Early Genes and Auxin Action

The plant hormone IAA (or auxin) is central to the control of plant growth and development. Processes governed by auxin in concert with other plant growth regulators include development of vascular tissues, formation of lateral and adventitious roots, control of apical dominance, and tropic responses (Went and Thimann, 1937). At the level of cellular physiology, auxin profoundly affects turgor, elongation, division, and cell differentiation, the major driving and shaping forces in morphogenesis and oncogenesis. The molecular mechanisms of auxin action are still unknown, although it is now well established that auxin modulates membrane function and gene expression (for review, see Napier and Venis, 1995). These biochemical changes, in turn, most likely affect fundamental aspects of plant morphology and physiology. However, a causal relationship between auxin-mediated alterations in gene expression or membrane function and a particular growth process has not yet been demonstrated. Despite its critical role in plant development and the immense volume of studies on the diverse auxin effects, understanding of the molecular mechanisms of auxin action remains one of the major challenges in plant biology. The signal transduction cascades leading from auxin perception to altered gene expression or membrane function hold the key in our attempts to elucidate the primary mechanism(s) of auxin action. An array of experimental strategies has been mounted to investigate auxin signaling pathways. The combination of biochemical, molecular, and genetic approaches will allow for significant new insights into how the hormone works in molecular terms (Fig. 1). One strategy employs genetics and reverse genetics to construct transgenic plants with perturbations in auxin homeostasis and to screen for mutants with defects in auxinrelated physiology. Transgenic plants expressing altered hormone levels have already resolved some longstanding questions in plant physiology. Mutant plants defective in auxin responses will rejuvenate and stimulate research by identifying novel genes involved in hormone perception, signal transduction, and physiological responses (for review, see Hobbie and Estelle, 1994; Klee and Romano, 1994). The first significant result (to our knowledge) of this approach was the cloning of the AXR1 gene, which encodes a protein related to the ubiquitin-activating enzyme El (Leyser et al., 1993). Although AXRl is probably not a functional El homolog, it is nonetheless an exquisite example of the potential of molecular genetics to connect the unexpected. The biochemical strategy is based on the identification of auxin receptors and subsequent isolation of interacting components. The search for auxin receptors has led to the discovery of a number of soluble and membranebound proteins that bind auxin with moderate but physiologically relevant affinity. Their functional role in auxin signaling is still unclear and is a major target of current research (for review, see Jones, 1994; Napier and Venis, 1995). Auxin-regulated genes provide yet another source of molecular tools to dissect auxin action. The hormone modulates gene expression in a wide variety of plant tissues and cell types over a broad period of time (for review, see Guilfoyle, 1986; Theologis, 1986). However, early genes selectively induced as a primary response to auxin and prior to the initiation of cell growth are likely candidates to play a pivotal role in mediating growth-stimulating effects of the hormone. This review focuses on recent advances in our knowledge on early auxin-inducible gene expression and possible functions of the polypeptides encoded.