Metabolite profile and genes/proteins expression in β-citraturin biosynthesis during fruit ripening in Chinese raspberry (Rubus chingii Hu)

Carotenoids are one of the most abundant pigments in raspberries. Rubus chingii Hu, indigenous to China, is traditionally consumed for health benefits. However, the carotenoid composition and pathways of R. chingii have not yet been studied. In this study, the components of carotenoids and genes/proteins involved in their biosynthesis were investigated during four fruit ripening phases via LC-MS/MS. Zeaxanthin, β-citraurin and its esters, first identified in Rubus, gradually accumulated during fruit maturation. These compounds, rather than anthocyanins, were responsible for the ripe fruit coloration. In carotenoid metabolism, upstream synthesis genes of RcPSY2 (CL1406.Contig2), RcPDS1 (CL7625.Contig2), RcZDS1 (CL590.Contig6) and RcCRTISO1 (CL6919.Contig2) were up-regulated in gene/protein expression to accelerate carotene biosynthesis. Downstream genes of RcLUT5CHYB/CYP97A (CL8884.Contig3) and RcCHYB/BCH (CL7966.Contig1) were up-regulated in gene/protein expression, while RcCHYE/CYP97C (CL9380.Contig1/2) were maintained at low levels. RcLCYE (Unigene19570) was down-regulated while RcLCYB (CL7586.Contig1) was up-regulated and then down-regulated. These differential gene/protein expressions between LCYB and LCYE, and between CHYE and CHYB led to zeaxanthin accumulation by elevating its biosynthetic enzymes and lowering enzymes for lutein biosynthesis. In apocarotenoid biosynthesis, RcCCD (CL1310.Contig3) was up-regulated in gene/protein expression, which raised the content of β-citraurin and its esters. Additionally, these genes/proteins diverged into different subgroups with distinct pattens of expression, suggesting their difference in function. For example, RcPSY1/3, RcZDS2, and RcCRTISO2/3 genes were expressed at very low levels, suggesting that they may be active in other tissues rather than in fruit. The mechanism of zeaxanthin and β-citraurin biosynthesis is responsible for fruit coloration, which is completely novel to Rubus.