Intercompartmental streaming in Aspergillus

Abstract

Filamentous fungi colonize dead or living substrates by means of a network of hyphae called a mycelium. They secrete enzymes to extracellularly degrade organic polymers. The degradation products are taken up by the fungus to serve as nutrients. Hyphae grow at their apices and branch sub-apically. They are compartmentalized by septa that contain a central pore. This pore allows for inter-compartmental streaming of nutrients and even organelles. Therefore, the cytoplasm within the mycelium is generally regarded as continuous. I showed that cytoplasmic streaming of the reporter protein GFP occurred in both directions at a rate of 15 µm s-1. The rate decreased when the reporter was fused to the cytosolic protein GPD. Probably, GPD is part of a protein complex that includes membrane proteins. Membrane docking and release could explain the decreased velocity. Woronin bodies plug septa when hyphae are mechanically injured. This prevents cytoplasmic bleeding. Woronin bodies are specialized peroxisomes that contain a core of the Hex1 protein. In my Thesis I show that about 35% of the septa are already plugged in intact growing hyphae of Aspergillus. Plugging was shown to be a reversible process. A strain in which the hex1 gene was deleted could no longer plug septa. Interestingly, when Aspergillus niger was subjected to low or high temperature or to hypertonic conditions 75-90% of the septa were plugged. Septal plugging after cold shock was shown to be reversible. Previously, it was shown that the glucoamylase (glaA) gene is heterogeneously expressed in the A. niger mycelium. In this Thesis it is shown that glaA is no longer heterogeneously expressed in a hex1 deletion strain. The loss of heterogeneity can be explained by uninterrupted cytoplasmic streaming of RNA and or protein. Taken together, it is shown for the first time that the cytoplasm of a vegetative mycelium of Aspergillus is not a continuous system.