Plant Hormone (Part-1)- Auxin

Plant hormones are naturally occurring organic substances that affect physiological processes. There are five major groups of plant hormones, such as auxins, gibberellins, cytokinins, abscisic acid and ethylene. In this presentation auxin is described with its biosynthesis, transport, pathways and physiological effects.

1. Auxin Dr. Emasushan Minj Assistant Professor St. Xavier’s College, Mahuadanr , Latehar Nilamber Pitamber University

2. Plant hormones “Plant hormones are small organic compounds , synthesized by specific plant cells/tissues, active in low concentration and promote or inhibit growth and developmental processes .” Naturally occurring organic substances that affect physiological processes at low concentration. Ablity to make use of their action locally or at a distance

3. Types of plant hormones The concept of plant hormones originates from a classical experiment on phototropism, carried out by Charles Darwin and his son Francis in 1880 , using oat seedlings exposed to a lateral light source results bending. Based on function or chemical structure, there are five major groups of plant hormones- such as auxins, gibberellins, cytokinins, abscisic acid and ethylene . In addition, there is a variety of other plant hormones including the brassinosteroids, polyamines, jasmonic acid, salicylic acid and others .

4. Introduction Auxin is the first plant hormone was discovered by Frits Went - as a growth promoting chemical in the tip of oat ( Avena sativa ) coleoptiles. Promoted the elongation of the coleoptile and Eventually named as auxin (derived from the Greek word auxein , meaning to increase ).

5. Biosynthesis and transport Various naturally occurring auxins are known as IAA ( indole 3-acetic acid) IBA ( indole 3-butyric acid) PAA (phenyl acetic acid)

6. The principal naturally occurring auxin in higher plants- Indole-3-acetic acid (IAA) There are more than 200 auxin analogs with different chemical structures known to have the common auxin activity. The only common features shared by these compounds are an unsaturated planar aromatic ring structure and a carboxyl side chain .

7. Major (primary) sites for IAA synthesis Shoot apical meristem Young leaves Developing fruits and seeds Intracellularly it is found in the chloroplast and cytosol . IAA occurs in both conjugated and free forms . Conjugated forms are found in both- high (such as glycoproteins ) and low-molecular-weight compounds (such as glucose ) IAA conjugates are located exclusively in the cytosol .

8. Pathways for the biosynthesis of IAA- Tryptophan-dependent pathways Tryptophan-independent pathway Tryptophan-dependent pathways Tryptophan serve as precursor of auxin biosynthesis because of its structural similarity. The indole-3-pyruvic acid pathway is the most common tryptophan dependent pathway.

9. Overall, the conversion of tryptophan to IAA involves: Deamination of tryptophan, ( Catalyzed by trp transaminase ). Decarboxylation of indole-3-pyruvic acid ( Catalyzed by indole-3-pyruvic acid decarboxylase ). Oxidation of indole-3-acetaldehyde ( Catalyzed by indole-3-acetaldehyde dehydrogenase ).

10. Tryptophan-independent pathway In addition to the tryptophan-dependent pathways, recent genetic studies have provided evidence that plants can synthesize IAA via one or more tryptophan-independent pathways. This route doesn’t involve tryptophan directly as a precursor to the formation of auxin . The precise pathway for tryptophan-independent IAA synthesis is not known.

11. Physiological effects Cell elongation: Auxins stimulate - cell elongation/expansion in stems and coleoptiles. According to the acid growth hypothesis- Auxin promotes elongation growth by increasing cell wall extensibility. Auxin also stimulates proton pumping by activating (directly or indirectly) proton pump , H+- ATPase , present on the plasma membrane. As a result, the pH of the cell wall falls as low as 4.5 .

12. The higher concentration of auxin on the shaded side causes cells to elongate more rapidly resulting in bending of the stem tip towards the unilateral light . Auxin causes elongation probably : By increasing osmotic solutes, By wall pressure, By increasing permeability to water, By inducing synthesis of specific DNA dependent new m-RNA and specific enzymic proteins

13. Cell differentiation: The differentiation of relative amounts of xylem and phloem are regulated by auxin concentration. In general, high auxin concentrations induce the differentiation of xylem and low auxin concentrations induce phloem differentiation . Rooting : Auxin inhibits elongation of the primary root , but stimulates root initiation on stem cuttings ( adventitious roots ) and lateral root development . Lateral roots are commonly found above the root hair zone and originate from cells present in the pericycle . Auxin stimulates these pericycle cells to divide . The dividing cells gradually give rise to the lateral root.

14. Apical dominance : In most plants, the growing apical bud inhibits the growth of lateral buds —a phenomenon called apical dominance . Removal of the shoot apex usually results in the growth of lateral buds. In the presence of apical bud, the outgrowth of the lateral bud is inhibited by auxin that is transported basipetally from the apical bud. Fruit set : Auxin promotes fruit development and rapid the growth of young fruit . After fertilization, fruit growth may depend on auxin produced in developing seeds. Auxin is produced in the endosperm and embryo of developing seeds .

15. In some plant species, seedless fruits may be induced by treatment of the unpollinated flowers with auxin . The production of such seedless fruits is called parthenocarpy . Tropic response: Auxin induces differential growth in plant organs under the influence of directional stimuli (i.e. light, gravity). This is termed as tropic response.

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