Gibberellins

Gibberellin's story began with a crop disease, not a growth study - Japanese rice growers had long known 'Bakanae' (foolish seedling) disease, which Kurosawa traced in 1926 to the fungus Gibberella fujikuroi; the active compound responsible was separated and named gibberellin by Yabuta in 1935, crystallised by Yabuta and Sumiki in 1938, renamed gibberellic acid by Brain et al. in 1955, and had its chemical structure worked out by Cross et al. in 1961. Over 100 distinct gibberellins (GA1, GA2, GA3 and so on) have since been identified from both fungi and higher plants, with GA3 the first discovered. In plants, gibberellins are produced mainly in the embryo, roots and young leaves near the growing tip, and immature seeds are particularly rich in them. Chemically they are terpenoid derivatives, built from a five-carbon isoprenoid precursor (Isopentenyl Pyrophosphate) via acetate, and every gibberellin shares a common gibbane ring structure. Unlike auxin, gibberellin transport is non-polar, moving mainly through the phloem with some lateral movement through the xylem. Their activity is classically measured with the dwarf pea bioassay, where GA-treated seedlings show extended epicotyl growth compared with untreated controls. Physiologically, gibberellins cause dramatic stem elongation through both cell division and cell elongation, and famously trigger bolting (Figure 15.13) - the sudden internode elongation and subsequent flowering seen when genetically dwarf rosette plants are treated with GA; they also break tuber dormancy in potato and can substitute for the cold exposure that biennials otherwise need before flowering. Agriculturally, they in …