Ovary-derived precursor gibberellin A9 is essential for female flower development in cucumber

Gibberellins (GAs) are hormones that control many aspects of plant development, including flowering. It is well known that stamen is the source of GAs that regulate male and bisexual flower development. However, little is known about the role of GAs in female flower development. In cucumber, high levels of GA precursors are present in ovaries and high levels of bioactive GA₄ are identified in sepals/petals, reflecting the expression of GA 20-oxidase and 3-oxidase in these organs, respectively. Here, we show that the biologically inactive precursor GA₉ moves from ovaries to sepal/petal tissues where it is converted to the bioactive GA₄ necessary for female flower development. Transient expression of a catabolic GA 2-oxidase from pumpkin in cucumber ovaries decreases GA₉ and GA₄ levels and arrests the development of female flowers, and this can be restored by application of GA₉ to petals thus confirming its function. Given that bioactive GAs can promote sex reversion of female flowers, movement of biologically inactive precursors, instead of the hormone itself, might help to maintain floral organ identity, ensuring fruit and seed production.