characterized using the reverse genetic approach. In rice, OsCCS52A is highly expressed in seedlings, flowers, immature panicles and 15 DAP kernels. Localization studies revealed that OsCCS52A is a nuclear protein. OsCCS52A interacts with OsCdc16 in yeast. In addition, overexpression of OsCCS52A inhibits mitotic cell division and induces endoreduplication and cell elongation in fission yeast. The homozygous mutant exhibits dwarfism and smaller seeds. Further analysis demonstrated that endoreduplication cycles in the endosperm of mutant seeds were disturbed, evidenced by reduced nuclear and cell sizes. Taken together, these results suggest that OsCCS52A is involved in maintaining normal seed size formation by mediating the exit from mitotic cell division to enter the endoreduplication cycles in rice endosperm.

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