Light is emerging as a central regulator of plant immune responses against herbivores and pathogens.Solar UV-B radiation plays an important role as a positive modulator of plant defense.However,since UV-B photons can interact with a wide spectrum of molecular targets in plant tissues,the mechanisms that mediate their effects on plant defense have remained elusive.Here,we show that ecologically meaningful doses of UV-B radiation increase Arabidopsis resistance to the necrotrophic fungus Botrytis cinerea and that this effect is mediated by the photoreceptor UVR8.The UV-B effect on plant resistance was conserved in mutants impaired in jasmonate (JA) signaling (jar1-1 and P35S:JAZ10.4) or metabolism of tryptophan-derived defense compounds (pen2-1,pad3-1,pen2 pad3),suggesting that neither regulation of the JA pathway nor changes in levels of indolic glucosinolates (iGS) or camalexin are involved in this response.UV-B radiation,acting through UVR8,increased the levels of flavonoids and sinapates in leaf tissue.The UV-B effect on pathogen resistance was still detectable in tt4-f,a mutant deficient in chalcone synthase and therefore impaired in the synthesis of flavonoids,but was absent in fah1-7,a mutant deficient in ferulic acid 5-hydroxylase,which is essential for sinapate biosynthesis.Collectively,these results indicate that UVR8 plays an important role in mediating the effects of UV-B radiation on pathogen resistance by controlling the expression of the sinapate biosynthetic pathway.