Expression of a Gibberellin 2-Oxidase Gene around the Shoot Apex Is Related to Phase Transition in Rice

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Abstract
A major catabolic pathway for gibberellin (GA) is initiated by 2β-hydroxylation, a reaction catalyzed by GA 2-oxidase. We have isolated and characterized a cDNA, designated Oryza sativa GA 2-oxidase 1 (OsGA2ox1) from rice (Oryza sativa L. cv Nipponbare) that encodes a GA 2-oxidase. The encoded protein, produced by heterologous expression inEscherichia coli, converted GA1, GA4, GA9, GA20, and GA44 to the corresponding 2β-hydroxylated products GA8, GA34, GA51, GA29, and GA98, respectively. Ectopic expression of theOsGA2ox1 cDNA in transgenic rice inhibited stem elongation and the development of reproductive organs. These transgenic plants were deficient in endogenous GA1. These results indicate that OsGA2ox1 encodes a GA 2-oxidase, which is functional not only in vitro but also in vivo. OsGA2ox1was expressed in shoot apex and roots but not in leaves and stems. In situ hybridization analysis revealed that OsGA2ox1 mRNA was localized in a ring at the basal region of leaf primordia and young leaves. This ring-shaped expression around the shoot apex was drastically decreased after the phase transition from vegetative to reproductive growth. It was absent in the floral meristem, but it was still present in the lateral meristem that remained in the vegetative phase. These observations suggest that OsGA2ox1 controls the level of bioactive GAs in the shoot apical meristem; therefore, reduction in its expression may contribute to the early development of the inflorescence meristem.