Indole-3-Acetic Acid

IAA is the most widely distributed, naturally occurring auxin in vascular plants, dicots, monocots, gymnosperms, and ferns. There are also reports of IAA being present in mosses and liverworts, as well as in some green algae (e.g., Caulerpa). IAA is a weak acid with a pH of 4.85. It occurs in dissociated state at neutral pH solutions. IAA is involved in nearly every aspect of plant growth and development, from embryo to adult reproductive plant. The processes regulated include pattern formation in embryo development, induction of cell division, stem and coleoptile elongation, apical dominance, induction of rooting, vascular tissue differentiation, fruit development, and tropic movements such as bending of shoots toward light or of roots toward gravity.

Fig. 8.1 Chemical structure of four endogenous auxins. Indole-3-acetic acid (IAA), indole-3butyric acid (IBA), 4-chloroindole-3-acetic acid (4-Cl-IAA), and phenylacetic acid (PAA)

It is difficult to unambiguously define typical “auxin activity.” Auxin displays morphogenic properties that are modulated by the environment and defined by dynamic changes in its perception and signal transduction. This machinery has been intensively studied during the past decade and includes effects that are either dependent or independent of gene expression (Bhalerao and Bennett 2003). Thus, “auxin action” may be understood as the sum of all these processes (Simon and Petrasek 2011). Later research convincingly demonstrated that auxin is required together with other plant hormones for both cell division and oriented cell expansion (PerrotRechenmann 2010), influencing all aspects of plant development (Vanneste and Friml 2009).

The isolation of plant mutants related to auxin showed that the modification of the regulation of auxin biosynthesis, transport, or signaling generates severe alterations in many aspects of plant development. For example, the auxin overproducer mutant Yucca leads to defects in vascular tissue formation (Cheng et al. 2007). Disruption in auxin transport, in the mutant pin1, leads to defects in floral development (Okada et al. 1991). Finally, mutation in auxin signaling can trigger a global dwarfism as for the auxin-resistant axr112 mutant (Lincoln et al. 1990), the absence of root formation as for the monopteros (mp) mutant (Hamann et al. 2002), or even embryo lethality as for the abp1 null mutant (Chen et al. 2001). This demonstrates that in plants, the phytohormone auxin plays a central role in plant growth and development.

Auxin is considered as a morphogen since it regulates the development in a dosedependent manner (Bhalerao and Bennett 2003). It highlights the importance of auxin gradients and the necessity of a subtle regulation of auxin concentration at the scale of organ, tissues, or even cells. To achieve such regulation, plants have developed various mechanisms aimed at controlling auxin homeostasis and the dynamics of auxin redistribution. In addition, various tissues exhibit distinct sensitivity to auxin, thus reflecting that the responsiveness (perception and signaling) is also tightly modulated (Tromas and Perrot-Rechenmann 2010).

Auxin-Binding Soluble Proteins

Shishova and Lindberg (2010) reported that for more than 100 years, the most intriguing question in plant physiology has been how IAA might trigger such enormous variety in physiological responses. According to recent knowledge, a broad spectral activity is observed, which might correlate with changes in the number and properties of auxin receptors. These proteins are responsible for recognition of the hormone and the initiation of further signal transduction chains, resulting in a specific physiological response. Thus, one of the main properties of the auxin receptor is its capability to bind auxin. An investigation of auxin-binding sites in plant cells started almost 30 years ago (Hertel et al. 1972). It has showed a heterogeneity of these sites both in affinity and localization. So, the pool of plant cell auxin-binding proteins (ABPs) consists of two groups: soluble and membrane-bound proteins.

Early biochemical investigations identified a number of auxin-binding soluble proteins such as 1,3-glucanase (MacDonald et al. 1991), β-glucosidase (Campos et al. 1992), glutathione S-transferase (Bilang et al. 1993), and superoxide dismutase (Feldwisch et al. 1994). Two soluble ABPs with a relatively low affinity for IAA were purified and reported to stimulate RNA synthesis in isolated nuclei (Kikuchi et al. 1989). Later, it was shown that one of these protein-bound RNA polymerase II had DNA-binding activity (Sakai 1992). Another polypeptide, a 65-kDa protein, was also found to have a nuclear localization (Prasad and Jones 1991). A soluble ABP 44-kDa protein showed a close link to auxin effects on elongation growth and high affinity labeling of chlorinated auxins (Reinard and Jacobsen 1995; Reinard et al. 1998).

< Prev   CONTENTS   Next >