Cellular signaling: new insights into the action of the plant growth hormone auxin.

The auxin indole-3-acetic acid (IAA) has long been recognized as an important hormone regulating a wide array of responses in the growth and development of plants (1, 2). It is required throughout the life cycle for cell division, elongation, and differentiation of vascular tissues as well as initiation of buds and lateral roots and control of lateral branching. It mediates the responses of plants to light and gravity (3-5). Until quite recently, all known substances with auxin activity have been small, amphipathic, organic compounds, usually with an aromatic ring system bearing a side chain with a polar group (1, 6, 7). Small molecules such as auxin do not convey much information and are thought therefore to interact in target cells with a receptor. The kind of receptor, its location, and the extent to which it is saturated specify an appropriate developmental message telling particular cells to divide, differentiate into tracheids, repair a wound, organize a meristem, and so forth. Until recently, knowledge of the actual receptors for auxin, their location, and how they evoke specific responses has eluded us. In this issue, Thiel et al. (8) report the startling discovery that a synthetic peptide, comprising the terminal 14 residues of the consensus sequence at the C terminus of the major auxin-binding protein (ABP) in plants, rapidly and reversibly modulates the activity. of K+ channels in the plasma membrane of guard cells. In this report, intact guard cells make their debut as a system for investigating an early response to auxin-in this case, the turgor-dependent regulation of stomatal aperture. This report provides elegant confirmation for the long-held view that the initial action of auxin occurs at the plasma membrane and that a small, soluble, ABP that is present on the exterior surface of the cell serves as an auxin receptor and initiates signal transduction. These authors have also been able to identify one of the initial steps coupling interaction of auxin at its receptor on the plasma membrane to changes in the activity of both inwardand outwardrectifying K+ channels. For comparison with what follows, a very condensed summary of our present conception of the much-studied process of cell elongation will be useful. In auxininduced cell extension, the hormone activates a cascade of specific genes beginning as early as 5-7 min after application (9). At about the same time, net H+ secretion also increases, presumably reflecting the synthesis and incorporation of additional molecules of H+-ATPase in the plasma membrane (10, 11); however, because gene expression can be shown to proceed independently of proton secretion, it is thought to be the primary response to auxin in this system (9, 12). The outcome of H+ secretion is an increase in the acidity of the cell walls that activates enzymatic loosening of the structure of the wall and enables the turgor of the cell to extend the walls (12-14). Until recently the identity and function of the proteins encoded by the auxin-regulated genes were unknown. The recent work of Theologis and his colleagues (15, 16) identifying conserved regions in the 5' flanking regions of three of these genes from peas (PS-IAA4/5 and PS-IAA6) as auxin response elements and recognizing the protein products of auxin-induced genes as short-lived, nuclear-localized, regulatory proteins sharing conserved DNA-binding domains with other auxinregulated genes (17) represents a notable breakthrough.