Transient and sustained increases in inositol 1,4,5-hisphosphate precede the differential growth response in gravistimulated maize pulvini

The internodal maize pulvinus responds to gravistimulation with differentia l cell elongation on the lower side, As the site of both graviperception an d response, the pulvinus is an ideal system to study how organisms sense ch anges in orientation. We observed a transient 5-fold increase in inositol 1 ,4,5-trisphosphate (IP3) within 10 s of gravistimulation in the lower half of the pulvinus, indicating that the positional change was sensed immediate ly. Over the first 30 min, rapid IP3 fluctuations were observed between the upper and lower halves. Maize plants require a presentation time of betwee n 2 and 4 h before the cells on the lower side of the pulvinus are committe d to elongation. After 2 h of gravi-stimulation, the lower half consistentl y had higher IP3, and IP3 levels on the lower side continued to increase up to approximate to 5-fold over basal levels before visible growth, As bendi ng became visible after 8-10 h, IP3 levels returned to basal values, Additi onally, phosphatidylinositol 4-phosphate 5-kinase activity in the lower pul vinus half increased transiently within 10 min of gravistimulation, suggest ing that the increased IP3 production was accompanied by an up-regulation o f phosphatidylinositol 1,5-bisphosphate biosynthesis. Neither IP3 levels no r phosphatidylinositol 4-phosphate 5-kinase activity changed in pulvini hal ves from vertical control plants. Our data indicate the involvement of IP3 and inositol phospholipids in both short- and long-term responses to gravis timulation. As a diffusible second messenger, IP3 provides a mechanism to t ransmit and amplify the signal from the perceiving to the responding cells in the pulvinus, coordinating a synchronized growth response.