Transcription factor CitERF36 promotes sucrose accumulation in citrus fruits via activating a sucrose transporter and suppressing a vacuolar invertase

Sugar is crucial for fruit flavor and nutrition, directly influencing the sensory perception and acceptance of consumers. Fruit sugar accumulation primarily depends on sugar metabolism and transport; however, the molecular regulatory mechanisms in citrus have not been fully elucidated. Physiological and metabolic analyses were performed on the 'Ehime 30' and 'Harumi' hybrid population, which exhibited stable transgressive levels of total soluble solids (TSS) and sucrose. High- (#13, #107) and low- (#237, #290) TSS and sucrose fruits were selected for transcriptional analyses. The results showed that CitERF36 exhibited significantly higher expression in high-TSS and sucrose fruits. Moreover, in this population, a significant positive correlation was observed between CitERF36 expression and both TSS and sucrose content. Localized to the nucleus, CitERF36 showed markedly higher expression in fruit than in other organs during development. In CitERF36-overexpressing tomato fruits, sucrose content was significantly elevated, whereas silencing CitERF36 in citrus led to a notable reduction. Through DNA affinity purification-sequencing analysis, Y1H, electrophoretic mobility shift assay, and dual-luciferase assays, CitERF36 was shown to activate CitSUT2 expression and repress CitvINV3 expression. The CitSUT2 locus, encoding CitSUT2.1 and CitSUT2.2 in mandarin, exhibited proton-coupled sucrose transport activity in Xenopus laevis oocytes and mediated sucrose accumulation in genetically transformed tomato and citrus. Additionally, CitvINV3, localized to the vacuole, showed the highest invertase activity at pH 5 in yeast complementation assays and facilitated the conversion of sucrose to hexose using transformation in citrus. These findings reveal a novel regulatory mechanism influencing fruit sucrose transport and metabolism, providing a theoretical basis for enhancing citrus fruit quality.