3′‐UTR Hypermethylation and ABA Signaling Coordinate Transcriptional Activation of SWEET14 for High Sugar Accumulation in Citrus

Sugar accumulation is pivotal for fruit quality, yet the molecular regulation of this process remains elusive. By comparing transcriptomes of sweet lemon (Citrus limon, SL, high sugar level) and Eureka (EL, trace amount of sugar), we identified the sucrose transporter CliSWEET14 as a key driver of sugar accumulation. Methylome atlas showed that the 26-bp sequence within 3'-untranslated region (UTR) of CliSWEET14-SL was substantially hypermethylated relative to CliSWEET14-EL. Notably, the 3'-UTRs of both genes formed chromatin loops with their native promoters. ClibHLH105 bound to unmethylated CACGAT sequence in the loop and repressed CliSWEET14-EL. In contrast, hypermethylation of the CACGAT motif disrupted ClibHLH105 interaction and repression on CliSWEET14-SL. Additionally, CliSWEET14-SL was positively regulated by CliNAC56, which was further activated by CliABF3. Both CliNAC56 and CliABF3 regulated sucrose accumulation in a CliSWEET14-dependent manner. Moreover, the regulatory cascade CliABF3-CliNAC56-CliSWEET14 in SL was activated by ABA (abscisic acid) to a greater degree due to higher ABA level. Taken together, 3'-UTR hypermethylation-mediated relief of transcriptional repression and stronger activation by ABA signaling robustly boost CliSWEET14 expression and sucrose accumulation. These findings substantially advance our understanding of sugar accumulation in fruit and provide novel epigenetic and transcriptional targets for improving fruit quality in citrus and beyond.