Plasma membrane ultrastructural changes caused by mechanical stress in the formation of extracellular ice as a primary cause of slow freezing injury in fruit-bodies of Basidiomycetes ( Lyophyllum ulmarium (Fr.) Kühner)

Abstract

By slow freezing, hyphae, which constitute the fruit-body cap of Lyophyllum ulmarium (Fr.) Kühner, were severely deformed by the formation of extracellular ice. The plasma membrane of hyphae showed distinct intramembrane particle aggregation only in the local regions where the inner surfaces of the plasma membrane in the same hyphae were brought into direct contact as a result of the cellular deformation resulting from the formation of extracellular ice. However, no intramembrane particle aggregation occurred in the plasma membrane regions without such a direct contact. Osmotic manipulation of hyphae without freezing caused neither distinct cellular deformation nor intramembrane particle aggregation in plasma membrane. The percentage of hyphae showing intramembrane particle aggregation in the plasma membrane of slowly frozen samples showed a close relation with the percentage damage as measured by the ion-leakage method after thawing. From these results, it was suggested that the primary cause of slow freezing injury in hyphae was due to mechanical stress by the formation of extracellular ice, which brought about lethal plasma membrane changes.