Plants rely on a diverse set of small-molecule hormones to regulate every aspect of their biological processes including development,growth,and adaptation.Since the discovery of the first plant hormone,auxin,hormones have always been at the frontier of plant biology.Although the physiological functions of most plant hormones have been studied for decades,the last 15-20 years have seen dramatic progress in our understanding of the molecular mechanisms of hormone actions.The publication of the whole-genome sequences of the model systems of Arabidopsis and rice,together with the advent of multidisciplinary approaches,has opened the door to successful experimentation on plant hormone actions.