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The Class III Gibberellin 2-Oxidases AtGA2ox9 and AtGA2ox10 Contribute to Cold Stress Tolerance and Fertility.

Many developmental processes in plants are regulated by GA hormones. GA homeostasis is achieved via complex biosynthetic and catabolic pathways. GA catabolic enzymes include GA 2-oxidases that are classified into three classes. Members of class III GA 2-oxidases typically act on GA precursors containing a C20-skeleton. Here, we identified two further members of this class of GA 2-oxidases, namely AtGA2ox9 and AtGA2ox10, in the Arabidopsis (Arabidopsis thaliana) genome. Both genes encode enzymes that have functional similarities to AtGA2ox7 and AtGA2ox8, which are class III GA 2-oxidases that 2β-hydroxylate C20-GAs. Previously unknown for GA 2-oxidases, AtGA2ox9 performs 2α-hydroxylation of C19-GAs and harbors putative desaturating activity of C20-GAs. Additionally, AtGA2ox9 and AtGA2ox10 exhibit GA 20-oxidase activity. AtGA2ox9 oxidizes carbon-20 to form tricarboxylic acid C20-GAs, whereas AtGA2ox10 produces C19-GA9. AtGA2ox9 transcript levels increase after cold treatment and AtGA2ox10 is expressed mainly in the siliques of Arabidopsis plants. Atga2ox9 loss-of-function mutants are more sensitive to freezing temperatures, whereas Atga2ox10 loss-of-function mutants produce considerably more seeds per silique than wild-type plants. We conclude that in Arabidopsis, AtGA2ox9 contributes to freezing tolerance and AtGA2ox10 regulates seed production.