Investigation of Sugar Metabolism in Pear (Pyrus bretschneideri) Fruits using Integrated Metabolomics

Sugars play an important role in maintaining fruit quality and nutritional value. Sugars also serve as precursors of other compounds contributing to fruit flavor including organic acids, amino acids, and volatile compounds. Therefore, sugars are the key metabolites connecting pathways of primary and secondary metabolisms in the fruit quality development. Sugar composition in fruits is often affected by factors related to genetics, physiology, ecology, etc. Sugar metabolism is regulated by a complex metabolic network. Metabolomic approaches have increasingly been used to gain insight of metabolic composition in plants. Metabolomics, when combined with transcriptomics and genomics, can provide a powerful tool for better understanding of a biological system and has now been successfully performed on a diverse array of plant species, including fruit tree species. Pyrus is an important genus in the Rosaceae family. Pear production has reached 23.90 million tons in 2011, ranked second to apple (Malus) worldwide. China is the largest pear producer with about 15.95 metric tons (Mt) or 67.7% of the total production in 2011 (FAOSTAT, 2012). In this study, sugar metabolism in pear (Pyrus bretschneideri Rehd. cv. Dangshansuli) during fruit development and maturation was invetigated using untargeted (GC-MS) and targeted (HPLC) metabolic profiling analyses. Data were analyzed using a variety of chemometric analyses including principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) to explore changes of carbon-flux in pear fruits at different developmental stages. Digital gene expression (DGE) profiling during fruit development was analyzed using the pear assembly transcriptome developed in our previous study. With the integration of metabolic profiling and DGE profiling, a detailed network within metabolites, genes, or between metabolites and genes were constructed. Several functional genes and their regulators closely related to sugar metabolism in pear fruits were identified. A natural mutant of P. bretschneideri was found. This mutant contains a higher sugar content than its wild type (WT). The metabolic profiling and DGE profiling were used to compare sugar metabolism between mutant and WT fruits. Our findings indicated that the integrated metabolomics is a useful tool to generate important and interesting insight of many biological processes, which will be of great significance in exploring sugar metabolism and its functional genes or regulators in pear fruits. It will facilitate enhancement of breeding germplasm and improvement of fruit quality in pear or other  related species.