# Biosynthetic Pathways of Hormones in Plants
## 5 Auxins
In addition, overexpression of the _IAM_ gene that encodes the enzyme tryptophan-2-mono-oxygenase, which is involved in the conversion of Trp into IAM, increases the content of IAA and results in high-auxin phenotypes being present in plants, leading to increased hypocotyl elongation under light, severe rosette leaf epinasty, and apical dominance. Thus, plants are able to transform IAM into IAA, as has been shown in in vivo studies performed using _A. thaliana_[71]. IAM, which is an intermediate of auxin biosynthesis, can also be detected in many other plant species, regardless of whether they are monocots or dicots [66]. The indole-3-pyruvate acid (IPA) pathway occurs using intermediate IPA, which is crucial for auxin biosynthesis and plant development [60]. Tryptophan Aminotransferase of _Arabidopsis_ 1 (TAA1) is a member of aminotransferases that can convert tryptophan into IPA. These enzymes have been identified in many plants. This observation indicates that this pathway is highly conserved. Mutations in the _TAA1_ gene that encodes this protein lead to a high reduction in IAA levels. Thus, IPA-dependent IAA biosynthesis is an important route for the biosynthesis of free IAA. The TAA1 protein belongs to the superfamily of enzymes  
Figure 5: The biosynthetic pathways of auxins (phytohormes are green). that is dependent on the \(\alpha\) class of PLP and exhibits Trp aminotransferase activity, which uses L-Trp, but not D-Trp, as a substrate. This enzyme can also react with other amino acids, such as L-phenylalanine, tyrosine, leucine, alanine, methionine, and glutamine [72]. Tryptophan can also be converted into TAM through the action of cytosolic Trp decarboxylase (TDC). TAM is a protoalkaloid that is involved in an early step of the biosynthetic pathway that influence terpenoid indole alkaloids [66]. TDC has been functionally and structurally characterized. It participates in synthesis of indole alkaloids and serotonin in many plants. However, TDC members probably do not fully contribute to IAA biosynthesis. As observations show, overexpression of the _TDC_ gene in _Catharanthus roseus_ has been indicated to generate increased levels of TAM, but not IAA, accumulation in tobacco (_Nicotiana tabucum_) [73]. Moreover, genetically modified rice plants with overexpression of the _TDC_ gene displayed a typical phenotype and contained serotonin levels that were 25 times higher in leaves and 11 times higher in seeds than those found in their wild-type equivalents [74]. In rice, TDC can be involved in the production of metabolites that are derivatives of the TAM-inducing disease known as Sekiguchi lesions or the accumulation of serotonin, which is a well-known neurotransmitter in mammals and a signalling molecule that is present in many plants [75]. Another possible pathway involved in the synthesis of IAA from Trp consists of the conversion of indole-3-acetaldoxime into indole-3-acetonitrile (IAN). In this reaction, amino nitrogen is removed from tryptophan via hydrolysis of the nitrile to form acid [76]. Indole-3-acetaldoxime (IAOX) is a precursor of the biosynthesis pathway of IAA and its conjugates, and it has been proposed as the branch point between the biosynthesis of IAA and indole-3-methyl glucosinolate [77]. Studies of _A. thaliana_ mutant lines show that IAOX is a key intermediate involved in IAA biosynthesis. This compound can be converted into IAN by eliminating one molecule of water. IAN is subsequently transformed into IAA by a family of nitrilases. IAOX can be hydrolyzed to form indole-3-acetaldehyde, and the aldehyde can then be converted into IAA via an action of two enzymes: aldehyde oxidase (AO1) or aldehyde dehydrogenase. AO1 is important enzyme for auxin biosynthesis [78]. The enzyme AO1 shows substrate specificity to indole-3-acetaldehyde, and the regulation of the activity of this enzyme is unaffected by the high levels of IAA.