Plant Growth and Development Test - 41

Foolish seedling disease or Bakanae disease is a disease of rice seedlings, which was caused by a fungal pathogen; Gibberella fujikuroi. 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 were later identified as Gibberellic acid.

Senescence refers to a number of controlled processes which finally culminate in death of the plant part. Senescence is a controlled process and is generally responsible for conservation of precious part nutrients like nitrogen and phosphorous. The senescing plant part is separated from plant due to formation of an abscission layer which finally separated the plant part.

Gibberellins are a kind of growth promoting plant growth regulators (PGRs). There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants. They are denoted as GA$$_{1}$$, GA$$_{2}$$, GA$$_{3}$$ and so on. However, gibberellic acid (GA$$_{3}$$) was one of the first gibberellins to be discovered and remains the most intensively studied form. All gibberellic acids 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. Ability to increase length is the most important characteristic feature of gibberellins and gibberellins cause increase in length of all aerial parts. Gibberellins, cause fruits like apple to elongate and improve its shape. They also delay senescence. 

A Japanese scientist, Eiichi Kurosawa, identifies the gibberellins in Fusarium in 1926 during his study on the "foolish seedling" disease in the rice. They regulate growth and influence various developmental processes in the presence of auxin. Hence option D is correct.

Ethylene is a simple gaseous plant growth regulator (PGR). It is synthesised in large amounts by tissues undergoing senescence and ripening fruits. Ethylene promotes senescence and abscission of plant organs especially of leaves and flowers. Ethylene is highly effective in fruit ripening. It enhances the respiration rate during ripening of the fruits. This rise in rate of respiration is called as respiratory climacteric.

Auxins are growth promoting phytohormones. The most important naturally occurring auxin is IAA or indoleacetic acid. GA refers to gibberellic acid, ABA to abscisic acid and kinetin to cytokinin.

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 (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.

Gibberellins are involved in seed germination in certain plants. In a classic experiment involving barley seed germination, researchers showed that the release of gibberellin from the embryo triggers the synthesis of $$\alpha$$ -amylase, an enzyme that digests starch in the endosperm. As a result, glucose becomes available for absorption by the embryo. Although enzymes mobilize starch reserves in many types of seeds, gibberellin control of seed enzymes appears restricted to cereals and other grasses. In addition to mobilizing food reserves in newly germinated grass seeds, application of gibberellins substitutes for low-temperature or light requirements for germination in seeds of plants such as lettuce, oats and tobacco.

Gibberellins are involved in seed germination in certain plants. In a classic experiment involving barley seed germination, researchers showed that the release of gibberellin from the embryo triggers the synthesis of $$\alpha$$-amylase, an enzyme that digests starch in the endosperm. As a result, glucose becomes available for absorption by the embryo. Although enzymes mobilize starch reserves in many types of seeds, gibberellin control of seed enzymes appears restricted to cereals and other grasses. In addition to mobilizing food reserves in newly germinated grass seeds, application of gibberellins substitutes for low-temperature or light requirements for germination in seeds of plants such as lettuce, oats and tobacco.

Ethylene is a simple gaseous plant growth regulator (PGR). It is synthesised in large amounts by tissues undergoing senescence and ripening fruits. Influences of ethylene on plants include horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings. Ethylene promotes senescence and abscission of plant organs especially of leaves and flowers. Ethylene is highly effective in fruit ripening. It enhances the respiration rate during ripening of the fruits. This rise in rate of respiration is called as respiratory climactic.