Evidence of an auxin‐mediated phosphoinositide turnover and an inositol (1,4,5)trisphosphate effect on isolated membranes of Daucus carota L.

Abstract

Microsomal membranes from carrot suspension cells were phosphorylated in vitro with [γ‐³²P]ATP. In the presence of submicromolar concentrations of the natural auxin indoleacetic acid (IAA), a rapid, but transient decrease of the [³²P] label could be detected in the phospholipid extracts of the membranes. The phytohormone effect was not the result of an inhibition of the lipid phosphorylation reactions, but was caused by a simultaneous release of water‐soluble compounds, which, according to their chromatographic properties, were assumed to contain inositol polyphosphates. Although the [³²P]‐labeled lipids, as well as the inositol polyphosphates, were not identified unequivocally by chemical analysis, these findings point to an auxin‐mediated control of a phosphoinositidase C‐like reaction similar to the hormone‐stimulated phosphoinositide response in animals. Exogenously applied inositol (1,4,5)trisphosphate [(1,4,5)IP₃] was found to release ⁴⁵Ca²⁺ from preloaded membrane vesicles of carrot cells. Both the detection of the auxin‐stimulated phosphoinositide response and the (1,4,5)IP₃‐mediated Ca²⁺ release on isolated cell membranes offer new experimental approaches for the identification of the putative auxin receptor and its signal transduction pathway.