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

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 μm2); 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.