Figure 4.

F-actin and microtubules in growth of b-dependent hyphae. (A) Under natural conditions compatible mating partners provide the two parts of the heterodimeric transcription factor bE/bW that triggers formation of a b-dependent dikaryotic hypha (Banuett, 1995). Strain AB33 contains both b genes under the control of the nar promoter that is induced upon shift to nitrate-containing medium, which results in filamentous growth (Brachmann et al., 2001), control). Similar to the dikaryotic b-dependent hyphae on the plant surface, this induced b-dependent filament grows by tip extension and forms empty sections (control, arrow). (B) In the presence of 5 μM latrunculin A only very few elongated cells were formed (LatA), and most cells remained in their yeast-like stage. (C) In the presence of 10 μM benomyl cells occasionally show multiple budding, but also form filaments. However, these filaments remain shorter, and empty parts were often formed at the tip of hyphae (inset). Bars, 20 μm. (D) Hyphal growth was extremely sensitive to disruption of F-actin by latrunculin A (IC₅₀ « 0.1 μM). In contrast, b-dependent hyphae tolerated low amounts of benomyl, but the number of long hyphae that formed empty sections dramatically decreased at a concentration higher than 1.5 μM. However, most cells were able to form short hyphae in the presence of benomyl, whereas a significant portion grew bipolar or was branched (aberrant).