How Does Auxin Turn On Genes?

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Abstract
The plant hormone auxin (or IAA) plays a key role in a wide variety of growth and developmental processes. At the cellular level, auxin acts as a signal for division, exten- sion, and differentiation during the course of the plant life cycle. At the whole-plant level, auxin plays an important role in root formation, apical dominance, tropism, and senescence. The question is how does such a simple mole- cule regulate such a plethora of responses within an assort- ment of cells, tissues, and organs of plants? The answer to this question requires an understanding of auxin perception, signal transduction, and gene regulation. At this time, little is known about how auxin is recognized as a hormone by plant cells or what receptor molecules are involved in this recognition. Although several classes of auxin-binding proteins have been identified and character- ized (for review, see Napier and Venis, 1995), it is not clear which if any of these function as receptors in signal trans- duction pathways that target the nucleus and regulate auxin-responsive gene expression. Likewise, the auxin sig- nal transduction pathway involved in early auxin- regulated gene expression is currently a mystery. It is possible that one or several classes of auxin receptors and auxin signal transduction pathways exist in plant cells, and that these receptors and pathways are not uniformly dis- tributed among different cell types and tissues. Multiple types of auxin receptors and signal transduction pathways could account for some of the diversity observed in differ- ent tissues and organs that respond to auxin in a variety of ways. Whatever the auxin receptors and signal transduction pathways, it is clear that exogenously applied auxin can rapidly and specifically alter the expression of selected genes in different tissues and organs. Responses at the gene-expression level can be detected as early as 2 to 3 min after auxin application (for review, see Guilfoyle, 1998), and genes that are activated or repressed in this brief time are referred to as primary or early auxin-responsive genes, a number of which have been identified and characterized. These genes and their expression have been discussed in recent reviews (Abel and Theologis, 1996; Guilfoyle, 1998) and will not be elaborated on here. This Update focuses on cis-acting elements (i.e. DNA sequences that confer auxin responsiveness to a promoter) and trans-acting factors (i.e. transcription factors that bind to the cis-acting elements) involved in the regulation of plant genes that respond rapidly and specifically to auxin.