Identification and characterization of unique 5-hydroxyisoflavonoid biosynthetic key enzyme genes in Lupinus albus.

KEY MESSAGE: 5-Hydroxyisoflavonoids, no 5-deoxyisoflavonoids, in Lupinus species, are due to lack of CHRs and Type II CHIs, and the key enzymes of isoflavonoid biosynthetic pathway in white lupin were identified. White lupin (Lupinus albus) is used as food ingredients owing to rich protein, low starch, and rich bioactive compounds such as isoflavonoids. The isoflavonoids biosynthetic pathway in white lupin still remains unclear. In this study, only 5-hydroxyisoflavonoids, but no 5-deoxyisoflavonoids, were detected in white lupin and other Lupinus species. No 5-deoxyisoflavonoids in Lupinus species are due to lack of CHRs and Type II CHIs. We further found that the CHI gene cluster containing both Type I and Type II CHIs possibly arose after the divergence of Lupinus with other legume clade. LaCHI1 and LaCHI2 identified from white lupin metabolized naringenin chalcone to naringenin in yeast and tobacco (Nicotiana benthamiana), and were bona fide Type I CHIs. We further identified two isoflavone synthases (LaIFS1 and LaIFS2), catalyzing flavanone naringenin into isoflavone genistein and also catalyzing liquiritigenin into daidzein in yeast and tobacco. In addition, LaG6DT1 and LaG6DT2 prenylated genistein at the C-6 position into wighteone. Two glucosyltransferases LaUGT1 and LaUGT2 metabolized genistein and wighteone into its 7-O-glucosides. Taken together, our study not only revealed that exclusive 5-hydroxyisoflavonoids do exist in Lupinus species, but also identified key enzymes in the isoflavonoid biosynthetic pathway in white lupin.