A role for inositol 1,4,5-trisphosphate in gravitropic signaling and the retention of cold-perceived gravistimulation of oat shoot pulvini

Abstract

Plants sense positional changes relative to the gravity vector. To date, the signaling processes by which the perception of a gravistimulus is linked to the initiation of differential growth are poorly defined. We have investigated the role of inositol 1,4,5-trisphosphate (InsP3) in the gravitropic response of oat (Avena sativa) shoot pulvini. Within 15 s of gravistimulation, InsP3 levels increased 3-fold over vertical controls in upper and lower pulvinus halves and fluctuated in both pulvinus halves over the first minutes. Between 10 and 30 min of gravistimulation, InsP3 levels in the lower pulvinus half increased 3-fold over the upper. Changes in InsP3 were confined to the pulvinus and were not detected in internodal tissue, highlighting the importance of the pulvinus for both graviperception and response. Inhibition of phospholipase C blocked the long-term increase in InsP3, and reduced gravitropic bending by 65%. Short-term changes in InsP3 were unimpaired by the inhibitor. Gravitropic bending of oat plants is inhibited at 4°C; however, the plants retain the information of a positional change and respond at room temperature. Both short- and long-term changes in InsP3 were present at 4°C. We propose a role for InsP3 in the establishment of tissue polarity during the gravitropic response of oat pulvini. InsP3 may be involved in the retention of cold-perceived gravistimulation by providing positional information in the pulvini prior to the redistribution of auxin.