Glucosinolates are sulphur and nitrogen containing compounds derived from different proteinogenic amino acids. These secondary metabolites occur only in a few plant families including the Brassicaceae. This family includes important crop plants like oil seed rape and many famous vegetables like cabbage, horseradish, mustard and rocket.
Glucosinolates are responsible for the typical sharp taste in these edibles. These compounds themselves are not biological active, but if they are hydrolysed by myrosinases different breakdown products are produced which contribute to plant defence.
Glucosinolates are well known for their defence capacity against herbivores, but in the last years studies could show that they are also important against microbial pathogens. Thereby glucosinolates can also contribute to control microbial infectious disease in humans and some special glucosinolate breakdown products even support cancer prevention.
Spodoptera littoralis choice assay with mutants which have reduced levels of indolic glucosinolat
Glucosinolates as phytoanticipins are constitutively present in the plant, but after herbivore or pathogen attack different types of glucosinolates are induced. Especially the tryptophan derived (indolic) glucosinolates react sensitive to the diverse types of hormone signalling cascades upon pathogens. These different chemical defence compounds are important for the resistance against the different aggressors. The biosynthesis of glucosinolates in the genetic model plant Arabidopsis thaliana is well understood, but the comprehension of the transcriptional regulation of these compounds was still fragmentary.
We show in our recent publication (Frerigmann and Gigolashvili 2014) that three homologous MYB transcription factors (MYB34, MYB51 and MYB122) integrate the diverse signals and regulate the complete biosynthesis to the indolic glucosinolate glucobrassicin. A loss-of-function mutant for all three MYB genes shows a drastic reduction of indolic glucosinolate gene expression and is thereby devoid of all indolic glucosinolates. Furthermore we could show that the three MYBs themselves are regulated differentially upon treatment with defence hormones like jasmonate or salicylic acid and are thereby responsible for the production of the distinct indolic glucosinolates corresponding to the defence strategy of the particular pathogen.
In another related study we could show that these MYB transcription factors are not solely sufficient to induce gene transcription (Frerigmann et al., 2014). We identified with a yeast-two-hybrid approach the bHLH transcription factor bHLH05/MYC3 as an interaction partner of MYB51. In addition to that, further protein interaction studies revealed that also the three homologous bHLH transcription factors MYC2, MYC4 and MYC5 interact with MYB51 and moreover with all six MYB transcription factors for the regulation of glucosinolates. Thus the bHLH proteins act on a hierarchical higher order of regulation. They represent a kind of platform for the MYB transcription factors so that the bHLH triple mutant (which lacks the three main homologs) contains no longer glucosinolates. That means that in this mutant all kinds of glucosinolates are missing and not only the indolic glucosinolates like in the myb34/51/122 triple loss-of-function mutant.
Our work in the framework of CEPLAS contributes to the deeper understanding of the regulation of secondary metabolites in response to different microbes and might be used in the future to generate synthetic plants which have altered levels of these defence compounds.
Contribution by Henning Frerigmann, Cologne Biocenter
Under the heading Planter’s Punch we present each month one special aspect of the CEPLAS research programme. All contributions are prepared by our young researchers.
Frerigmann H, Berger B, & Gigolashvili T (2014) bHLH05 is an Interaction Partner of MYB51 and a Novel Regulator of Glucosinolate Biosynthesis in Arabidopsis. Plant physiology. [Abstract]
Frerigmann H & Gigolashvili T (2014) MYB34, MYB51 and MYB122 Distinctly Regulate Indolic Glucosinolate Biosynthesis in Arabidopsis thaliana. Molecular plant 10.1093/mp/ssu004. [Abstract]