Systematic transcriptomic and metabolomic analysis of walnut (Juglans regia L.) fruit to trace variations in antioxidant activity during ripening

Abstract

Walnut is considered as a good source of antioxidants. However, the molecular mechanism for the composition and accumulation of antioxidant-related metabolites in walnut remains elusive. In this study, a comprehensive transcriptomic and metabolomic analysis was carried to reveal the metabolic dynamics, antioxidant activity variations and underlying regulatory networks during the development of walnut kernel. The results demonstrated that walnut kernel goes through the stages of cell division and proliferation, nutrient accumulation, and nutrient stabilization and maintenance, and the antioxidant activity first decreases and then increases during development. Metabolomic analysis identified a total of 176 metabolites in walnut. Hydrolyzed tannins (HT), coumarins and their derivatives (COA) are significantly positively correlated with the antioxidant activity. Phenylpropane metabolism is the key pathway for HT and COA synthesis, and CHI, CHS, F3H and UGT are the key regulatory enzymes. Sugars and organic acids provide energy and precursor for the accumulation of nutrients in walnut kernel through sugar metabolism and the tricarboxylic acid cycle (TCA). Large amounts of amino acids are accumulated at the nutrient accumulation stage, providing precursors for the formation of proteins and flavonoids during the ripening of walnut. Our results provide new insights into the metabolic regulation of antioxidant activity during walnut ripening.