GIBBERELLIN 3-OXIDASE genes regulate height and grain size in bread wheat.

Plant gibberellin (GA) concentrations are tightly regulated to optimize growth and development. GA 3-oxidases (GA3OX) catalyse a key GA biosynthesis step, converting precursor GAs into bioactive forms. We characterized seven GA3OX homologues in bread wheat (Triticum aestivum L.): a homoeologous triad of GA3OX2 genes expressed in vegetative and reproductive tissues, and four others (a homoeologous triad of GA3OX3 genes plus GA1OX1-B1) expressed predominantly in grains. ga3ox2 mutants are severely dwarfed and infertile due to very low bioactive GA concentrations, indicating that GA3OX2 is essential for normal wheat development. By contrast, ga3ox3 mutants have lower bioactive GA concentrations in grains, reducing grain size and weight, whereas ga1ox1 mutants accumulate high concentrations of bioactive GAs, producing larger grains. Unexpectedly, ga3ox3 and ga1ox1 alleles also affected height, possibly reflecting GA transport to vegetative tissues. Natural variation in adjacent GA3OX3-B1 and GA1OX1-B1 genes was associated with differences in grain size and weight, suggesting that a haplotype associated with larger grains was selected during modern breeding. Our study shows that the wheat GA3OX family has diversified roles, with GA3OX2 required for general growth and GA3OX3/GA1OX1 modulating GA concentrations during grain development. These findings highlight opportunities to exploit variation in GA biosynthetic pathways for wheat improvement.