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

Abstract

Indole-3-acetic acid (IAA) plays a critical role in regulating numerous aspects of plant growth and development. While there is much genetic support for tryptophan-dependent (Trp-D) IAA synthesis pathways, there is little genetic evidence for tryptophanindependent (Trp-I) IAA synthesis pathways. Using Arabidopsis, we identified two mutant alleles of ISS1 (Indole Severe Sensitive) that display indole-dependent IAA overproduction phenotypes including leaf epinasty and adventitious rooting. Stable isotope labeling showed that iss1, but not WT, uses primarily Trp-I IAA synthesis when grown on indole-supplemented medium. In contrast, both iss1 and WT use primarily Trp-D IAA synthesis when grown on unsupplemented medium. iss1 seedlings produce 8-fold higher levels of IAA when grown on indole and surprisingly have a 174-fold increase in Trp. These findings indicate that the iss1 mutant's increase in Trp-I IAA synthesis is due to a loss of Trp catabolism. ISS1 was identified as At1g80360, a predicted aromatic aminotransferase, and in vitro and in vivo analysis confirmed this activity. At1g80360 was previously shown to primarily carry out the conversion of indole-3-pyruvic acid to Trp as an IAA homeostatic mechanism in young seedlings. Our results suggest that in addition to this activity, in more mature plants ISS1 has a role in Trp catabolism and possibly in the metabolism of other aromatic amino acids. We postulate that this loss of Trp catabolism impacts the use of Trp-D and/or Trp-I IAA synthesis pathways.KEYWORDS Arabidopsis thaliana; auxin; ISS1/VAS1; phenylpropanoids; tryptophan metabolism I N 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 t...

Localization and interactions between Arabidopsis auxin biosynthetic enzymes in the TAA/YUC-dependent pathway

“…Multiple pathways enhance the complexity of auxin biosynthesis. Parallel tryptophandependent and -independent pathways (Woodward and Bartel, 2005;Wang et al, 2015;Pieck et al, 2015;Kasahara 2015) act in different organs, developmental stages and environmental conditions (Normanly and Bartel, 1999;Östin et al, 1999). All these different routes can be independently and differentially regulated to build a metabolic network capable of dynamic changes to keep up auxin homeostasis or supply auxin maxima for local demands.…”

Loss of function of the Pad-1 aminotransferase gene, which is involved in auxin homeostasis, induces parthenocarpy in Solanaceae plants

“…VAS1 protein catalyzes the conversion of IPyA to Trp, which is the reverse reaction of TAA1/TARs and is thought to be involved in auxin homeostasis. Pieck et al (34) suggested that in addition to this reaction, VAS1 had a role in Trp catabolism and metabolism of other aromatic amino acids. Our biochemical analysis showed that the recombinant Pad-1 protein catalyzed the conversion of IPyA to Trp in vitro (SI Appendix, Fig.…”

The Role of Auxin-Ethylene Crosstalk in Orchestrating Primary Root Elongation in Sugar Beet

“…Inhibition of auxin transport using auxin transport mutants (e.g., pin2 and aux1 ) results in an ethylene insensitive root growth due to the lack of crosstalk possibilities (Ruzicka et al, 2007). Another possible point of auxin-ethylene crosstalk is the enzyme VAS1 which regulates both auxin and ethylene production (Zheng et al, 2013; Pieck et al, 2015). …”