Tryptophan is the precursor of IAA and zinc is
responsible for its synthesis. IAA is the example of natural
auxin.
please explain me.please quick
Answers
Answer:
lndole-3-acetic acid (IAA), the most important natural auxin in plants, is mainly synthesized from the amino acid tryptophan (Trp). Recent genetic and biochemical studies in Arabidopsis have unambiguously established the first complete Trp-dependent auxin biosynthesis pathway. The first chemical step of auxin biosynthesis is the removal of the amino group from Trp by the TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS (TAA) family of transaminases to generate indole-3-pyruvate (IPA). IPA then undergoes oxidative decarboxylation catalyzed by the YUCCA (YUC) family of flavin monooxygenases to produce IAA. This two-step auxin biosynthesis pathway is highly conserved throughout the plant kingdom and is essential for almost all of the major developmental processes. The successful elucidation of a complete auxin biosynthesis pathway provides the necessary tools for effectively modulating auxin concentrations in plants with temporal and spatial precision. The progress in auxin biosynthesis also lays a foundation for understanding polar auxin transport and for dissecting auxin signaling mechanisms during plant development.
INTRODUCTION
Auxin has long been recognized as a hormone essential for almost every aspect of plant growth and development (Zhao, 2010). However, an understanding of its biosynthetic mechanisms in plants had remained elusive until very recently. For a long time, the physiological roles of auxin were mainly inferred from studies on how plants responded to exogenous auxin treatments. These studies were also the foundation for elucidating the auxin signaling and polar transport mechanisms. However, to precisely define the physiological roles of auxin, we need to characterize auxin deficient mutants, a goal that becomes feasible only when we understand how auxin is synthesized in plants. Understanding of auxin biosynthesis will also reveal the sites of auxin production in plants, thereby allowing us to define auxin sources/sinks and to better understand polar auxin transport. Knowledge in auxin biosynthesis will greatly facilitate our understanding of the molecular mechanisms by which auxin controls various developmental processes. Progress in auxin biosynthesis research lays the foundation for improving agriculturally important traits such as branching and flower development by allowing us to regulate auxin levels in specific tissues/cells. Therefore, a clear understanding of auxin biosynthesis will ultimately have many significant impacts on agriculture and will also greatly extend our knowledge of fundamental plant biology.
Auxin biosynthesis can be divided into two general categories: de novo auxin biosynthesis and the release from auxin conjugates [see recent reviews (Normanly, 2010; Ludwig-Muller, 2011; Mano and Nemoto, 2012; Brumos et al., 2013; Ljung, 2013; Zhao, 2013; Tivendale et al., 2014)]. Indole-3-acetic acid (IAA), the main natural auxin in plants, exists in both free and conjugated forms. Free IAA is the active form of auxin and the conjugated auxins are considered storage forms or intermediates destined for degradation (Woodward and Bartel, 2005; Korasick et al., 2013). Free IAA can be released from IAA conjugates such as IAA esters, IAA-sugar, and IAA-amino acid conjugates by hydrolysis (Davies et al., 1999; Rampey et al., 2004; Ludwig-Muller, 2011; Korasick et al., 2013). Free IAA can also be produced from indole-3-butyric acid by a process similar to fatty acid β-oxidation in the peroxisomes (Zolman et al., 2000; Zolman et al., 2008). In this chapter, I focus on the recent progresses in de novo auxin biosynthesis. Mechanisms regarding the release of free auxin from conjugates and IBA have been reviewed elsewhere (Woodward and Bartel, 2005; Ludwig-Muller, 2011; Korasick et al., 2013).
Trp is a known precursor for auxin biosynthesis and it has been demonstrated that feeding plants with labeled Trp leads to the production of labeled IAA (Wright et al., 1991; Normanly et al., 1993). Two decades ago, isotope-labeling experiments in combination with using Trp biosynthetic mutants led to the proposal that IAA is also synthesized in a Trp-independent fashion (Wright et al., 1991; Normanly et al., 1993). So far, however, the molecular components of the Trp-independent pathway have not been identified. In this chapter, I will not discuss the Trp-independent auxin biosynthesis pathway. Instead, I will concentrate on the discovery of the first complete plant auxin biosynthetic pathway in which Trp is converted into IAA in two steps using indole-3-pyruvate (IPA) as the intermediate (Figure 1)