Phytohormones affect differentiation and growth of plant cells causing permanent, irreversible changes in the activities of cells. In contrast to animals — where a hormone often acts quite specifically on particular target tissues — phytohormones can affect almost all tissues in the plant. Auxins were the first plant hormones to be discovered and influence plant growth in a flow from shoot apex to root tip, but how this is translated into growth has been unclear. In the February 13 Nature, Fu Xlangdong and Nicholas P. Harberd at the John Innes Centre, Norwich, UK, show that auxin is necessary for root growth mediated by the phytohormone gibberellin (GA), and is further regulated by GA antagonism of the nuclear DELLA proteins (Nature, 421:740-743, February 13, 2003).

Xlangdong and Harberd investigated the role of auxins in the regulation of root growth in Arabidopsis. They showed that root growth...

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