CELLULAR MECHANISMS UNDERLYING GROWTH ASYMMETRY DURING STEM GRAVITROPISM

Plant stems respond to gravitropic stimulation with a rapid, local and reversible change in cell growth rate (elongation), generally on both the upper and lower sides of the stem. The cellular and biochemical m echanisms for this differential growth are reviewed. Considerable evid ence implicates an asymmetry in wall pH in the growth response. The st rengths and weaknesses of the wall ''loosening enzyme'' concept are re viewed and the possibility of expansin involvement in the bending resp onse of stems is considered. Also discussed is the possibility that wa ll stiffening processes, e.g. phenolic coupling driven by oxidative bu rsts or altered orientation of newly deposited cellulose, might mediat e the growth responses during gravitropism.