Abstract：Brassinosteroid (BR) is one of the most important hormones which regulate plant growth and development. To clarify the regulatory function of BR in rice (Oryza sativa) cell development, rice calli and suspension cells treated with different concentrations of exogenous BR and the mutant OsBZR1-OE in which the endogenous BR signal enhanced was used to analyze the effect of BR on the volume and cell distribution in rice callus, the shape and size of the single cell suspension, and we observed the cells in OsBZR1-OE transgenic rice seedlings, analyzed expression level of the key genes in regulating cell division and the accumulation of cytoskeletal protein (F-actin) which plays an important role in cell division. We found that low concentration (1×10-10 mol/L) BR could increase the calli volume but the high concentration (1×10-6 mol/L) BR blocked the growth of rice calli. Consistent with calli size, the rice calli treated with high concentrations of BR showed a high density nuclei distribution pattern (cell volume was small) whereas the calli treated with low concentrations of BR showed a low density nuclei distribution pattern (cell volume was large). In addition, BR treatments of single suspension cells showed similar results, low concentration of BR promoted the cell elongation but the high concentration blocked the cell elongation. Moreover, the length of hypocotyl cells in OsBZR1-OE transgenic rice plants (in which the BR signaling was enhanced) was shorter than that in wild type rice. Propidium iodide(PI) and fluorescein isothiocyanate isomerⅠ(FITC)-phalloidin staining showed that BR treatment could promote the division of suspension and cytoskeleton, and the promoted effect of the high concentration BR treatment was more obvious. Two dimensional electrophoresis results indicated that BR could promote the accumulation of actin cytoskeleton protein. RT-PCR results showed that BR could increase the expression of CDC48 (cell division cycle protein 48) gene which can promote mitosis and CYCD2 (cyclin D2) gene which can shorten the cell cycle. Taken together, these results suggested that low concentration of BR could promote the elongation of rice cells but high concentration inhibited the elongation of rice cells via affecting the expression of cytoskeleton protein F-actin and the cell cycle regulatory genes CDC48 which can promote mitosis and CYCD2 which can shorten the cell cycle. These results provide a theoretical basis for the application of BR in control of the rice height and increasing the lodging resistance.
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