Identification of Anthocyanin Composition and Functional Analysis of an Anthocyanin Activator in Solanum nigrum Fruits

Solanum nigrum fruits have been conventionally used in beverages due to their nutritional substances such as minerals, vitamins, amino acids, proteins, sugars, polyphenols, and anthocyanins. The characterization of components and regulatory mechanism of anthocyanins in S. nigrum fruits have rarely been reported. In this study, we determined that the peel and flesh of S. nigrum fruits shared similar HPLC profiles but different contents and total antioxidant activities for anthocyanins. After an efficient purification method, mainly including extraction with pH 1.0 distilled water and then desorption with pH 1.0 95% ethanol after a DM-130 resin adsorption step to obtain more pure anthocyanin extracts, the purity of anthocyanins extracted from S. nigrum fruits reached 56.1%. Moreover, eight anthocyanins from S. nigrum fruit were identified with HPLC-MS/MS for the first time. A typical R2R3-MYB transcription factor gene, SnMYB, was also cloned for the first time by rapid amplification of cDNA ends (RACE)-PCR from S. nigrum. Moreover, the contents of anthocyanins were shown to correlate well (r = 0.93) with the expression levels of SnMYB gene during the fruit's developmental stages. Most significantly, SnMYB gene successfully produced high anthocyanin content (1.03 mg/g) when SnMYB gene was transiently expressed in tobacco leaves. Taken together, S. nigrum fruits are a promising resource for anthocyanin extraction, and SnMYB gene is an activator that positively regulates anthocyanin biosynthesis in S. nigrum.