Regulators of fungal asexual development FlbE and FlbD orchestrate in vegetative hyphae the long-distance tip-to-nucleus dynamics of the transcription factor FlbB

Asexual spores are the main vehicle for dissemination of filamentous fungi. In the model filamentous fungus Aspergillus nidulans asexual sporulation (conidiation) is induced by a transduction mechanism that connects signals received at the tip of hyphae with the transcriptional activation of brlA, the first conidiation-specific gene. Here we describe that the tip-to-nucleus dynamics of the transcription factor FlbB is controlled in space and time by the interactions with developmental regulators FlbE and FlbD. The former enables the apical localization of FlbB, and the role of its functional domains is characterized. Expression of an FlbE::FlbB::GFP chimera enables apical localization but inhibits nuclear accumulation, blocking conidiation. The insertion of a T2A viral peptide between FlbE and FlbB, which causes the split of a single mRNA (flbE::mrfp::t2A::flbB::gfp) into two proteins (FlbE::mRFP::T2A and FlbB::GFP), partially restores the nuclear accumulation of FlbB and conidia production. This suggests that, once at the tip, FlbB/FlbE interaction is apparently inhibited to initiate a basipetal transport to nuclei. We have also analyzed the dependence of this movement on microtubules, dynein and importin-α KapA. The nuclear levels of FlbB decrease with those of FlbD, but increase in the absence of the repressor of asexual development, NsdD. Overall, results show a dynamic interaction pattern of FlbB with other regulators of development in order to control the induction of conidiation