Actin cytoskeleton and organelle movement in the sporangiophore of the zygomycete Phycomyces blakesleeanus

Abstract

Growth, photo- and gravitropism of sporangiophores of the zygomycete Phycomyces blakesleeanus occur within the apical growing zone, a cylindrical structure (diameter about 100 μm) that reaches about 1.5-2.5 mm below the tip and has growth rates up to 50 μm·min(-1) . To better understand morphogenesis and growth of the giant aerial hypha, we investigated with confocal microscopy and inhibitors the actin cytoskeleton and by in-vivo particle tracking the associated organelle movement. We found stage-1 sporangiophores (without sporangium) possess an actin cytoskeleton with polar zonation. (i) In the apex, abundant microfilaments without preferential orientation entangled numerous nuclei as well as a conspicious complex of some 200 lipid globules. Microfilament patches (≈ 1.6-μm diameter) are clustered in the tip and were found in the apical cortex, whereas short, curved microfilament bundles (≈ 2.3-μm long) prevailed in the subapex. (ii) In a transition zone downwards to the shaft, the microfilaments rearranged into a dense mat of longitudinal microfilaments that was parallel close to the periphery but more random towards the cell centre. Numerous microfilament patches were found near the cortex (≈ 10/100 μm(2) ); their number decreased rapidly in the subcortex. In contrast, the short, curved microfilament bundles were found only in the subcortex. (iii) The basal shaft segment of the sporangiophore (with central vacuole) exhibited bidirectional particle movement over long distances (velocity ≈ 2 μm·s(-1) ) along massive longitudinal, subcortical microfilament cables. The zonation of the cytoskeleton density correlated well with the local growth rates at the tip of the sporangiophore, and appears thus as a structural prerequisite for growth and bending.

The sporangiophore of Phycomyces blakesleeanus: a tool to investigate fungal gravireception and graviresponses

“…) are in continual non‐Brownian motion; the requisite motor molecules are presently unknown, but the fact that they occur within a dense actin mesh cage indicates that they are powered by an actomyosin system (Grolig et al . ; this volume). Electron microscopy shows that the globules have different degrees of electron density.…”

“…It appears necessary to investigate in greater detail whether the nuclei of Phycomyces , which are embedded in the apical actin network (Grolig et al . ), could similarly contribute to gravitropic bending. Even though physiologists have been aware for a long time that flexure and wall stress both evoke bending responses (see above), these phenomena have not always been considered to be intrinsic parts of gravitropism. Mainstream gravitropism research was largely centred on the statolith–receptor paradigm, and wall stress was viewed at best as an epi‐phenomenon.…”

“…) are encased within a dense mesh of actin filaments that fills the dome‐like structure of the apex (Grolig et al . ; this volume), a property that makes it a good candidate for a tensegrity‐mediated mechanism (Ingber ; Yamaoka et al . ) of gravireception.…”

“…1, left and centre). The CLG is not surrounded by a membrane, but rather represents an aggregate of lipid globules that are loosely held together in a cage made of an actin mesh (Grolig et al 2014;this volume). Sporangiophores lacking the CLG display a greatly reduced gravitropic response (Grolig et al 2004).…”

Plant biology in space

“…Using imaging tools, Grolig et al . () report on the role of actin in organelle movement in the sporangiophore of the zygomycete Phycomyces blakesleeanus .…”