Cytoplasmic continuity revisited: closure of septa of the filamentous fungus Schizophyllum commune in response to environmental conditions

Abstract

Background: Mycelia of higher fungi consist of interconnected hyphae that are compartmentalized by septa. These septa contain large pores that allow streaming of cytoplasm and even organelles. The cytoplasm of such mycelia is therefore considered to be continuous.

Methodology/principal findings: Here, we show by laser dissection that septa of Schizophyllum commune can be closed depending on the environmental conditions. The most apical septum of growing hyphae was open when this basidiomycete was grown in minimal medium with glucose as a carbon source. In contrast, the second and the third septum were closed in more than 50% and 90% of the cases, respectively. Interestingly, only 24 and 37% of these septa were closed when hyphae were growing in the absence of glucose. Whether a septum was open or closed also depended on physical conditions of the environment or the presence of toxic agents. The first septum closed when hyphae were exposed to high temperature, to hypertonic conditions, or to the antibiotic nourseothricin. In the case of high temperature, septa opened again when the mycelium was placed back to the normal growth temperature.

Conclusions/significance: Taken together, it is concluded that the septal pores of S. commune are dynamic structures that open or close depending on the environmental conditions. Our findings imply that the cytoplasm in the mycelium of a higher fungus is not continuous per se.

Figure 1. Glass bottom culture dish used to assess plugging of septa in S. commune.

In most cases S.commune was grown in a thin layer of agar medium overlaid with minimal medium (A). However, liquid medium was not added when it was assessed whether septa were open or closed in the absence of glucose in the medium. In this case, the culture dish contained distinct patches of minimal medium with or without glucose, which were separated by a gap of 5 mm (B). (C) Magnification of boxed area in (A) and (B) showing a hypha with the first and second septum as referred to in the text.

Figure 2. A septum is open when cytoplasm moves through the septum after laser dissection of a neighbouring compartment.

Compartments before (A, E) and after (B–D, F–H) dissection of a hypha with a closed (A–D) and an open (E–H) septum. Cytoplasmic flow in E–H is indicated by the movement of a vacuole (arrows) through the septum (*). Dots represent the position of the laser.