Auxin, a class of plant hormones which affects a wide array of growth and developmental processes including cell elongation and cell division, alters gene expression in a very rapid, selective, and dramatic way. The relative level of some mRNAs decreases several fold, while that of other mRNAs increases many fold. These changes are mediated, at least in some cases, by very fast (within 5-10 min) modulation by auxin of transcription as measured by run-off transcription assays using nuclei isolated from control and auxin-treated tissues. Rapid turnover of mRNAs following auxin treatment also contributes to large changes in steady state concentration in some cases. The data are suggestive of multiple and complex mechanisms of regulation of expression of those genes which have been studied, using cloned cDNAs for direct quantitation of mRNA steady state levels and relative transcription rats. While there is no definitive evidence that auxin-regulated gene expression mediates any of the growth responses effected by auxin, several lines of evidence are supportive of a very close relationship between these processes. The working hypothesis is that there is a causal relationship between the effects of auxin on gene expression and at least some of the physiological and growth responses to auxin.