Abstract
Chalcone isomerase (CHI) is a key enzyme in flavonoid biosynthesis. In plants, CHIs occur in multigene families, and they are divided into four types, types I–IV. Type I and II CHIs are bona fide CHIs with CHI activity, and type III and IV CHIs are non-catalytic members with different functions. Rice contains seven CHI family genes (OsCHIs). Molecular analysis suggested that OsCHI3 is a type I CHI, and the other OsCHIs were classified into types III and IV. To elucidate their biochemical functions, OsCHI1, OsCHI3, OsCHI6, and OsCHI7 were expressed in Escherichia coli, and the recombinant OsCHI proteins were purified. An activity assay of recombinant OsCHIs showed that OsCHI3 catalyzed the isomerization of naringenin chalcone and isoliquiritigenin, whereas the other recombinant OsCHIs had no CHI activity. OsCHI3 also exhibited a strong preference to naringenin chalcone compared to isoliquiritigenin, which agrees well with the catalytic properties of type I CHIs. These results ascertain OsCHI3 to be a bona fide CHI in rice. OsCHI3 and the other OsCHIs were expressed constitutively throughout the rice growth period and different tissues. OsCHI3 expression was induced immediately in response to ultra-violet (UV) stress, suggesting its involvement in the biosynthesis of sakuranetin, a flavonoid phytoalexin in rice.
Highlights
Flavonoids are a major group of plant secondary metabolites that play a role in pigmentation, development, and reproduction as well as participate in defense against biotic and abiotic stresses as UV-protectants, antioxidants, and antimicrobial agents [1,2,3]
A search of the MSU Rice Genome Annotation Project (RGAP) database [28] indicated that the rice Chalcone isomerase (CHI) family is comprised of seven members designated as OsCHI1–OsCHI7 (Table 1)
Despite extensive attempts to clone OsCHI3 cDNA from diverse rice tissues, only a smaller variant was cloned from rice leaves
Summary
Flavonoids are a major group of plant secondary metabolites that play a role in pigmentation, development, and reproduction as well as participate in defense against biotic and abiotic stresses as UV-protectants, antioxidants, and antimicrobial agents [1,2,3]. They are health-beneficial phytonutrients in the human diet, with diverse biological activities, including hepatoprotective, anti-inflammatory, antibacterial, and anticancer properties [4,5]. Expression patterns of six non-catalytic OsCHI family members in different tissues and developmental stages were examined, and their possible role was discussed
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