Transcriptome analysis provides new insight into the molecular mechanisms in regulating carotenoid accumulation in pericarp of fruits from two pummelo cultivars Citrus grandis (L.) Osbeck

Abstract Background Carotenoid improves fruit external quality and benefits to human health. Although the mechanism underlining carotenoid metabolism is clear, the regulation of carotenoid accumulation remains poorly understand. Recently, ‘Huangbao’ pummelo was selected from the bud mutation of ‘Guanxi’ pummelo (yellow pericarp). The pericarp of mutant-type fruits is characterized by red colour development during fruit ripening stage. The aim of the study is to explore the molecular mechanisms in regulating pericarp colouration based on transcriptomic profile analysis of the two cultivars. Results Lycopene content in the pericarp of ripe fruits from ‘Huangbao’ (160.29 µg g-1 FW) was significantly higher than that of from ‘Guanxi’ (0.91µg g-1 FW). Lycopene is a main contributor for pericarp colouration. 929 differentially expressed genes (DEGs) were indentified through transcriptional profiling in the pericarp of ripe fruits from the two cultivars. Expression levels of the Genes encoding for fructose-bisphosphate aldolase, enolase and pyruvate kinase isozyme were significantly higher in ‘Huangbao’ than that of in ‘Guanxi’, which possibly promote 3-phosphate-glyceraldehyde and pyruvic acid biosyntheses. This two productions act as precursors for the biosyntheses of Isopentenylpyrophosphate (IPP) and Dimethylallyl pyrophosphate (DMAPP) in Methylerythritol phosphate (MEP) pathway. Genes ecoding for phytoene synthase and prolycopene isomerase were unexpected down-regulated in ‘Huangbao’. Gibberellin-2-β-dioxygenase (GA2OX) gene was specially expressed in ‘Huangbao’, while the transcript amounts of genes related to auxin response and auxin transport as well as several negative regulators in ethylene signaling in ‘Huangbao’ were all down regulated. Additionally, Transcription factors (DELLA, NAC, MADS-box, WRKY) and post translational modification proteins (histone acetyltransferase and E2 ubiquitin-conjugating enzyme) involved in regulating ethylene and carotenoid metabolisms were also differentially expressed in the two pummelo cultivars. Conclusions The main differences in the carotenoid metabolism in the pericarp of ripen fruits from ‘Huangbao’ and ‘Guanxi’ were observed in the carotenoid precursor biosynthetic pathway. Differentially expressed genes in gibberellin, auxin and ethylene metabolisms and plant hormone signaling pathways, as well as several transcription factors and post translational modification protein genes involved in regulating ethylene and carotenoid metabolisms provide targets for further exploration in revealing mechanisms underlying fruit colour development.