Relationship between cell wall structure and freezing behavior of woody cells.

本研究阐明了细胞壁特性在木质部射线薄壁细胞适应低温中的重要作用。首先，通过低温扫描电镜观察软木木质部射线薄壁细胞的冻结行为，发现细胞壁的宏观结构差异对其冻结行为有明显的影响。结果表明，未木质化的薄细胞壁构成的木质部射线薄壁细胞（Sciadopitys verticillata, Pinus densiflora, p.p parifilora）对细胞外冷冻的反应较好，而木质化的厚细胞壁构成的木质部射线薄壁细胞（Cryptomeria japoriica, Larix leptolepis）对深度过冷冷冻的反应较好。其次，我们发现细胞壁的微观结构差异也影响了由厚细胞壁构成的木质部射线薄壁细胞的冻结行为，而电镜观察没有发现这些结构差异。例如，白桦木质部射线薄壁细胞在夏季对过冷反应，而在冬季对胞外冷冻反应，但电镜观察未发现细胞壁结构的季节性变化。为了阐明细胞壁特性在冻结行为下的季节变化机制，我们研究了细胞壁结合蛋白的季节变化，发现低温诱导的外胞体蛋白LIAP29、28和23有明显的积累。我们目前正在继续对这些蛋白质进行生化分析。

Present research clarified important role of cell wall properties in the adaptation of xylem ray parenchyma cells to subzero temperatures. First, we showed that macroscopic structural differences in the cell walls affected distinctly to the freezing behavior by examing freezing behavior of xylem ray parenchyma cells of softwood species with observation by cryo-scanning electron microscopy. It was shown that xylem ray parenchyma cells constructed of unlignified thin cell walls with soft appearance (Sciadopitys verticillata, Pinus densiflora, P.parifilora) responded to freezing by extracellular freezing, while xylem ray parenchyma cells constructed of lignified thick cell walls with rigid appearance (Cryptomeria japoriica, Larix leptolepis) responded to freezing by deep supercooling. Second, we showed that microscpic structural differences in the cell walls, which were not detected by observation with electron microscopy, also affected to the freezing behavior in xylem ray parenchyma cells constructed of thick cell walls with rigid appearance. For example, xylem ray parenchyma cells of birch responded to freezing by supercooling in summer but they responded to freezing by extracellular freezing in winter, although no seasonal alteration of cell wall structures was detected by electron microscope observations. For clarifying the mechanisms of seasonal alteration of cell wall properties upon freezing behavior, we examined seasonal change of the cell wall-binding proteins and found distinct accumulation of Lowtemperature-Induced Apoplastic Proteins LIAP29, 28 and 23. We are currently continuing the biochemical analysis of these proteins.