Gibberellin and spermidine synergistically regulate polyamine metabolism during the development of Rhododendron flowers

Polyamines (PAs) are involved in various developmental processes, especially plant flowering. Their significant influences have been established; however, the exact mechanism by which PAs regulate flowering remains unclear. To explore PA metabolism in plant flowering, gibberellic acid (GA3, 0~2400 mg L−1) and spermidine (Spd, 0~1 mM) were applied alone or in combination during the early stage of flower bud formation in Rhododendron simsii. The application of GA3 alone advanced initial flowering, while that of Spd alone delayed initial flowering. Interestingly, GA3 and Spd applied in combination advanced initial flowering by 2 days. Furthermore, from stage 1 to 2, endogenous PA levels and the soluble conjugated and insoluble bound fractions of PAs and key enzymes (e.g., diamine oxidase, arginine decarboxylase, ornithine decarboxylase and S-adenosylmethionine decarboxylase) increased, and the level of PA oxidase decreased. These findings revealed that exogenous GA3 and Spd delay flower senescence by improving PA biosynthesis and preventing PA degradation. Moreover, exogenous GA3 and Spd enhanced the levels of endogenous PA and GA3, while the conversion of free PAs to soluble conjugated and insoluble bound forms delayed Rhododendron senescence. Overall, our findings reveal a potential positive feedback mechanism by which higher endogenous PA contents and the combined effects of exogenous GA3 and Spd synergistically delay Rhododendron senescence by enhancing PA biosynthesis and converting free PA to soluble conjugated and insoluble bound forms, thus reducing PA degradation during flower senescence.