Differences in the enzymatic hydrolysis of glucosinolates increase the defense metabolite diversity in 19 Arabidopsis thaliana accessions

Abstract

Plants of the order Brassicales produce glucosinolates (GS), a group of secondary metabolites that are part of an elaborate defense system. But it is not the GS itself rather its enzymatic hydrolysis products that cause the bioactive effects protecting the plants against pests and pathogens. Thus the enzymatic hydrolysis and a variety of additional influential factors determine the structural outcome of the GS degradation process. To evaluate the possible diversity of defense metabolites a range of 19 Arabidopsis thaliana accessions were selected showing divergence in their geographical origin, in their phenotype, and in their GS profile. These particular accessions accumulate several alkenyl GS, hydroxyalkyl GS, methylthioalkyl GS, and methylsulfinylalkyl GS in their rosette leaves whereas the indole GS contents are relatively invariant, as analyzed by UHPLC-DAD. After tissue disruption the enzymatic formation of GS hydrolysis products was examined and breakdown products were identified and quantified by GC-MS. Great differences in the amount and structure of volatile enzymatic degradation products could be observed in the different accessions, with strong variation in formation of epithionitriles, nitriles, and isothiocyanates. The occurrence of specific GS hydrolysis products was put in relation to relative gene expression profiles of myrosinases and specifier proteins as measured by RT-qPCR, and in relation to relative protein abundance of epithiospecifier protein. Dependent on the different GS profiles and reliant on degradation protein abundance and composition the ecotypes strongly varied in their ability to form isothiocyanates, nitriles and epithionitriles, thus increasing the plants' equipment of defense metabolites.

Keywords: Arabidopsis; Defense metabolite; Epithionitrile; Gene expression; Glucosinolate hydrolysis; Isothiocyanate; Nitrile.