Plant Hormone: Auxin Biosynthesis with steps, pathways and diagrams II Plant Physiology II

 Biosynthesis of Auxins

Site Of Synthesis

IAA is synthesized in meristems, young leaves, and developing fruits and seeds. IAA biosynthesis is associated with rapidly dividing and growing tissues, especially in shoots. Although virtually all plant tissues appear to be capable of producing low levels of IAA, shoot apical meristems and young leaves are the primary sites of auxin synthesis.

Root apical meristems are also important sites of auxin synthesis, especially as the roots elongate and mature, although the root remains dependent on the shoot for much of its auxin. Young fruits and seeds contain high levels of auxin, but it is unclear whether this auxin is newly synthesized or transported from maternal tissues during development.

Biosynthesis

Precursor: Tryptophan (An Aromatic Amino Acid)

Synthesized by Tryptophan dependent and Tryptophan independent pathways

Multiple pathways exist for the biosynthesis of IAA. IAA is structurally related to the amino acid tryptophan, and to the tryptophan precursor indole-3-glycerol phosphate, both of which can serve as precursors for IAA biosynthesis. Molecular genetics and radioisotope labelling studies have been used to identify the enzymes and intermediate molecules involved in tryptophan-dependent IAA biosynthesis, and the order in which they function. Multiple biosynthetic pathways using tryptophan as a precursor have been shown to produce IAA in plants, and a bacterial pathway of tryptophan dependent IAA biosynthesis has also been identified. Auxin can be covalently bound to both high and low molecular weight compounds, particularly in seeds and storage organs such as cotyledons. IAA can be conjugated to many different low molecular weight compounds like amino acids or sugars, or to high molecular weight molecules like peptides, complex glycans (multiple sugar units), or glycoproteins. IAA is rapidly released from many, but not all, conjugates by enzymatic processes. Those conjugates that can release free auxin serve as reversible storage forms of the hormone.

Plants use both tryptophan (Trp)-dependent and Trp-independent routes to synthesize IAA; several Trp-dependent pathways have been suggested.

1.The IPA pathway [Trp -> IPA  -> indole-3-acetaldehyde (IAAld) -> 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.

2.The IAM pathway [Trp -> IAM -> IAA] is a second microbial pathway that also may act in plants. In Agrobacterium tumifaciens and Pseudomonas syringae, for example, Trp monooxygenase (IaaM) converts Trp to IAM, and an IAM hydrolase (IaaH) converts IAM to IAA (Patten and Glick, 1996). IAM lacks auxin activity in arabidopsis, which allows the iaaH gene to be used as a screenable marker that confers IAM sensitivity.

3.A tryptamine (TAM) pathway [Trp -> TAM ->N-hydroxyl-TAM -> indole-3-acetaldoxime (IAOx) -> IAAld -> IAA] could also convert Trp to IAA. Trp decarboxylase converts Trp to tryptamine in the first committed step in the biosynthesis of Catharanthus roseus monoterpenoid indole alkaloids. The arabidopsis genome contains potential Trp decarboxylase genes, but the encoded enzymes have not been characterized, and tryptamine has not been identified in arabidopsis.

4.Nitrilases that can hydrolyse IAN to IAA are found in several plant families, including crucifers and grasses. These enzymes are encoded by the arabidopsis NIT genes.

Trp independent pathway:
Analyses of trp mutants reveal Trp-independent IAA biosynthesis. In addition to the proposed Trp-dependent IAA biosynthetic pathways analyses of Trp biosynthetic mutants demonstrate that plants also can synthesize IAA without using a Trp intermediate. 

IAA is degraded by multiple pathways

To be effective hormones must be short-lived and should not accumulate over time. 

Auxin catabolism ensures the degradation of active hormone when the concentration exceeds the optimal level or when the response to the hormone is complete. 

Like IAA biosynthesis, the enzymatic breakdown (oxidation) of IAA involves more than one pathway. On the basis of isotopic labeling and metabolite identification, two oxidative pathways are probably involved in the controlled degradation of IAA. In one pathway, the indole moiety of IAA is oxidized to form oxindole-3-acetic acid (OxIAA) and subsequently, OxIAA-glucose (OxIAA-Gluc). In another pathway, IAA-aspartate conjugates are oxidized to OxIAA.