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 <i>EjAGL18</i> increased during fruit ripening, opposite to the accumulation pattern of malic acid in loquat. The transient overexpression of <i>EjAGL18</i> in loquat fruit downregulated malic acid accumulation and the transcriptional level of the tonoplast dicarboxylate transporter <i>EjtDT1</i>. Conversely, silencing <i>EjAGL18</i> in loquat fruit upregulated the malic acid content and <i>EjtDT1</i> expression level. Dual-luciferase assays and yeast one-hybrid experiments further confirmed that EjAGL18 could bind to the promoter of <i>EjtDT1</i> and repress its transcriptions. Furthermore, the transient overexpression of <i>EjtDT1</i> in loquat fruit increased the malic acid content. These results revealed that EjAGL18 negatively regulates malic acid content by repressing <i>EjtDT1</i> 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.