Hormonal crosstalk during the reproductive stage of Coffea arabica: interactions among gibberellin, abscisic acid, and ethylene.

Abstract

Main conclusion: The application of gibberellin and abscisic acid in coffee plants resulted in increased floral bud formation and fruit production by regulating key genes involved in flowering and hormonal biosynthesis pathways. Despite ongoing efforts, understanding hormonal regulation in perennial and woody species with complex phenological cycles, such as Coffea arabica L., remains limited. Given the global importance of coffee, identifying the main regulators of reproductive development is crucial to guarantee production, especially in face of climate change. This study investigated the effects of gibberellin (GA) and abscisic acid (ABA) at different concentrations (5, 25 and 100 ppm) in the reproductive development of C. arabica. Phenological analyses, molecular identification of genes involved in GA and ABA biosynthesis, degradation, and signaling, as well as gene expression profiling in leaves and floral buds during floral induction and development, were conducted. Promoter analysis of CaFT, quantification of 1-aminocyclopropane-1-carboxylate (ACC), enzymatic activity of ACC oxidase (ACO), and ethylene content were also assessed. Results showed that GA irrespective of concentration and ABA at 25 ppm applied during the main period of floral induction (March) significantly increased the number of floral buds, with ABA also accelerating the development. Similarly, applying these regulators in plants with floral buds at more advanced stages (August) increased the number of floral buds and fruit production in the GA (5 and 100 ppm) and ABA (25 and 100 ppm) treatments. Phylogenetic and molecular analyses identified genes related to GA and ABA biosynthesis, degradation, and signaling in coffee plants. GA and ABA treatments affected the expression of genes related to floral induction and organ formation, such as CaDELLA in March, which may relate to the increased number of floral buds. Moreover, in August, plants treated with 5 and 100 ppm GA and 100 ppm ABA showed up-regulation of CaFT1 expression, likely due to the down-regulation of CaCO during this period. In addition to GA-ABA interactions, our results suggest that GA promotes ACC accumulation in leaves in August, which may act as a mobile signal transported to floral buds, where its conversion to ethylene could regulate anthesis, highlighting a GA-ACC-ethylene interaction in coffee flowering. However, no significant differences in ethylene biosynthesis were observed in March with the application of these hormones, underscoring the incipient role of ethylene during floral induction in coffee. These results suggest reciprocal regulation of floral development by GA-ABA pathways in a dose-dependent manner and interacting with other hormonal pathways such as the ethylene biosynthesis in leaves and floral buds. These findings provide new insights into the hormonal regulation of coffee flowering, guiding field practices and breeding programs to maximize coffee production.