AP3 promotes the synthesis of condensed tannin in fruit by positively regulating ANR expression

Condensed tannins are often found in fruits and nuts and have an astringent flavor. The synthesis pathway of condensed tannins is already clear, but few related regulatory factors have been explored. Previous studies about MADS-box transcription factors have mainly focused on the regulation of floral organ development. Recent studies have shown that MADS-box are also involved in fruit development, maturation, and quality. The fruit of Quercus fabri is rich in starch and nutrients in its kernel but is difficult to eat directly because of its high condensed tannin content. This study identified and functionally characterized the MADS-box transcription factor QfAP3 in Q. fabri. Functional analysis based on overexpression in Micro-Tom showed that QfAP3 promoted condensed tannin synthesis. By analyzing the expression trend of key genes in the condensed tannin synthesis pathway in Micro-Tom plants, we found that the expression trend of ANR was consistent with that of QfAP3, and QfAP3 could bind to the promoter of ANR and positively regulate it. This study has discovered new functions of MADS-box transcription factors in fruit quality formation, developed new regulatory factors for the synthesis pathway of condensed tannin, and provided a biotechnological method that can effectively reduce astringency in fruit.