Anti Gibberellins

The origins of gibberellin research can be traced back to the late nineteenth century in Japan, with the demonstration that a rice disease characterised by symptoms such as excessive seedling elongation and infertility, among others, was caused by fungal infection (Hori 1898). The active growth-promoting principle was named gibberellin after the perfect (reproductive) form of the fungus, Gibberella fujikuroi, after culture filtrates of the fungal pathogen were later shown to reproduce the symptoms in rice. 

Depending on where the disease was found, Japanese farmers gave it various names, the most well-known of which was “bakanae,” which translates as silly seedling.Phinney has thoroughly reviewed the early research that led to the discovery, isolation, and structural determination of gibberellins, as well as the realisation that these compounds may be endogenous growth regulators in plants (1983). Photographs of the principal scientists involved in this research can be found in his review. While work on gibberellins was restricted to Japan prior to 1945, some coverage of this research was available to the West in the 1930s through Chemical Abstracts, but did not pique interest.

Definition

Gibberellin is a plant hormone found in seeds, young leaves, and roots. The name is derived from Gibberella fujikuroi, a phylum Ascomycota hormone-producing fungus that causes excessive growth and poor yield in rice plants. Gibberellins appear to stimulate the growth of main stems, especially when applied to the entire plant. They are also involved in the bolting (elongation) of rosette plants (e.g., lettuce) in response to environmental stimuli such as prolonged periods of sunlight.

Gibberellic acid, a gibberellin found in both higher plants and fungi, is a horticultural and home gardening product that is commercially available. Miniature applications can turn bush beans into pole beans or dwarf corn into regular corn. 

The most common application has been in grape production. Gibberellin is commonly used in the culture of the ‘Thompson Seedless’ (‘Sultanina’) grape cultivar to increase the fruit size, and it is also used to induce seedlessness in certain other grape varieties.

Gibberellins in higher plants

Numerous reports on the effects of gibberellin on plants appeared in the literature from the mid to late 1950s. The ability of gibberellic acid to induce bolting and flowering in a number of biennial rosette species and to rescue the growth defect in dwarf mutants of pea (Brian and others 1954; Brian and Hemming 1955) and maize (Phinney 1956) was particularly noteworthy (Lang 1956; Wittwer and others 1957).

 At the time, auxin was the only known endogenous plant growth regulator, but the remarkable properties of gibberellins prompted the hypothesis that they may also be found naturally in plants.

Margaret Radley, working with Percy Brian at the Akers laboratory, was inspired by the possibility that dwarf peas lacked gibberellin and decided to test extracts of tall peas on dwarf peas to see if they produced a similar growth response as gibberellic acid (Radley 1956). Bernard Phinney and colleagues at UCLA used dwarf maize in bioassays to show that extracts from a variety of plant species contained gibberellin-like substances in similar experiments (Phinney and others 1957). 

Jake MacMillan and P.J. Suter isolated 2 mg of gibberellin A1 from 87.3 kg of immature seeds of runner bean (Phaseolus multiflorus, later reclassified as Phaseolus coccineus) to provide the first definitive evidence for the presence of gibberellins in plants (MacMillan and Suter 1958).

Cytokinins

The effect of cytokinins was first discovered when the liquid endosperm of coconuts was added to developing plant embryos in culture and stimulated their growth. Cytokinin, a hormone that promotes cytokinesis, was discovered to be the stimulating growth factor (cell division). To date, nearly 200 naturally occurring or synthetic cytokinins have been identified. Cytokinins are found in abundance in growing tissues such as roots, embryos, and fruits, where cell division occurs. 

Cytokinins that have been shown to delay senescence in the leaf tissues, promote mitosis, and stimulate meristem differentiation in the shoots and the roots. Many effects on plant development are influenced by cytokinins, either alone or in combination with auxin or another hormone.

Auxins

The word auxin comes from the Greek word auxein, which means “to grow.” In phototropism and gravitropism, auxins are the primary hormones responsible for cell elongation. They also regulate meristem differentiation into vascular tissue and promote leaf development and arrangement. While many synthetic auxins are used as herbicides, the only naturally occurring auxin with physiological activity is indole acetic acid (IAA). 

Auxins produced in the apical meristem cause apical dominance (inhibition of lateral bud formation). Other plant responses under direct or indirect control of auxins include flowering, fruit setting and ripening, and inhibition of abscission (leaf falling). Auxins also serve as a relay for the effects of blue light and the red/far-red reactions.

Conclusion

The origins of gibberellin research can be traced back to the late nineteenth century in Japan, with the demonstration that a rice disease characterised by symptoms such as excessive seedling elongation and infertility, among others, was caused by fungal infection. The active growth-promoting principle was named gibberellin after the perfect form of the fungus, Gibberella fujikuroi, after culture filtrates of the fungal pathogen were later shown to reproduce the symptoms in rice. Phinney has thoroughly reviewed the early research that led to the discovery, isolation, and structural determination of gibberellins, as well as the realisation that these compounds may be endogenous growth regulators in plants. Gibberellins appear to stimulate the growth of main stems, especially when applied to the entire plant.