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

Plants sense positional changes relative to the gravity vector. To date, th e signaling processes by which the perception of a gravistimulus is linked to the initiation of differential growth are poorly defined. We hare invest igated the role of inositol 1,4,5-trisphosphate (InsP(3)) in the gravitropi c response of oat (Avena sativa) shoot pulvini. Within 15 s of gravistimula tion, InsP(3) levels increased 3-fold over vertical controls in upper and l ower pulvinus halves and fluctuated in both pulvinus halves over the first minutes. Between 10 and 30 min of gravistimulation, InsP(3) levels in the l ower pulvinus half increased 5-fold over the upper. Changes in InsP(3) were confined to the pulvinus and were not detected in internodal tissue, highl ighting the importance of the pulvinus for both graviperception and respons e. Inhibition of phospholipase C blocked the long-term increase in InsP(3), and reduced gravitropic bending by 65%. Short-term changes in InsP(3) were unimpaired by the inhibitor. Gravitropic bending of oat plants is inhibite d at 4 degreesC; however, the plants retain the information of a positional change and respond at room temperature. Both short- and long-term changes in InsP(3) were present at 4 degreesC. We propose a role for InsP(3) in the establishment of tissue polarity during the gravitropic response of oat pu lvini. InsP(3) may be involved in the retention of cold-perceived gravistim ulation by providing positional information in the pulvini prior to the red istribution of auxin.