FORMATION OF MULTIPLEX LAMELLAE BY EQUILIBRIUM SLOW FREEZING OF CORTICAL PARENCHYMA CELLS OF MULBERRY AND ITS POSSIBLE RELATIONSHIP TO FREEZING TOLERANCE

摘要

Cortical parenchyma cells of mulberry (Morus bombycis Koidz. cv. Goroji) become extremely cold hardy in winter and can tolerate equilibrium freezing below -30 degrees C and subsequent immersion into liquid nitrogen. We show in this ultrastructural study that, in these extremely cold hardy cortical parenchyma cells of mulberry collected in winter, initiation of freezing at -5 degrees C resulted in the formation of multiplex lamellae (MPL) that completely covered the area beneath the plasma membrane. The MPL were produced by fusion of pre-existing vesicular endoplasmic reticulum (ER), via a reticular ER network. The completed MPL were composed of a parallel array of sheet-like ER cisternae. This structural reorganization of the ER was completed within 10 min upon freezing at -5 degrees C and was quickly reversed upon thawing. The same structural reorganization of the ER was produced by osmotic dehydration of the cortical tissues with a 2.7 osmol sorbitol solution at 20 degrees C. Thus; the structural reorganization of the ER upon freezing was, in fact, produced by dehydration. In winter samples, the formation of MPL with the initiation of freezing completely inhibited close apposition of membranes upon deep freezing that has been reported to be a cause of freezing injury via the production of ultrastructural changes in the plasma membrane. Similar but more or less incomplete MPL were produced by freezing or osmotic dehydration in cortical parenchyma cells collected in spring and autumn. and these MPL partly inhibited close apposition of membranes. MPL were not produced in the cells of mulberry collected in summer and close apposition of membranes occurred upon deep freezing. We speculate that the formation of MPL with the initiation of freezing might play a specific role in inhibiting the close apposition of membranes due to the specific nature of the cisternal membranes and might, consequently, be responsible for the high freezing tolerance of winter cells. [References: 51]