Metabolomics of capsicum ripening reveals modification of the ethylene related-pathway and carbon metabolism

Capsicum (Capsicum annuum L. cv. Aries) is a non-climacteric bell-pepper fruit, exhibiting limited ethylene and respiration levels during ripening. In contrast to climacteric fruit, such as tomato which is largely dependent upon ethylene to ripen, the regulation of non-climacteric ripening is still inadequately understood. A metabolomics approach was used to identify differentially abundant compounds between ripening stages with the aim of elucidating metabolic pathways involved in the regulation of non-climacteric ripening. Metabolite profiling using gas chromatography-mass spectrometry (GC-MS) was initially employed to screen potential metabolite differences among three ripening stages (Green, Breaker Red 1 and Light Red). Targeted analyses using liquid chromatography-mass spectrometry (LC-MS) or enzymatic assays were subsequently employed to characterise selected metabolites in more ripening stages. Starch, sugars and their derivatives were significantly modified during ripening which may affect the abundance of some glycolysis intermediates and consequently other metabolic pathways involving amino acids, colour and pungency precursors, and tricarboxylic acid (TCA) cycle intermediates. Furthermore, metabolites closely related to ethylene production such as cysteine and methionine gradually increased between the ripening stages, whereas putrescine significantly decreased during ripening, suggesting that some parts of the ethylene pathway may still be functional in this non-climacteric fruit. Thus, this study which utilised both profiling and targeted metabolomics, has identified a wide range of metabolites which are involved in various biochemical pathways and highlights the overall metabolic shifts during non-climacteric capsicum ripening.