Integrated analysis of metabolites and proteins reveal aspects of the tissue-specific function of synthetic cytokinin in kiwifruit development and ripening.

UNLABELLED: Fruit development and ripening depends on highly coordinated phyto-hormonal activities. Although the role of synthetic cytokinin N-(2-chloro-4-pyridyl)-N'-phenylurea (CPPU) in promoting fruit growth has been established, knowledge regarding the underlying mechanism is still lacking. Here, we characterize the effect of CPPU application 20d after full bloom at pre- and post-harvest biology of kiwifruit (Actinidia deliciosa [A. Chev.] C.F. Liang et A.R. Ferguson var. deliciosa cv. 'Hayward'). Data revealed that CPPU stimulates kiwifruit growth through the enlargement of small cells. During fruit development, the abundance of 16 proteins that are mainly related to defence was increased by CPPU while CPPU altered the expression of 19 polar metabolites in outer pericarp. Sugar homeostasis, cell wall modifications, TCA cycle and myo-inositol pathway were mostly affected by CPPU in kiwifruit during development. Upon postharvest ripening at 20°C following 2months of cold storage (0°C), CPPU suppressed ethylene production and retained central placenta softening, indicating that CPPU induced tissue-dependent disturbances in climacteric ripening. Nineteen central placenta proteins and up to 15 metabolites of outer pericarp and central placenta tissues were affected by CPPU in ripened kiwifruits. These observations amplified our understanding in the regulation of fruit development and ripening by exogenously supplied cytokinins. BIOLOGICAL SIGNIFICANCE: This study demonstrates that CPPU application, apart from fruit development, influenced also the kiwifruit climacteric ripening behaviour. An insight on the action of CPPU during kiwifruit development is provided, showing that it is partially based on a general stimulation of TCA cycle and myo-inositol pathway along with alternation in sugar and cell wall metabolism. Data also revealed that CPPU regulates ethylene biosynthesis and influences central placenta softening, indicating that this tissue may play a prominent role in kiwifruit ripening. Also, this work provides a first characterization of the ripening-affected central placenta proteins that offers insights into kiwifruit ripening. The current study provides a baseline of information for understanding the metabolic processes that are regulated by exogenous cytokinin during fruit development and ripening.