Academic journal article Genetics

Auxin and Tryptophan Homeostasis Are Facilitated by the ISS1/VAS1 Aromatic Aminotransferase in Arabidopsis

Academic journal article Genetics

Auxin and Tryptophan Homeostasis Are Facilitated by the ISS1/VAS1 Aromatic Aminotransferase in Arabidopsis

Article excerpt

IN plants, the aromatic amino acids tryptophan (Trp), tyrosine (Tyr), and phenylalanine (Phe) are used for the synthesis of proteins and as precursors to a variety of specialized metabolites. While most secondary metabolites help protect the plant against abiotic and biotic stress (Tzin and Galili 2010), some such as Trp-derived indole-3-acetic acid (IAA) are essential growth regulators (Woodward and Bartel 2005). IAA, the primary auxin in plants, functions in establishing cell polarity during embryogenesis (Weijers et al. 2005; Kleine-Vehn et al. 2008), determination of leaf patterning (Bainbridge et al. 2008), and initiation of lateral roots and shoots (Celenza et al. 1995; Peret et al. 2009).

There are two general routes proposed for IAA biosynthesis in plants: from a Trp-dependent (Trp-D) pathway or through an indolic precursor of Trp in a Trp-independent (Trp-I) pathway (Woodward and Bartel 2005; Tivendale et al. 2014) (Figure 1). Within the Trp-D IAA biosynthetic pathway, there are three established pathways in plants that lead to IAA production: (i) the indole-3-pyruvic acid (IPA) pathway (ii) the indole-3-acetaldoxime (IAOx) pathway, and (iii) the indole-3-acetamide (IAM) pathway (Figure 1) (Ljung 2013; Zhao 2014).

In the IPA pathway, the TAA1 family of Trp aminotransferases converts Trp into IPA (Stepanova et al. 2008; Tao et al. 2008; Yamada et al. 2009; Zhou et al. 2011) followed by the direct conversion to IAA by the YUCCA (YUC) family of flavin monooxygenase-like (FMO) enzymes (Mashiguchi et al. 2011; Won et al. 2011; Zhao 2012). Both TAA1 and YUC gene families are highly conserved across the plant kingdom and TAA1 and YUC genes are coexpressed in a spatial and temporal manner (Stepanova et al. 2011) consistent with TAA1 and YUC being the primary route for IAA biosynthesis.

In Arabidopsis, IAOx is produced by CYP79B2 and CYP79B3 and is an intermediate in the synthesis of IAA and two classes of defense compounds, indole-glucosinolates (IGs) and camalexin (Hull et al. 2000; Zhao et al. 2002; Glawischnig et al. 2004; Sugawara et al. 2009). Disruption of the metabolic flux from IAOx to IGs in the sur1-1 (Mikkelsen et al. 2004) or sur2-1 (Morant et al. 2010) mutant causes an increase in conversion of IAOx to IAA, supporting this pathway as a route to IAA. However, IAOx is likely a minor contributor to overall IAA production under most conditions because the double cyp79B2 cyp79B3 mutant produces no IAOx and appears wild type (WT) (Zhao et al. 2002; Sugawara et al. 2009). In addition, IAOx production appears to be limited to the Brassicaceae family (Sugawara et al. 2009; Nonhebel et al. 2011).

For the IAM pathway, IAM is produced from Trp, perhaps by a monooxygenase, similar to what is used by bacteria (Woodward and Bartel 2005; Ljung 2013), although in Arabidopsis the IAOx pathway may also produce IAM (Sugawara et al. 2009). In Arabidopsis IAM can be converted to IAA in vitro by IAM hydrolase encoded by AMI1 (Pollmann et al. 2003). While the IAM pathway has been proposed to existthroughouttheplantkingdom(Manoet al. 2010), its importance in overall IAA metabolism remains to be characterized genetically.

Evidence of the Trp-I IAA synthesis pathway comes primarily from stable isotope labeling studies of Trp auxotrophs in maize (Wright et al. 1991) and Arabidopsis (Normanly et al. 1993), IAA homeostasis mutants (Quint et al. 2009) and in wild-type Lemna gibba under environmental perturbations (Rapparini et al. 2002). Indole or indole-3glycerol phosphate, precursors of Trp, have been proposed as the branch point to Trp-I IAA biosynthesis (Ouyang et al. 2000) and recent work suggests that a cytosolic tryptophan synthase a-subunit called indole synthase (INS) participates in Trp-I IAA synthesis in embryogenesis in Arabidopsis (Wang et al. 2015). Indirect evidence comes from analysis of the Arabidopsis alf3-1 mutant, which produces lateral roots that arrest growth and die after the initial stages of lateral root formation (Celenza et al. …

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