Loss of MYB34 transcription factor that controls feedback loop in indole glucosinolate biosynthesis supports backward evolution of this pathway within Camelineae tribe

Glucosinolates are specialized defensive metabolites characteristic for the Brassicales order. Among them aliphatic and indolic glucosinolates (IGs) are usually highly abundant in the species from Brassicaceae family. The exception from this trend is constituted by species representing a subclade of Camelineae tribe, including Capsella and Camelina genera, which have reduced capacity to produce and metabolize IGs. Our study addresses contribution of specific glucosinolate-related MYB transcription factors to this unprecedented backward evolution of IG biosynthesis. To this end we performed phylogenomic and functional studies of respective MYB proteins. Obtained results revealed weakened conservation of glucosinolate-related MYB transcription factors, including loss of functional MYB34 protein, in the investigated species. We showed that introduction of functional MYB34 from Arabidopsis thaliana partially restores IG biosynthesis in Capsella rubella indicating that loss of this transcription factor contributes to the backward evolution of this metabolic pathway. Finally, we performed analysis of the impact of particular myb mutations on the feedback loop in IG biosynthesis, which drives auxin overproduction, metabolic dysregulation and strong growth retardation caused by mutations in IG biosynthetic genes. This uncovered unique function of MYB34 among IG-related MYBs in this feedback regulation and consequently in IG conservation in Brassicaceae plants.