Expression of the Auxin-Inducible GH3 Promoter/GUS Fusion Gene as a Useful Molecular Marker for Auxin Physiology

Abstract

Expression of the GH3 promoter/GUS reporter (GH3/ GUS) gene in transgenic tobacco is silent in most vegetative tissues and organs, but can be specifically induced by auxin in all cell types throughout the plant, indicating that auxin is a limiting factor for activation of the GH3/GUS gene. In the absence of exogenous auxin, expression of the GH3/ GUS gene either correlates with physiological events which are presumably mediated by auxin or occurs in cells where relatively high levels of endogenous auxin are expected. For example, the GH3/GUS gene is expressed in dormant lateral buds, tips of adventitious roots, and stem parenchyma cells where adventitious roots are initiating. The GH3/GUS gene is activated at the cut surface nearest the tip of a leaf from which a strip of tissue has been excised, and in the basal end of an excised shoot segment. Also, the GH3/GUS gene is expressed predominantly in the lower side of a gravistimulated shoot and in the dark side of a unilateral light-stimulated shoot. Furthermore, the expression pattern of the GH3/GUS gene is similar to the distribution pattern of 14C-IAA that is applied at the apical end of the gravistimulated shoot. Thus, the results reported here provide additional support for a crucial role of auxin in shoot gravi- and photo-tropisms and adventitious root formation. Furthermore, the data presented in this report also indicate that expression of the GH3/GUS gene is an excellent molecular marker for studying auxin transport and for monitoring changes in either auxin concentration or cellular sensitivity to auxin in planta.