Gibberellins Dr Abdul Latif Khan

Gibberellins: Regulators of Plant Height and Seed Germination - Plant growth and development are regulated by several hormones acting both individually and in combination. - Gibberellins (GAs) are the second group of plant hormones to be characterized. - At least 136 naturally occurring GAs have been identified. - The GAs all share a similar chemical structure but few of them have intrinsic biological activity (bioactive) - Many of the GAs are either precursors of the bioactive GAs or deactivated products of these precursors.

- In any given plant, there are only a few bioactive GAs and their levels are correlated with stem length. - Gibberellins are best known for their promotion of stem elongation. - GA-deficient mutants have dwarf phenotypes. - Mendel’s study on pea plants indicated that there is a single gene locus (of two alleles, tall/dwarf) that can control the level of bioactive GA and hence stem length. - Recent molecular genetic approaches revealed that the biosynthesis of GAs is under strict genetic, developmental, and environmental control.

1- Gibberellins: Their Discovery and Chemical Structure Gibberellins were discovered by studying a disease of rice - GAs had been discovered in the 1930s in Japan. - Rice farmers had long known of a fungal disease (termed “foolish seedling”) that caused rice plants to grow too tall and eliminated seed production. - Plant pathologists found that these symptoms in rice were induced by a chemical (or chemicals) secreted by a pathogenic fungus, Gibberella fujikuroi, that had infected the plants.

- Japanese scientists cultured the fungus, Gibberella fujikuroi, in the laboratory and analyzed its culture filtrate. - In the 1930s, Japanese scientists were able to obtain impure crystals of two compounds with plant growth-promoting activity from the fungus. - (the two compounds isolated back then by the Japanese were later shown to be mixtures) - However, because one of these compounds was isolated from the fungus Gibberella, it was named gibberellin A.

Gibberellic acid was first purified from Gibberella culture filtrate - In the 1950s, two research groups (Britain & US) purified a compound from Gibberella culture filtrates and elucidated its chemical structure, they named it gibberellic acid. - At about the same time, Japanese scientists separated and characterized three different gibberellins from the original gibberellin A sample, named them gibberellin A1 (GA1) gibberellin A2 (GA2), and gibberellin A3 (GA3). - The Japanese scientists’ GA3 was later shown to be identical to the gibberellic acid isolated by the U.S. and British scientists.

- GA3 (gibberellic acid) is the principle component present in Gibberella cultures - GA3 is the most frequently produced GA in commercial industrial-scale fermentations of Gibberella, for agronomic, horticultural, and other scientific use. - GA3 shows spectacular responses in the elongation growth of dwarf and rosette plants, particularly in dwarf and rosette genotypes. - (see figures 20.1 and 20.2)

GA from plant extracts: - Application of GA3 derived from the fungus increases the height of dwarf mutants. - Do wild-type plants contain their own endogenous GA? - Bioassay of the extracts of a variety of plant species showed that GA-like substances were indeed present.

- Higher concentrations were found in immature seeds than in vegetative tissues (i.e. the plant material of choice for GA extraction is the immature seeds) - In 1958, GA1 was the first GA to be identified from a plant extract; it was from immature seeds of the bean, Phaseolus coccineus.

2- Effects of Gibberellins on Growth and Development All gibberellins are based on an ent-gibberellane skeleton 2- Effects of Gibberellins on Growth and Development

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