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 < 0.01) between the change in CsAPD2 transcript level and citric acid content. Additionally, when drought stress was applied to citrus trees, both the CsAPD2 transcript level and the citric acid content were significantly enhanced in fruit JS. Taken together, the present study results suggested that CsAPD2 possibly participates in the dynamic change of citric acid accumulation in citrus fruits.