Modification of gibberellin production and plant development in Arabidopsis by sense and antisense expression of gibberellin 20-oxidase genes

Gibberellin (GA) 20-oxidase catalyses consecutive steps late in GA biosynth esis in plants. In Arabidopsis, the enzyme is encoded by a gene family of a t least three members (AtGA20ox1, AtGA20ox2 and AtGA20ox3) with differentia l patterns of expression. The genes are regulated by feedback from bioactiv e GAs, suggesting that the enzymes may be involved in regulating GA biosynt hesis. To investigate this, we produced transgenic Arabidopsis expressing s ense or antisense copies of each of the GA 20-oxidase cDNAs. Over-expressio n of any of the cDNAs gave rise to seedlings with elongated hypocotyls; the plants flowered earlier than controls in both long and short days and were 25% taller at maturity. GA analysis of the vegetative rosettes showed a tw o- to threefold increase in the level of GA(4), indicating that GA 20-oxida se normally limits bioactive GA levels. Plants expressing antisense copies of AtGA20ox1 had short hypocotyls and reduced rates of stem elongation. Thi s was reflected in reduced levels of GA(4) in both rosettes and shoot tips. In short days, flowering was delayed and the reduction in the rate of stem elongation was greater. Antisense expression of AtGA20ox2 had no apparent effects in long days, but stem growth in one transgenic line grown in short days was reduced by 20%. Expression of antisense copies of AtGA20ox3 had n o visible effect, except for one transgenic line that had short hypocotyls. These results demonstrate that GA levels and, hence, plant growth and deve lopment can be modified by manipulation of GA 20-oxidase expression in tran sgenic plants.