The MADS-Box Transcription Factor EjAGL18 Negatively Regulates Malic Acid Content in Loquat by Repressing EjtDT1

Malic acid is the major organic acid in loquat fruit, contributing to the sourness of fruit and affecting fruit flavor. However, the transcriptional regulation of malic acid in loquat is not well understood. Here, we discovered a MADS-box transcription factor (TF), EjAGL18, that regulated malic acid accumulation in loquat. EjAGL18 is a nucleus-localized TF without transcriptional activity. The expression of EjAGL18 increased during fruit ripening, opposite to the accumulation pattern of malic acid in loquat. The transient overexpression of EjAGL18 in loquat fruit downregulated malic acid accumulation and the transcriptional level of the tonoplast dicarboxylate transporter EjtDT1. Conversely, silencing EjAGL18 in loquat fruit upregulated the malic acid content and EjtDT1 expression level. Dual-luciferase assays and yeast one-hybrid experiments further confirmed that EjAGL18 could bind to the promoter of EjtDT1 and repress its transcriptions. Furthermore, the transient overexpression of EjtDT1 in loquat fruit increased the malic acid content. These results revealed that EjAGL18 negatively regulates malic acid content by repressing EjtDT1 in loquat. This study broadens the understanding of the MADS-box TF's regulatory mechanisms in malic acid and provides new insights into fruit flavor improvement in loquat.