The Hormone Concept in Plants and Discovery of Auxin, Slides of Plant physiology

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Auxin

Aman Bisht

The Maharaja Sayajirao University of Baroda

This presentation is referenced from: Plant physiology by Hopkins.

The Hormone Concept in Plants ● (^) The concepts of hormones arouse through mammalian physiology. A huge amount of studies were done to understand hormones, their action, their relation to disease, etc. ● (^) In 1905 , British physician E. H. Starling introduced the term hormone. It means to excite or arouse. ● (^) The first application of hormone concept can be traced back to observations of of Duhamel du Monceau in 1758. He observed the formation of roots on the swellings that occur above girdle wounds that interrupted the phloem tissues around the stems of woody plants. ● (^) German botanist Julius Sachs postulated specific organ forming substances in plants. He postulated that root forming substance, produced in leaves would migrate down in the stem to initiate root formation. ● (^) It was the research of Darwin and F. W. Kent that would describe a hormone like substance as a causative agent for plant growth towards light. ● (^) It was H. Fitting that introduced the term hormone in the plant physiology literature.

Observations of Duhamel du Monceau Girdling on a plant (^) Root formation in girdled stem

The Hormone Concept in Plants ● (^) Hormone pool and the availability of hormone. ● (^) Hormone pool is the total hormone available to an organism. The amount of hormone available will be governed by the rate at which an active hormone molecule enter and exit the hormone pool. ● (^) The entry of hormones can be: ● (^) Through denovo synthesis of the hormone. ● (^) Retrieval of a active hormone from an inactive storage form. (See – Slide 21,22) ● (^) Transport to hormone pool from elsewhere. ● (^) The removal of hormone is through: ● (^) Oxidation or some other form of chemical degradation. ● (^) Synthesis of irreversible deactivated conjugate. (See – Slide 25)

Auxin is Distributed Throughout the Plant ● (^) Auxin (means to increase ) is an essential plant hormone. It was the first plant hormone to be discovered. It plays an important role in plant response – the enlargement of plant cells. ● (^) It is synthesized in meristematic regions and other actively growing organs such as coleoptile apices, root tips, germinating seeds and the apical buds of growing stems. This hormone is actively distributed throughout the plant. Auxin distribution in an oat seedling ( Avena sativa ), showing higher concentrations of hormone in the actively growing coleoptile and root apices.

Discovery of Auxin ● (^) The beginnings of this research can be traced back to Darwin in his book The Power of Movements in Plants. ● (^) One of the movements he studied was the tendency of canary grass ( Phalaris canariensis ) seedlings to bend towards light coming from the window. This phenomenon we now know as phototropism. ● (^) The primary leaves of grass seedlings are enclosed in a hollow sheath like structure called coleoptile. This structure protect the leaves as they grown through the soil. ● (^) Darwin observed that coleoptile, like stem grow towards the light source. The growth would happen in a slight curvature, bending towards the light source. However this curvature did not happen if the tip of the coleoptile was removed or covered to exclude light. ● (^) Darwin concluded that the phototropic signal was perceived by the tip.

Discovery of Auxin Darwin said that “when the seedlings are freely exposed to lateral light, some influence is transmitted from the upper to the lower part, causing the latter to bend.” Darwin experiment Boysen-Jensen experiment

Discovery of Auxin ● (^) The active substance was first isolated in 1928 by F. W. Went. ● (^) Went removed the apex of oat ( Avena sativa ) coleoptile and stood the apical pieces in small blocks of agar. He allowed the component to diffuse from tissue to agar block and placed the agar block asymmetrically in a freshly decapitated coleoptile. The substance then diffused from the block into the coleoptile, preferentially stimulating elongation of the cells on the side of the coleoptile below the agar block. Curvature of the coleoptile was due to differential cell elongation on the two sides. ● (^) The curvature proved to be proportional to the amount of active substance in the agar. ● (^) This came to be known as Avena Curvature test. (^) (See – Slide 14,15) Because of his incredible work, intensive efforts were put to isolate the compound. One particularly active compound, indole-3-acetic acid (IAA), was isolated from human urine in 1934. At the same time, IAA was isolated from yeast extracts, following year, from cultures of Rhizopus suinus. IAA was isolated from immature corn kernels in 1946.

Discovery of Auxin Avena Curvature Test Auxin preferentially stimulates cell growth when the agar block is placed asymmetrically

The Principal Auxin in Plants is Indole-3-Acetic Acid (IAA) ● (^) Indole-3-Acetic acid is the most widely distributed natural auxin. There are several naturally occurring indole derivatives that express auxin activity like, I ndole-3-Ethanol, Indole-3- Acetaldeyhde and Indole-3-Acetonitrile. However, most of these compounds occur as a precursors to IAA. (^) (For use of Indole-3-Acetonitrile See – Slide 22) ● (^) Examples of other natural auxins - Indole-3-Butyric acid (IBA), 4-Chloroindole-3- Acetic acid (a chlorinated derivative present in legumes) and Phenylacetic acid. ● (^) The unifying character of these molecules is that they have an acidic side chain on an aromatic ring. ● (^) Amount of IAA in plants depends upon various numbers of factors like the type of tissue or the age of the tissue. For example, ● (^) In vegetative tissue, the IAA concentration ranges from 1μg and 100 μg (5.7 to 570 nanomoles) kg − . ● (^) The seeds have much higher amount of IAA in comparison (for rapid growth).

The Principal Auxin in Plants is Indole-3-Acetic Acid (IAA) Naturally occurring auxins Synthetic auxins Active acidic Side chain

IAA is Synthesized from the Amino Acid L- Tryptophan ● (^) IAA synthesis occurs in three steps.

1. The removal of amino group from tryptophan side chain. The product is Indole-3- Pyruvic acid (IPA). This reaction is catalyzed by tryptophanamino transferase. This enzyme removes amino group from variety of tryptophan structural analogs like phenylalanine and tyrosine.
2. The second step is decarboxylation of IPA into Indole-3-Acetaldehyde (IAAld). This is carried by the enzyme Indole-3-Pyruvate decarboxylase.
3. In the last step IAAld is oxidized to Indole-Acetic acid (IAA). This reaction is carried out by NAD dependent Indole-3-Acetaldehyde oxidase. Note that IAAld may also be reversibly reduced to indole-3-ethanol.
4. Finally, IAA can be reversibly converted to Indole-3-Butyric acid (IBA).

IAA is Synthesized from the Amino Acid L- Tryptophan Tryptophanamino transferase Indole-3- Acetaldehyde oxidase Indole-3-Pyruvate decarboxylase NAD dependent