Changes in starch content in oat (Avena sativa) shoot pulvini during the gravitropic response

In order to determine if components of the signal transduction pathway are involved in starch metabolism during the gravitropic response, the effects of inhibitors of phosphoprotein phosphatases and protein kinases (OA), and calcium channel blockers (LaCl3), on gravitropic bending and starch levels in gravisensitive node/pulvini of oat shoots were examined. Among the compo unds tested, okadaic acid (OA) and lanthanum chloride (LaCl3) showed the st rongest inhibitory effects on the negative gravitropic curvature response i n oat shoot node:pulvini. At the same time, they caused a rapid loss of sta rch in graviresponding pulvini based on a quantitative analysis of starch l evels in the bending tissues over 48 h periods. These two compounds act ini tially to block the net increase in starch content that occurs during the e arly stages (0-9 h) in graviresponding oat shoot pulvini. As a result, star ch levels drop precipitously in shoots treated with OA and LaCl3, starting at time zero of gravistimulation by reorientation. These findings suggest t hat protein dephosphorylation and calcium play a role in starch metabolism in oat shoot pulvini in response to a gravistimulation signal. They also in dicate that the amount of starch present in the chloroplast gravisensors in oat shoot pulvini may determine the rate of upward bending in gravirespond ing pulvini.