Hormone-Induced Gene Expression During Gravicurvature of <Emphasis Type="Italic">Brassica</Emphasis> Roots

To investigate the spatial and temporal dependence of hormonal regulation during gravitropism, we compared the effects of root cap application of indole-3-acetic acid (IAA) and abscisic acid (ABA) with gene expression changes occurring naturally during gravitropic reaction of Brassica rapa roots. The expression of auxin, ABA, and metabolism-related genes in the tip, elongation zone, and maturation zone varied with time, location, and hormone concentration and characterized polar auxin transport. IAA was transported readily shootward and inhibited growth more than ABA. Expression of PIN3 and IAA5 in the elongation zone showed downregulation on the convex but upregulation on the concave side. Both PIN7 and IAA5 responded near maximally to 10^?8 M IAA within 30 min, suggesting that auxin activates its own transport system. Ubiquitin 1 (UBQ1) responded after a lag time of more than 1 h to IAA. The metabolic control gene Phosphoenolpyruvate carboxylase 1 (PEPC1) was more sensitive to ABA but upregulated by high concentrations of either hormone. The time course and duration of gene activation suggests that ABA is not involved in gravitropic curvature, differential elongation is not simply explained by IAA-induced upregulation, and that reference genes are sensitive to auxin.