AoSO protein accumulates at the septal pore in response to various stresses in the filamentous fungus Aspergillus oryzae.

Filamentous ascomycetes form hyphal networks that are compartmentalized by septa which have a perforated pore allowing the passage of cytoplasm and organelles between adjacent hyphal compartments. Thus, the septal pore may play an important role in the organized growth of multicellular organisms. Upon hyphal injury, the septal pore is plugged by a wound-healing organelle, known as the Woronin body, to prevent excessive cytoplasmic leakage. However, the movement of proteins towards the septal pore in response to stress has not been extensively studied in filamentous fungi. In this study, we identified an Aspergillus oryzae protein, AoSO, which is homologous to the Neurospora crassa SO protein that was reported to accumulate at the septal pore in aging hyphae. The DeltaAoso strain showed excessive cytoplasmic leakage upon hyphal injury similar to the Woronin body-deficient strain DeltaAohex1. Cellular localization studies using EGFP showed that AoSO accumulated at the septal pore adjacent to the injured compartment, while it was dispersed throughout the cytoplasm under normal growth conditions. These results indicate that AoSO plays a role in preventing excessive cytoplasmic leakage upon hyphal injury by accumulating at the septal pore. Furthermore, AoSO accumulated at the septal pore in response to various stresses, including low and high temperature, extreme acidic and alkaline pH, and nitrogen and carbon depletion. Physical stress induced by pulse laser treatment on a hyphal region at a distance from the septum caused accumulation of the AoSO protein at the septal pore within only a few minutes. This study presents a novel behavior in which a filamentous fungal protein relocalizes to the septal pore in response to various stresses.