Involvement of cell wall-bound phenolic acids in decrease in cell wall susceptibility to expansins during the cessation of rapid growth in internodes of floating rice

The cell walls in the elongating zone of submerged floating rice internodes show high susceptibility to expansins. When internode sections corresponding to such an elongation zone were incubated for 24 h under osmotic stress conditions produced by treatment with 100 mM polyethylene glycol 4000 (PEG), the cell wall susceptibility to expansins remained at its initial level, while the susceptibility of internode sections incubated under unstressed conditions decreased considerably during the same period. The contents of polysaccharides and phenolic acids as ferulic, diferulic and p-coumaric acids in the cell walls of internode sections increased substantially under unstressed conditions, but the increases were almost completely prevented by osmotic stress. Ferulic acid applied to internode sections under osmotic stress reduced the susceptibility of the cell walls to expansins and increased the levels of ferulic and diferulic acids in the cell walls, with little effect on the accumulation of polysaccharides. In contrast, applied p-coumaric acid increased the level of p-coumaric acid in the cell walls without a change in the levels of ferulic and diferulic acids but did not reduce the susceptibility to expansins. These results suggest that the deposition of ferulic and diferulic acids is a primary determinant in regulating the reduction of the susceptibility of cell walls to expansins in floating rice internodes.