Abstract
Many plants living under subzero temperatures in winter increase freezing tolerance by exposure to non-freezing temperature, which is known as cold acclimation. In cold-acclimated cells, unique cryobehaviors of the plasma membrane and endoplasmic reticulum (ER) have been reported but their physiological meaning or mechanism is largely unknown. Allium fistulosumis a cold-hardy Welshonion which survives winter of-40℃ in Saskatchewan, Canada, andintact cells inthe single epidermal layer, which is easily peeled from leaf sheath, were observed. The cryobehavior of ER in these epidermal cells that were stained with ER-selective fluorescent dye (ER-Tracker) was observed using a confocal fluorescent microscope with cryostage. According to our observations, cold acclimation increased ER volume per cell and extracellular freezing induced ER vesiculation through the breakdown of the ER network. Freeze-induced ER vesicles in cold-acclimated cells were larger and more abundant than those in non-acclimated cells. ER vesiculation may be associated with extracellular calcium because freeze-induced ER vesicles tended to be more abundant in the presence of calcium than in the absence of calcium. Furthermore, ER vesiculation also occurred in Arabidopsis root cells, suggesting a possibility that ER vesiculationis conserved in monocotyledonous and dicotyledonous plants. After thawing, the ER network was recovered only in cold-acclimated cells, suggesting that the dynamics of ER during freeze/thaw cycles are associated with freezing tolerance.
