The lipid phase of the thylakoid membrane is mainly composed of the galactolipids mono- and digalactosyl diacylglycerol (MGDG and DGDG, respectively). It has been known since the late 1960s that MGDG can be acylated with a third fatty acid to the galactose head group (acyl-MGDG) in plant leaf homogenates. In certain brassicaceous plants likeArabidopsis thaliana, the acyl-MGDG frequently incorporates oxidized fatty acids in the form of the jasmonic acid precursor 12-oxo-phytodienoic acid (OPDA). In the present study we further investigated the distribution of acylated and OPDA-containing galactolipids in the plant kingdom. While acyl-MGDG was found to be ubiquitous in green tissue of plants ranging from non-vascular plants to angiosperms, OPDA-containing galactolipids were only present in plants from a few genera. A candidate protein responsible for the acyl transfer was identified inAvena sativa(oat) leaf tissue using biochemical fractionation and proteomics. Knockout of the orthologous gene inA. thalianaresulted in an almost total elimination of the ability to form both non-oxidized and OPDA-containing acyl-MGDG. In addition, heterologous expression of theA. thalianagene inE. colidemonstrated that the protein catalyzed acylation of MGDG. We thus demonstrate that a phylogenetically conserved enzyme is responsible for the accumulation of acyl-MGDG inA. thaliana. The activity of this enzymein vivois strongly enhanced by freezing damage and the hypersensitive response.Significance Statement
Galactolipids are the most abundant lipids in the biosphere and essential components of photosynthetic membranes. A subfraction of galactolipids carries an extra acyl group attached to the head group. In this study, we identify a ubiquitous enzyme responsible for acylating the galactolipid head group and show that production of these acylated galactolipids is induced by tissue disruption as well as pathogen elicitation.