# Biosynthetic Pathways of Hormones in Plants
## 5 Auxins  
Auxins play a crucial role in the regulation of various aspects of plant growth and development. They are involved in controlling processes such as cell division, elongation, differentiation, tropisms (response to external stimuli), flowering, apical dominance (suppression of lateral bud growth), lateral root formation, senescence (aging), abscission (shedding of plant parts), and responses to environmental stresses [56]. The most well-studied auxin in plants is indole-3-acetic acid (IAA). The process of producing auxins in plants is highly intricate, and understanding this process can significantly enhance our comprehension of auxins' biological role and contribution to the regulation of plant growth and physiology [57]. Although different plant species employ distinct strategies to optimize their metabolic pathways, it seems that there could be shared mechanisms for auxin biosynthesis, given that IAA is an phytohormone essential to plant life. Auxins are primarily generated in the plant's apical meristems, young leaves, and flower buds, and they are then quickly transported throughout the plant via the phloem. Nonetheless, some research suggests that auxins may also be locally synthesized within the roots [58; 59; 60]. Two major routes have been assumed to contribute to de novo IAA biosynthesis in plants: the tryptophan (Trp)-dependent and Trp-independent pathways (Figure 5). Several pathways for Trp-dependent IAA biosynthesis have been proposed, including:  
* the indole-3-acetamide (IAM) pathway;
* the indole-3-pyruvate acid (IPA) pathway;
* theryptamine (TAM) pathway;
* the indole-3-acetaldoxime (IAOX) pathway [61; 62; 63; 64; 65]. Even though IAA was the first natural auxin to be identified, our understanding of the genes that encode all of the enzymes involved in auxin biosynthesis remains limited. It is also uncertain whether all of these pathways exist in all plant species [66]. Tryptophan, which is a precursor of the production of various indole-containing compounds in plants, including IAA, indole glucosinolates, phytoalexins, and tryptamine (TAM) derivatives, is synthesized from chorisinate through the action of indole-3-glycerol phosphate synthase in the chloroplast. The _A. thaliana_ genes _Anthranilate synthase_[_AtASAA_] and _AtASAA_ encode the \(\alpha\)-subunit of the enzyme that is responsible for catalyzing the initial reaction through the Trp synthesis pathway. Indole-3-glycerol phosphate synthase, on the other hand, facilitates the conversion of 1-(\(O\)-carboxyphenylamino)-1-deoxyribulose-5-phosphate into indole-3-glycerol phosphate [67; 68]. In hairy plant roots, IAA is synthesized from Trp via a reaction that consists of two steps. Tryptophan is first converted into IAM by the enzyme Trp-2-mono-oxygenase, and IAM is then transformed to IAA by indole-3-acetamide hydrolase [66]. This IAA biosynthetic pathway via IAM is a bacteria-specific synthesis because plant pathogens, such as _Pseudomonas_ and _Agrobacteriumrium_, have a large root-inducing plasmid and are able to induce hairy-root disease, which is characterized by a high root proliferation ratio from the place of infection, which occurs as a result of a high rate of IAA biosynthesis and accumulation [69]. _Pseudomonas_ and _Agrobacteriumrium_ use Trp-2-mono-oxygenase to convert Trp into IAM, which is then hydrolyzed into IAA. The Trp-dependent IAA biosynthesis pathway is the best-known and studied pathway used in this process [65]. It was reported that plants can also synthesize IAA via the IAM pathway. Therefore, enzymes amidases that are able to hydrolyze IAM into IAA have been identified in _A. thaliana_. This observation suggests that IAM may be a key substrate for IAA biosynthesis in plants [70].