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 leve1 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 critica1 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 nove1 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