Integrated metabolomic and transcriptomic analysis of the anthocyanin regulatory networks in red walnut

Abstract

Background Walnut is one of the most important dry fruit crops worldwide, typically green leaves and yellow-brown or gray-yellow seed coats. A specific walnut type, red walnut ‘RW-1’ with red leaves and seed coats was selected as plant material because of higher anthocyanins contents. Anthocyanins are important colorants with strong antioxidant activity, especially, benefic for human health. However, few studies focused on the molecular mechanism of anthocyanin biosynthesis in walnut. Results From the results of Widely Targeted Metabolome and anthocyanidin detection analysis, 395 substances, including 4 procyanidins and 26 anthocyanins, were identified from the red-leaf walnuts of RW-1 natural hybrid progenies (SR) and the green-leaf walnuts of RW-1 natural hybrid progenies (SG). Among these, all the anthocyanins in SR were significantly up-accumulated comparing with SG. Also, delphinidin 3-O-galactoside, cyanidin 3-O-galactoside, delphinidin 3-O-glucoside and cyanidin 3-O-glucoside were identified to the primary components of anthocyanidins because of the higher contents. It was noted that 9 anthocyanins including malvidin 3-O-galactoside, malvidin 3-O-arabinoside, cyanidin 3-O-(6-O-malonyl-beta-D-glucoside), delphinidin 3-O-glucoside, delphinidin 3,5-O-diglucoside (Delphin), peonidin 3-O-(6-O-malonyl-beta-D-glucoside), petunidin 3-O-(6-O-malonyl-beta-D-glucoside), petunidin 3-O-arabinoside and pelargonidin 3-O-(6-O-malonyl-beta-D-glucoside) were detected only in SR walnut. Furthermore, transcriptome analysis demonstrated that the expression of structural genes (C4H, F3H, F3’5’H and UFGTs), and four MYBs in anthocyanin biosynthetic pathway were significantly higher in SR walnut. Conclusions We identified the color formation of SR leaves is due to the accumulation of anthocyanins. And our results obtained the valuable information on the anthocyanin metabolites and candidate genes in anthocyanin biosynthesis, which provided new insights into the anthocyanin biosynthesis in walnuts.