Differential distribution of the endoplasmic reticulum network as visualized by the BipA-EGFP fusion protein in hyphal compartments across the septum of the filamentous fungus, Aspergillus oryzae.

We visualized the endoplasmic reticulum (ER) network by expression of the BipA-EGFP fusion protein in the filamentous fungus, Aspergillus oryzae, and focused on the spatial difference of the ER distribution throughout hyphae. The ER formed an interconnected network with motility and displayed a gradient distribution from the apical region. The ER was also found as a tubular network along the septum, which was formed soon after the completion of septation. Discontinuity of the ER network distribution was noticed between the adjacent compartments across the septum, suggesting that the cellular activities in these compartments were independently regulated although they are considered to communicate with each other through the septal pore. Moreover, the ER-visualized strain was subjected to a hypotonic shock, leading to hyphal tip bursting where the Woronin body plugs septal pores and prevents excessive loss of the cytoplasm. In the compartment adjacent to the burst apical tip, the ER network structure and motility were still retained. We also observed re-growth of hyphae from the plugged septa forming intrahyphal hyphae in which the ER network distribution, specialized for apical growth, was regenerated.