Abstract
Flavones, one of the largest classes of flavonoids in plants, have a variety of bioactivities and participate in the resistance response of plants to biotic and abiotic stresses. However, flavone synthase (FNS), the key enzyme for flavone biosynthesis, has not yet been characterized in mulberry. In this study, we report that the leaves of certain mulberry cultivars, namely BJ7, PS2, and G14, are rich in flavones. We identified a Fe2+/2-oxoglutarate-dependent dioxygenase from Morus notabilis (MnFNSI) that shows the typical enzymatic activity of a FNSI-type enzyme, and directly converts eriodictyol and naringenin into their corresponding flavones. Overexpression of MnFNSI in tobacco increased the flavones contents in leaves and enhanced the tolerance of tobacco to ultraviolet-B (UV-B) stress. We found that mulberry cultivars with higher flavones contents exhibit less UV-B induced damage after a UV-B treatment. Accordingly, our findings demonstrate that MnFNSI, a FNSI-type enzyme, is involved in the biosynthesis of flavones, which provide protection against UV-B radiation. These results lay the foundation for obtaining mulberry germplasm resources that are more tolerant to UV-B stress and richer in their nutritional value.
Highlights
Flavones are ubiquitous secondary metabolites in plants and are one of the largest subclasses of flavonoids [1]
As flavones may play a role in protecting plants against UV-B radiation, we further examined whether mulberry FNSI expression and flavones accumulation in the leaves are regulated by UV-B radiation
In our previous mulberry resource survey, we found that the leaves of some cultivars, including Baojing 7 (BJ7), Pisang 2 (PS2), and Gui 14 (G14), have high total flavonoid contents [28]
Summary
Flavones are ubiquitous secondary metabolites in plants and are one of the largest subclasses of flavonoids [1]. Our previous survey of mulberry resources revealed that some mulberry cultivars such as BJ7, PS2, and G14, contain higher total flavonoids contents in their leaves [28]. Our results show that MnFNSI is involved in the tolerance of plants to UV-B radiation, and that mulberry cultivars rich in flavones cope with UV-B stress more effectively than do those with lower flavones contents. These results lay the foundation for obtaining mulberry germplasm resources that are more resistant to UV-B stress and richer in nutritional value
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