A Mutant Gene That Increases Gibberellin Production in Brassica

A single gene mutant (elongated internode [ein/ein]) with accelerated shoot elongation was identified from a rapid cycling line of Brassica rapa. Relative to normal plants, mutant plants had slightly accelerated floral development, greater stem dry weights, and particularly, increased internode and inflorescence elongation. The application of the triazole plant growth retardant, paclobutrazol, inhibited shoot elongation, returning ein to a more normal phenotype. Conversely, exogenous gibberellin A3 (GA3) can convert normal genotypes to a phenotype resembling ein. The content of endogenous $\text{GA}_{1}$ and GA3 were estimated by gas chromatography-selected ion monitoring using $[{}^{2}\text{H}]\text{GA}_{1}$ as a quantitative internal standard and at day 14 were 1.5- and 12.1-fold higher per stem, respectively, in ein than in normal plants, although GA concentrations were more similar. The endogenous levels of $\text{GA}_{20}$ and $\text{GA}_{1}$, and the rate of $\text{GA}_{19}$ metabolism were simultaneously analyzed at day 7 by feeding $[{}^{2}\text{H}{}_{2}]\text{GA}_{19}$ and measuring metabolites $[{}^{2}\text{H}{}_{2}]\text{GA}_{20}$ and $[{}^{2}\text{H}{}_{2}]\text{GA}_{1}$ and endogenous $\text{GA}_{20}$ and $\text{GA}_{1}$, with $[{}^{2}\text{H}{}_{5}]\text{GA}_{20}$ and $[{}^{2}\text{H}{}_{5}]\text{GA}_{1}$ as quantitative internal standards. Levels of $\text{GA}_{1}$ and $\text{GA}_{20}$ were 4.6- and 12.9-fold higher, respectively, and conversions to $\text{GA}_{20}$ and $\text{GA}_{1}$ were 8.3 and 1.3 times faster in ein than normal plants. Confirming the enhanced rate of $\text{GA}_{1}$ biosynthesis in ein, the conversion of $[{}^{3}\text{H}]\text{GA}_{20}$ to $[{}^{3}\text{H}]\text{GA}_{1}$ was also faster in ein than in the normal genotype. Thus, the ein allele results in accelerated $\text{GA}_{1}$ biosynthesis and an elevated content of endogenous GAs, including the dihydroxylated GAs A1 and A3. The enhanced GA production probably underlies the accelerated shoot growth and development, and particularly, the increased shoot elongation.