The ectopic overexpression of a citrus gibberellin 20-oxidase enhances the non-13-hydroxylation pathway of gibberellin biosynthesis and induces an extremely elongated phenotype in tobacco

Transgenic plants of Nicotiana tabacum overexpressing a gibberellin (GA) 20‐oxidase cDNA (CcGA20ox1) from citrus, under the control of the 35S promoter, were taller (up to twice) and had larger inflorescences and longer flower peduncles than those of control plants. Hypocotyls of transgenic seedlings were also longer (up to 4 times), and neither the seedlings nor the growing plants elongated further after application of GA3. Hypocotyl and stem lengths were reduced by application of paclobutrazol, and this inhibition was reversed by exogenous GA3. The ectopic overexpression of CcGA20ox1 enhanced the non‐13‐hydroxylation pathway of GA biosynthesis leading to GA4, apparently at the expense of the early‐13‐hydroxylation pathway. The level of GA4 (the active GA from the non‐13‐hydroxylation pathway) in the shoot of transgenic plants was 3–4 times higher than in control plants, whereas that of GA1, formed via the early‐13‐hydroxylation pathway (the main GA biosynthesis pathway in tobacco), decreased or was not affected. GA4 applied to the culture medium or to the expanding leaves was found to be at least equally active as GA1 on stimulating hypocotyl and stem elongation of tobacco plants. The results suggest that the tall phenotype of tobacco transgenic plants was due to their higher content of GA4, and that the GA response was saturated by the presence of the transgene.