Isolation and Expression of Three Gibberellin 20-Oxidase cDNA Clones from Arabidopsis

Using degenerate oligonucleotide primers based on a pumpkin (Cucurbita maxima) gibberellin (GA) 20-oxidase sequence, six different fragments of dioxygenase genes were amplified by polymerase chain reaction from Arabidopsis thaliana genomic DNA. One of these was used to isolate two different full-length cDNA clones, At2301 and At2353, from shoots of the GA-deficient Arabidopsis mutant ga1–2. A third, related clone, YAP169, was identified in the Database of Expressed Sequence Tags. The cDNA clones were expressed in Escherichia coli as fusion proteins, each of which oxidized GA12 at C-20 to GA15, GA24, and the C19 compound GA9, a precursor of bioactive GAs; the C20 tricarboxylic acid compound GA25 was formed as a minor product. The expression products also oxidized the 13-hydroxylated substrate GA53, but less effectively than GA12. The three cDNAs hybridized to mRNA species with tissue-specific patterns of accumulation, with At2301 being expressed in stems and inflorescences, At2353 in inflorescences and developing siliques, and YAP169 in siliques only. In the floral shoots of the ga1–2 mutant, transcript levels corresponding to each cDNA decreased dramatically after GA3 application, suggesting that GA biosynthesis may be controlled, at least in part, through down-regulation of the expression of the 20-oxidase genes.