The ultrastructure of Polymyxa betae zoospore exit-tube differentiation

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Abstract
Zoosporangia of Polymyxa betae Keskin are plurilobate and develop cross walls. During ripening they show dark zones at the contact points with the host cell wall at the light microscope level. With the electron microscope these zones appear to correspond to single uninucleate compartments separated by a wall from the zoosporangium itself. These compartments give rise to exit tubes while the zoosporangial contents differentiate into zoospores. At the beginning of exit-tube formation, the thallus wall in contact with the host cell wall becomes thinner and softened. At the same time, thallus cytoplasm shows membrane-bound vesicles, the endoplasmic reticulum increases, and vacuoles enlarge. Vesicles are concentrated in the area of exit-tube formation and this suggests that they are involved both in host and thallus wall lysis. Wall microfibril profiles indicate that at this point mechanical forces have also been applied. A papilla, surrounded by a wall which appears to be continuous with the inner layer of the zoosporangium wall, is formed. The papilla elongates and becomes tubular, forming the zoospore exit tube. The same lytic and mechanical sequence described for thallus and host cell wall demolition at the site of exit-tube emission, also takes place in the demolition of zoosporangial cross walls.