The indolic compound auxin regulates virtually every aspect of plant growth and development, but its role in embryogenesis and its molecular mechanism

The plant hormone auxin (indole-3-acetic acid) has been shown to regulate a wide variety of plant processes, including tropisms, vascular development and formation of lateral shoots and roots (for reviews see Aloni, 1995; Malamy and Benfey, 1997; Chen et al., 1999). At the cellular level, auxin affects elongation, division and differentiation, but the mechanisms by which it produces these cellular effects remain poorly understood. The auxin-binding protein ABP1 appears to function as an auxin receptor regulating cell elongation (Jones et al., 1998); roles as receptors for other auxin-binding proteins have not been demonstrated. The products of the AXR1 (Leyser et al., 1993) and TIR1 genes (Ruegger et al., 1998) are likely to regulate auxin response through controlling protein modification by ubiquitin or ubiquitin-like molecules (del Pozo et al., 1998). Possible targets of such ubiquitinor ubiquitin-like modification are the proteins encoded by the Aux/IAA family of genes (Abel et al., 1994), short-lived, nuclear-localized proteins whose transcription is induced by auxin and which may encode transcriptional regulators (Abel and Theologis, 1996). The related Auxin Response Factor (ARF) genes encode proteins demonstrated to bind to and regulate transcription at auxinresponsive promoters (Ulmasov et al., 1999). The importance of these proteins in auxin response has been demonstrated by genetic approaches: three Arabidopsis mutants with defects in auxin-related processes, auxin-resistant3 (Rouse et al., 1998), short hypocotyl2 (Tian and Reed, 1999), and monopteros (Hardtke and Berleth, 1998) were found to have defects in Aux/IAA or ARF genes. There is also some evidence that the auxin signal transduction cascade may include components of a mitogen-activated protein kinase (MAPK) cascade (Kovtun et al., 1998; Mizoguchi et al., 1994), although not all experiments supports this model (Tena and Renaudin, 1998). The factors controlling embryonic development in plants are still poorly understood, but evidence suggests that shortrange or direct cell-cell interactions may play important roles (Laux and Jürgens, 1997). Putative auxin gradients established by auxin polar transport (Lomax et al., 1995) may be important for establishing bilateral symmetry and polarity during embryonic development (Schiavone and Cooke, 1987; Liu et al., 1993; Fischer and Neuhaus, 1996; Hardtke and Berleth, 1998), although the only direct evidence that auxin acts as a positional morphogen comes from studies of cambial development (Uggla et al., 1998). Auxin has been further implicated in embryonic pattern formation by characterizaton of three embryonic mutants. fass mutants (Torres-Ruiz and Jürgens, 1993) were shown to have defects in auxin homeostasis (Fisher et al., 1996). Homozygous monopteros mutants, with mutations in an Auxin Response Factor gene 23 Development 127, 23-32 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV0264