A temperature-sensitive auxin auxotroph not deficient in indole-3-acetic acid.

A temperature-sensitive variant of Hyoscyamus muticus L. expressing a lethal phenotype in both cultured cells and regenerated plants has been shown to be a conditional auxin auxotroph with an absolute requirement for an exogenous auxin at temperatures above 30° C but not at lower temperatures. The requirement was satisfied by indole-3-acetic acid (IAA) and all active auxin analogous tested. Despite this dependence on exogenous auxin at high temperatures, the variant nevertheless synthesises IAA under these conditions and contains a pool of free IAA equal to that of the wild type. Furthermore, there is no depletion of the cellular IAA pool on incubation at high temperature in the absence of added auxin, even during the expression of the auxin-auxotrophic phenotype. The characteristics of this variant indicate that auxin auxotrophy can be lethal at the plant level and raises questions about the cellular function and mode of action of auxin.