Identification and transcript analysis of CsAPD2 reveal its potential role in citric acid accumulation in citrus fruits

Abstract Aberrant pollen development gene 2 (APD2) encodes a plant-specific RING-finger ubiquitin-protein, which may play an important role in pollen development or fruit development and ripening. However, the role of ubiquitin-proteins in fruit quality formation is not clear. In this study, we successfully cloned an Arabidopsis APD2 homologous gene (CsAPD2) from the citrus genome. The full-length sequence of CsAPD2 has 1431 nucleotides, encoding a protein of 476 amino acids. This gene contains ten exons and nine introns with two transmembrane domains and one RING-finger domain. The tissue-specific transcript analysis showed that CsAPD2 was predominantly expressed in fruit juice sacs (JS), followed by young leaves. Moreover, temporal expression analysis in the JS of six citrus cultivars displayed a significantly positive correlation coefficient (0.939, P