SUPERROOT1 is not the only C-S lyase capable to contribute to glucosinolate biosynthesis in Arabidopsis thaliana

Glucosinolates (GLs) are sulfur-containing specialized metabolites from plants representing the Brassicales order. They can be derived from several amino acids through a complex biosynthetic pathway. The majority of steps on this pathway involve multiple amino acid-specific and/or redundant isoforms of the respective enzymes. According to the published results, the exception from this rule in the model plant Arabidopsis thaliana is Superroot1 (SUR1), which has been proposed to be the only isoform catalyzing cleavage of the C-S bond in S-alkyl-thiohydroximate intermediates derived from different amino acids. Notably, mutation in SUR1 not only disrupts GL biosynthesis but also leads to severe growth phenotypes and lethality, resulting from auxin overproduction and possibly from accumulation of toxic GL biosynthetic intermediates. In this study, we show that elevated auxin production in the sur1 plant depends on the MYB34 transcription factor, which is one of the MYBs controlling biosynthesis of Trp-derived GLs. Unlike its impact on IAA accumulation, the myb34 mutation only partially weakened the growth phenotype and did not prevent from the lethality of the sur1 mutation. Strikingly, we show that sur1 plants are still able to accumulate significant indolic GL amounts as compared with wild type plants, indicating that additional C-S lyase is involved in the biosynthesis of these specialized metabolites.