Abstract
Auxin regulates several aspects of plant growth and development. Auxin is unique among plant hormones for exhibiting polar transport. Indole-3-acetic acid (IAA), the major form of auxin in higher plants, is a weak acid and its intercellular movement is facilitated by auxin influx and efflux carriers. Polarity of auxin movement is provided by asymmetric localization of auxin carriers (mainly PIN efflux carriers). PIN-FORMED (PIN) and P-GLYCOPROTEIN (PGP) family of proteins are major auxin efflux carriers whereas AUXIN1/LIKE-AUX1 (AUX/LAX) are major auxin influx carriers. Genetic and biochemical evidence show that each member of the AUX/LAX family is a functional auxin influx carrier and mediate auxin related developmental programmes in different organs and tissues. Of the four AUX/LAX genes, AUX1 regulates root gravitropism, root hair development and leaf phyllotaxy whereas LAX2 regulates vascular development in cotyledons. Both AUX1 and LAX3 have been implicated in lateral root (LR) development as well as apical hook formation whereas both AUX1 and LAX1 and possibly LAX2 are required for leaf phyllotactic patterning.
Highlights
Genetic, molecular and pharmacological approaches have elegantly demonstrated that auxin regulates several aspects of plant growth and development including embryo (Steinmann et al, 1999; Wolters et al, 2011), root (Swarup et al, 2001, 2004, 2005), lateral root (LR) (Swarup et al, 2008; Péret et al, 2009a,b), leaf (Bainbridge et al, 2008; Guenot et al, 2012) and flower development
Auxin influx carriers have been implicated in regulating LR development (Marchant et al, 2002; De Smet et al, 2007; Swarup et al, 2008) Marchant et al (2002) demonstrated that AUX1 is expressed in the pericycle cells before the first periclinal division and the aux1 mutant displays a 50% reduction in the number of LRs (Hobbie and Estelle, 1995)
Mutation in aux1 results in apical shifting of the root hairs. aux1 mutants had 30 times higher frequency of double hair formation compared to wildtype. These results provided a clear link between auxin transport and the establishment of apical-basal epidermal polarity in Arabidopsis
Summary
Molecular and pharmacological approaches have elegantly demonstrated that auxin regulates several aspects of plant growth and development including embryo (Steinmann et al, 1999; Wolters et al, 2011), root (Swarup et al, 2001, 2004, 2005), lateral root (LR) (Swarup et al, 2008; Péret et al, 2009a,b), leaf (Bainbridge et al, 2008; Guenot et al, 2012) and flower development. AUXIN DISTRIBUTION: COMPLEX Use of auxin response reporters for example DR5 (Ulmasov et al, 1997) and IAA2 (Abel et al, 1994) and auxin sensors DII 28 (Brunoud et al, 2012) have provided great insight into auxin accumulation and distribution in plant tissues. These studies show that auxin gradients are crucial for several aspects of plant development including tropic responses, organ development and meristem size. AUXIN TRANSPORTERS: PROVIDING DIRECTION As per chemiosmotic polar diffusion hypothesis, the term first coined by Goldsmith (1977) based on the famous work of Rubery and Sheldrake (1974) and Raven (1975) cellular IAA movement is www.frontiersin.org
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