Bridging the Gap: Berberine Bridge Enzyme-like Proteins in Arabidopsis thaliana

In Arabidopsis thaliana 27 genes are encoding berberine bridge enzyme-like (AtBBE-like) proteins that belong to the flavoprotein superfamily. BBE-like enzymes are present in plants, fungi and bacteria. AtBBE-like enzymes play a role in stress-induced plant responses and developmental processes, especially in the breakdown of cell wall-derived molecules, such as oligogalacturonides and cellodextrins. We focus on AtBBE-like subgroup six, wich consist of five members: AtBBE-like 13, 15, 24, 25 and 26. Biochemical characterization of two members of AtBBE-like subgroup six revealed monolignols as adequate substrates. We therefore hypothesize a role of AtBBE-like enzymes from subgroup six in the regulation of the extracellular phenolics pool.
We initiated a multidimensional approach in order to reveal spatiotemporal expression patterns of AtBBE-like enzymes from subgroup six employing transcriptional reporter lines, translational reporter lines and loss-of-function lines. For all genes of AtBBE-like subgroup six, we found expression in roots showing very distinct expression patterns of the GUS reporter. Additionally, AtBBE-like 15, 24 and 26 show expression of the reporter in developing flowers. Exogenous auxin treatment led to a drastic change of the expression pattern in AtBBE-like 13 in seedling roots. Up to now, no loss-of-function line exhibited an obvious phenotype, pointing towards a very specific function.
Further phenotypic characterization of loss-of-function lines for the selected AtBBE-like, employment of fluorescent reporter lines for subcellular localization as well as metabolomics of mutant lines will widen our understanding of this deeply interesting group of flavoenzymes.