The role of clathrin in exocytosis and the mutual regulation of endo- and exocytosis in plant cells

Within the plant endomembrane system, the vesicle coat protein clathrin localizes to the plasma membrane (PM) and thetrans-Golgi Network/Early Endosome (TGN/EE). While the role of clathrin as a major component of endocytosis at the PM is well established, its function at TGN/EE, possibly in exocytosis or the vacuolar pathway, is a matter of debate. This shared function of clathrin also opens a question whether plant cells possess a homeostatic mechanisms that balance rates of opposite trafficking routes, such as endo- and exocytosis. Here we address these questions using lines inducibly silencingCLATHRIN HEAVY CHAIN(CHC). We find a relocation of exocytic soluble and integral membrane protein cargoes to the vacuole, supporting a function of clathrin in exocytosis. A comparison with lines overexpressing AUXILIN-LIKE1, where inhibition of CME precedes rerouting of secretory cargoes, does not support a homeostatic regulatory mechanism adjusting exocytosis to the rates of endocytosis. Complementary experiments reveal only minor and variably detectable reductions in the rates of CME in secretory mutants, also not indicative of a converse homeostatic mechanism adjusting rates of endocytosis to the rates of secretion.