XVI International Botanical Congess

We have used the pulvini of maize and oats to investigate the involvement of phosphoinositides in gravitropism. A rapid, transient increase in inositol-1,4,5-trisphosphate (IP3) was detected in both systems within 15 s of gravistimulation. In maize, the increase in IP3 was detected initially in the lower half of the pulvinus followed by a fluctuation of the IP3 levels in the upper and lower half for about 1 h compared to vertical controls. After 2 h of gravistimulation, the lower half had significantly higher IP3 that continued to increase to 5-fold above the control value and then decreased prior to visible growth. A similar sustained increase in IP3 was observed in the lower half of the oat pulvini only the time scale was shorter. Both short term and long term changes in IP3 during graviperception and response would enable the plant to distinguish between transient movements caused by the wind and permanent lodging. The initial spike would serve as an initiation signal and the gradual increase in IP3 would sustain the requisite biochemical processes that precede differential growth.