Cell wall permeability and its relationship to spore release in Achlya intricata

Abstract

Dynamically, the process of sporangial emptying in the zoosporic fungus Achlya intricata best fits a model based on the regulated dissipation of an osmotically generated pressure potential. Two testable predictions, derived from this hypothetical discharge mechanism, are (a) that the sporangial cell wall must be capable of maintaining a concentration gradient of some solute to depress the luminal osmotic potential, since the mature sporangium lacks a plasmalemma; and (b) that the emptying process should be inhibited by water stress. The permeability of the sporangial wall was estimated by monitoring the uptake of polyethylene glycol (PEG) molecules of different molecular weights. An aqueous solution of PEG-2000 contained molecules of the critical hydrodynamic volume, corresponding to a maximum pore size of ∼2.0 nm in the sporangial wall. Complete control (and inhibition above a critical solute concentration) over the emptying process was gained using polymers with M r 's in excess of 2000. The pressure potential within the sporangium was estimated, during discharge, to be ∼ + 10 −3 MPa. This value of pressure potential is consistent with that predicted from a mathematical model previously published.