Electron tomography of RabA4b- and PI-4Kβ1-labeled trans Golgi network compartments in Arabidopsis

The trans Golgi network (TGN) of plant cells sorts and packages Golgi products into secretory (SV) and clathrin-coated (CCV) vesicles. We have analyzed of TGN cisternae in Arabidopsis root meristem cells by cell fractionation and electron microscopy/tomography to establish reliable criteria for identifying TGN cisternae in plant cells, and to define their functional attributes. Transformation of a trans Golgi cisterna into a Golgi-associated TGN cisterna begins with cisternal peeling, the formation of SV buds outside the plane of the cisterna and a 30-35% reduction in cisternal membrane area. Free TGN compartments are defined as cisternae that have detached from the Golgi to become independent organelles. Golgi-associated and free TGN compartments, but not trans Golgi cisternae, bind anti-RabA4b and anti-phosphatidylinositol-4 kinase (PI-4K) antibodies. RabA4b and PI-4Kβ1 localize to budding SVs in the TGN and to SVs en route to the cell surface. SV and CCV release occurs simultaneously via cisternal fragmentation, which typically yields ∼30 vesicles and one to four residual cisternal fragments. Early endosomal markers, VHA-a1-green fluorescent protein (GFP) and SYP61-cyan fluorescent protein (CFP), colocalized with RabA4b in TGN cisternae, suggesting that the secretory and endocytic pathways converge at the TGN. pi4k1/pi4k2 knockout mutant plants produce SVs with highly variable sizes indicating that PI-4Kβ1/2 regulates SV size.