IAA Biosynthesis in A. thaliana
Explore and order pathway-specific siRNAs, real-time PCR assays, and expression vectors. View pathway information and literature references for your pathway.
  • Click on your proteins of interest in the pathway image or review below
  • Select your genes of interest and click "add selection"
  • When you have finished your gene selection, click "Find Products" to find assays, arrays, or create custom products
Download Image Terms of Use Download PPT
Pathway Navigator
IAA Biosynthesis in A. thaliana

Auxin is a classic Phytohormone involved in a myriad of Developmental and Environmental Processes: Embryo Patterning, Cell Division and Elongation, Vascular Differentiation, Lateral Root Initiation, Gravitropism, and Phototropism. Though many natural and synthetic compounds exhibit Auxin-like activity in bioassays, IAA (Indole-3-Acetic Acid) is recognized as the key Auxin in most plants. IAA gene family consists of over 20 members in Arabidopsis. The physiologically active form is the free acid, but IAA can also be found in different conjugated forms, including Ester-types with the Carboxyl group linked via Oxygen to a Sugar (for example Glucose) and Amide-types with the Carboxyl group forming an Amide (Peptide bond) to Amino Acids or Polypeptides. Plants use several pathways to synthesize IAA but none of the pathways is yet defined to the level of knowing each relevant gene, enzyme, and intermediate (Ref.1).

Basically, IAA is synthesized from Trp (Tryptophan) using Trp-dependent pathways and from an Indolic Trp precursor via Trp-independent pathways. Tryptophan is an Amino Acid essential for survival. The synthesis of Tryptophan is one of the most complicated of all Amino Acids. Trytophan synthesis is complex and involves 5 steps from Chorismate. Glutamate donates an Amine group in the first step of the pathway and Pyruvate is lost from Chorismate, to form Anthranilate. The enzyme, Anthranilate Synthase, encoded by ASA and ASB  in Arabidopsis, catalyzes the reaction. In the next step a Ribose Sugar is added, this eventually contributes to the 5-membered ring of Tryptophan. Anthranilate condenses with PRPP (5-Phosphoribosyl-Alpha-Pyrophospate) with the help of PAT1(Anthralinate Phosphoribosyl Transferase). Energy is contributed to the process in the form of hydrolysis of PRPP. This hydrolysis helps drive the addition of the ribose sugar in the second step of the reaction and in the process, N-(5`-Phosphoribosyl-Anthranilate is formed, which is then converted to 1-O-Carboxyphenylamino-1-Deoxyribulose Phosphate, by PAI (N-(5`-Phosphoribosyl)-Anthranilate Isomerase). 1-O-Carboxyphenylamino-1-Deoxyribulose Phosphate then undergoes a Decarboxylation reaction, catalyzed by the enzyme IGPS (Indole-3-Glycerol Phosphate Synthase) to form IGP (Indole-3-Glycerol Phosphate). The final step of Tryptophan biosynthesis is catalyzed by Tryptophan Synthase (encoded by TSA1, TSB1 and TSB2 in Arabidopsis), a Pyridoxal Phosphate enzyme. In presence of the enzyme TSA1, Indole-3-Glycerol Phosphate is converted to Indole. Indole once formed is catalyzed by TSB1 and TSB2 to form Tryptophan. Tryptophan is converted to IAA through a series of reactions (Ref.2).

Several Trp-dependent pathways, which are generally named after an intermediate, have been proposed for IAA biosynthesis: the IPA (Indole-3-Pyruvic Acid) pathway, the IAM (Indole-3-Acetamide) pathway, the TAM (Tryptamine) pathway, and the IAOx (Indole-3-Acetaldoxime) pathway. The IPA pathway [Trp ® IPA ® IAAld (Indole-3-Acetaldehyde) ® IAA] is important in some IAA-synthesizing microorganisms and may operate in plants as well. IPA is found in Arabidopsis seedlings, but genes encoding a Trp Aminotransferase that oxidatively transaminates Trp to IPA or an IPA Decarboxylase that converts IPA to IAAld have not been identified in plants. The final enzyme in the proposed IPA pathway is an IAAld specific AAO1 (Aldehyde Oxidase-1) protein. The identification of Arabidopsis AAO1 does not verify the existence of the IPA pathway, however, as IAAld may be an intermediate in other IAA biosynthetic pathways. The IAM pathway [Trp ® IAM ® IAA] is a second microbial pathway that may also act in plants. IAM lacks Auxin activity in Arabidopsis, which allows the iaaH gene to be used as a screenable marker that confers IAM sensitivity. Intriguingly, IAM is found in Arabidopsis seedlings at levels similar to free IAA, and an Arabidopsis AMI1 (Amidohydrolase) converts IAM to IAA in vitro (Ref.3).

TAM pathway [Trp ® TAM ® N-Hydroxyl-TAM ® IAOx (Indole-3-Acetaldoxime) ® IAAld ® IAA] also converts Trp to IAA. Tryptophan Decarboxylase converts Trp to Tryptamine. The Arabidopsis genome contains potential Trp Decarboxylase genes, but the encoded enzymes have not been characterized, and Tryptamine has not been identified in Arabidopsis. Yucca may catalyze a rate-limiting step in a Tryptamine pathway. Yucca encodes a Flavin monooxygenase that catalyzes the N-hydroxylation of Tryptamine, a proposed step in IAA biosynthesis. Yucca has 10 homologs in the Arabidopsis genome, some of which have been shown to be functionally redundant with Yucca. For example, overexpression of either Yucca or Yucca2 in Arabidopsis leads to IAA overproduction phenotypes, but loss-of-function mutants of Yucca and Yucca2 fail to display any developmental phenotypes, consistent with genetic redundancy in the biosynthetic pathway as had been proposed. The N-hydroxyl-TAM produced by Yucca is dehydrogenated to IAOx or dehydrogenated and hydrolyzed to IAAld. Enzymes that catalyze these conversions have not been identified. Indole-3-acetaldoxime is a precursor to Indolic Glucosinolates that can be converted to IAA. The IAOx pathway [Trp ® IAOx ® IAN (Indole-3-Acetonitrile) or IAAld ® IAA] is of particular interest in plants like Arabidopsis that make Indolic Glucosinolate secondary metabolites, because IAOx is the branch-point between Indole-3-Methylglucosinolate and IAA biosynthesis. Two Arabidopsis P450 Monooxygenases, CYP79B2 (Cytochrome-P450-79B2) and CYP79B3 (Cytochrome-P450-79B3), oxidize Trp to IAOx in vitro. A third P450 monooxygenase, CYP83B1 (Cytochrome-P450-83B1), converts IAOx to its N-oxide, the first committed step in Indole-3-Methylglucosinolate biosynthesis. IAOx N-Oxide is converted to IAN via S-(Indolylacetohydroximoyl)-L-cysteine, Indole-3-Thiohydroximate and Indole-3-Methyl Glucosinolate respectively. IAN is converted to IAA in presence of enzyme Nitrilase. Four Nitrilases genes, of which three encode enzymes capable of IAN metabolism, have been identified in Arabidopsis. Nitrilase and Yucca have been proposed to function in the same IAA biosynthetic pathway, with Yucca catalyzing the rate-limiting step upstream of the Nitrilase (Ref.3 & 4). In the Tryptophan-independent pathway, IGP is the putative precursor. In Arabidopsis, the greatest capacity to synthesize IAA de novo has been found in very young leafs, less than 0.5 mm in length, but all parts of the young Arabidopsis plant possess the capacity to synthesize IAA de novo. Plants can also obtain IAA by Beta-oxidation of IBA (Indole-3-Butyric Acid), a second endogenous Auxin, or by hydrolyzing IAA conjugates, in which IAA is linked to Amino Acids, Sugars or Peptides. IAA is known to regulate the expression levels of various genes; however, it is difficult to correlate the observed IAA-induced gene expression with particular plant growth and development responses (Ref.5).