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

目前的研究阐明了细胞壁特性在木质部射线薄壁细胞适应零下温度中的重要作用。首先，我们通过冷冻扫描电子显微镜观察了针叶树种木射线薄壁细胞的冻结行为，表明细胞壁的宏观结构差异对冻结行为有明显的影响。结果表明，未木质化的外观较软的薄细胞壁的木射线薄壁细胞通过细胞外冷冻反应，而木质化的硬质细胞壁的木射线薄壁细胞(柳杉、兴安落叶松)则通过深过冷反应。其次，我们发现细胞壁的显微结构差异也影响到木射线薄壁细胞的冷冻行为，这些差异是电子显微镜观察不到的，细胞壁厚，外观坚硬。例如，白桦木质部射线薄壁细胞在夏季以过冷反应，而在冬季以胞外冻结反应，尽管电子显微镜观察没有观察到细胞壁结构的季节性变化。为了阐明细胞壁性质对冻结行为的季节性变化的机制，我们检测了细胞壁结合蛋白的季节性变化，发现低温诱导的质外体蛋白LIAP29、28和23明显积累。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.