4 DISCUSSION
Flavonoids are important secondary metabolites in plants, and the biosynthesis of flavonoids is precisely controlled by many transcription factors. MYB11, MYB12, and MYB111 are close R2R3 homologous proteins, which acts partially redundantly to determine the accumulation and distribution of flavonol in Arabidopsis (Stracke et al., 2007). In the present study, six R2R3 NtMYB genes were cloned from the N. tabacum genome, and grouped into three pairs, which were subsequently named asNtMYB11 , NtMYB12 , and NtMYB111 , respectively. The NtMYB12b is a flavonol-specific regulator, while the NtMYB12a promotes the biosynthesis of both flavonol and anthocyanin in tobacco. Moreover, we find that NtMYB12a can directly induce the transcription ofNtSFAR4 , NtLOX6 , NtLOX5 , and NtGDSL2 genes, and indirectly promotes the transcription of NtGDSL1 ,NtGDSL3 , NtGDSL4 , and NtFAH1 genes, which leads to the reduction of FA content in tobacco leaves and seeds. We further show that high concentration of sucrose enhances the activity of NtMYB12a in promoting the dehydration of FA in tobacco.