Auxin induces rapid changes in phosphatidylinositol metabolites

The plant growth-hormone auxin (indole-3-acetic acid, IAA) is involved in regulating such diverse processes as cell elongation, cell division and differentiation. The sequence of events leading to the various phenomena is still poorly understood. Both changes in extra- and intracellular pH (refs 1-4) and selective transcription are known to be induced by auxin. Evidence for auxin receptors at the plasmalemma membrane has been reported, but the signal transduction pathway is not known, for this nor for other plant hormones. In animal cells, hydrolysis of inositolphospholipids is a major mechanism for transmembrane signalling in response to external stimuli such as hormones, growth factors, neurotransmitters, antigens or light (reviewed in refs 8-11). Here we report that auxin can generate transient changes in inositol-1,4,5-trisphosphate (Ins(1,4,5)P3) and inositol bisphosphate (InsP2) within minutes in Catharanthus roseus cells arrested in G1. These changes are accompanied by a redistribution within the polyphosphoinositide fraction. As the physiological response to auxin addition is to relieve the arrest in G1, we suggest that these effects are an element in the signal transduction of this plant hormone.