Promotion of leaf sheath growth by gibberellic acid in a dwarf mutant of rice

The mechanism of gibberellin (GA)-induced leaf sheath growth was examined using a dwarf mutant of rice (Oryza sativa L. cv. Tan-ginbozu) treated in advance with an inhibitor of GA biosynthesis. Gibberellic acid (GA(3)) enhanced the growth of the second leaf sheath, but auxins did not. Measurement of the mitotic index and cell size revealed that cell elongation rather than cell division is promoted by GA(3). Gibberellic acid increased the extensibility of cell walls in the elongation zone of the leaf sheath. It also increased the total amount of osmotic solutes including sugars in the leaf sheath, but did not increase the osmotic concentration of the cell sap, due to an accompanying increase in cell volume by water absorption. In the later stage of GA(3)-induced growth, starch granules completely disappeared from leaf sheath cells, whereas dense granules remained in control plants. These findings indicate that GA enhances cell elongation by increasing wall extensibility, osmotic concentration being kept unchanged by starch degradation.