MYB134-RNAi poplars show reduced tannin synthesis in leaves but not roots, and increased susceptibility to oxidative stress

The importance of the poplar MYB134 gene in controlling condensed tannin (CT) biosynthesis was tested by suppressing its expression using RNAi. MYB134-RNAi plants grew normally but showed a reduced accumulation of stress-induced CTs in leaves. RNA-seq analysis indicated that flavonoid- and CT-related genes, as well as additional CT regulators, were strongly and specifically down-regulated by MYB134 suppression. This confirmed that the primary MYB134 target is the leaf flavonoid and CT pathway. Root CT accumulation was not impacted by MYB suppression, suggesting that additional CT regulators are active in roots and emphasizing the complexity of the CT regulation in poplar. In order to test the effect of CT downregulation on oxidative stress resistance, leaves of MYB134-RNAi and control leaves were exposed to the reactive oxygen generator, methyl viologen. The MYB134-RNAi leaves sustained significantly more photosystem II damage, as seen in reduced chlorophyll fluorescence, compared to wild-type leaves. MYB134-RNAi leaves also contained more hydrogen peroxide, a reactive oxygen species, compared to the wild-type. Our data thus corroborate the hypothesis that CT can act as in vivo antioxidant and protect against oxidative stress. Overall, MYB134 was shown to be a central player in regulation of CT synthesis in leaves.