A lipid droplet - plasma membrane contact site is conserved across the angiosperm lineage

In Arabidopsis thaliana, a tethering complex of three proteins, SEED LIPID DROPLET PROTEIN 1 and 2 (SLDP1 and 2) and LIPID DROPLET PLASMA MEMBRANE ADAPTOR (LIPA), binds lipid droplets (LDs) to the plasma membrane (PM). While the physiological function remains unknown, it seems to be conserved across seed plants, as we here observed this tethering of LDs in seedlings of a variety of angiosperms. The analysis of LIPA and SLDP homologs indicated that these proteins emerged in the early period of land plant evolution before seed plants became dominant. LIPA and SLDP are, however, more strongly conserved among seed plants and share distinct amino acid motifs that could be involved in their interaction. We cloned LIPA and SLDP homologs from Arachis hypogaea, Glycine max and Zea mays and transiently expressed them in tobacco pollen tubes, a suitable model system. Here, the LIPAs and SLDPs localised to the PM and LDs, respectively, like their A. thaliana homologs. When coexpressed in pollen tubes, where LDs normally move freely in the cytosol, the LIPAs and SLDPs can tether LDs to the PM indicating that they can interact and constitute a LD-PM contact site. This interaction was even functional across taxon boundaries.