Secretion and endocytosis at the interface of plants and fungi

Abstract

Secretion products and the uptake of material from the outside of the protoplasts of plants and microbes may influence the mutual interaction in host-parasite systems. Organelles involved in secretion by rust fungi have been characterized using immuno-electron microscopy. Proteins with the C-terminal histidine - aspartic acid - glutamic acid - leucine tetrapeptide (HDEL) signal sequence accumulated in tubular vesicular complexes of the ER in the parasitic phase of the fungi. The tubular complexes were differentiated from smooth cisternae that probably correspond to the Golgi equivalent. In the plant, material was secreted into the papilla and at the same time, endocytosis of 1,3-β-glucans by clathrincoated vesicles occurred at the penetration site of the monokaryotic appressorium. During further development of monokaryotic haustoria, different wall layers were produced around these haustoria. Components of those layers were plant cell wall constituents most of which were also detected in the Golgi apparatus. One layer was rich in callose, which is supposed to be synthesized at the plasma membrane. Obviously, endo- and exo-cytosis occur permanently by host and parasite during fungal development within the plant.