Gibberellin and cytokinin are coordinately involved in regulating kiwifruit flowering.

Abstract

Flowering time critically affects kiwifruit (Actinidia deliciosa) yield and fruit quality. To uncover the molecular basis of early flowering, we compared the early-flowering cultivar 'Hanhong' with the late-flowering 'Guichang'. Phenological data showed 'Hanhong' completes flowering in eight weeks-two weeks earlier than 'Guichang'-and exhibits significantly higher endogenous gibberellin (GA) levels in both floral buds and open flowers. Integrated transcriptomic and proteomic analyses revealed strong enrichment of GA and cytokinin signaling pathways during floral development. While transcript levels of the DELLA gene REPRESSOR OF GA1-3 (AdRGA) remained stable, its protein abundance markedly declined from bud to flower stages in both cultivars, indicating post-transcriptional regulation. Functional assays in Arabidopsis confirmed that AdRGA overexpression delays flowering by suppressing FLOWERING LOCUS T (FT), establishing AdRGA as a negative flowering regulator. Conversely, Arabidopsis Histidine Phosphotransfer Protein (AdAHP), a cytokinin signaling component, was significantly upregulated at both mRNA and protein levels in 'Hanhong'. Ectopic expression of AdAHP in the ahp5-2 mutant accelerated flowering and enhanced expression of FT and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), confirming AdAHP as a positive regulator. Additionally, Gibberellin Insensitive Dwarf1 (AdGID1) displayed stage-specific expression-enriched during floral bud development but not at anthesis-suggesting a role in bud formation rather than floral transition timing. Together, our results demonstrate that coordinated GA and cytokinin signaling, mediated by AdRGA and AdAHP, fine-tunes kiwifruit flowering through differential regulation of FT and SOC1. This work provides a molecular framework for manipulating flowering time and identifies key genetic targets for kiwifruit breeding programs aimed at optimizing production.