Plant Growth and Development Test - 38

Kogl and Smit isolated auxin (indoleacetic acid) from human urine. Chemically, it was auxentriolic acid. Kogl, Erxleben and Smit isolated auxin b (auxenolonic acid) from corn germ oil. Later, they also isolated heteroauxin from human urine which was indole-3-acetic acid.

Gibberellins affect several reproductive processes in plants.They stimulate flowering, particularly in long-day plants. In addition, gibberellins substitute for the low temperature that biennials require before they begin flowering (vernalisation). If gibberellins are applied to biennials during their first year of growth, flowering occurs without exposure to a period of low temperature. 

Gibberellins are a kind of growth promotery plant growth regulators (PGR). There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants. They are denoted as GA1, GA2, GA3 and so on. However, gibberellic acid (GA3) was one of the first gibberellins to be discovered and remains the most intensively studied form. All GAs are acidic. They produce a wide range of physiological responses in the plants. Their ability to cause an increase in length of axis is used to increase the length of grapes stalks. Gibberellins, cause fruits like apple to elongate and improve its shape. They also delay senescence. Thus, the fruits can be left on the tree longer so as to extend the market period. GA3 is used to speed up the malting process in brewing industry.

Gibberellins are growth promoting phytohormones. Gibberellins promote stem elongation in many plants. When gibberellin is applied to a plant, particularly certain dwarf varieties, this elongation may be spectacular. Some corn and pea plants that are dwarfs as a result of one or more mutations grow to a normal height when treated with gibberellin. They stimulate seed germination by inducing hydrolytic enzymes like amylase in cereals. They are also used along with auxins in production of parthenocarpic fruits.

Cytokinin causes delay of senescence called Richmond Lang effect. It checks disappearance of chlorophyll and degradation of protein synthesis in leaves. 
Senescense of leaves leads to yellowing and finally leads to the fall from the plant.  If a young excised leaf is kept in water, it slowly changes its color to yellow and dies.  If such leaves are provided with cytokinin, the yellowing is significantly delayed and such an effect is called Richmond and Langs effect; named after the discoverers. 

During the 1940s and 1950s, researchers were trying to find substances that might induce plant cells to divide in tissue culture, a technique in which cells are isolated from plants and grown in a nutrient medium. They discovered that cells would not divide without a substance found in coconut milk. Because coconut milk has a complex chemical composition, investigators did not chemically identify the division-inducing substance for some time. Finally, researchers isolated an active substance from a different source, aged DNA from herring sperm. They called it cytokinin because it induces cytokinesis, or cytoplasmic division. In 1963, researchers identified the first plant cytokinin, zeatin, in corn. Since then, similar molecules have been identified in other plants. Biologists have also synthesized several cytokinins. Cytokinins are structurally similar to adenine, a purine base that is part of DNA and RNA molecules.

Gibberellins are growth promoting phytohormones. Gibberellins promote stem elongation in many plants. When gibberellin is applied to a plant, particularly certain dwarf varieties, this elongation may be spectacular. Some corn and pea plants that are dwarfs as a result of one or more mutations grow to a normal height when treated with gibberellin. Short-stemmed, high-yielding varieties of wheat have short stems because they have a reduced response to gibberellin. These varieties put less of their resources into stem height and more resources into grain production. Gibberellins are also involved in bolting, the rapid elongation of a floral stalk that occurs naturally in many plants when they initiate flowering.Gibberellins cause stem elongation by stimulating cells to divide as well as to elongate. The actual mechanism of cell elongation differs from that caused by auxin, however. Auxins like indole acetic acid (IAA) induced cell elongation involves the acidification of the cell wall. In gibberellin-induced cell elongation, cell wall acidification does not occur; biologists do not yet know how gibberellins modify cell wall properties to cause cell elongation.

The discovery of each of the five major groups of plant growth regulators (PGRs) have been accidental. The bakane (foolish seedling) a disease of rice seedlings, was caused by a fungal pathogen Gibberalla fujikuroi. Japanese scientist, E. Kurosawa reported the appearance of symptoms of the disease in uninfected rice seedlings when they were treated with sterile filtrates of the fungus. The active substances in the extract were later identified as gibberellic acid.

In the 1920s, a Japanese biologist (E. Kurosawa) was studying a disease of rice in which the young rice seedlings grew extremely tall and spindly, fell over and died. The cause of the disease was a fungus (Gibberella fujikuroi) that produces a chemical substance named as gibberellin. Not until after World War II did scientists in Europe and North America learn of the work done by the Japanese scientists. During the 1950s and 1960s, studies in the United States and Great Britain showed that gibberellins are produced by healthy plants as well as by the fungus like Fusarium.