IAA or Indole-3-Acetic Acid is the most important plant growth hormone. Chemically, it is heterocyclic compound, a phytohormone or also known as plant auxin. It is produced by plants and soil microorganisms, especially plant associated bacteria. If a plant produces IAA then what is the need to produce it by bacteria? This is because the rhizobacteria maintain positive interactions with the host plant by supplying IAA and stimulating its growth; in return they utilize plant root exudates as a source of carbon and energy. Plant host also like to depend on its rhizobacterial flora for IAA instead of wasting the metabolic energy for the synthesis of IAA. Many plant growth promoting rhizobacteria (PGPR) produce IAA. For example, Pseudomonas, Bacillus, Acetobacter, Rhizobium, Burkholderia, Azotobacter, Azospirillum and some fungal genera like Verticillum, Colletotrichum, Endophylum and Ustilago. These microbes are able to produce IAA in pure culture conditions in presence or absence of precursor substrate. IAA is also synthesized chemically.
Biosynthetic pathways: IAA is synthesized chemically by reaction of Indole with Glycolic acid at 250˚C. Biologically IAA is synthesized from the universal precursor, amino acid tryptophan. Tryptophan is the principle component of plant root exudates and rhizobacteria utilize this exuded tryptophan to synthesize IAA for the plant. Like microbes, plants too can synthesize IAA in absence of precursor molecule. The different IAA biosynthesis pathways are Indole-3-Acetamide, Indole-3-Pyruvate, Indole-3-Acetonitrile, Tryptamine pathway and Tryptophan side chain pathway. These pathways were named according to intermediate produced during IAA synthesis. Thus Indole-3-Acetonitrile or Tryptamine is the intermediate produced in that particular pathway leading to synthesis of IAA. Rhizobacteria uses one or more type of pathways to produce IAA but tryptophan dependent pathway predominates. Biosynthesis of IAA involves enzyme regulated mechanisms like deamination, carboxylation, oxidation and decarboxylation.
IAA genes (iaa): In rhizobacteria the IAA production is chromosomally or plasmid encoded. Study of spontaneous IAA nonproducing mutants is useful to map and characterize iaa genes. The conjugation, Transformation or Curing (with mutagens like ethidium bromide) are useful to determine the presence of iaa genes on plasmid. Further the cloning, sequencing and expression studies of these gene loci can be done by employing suitable protocol.
Laboratory detection: The most common assay for detection of IAA production is colorimetric determination using Salkowaskii reagent. It is also detected by using nitrocellulose or nylon membrane on agar plates using the same reagent as color indicator. In the second method, screening of large number of bacteria for IAA production is possible and purified extract or cell free supernatant is not required for the analysis. In addition to these routine assays, spectrophotometry, chromatography techniques like TLC, HPLC or GC are also employed for estimation of IAA and its derivatives.
Physiological role of IAA: Like any hormone is functional, IAA actively stimulates plant growth. Its most important function is to break apical dominance and increases the growth of main stem. It plays potent role in many other important plant developmental growth processes such as cell division and cell expansion, vascular tissue differentiation, root initiation, gravitropic and phototropic responses, flowering, fruit ripening, leaf senescence and abscission of leaves and fruit. It also regulates cell permeability and legume root nodulation efficiency of Rhizobium and Bradyrhizobium spp. The root hair curling is the first response of plant host for the invasion of beneficial nodulating bacteria. Root nodulating bacteria fix atmospheric nitrogen living symbiotically within root nodules of their legume host plants. The induction of cell division, expansion and tissue differentiation results in increased plant growth what we refer as plant growth promotion. It is also used in tissue culture media as a rooting hormone for rooting the plant explants. In nurseries, it is used to generate roots in cuttings and seedlings. The most IAA producing microorganisms can produce analogs or derivatives of IAA such as IBA (Indole-3-Butyric Acid) and NAA (1-Naphthalene Acetic Acid) either in vitro or in vivo condition. They also are plant growth stimulators but are very stable in comparison to IAA which is heat and light unstable and readily oxidized by regulatory enzyme IAA oxidase present in soil or produced by bacterium itself. Two synthetic analogs of IAA are well known to us; that are 2,4-D (2,4-Dichlorophenoxyacetic acid) and 2,4,5-T (2,4,5-Trichlorophenoxyacetic acid). Because of stability of natural and synthetic analogs of IAA, they are routinely used in horticultural practices mostly as herbicides but they have their own disadvantages. IAA is required in very small amounts to work out so many plant growth functions. In fact enhanced plant growth responses are also indicators of healthy plant. Increased IAA supply by rhizobacteria or overproduction of IAA in plant may result in hypertrophy or hyperauxiny. Hypertrophic growth is like cancerous cells where plant cell undergo uncontrolled cell division and growth. This results in the formation of tumor like growth or crown gall disease of roots, leaves, stems or inflorescences. Hyperauxiny is also an indication of plant infection by a phytopathogen. Such phytopathogenic microorganisms are responsible for wilting, smut diseases of plants. The disease is exaggerated when IAA acts in conjunction with tumor inducing substances as produced by Agrobacterium tumefaciens. The crown gall disease induced by phytopathogenic Agrobacterium strains is classical example of hyperauxiny or hypertrophy.
IAA producing rhizobacteria are important constituents of biofertilizers and extensively used to increase crop growth and subsequent yield.
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