Cell-to-cell transport via motile tubules in growing hyphae of a fungus.

The system of pleiomorphic, motile tubules and vacuoles in growing hyphal tips of Pisolithus tinctorius has been shown to play a role in intracellular transport. Here we show that the same system also exchanges material between adjacent cells. This exchange is most obvious between terminal and penultimate cells following nuclear division in the tip cell and just before dissolution of the cell wall between the clamp connection and penultimate cell. At this stage the two new dolipore septa are complete. The process was studied in living hyphae using confocal and conventional fluorescence microscopy. Tubules could move in either direction across the septum and often extended and retracted several times and penetrated for some distance (e.g. 40 microns) into the receiving cell. Movements appeared co-ordinated and during the exchange tubules transiently interconnected vacuoles in adjacent cells and by peristaltic movements appeared to transfer material between them. The fluorescent tubules occupied a specific plane in the vicinity of the septum and remained in this plane for the duration of their movement, suggesting that their orientation and direction of movement is controlled. In freeze-substituted hyphae, tubular cisternae of similar dimensions to fluorescent tubules passed through the parenthesome pores perpendicular to the septum and in some cases entered the mouth of the septal pore. This indicates that the septal pore is of an appropriate dimension to accommodate the tubules and that they can cross the septal pore to exchange material between vacuole systems of adjacent cells. This is the first direct demonstration of such intercellular transport via a sub-cellular compartment.