Extended darkness induces internal turnover of glucosinolates in Arabidopsis thaliana leaves

Prolonged darkness leads to carbohydrate starvation, and as a consequence plantsdegrade proteins and lipids to oxidize amino acids and fatty acids as alternative substratesfor mitochondrial ATP production. We investigated, whether the internal breakdown of glucosinolates,a major class of sulfur-containing secondary metabolites, might be an additionalcomponent of the carbohydrate starvation response in Arabidopsis thaliana (A.thaliana). The glucosinolate content of A. thaliana leaves was strongly reduced after sevendays of darkness. We also detected a significant increase in the activity of myrosinase, theenzyme catalyzing the initial step in glucosinolate breakdown, coinciding with a stronginduction of the main leaf myrosinase isoforms TGG1 and TGG2. In addition, nitrilase activitywas increased suggesting a turnover via nitriles and carboxylic acids. Internal degradationof glucosinolates might also be involved in diurnal or developmental adaptations of theglucosinolate profile. We observed a diurnal rhythm for myrosinase activity in two-week-oldplants. Furthermore, leaf myrosinase activity and protein abundance of TGG2 varied duringplant development, whereas leaf protein abundance of TGG1 remained stable indicatingregulation at the transcriptional as well as post-translational level.