Ishikawa Masaya, Ide Hiroyuki, Yamazaki Hideyuki, Murakawa Hiroki, Kuchitsu Kazuyuki, Price William S, Arata Yoji
Division of Plant Sciences, National Institute of Agrobiological Sciences, Kan'nondai 2-1-2, Tsukuba, Ibaraki, 305-8602, Japan.
Imaging Frontier Center, Tokyo University of Science, Yamazaki 2641, Noda, Chiba, 278-8510, Japan.
Plant Cell Environ. 2016 Dec;39(12):2663-2675. doi: 10.1111/pce.12813. Epub 2016 Oct 13.
How plant tissues control their water behaviours (phase and movement) under subfreezing temperatures through adaptative strategies (freezing behaviours) is important for their survival. However, the fine details of freezing behaviours in complex organs and their regulation mechanisms are poorly understood, and non-invasive visualization/analysis is required. The localization/density of unfrozen water in wintering Cornus florida flower buds at subfreezing temperatures was visualized with high-resolution magnetic resonance imaging (MRI). This allowed tissue-specific freezing behaviours to be determined. MRI images revealed that individual anthers and ovules remained stably supercooled to -14 to -21 °C or lower. The signal from other floral tissues decreased during cooling to -7 °C, which likely indicates their extracellular freezing. Microscopic observation and differential thermal analyses revealed that the abrupt breakdown of supercooled individual ovules and anthers resulted in their all-or-nothing type of injuries. The distribution of ice nucleation activity in flower buds determined using a test tube-based assay corroborated which tissues primarily froze. MRI is a powerful tool for non-invasively visualizing unfrozen tissues. Freezing events and/or dehydration events can be located by digital comparison of MRI images acquired at different temperatures. Only anthers and ovules preferentially remaining unfrozen are a novel freezing behaviour in flower buds. Physicochemical and biological mechanisms/implications are discussed.
植物组织如何通过适应性策略（冷冻行为）在亚冰点温度下控制其水分行为（相态和运动）对其生存至关重要。然而，复杂器官中冷冻行为的精细细节及其调控机制尚不清楚，需要进行非侵入性可视化/分析。利用高分辨率磁共振成像（MRI）对处于亚冰点温度的越冬佛罗里达山茱萸花芽中未冻水的定位/密度进行了可视化。这使得能够确定组织特异性的冷冻行为。MRI图像显示，单个花药和胚珠保持稳定过冷状态至-14至-21°C或更低。在冷却至-7°C期间，其他花组织的信号下降，这可能表明它们发生了细胞外冷冻。显微镜观察和差示热分析表明，过冷的单个胚珠和花药突然破裂导致其全有或全无类型的损伤。使用基于试管的测定法确定的花芽中冰核活性的分布证实了哪些组织主要发生冷冻。MRI是一种用于非侵入性可视化未冻组织的强大工具。通过对在不同温度下采集的MRI图像进行数字比较，可以定位冷冻事件和/或脱水事件。只有花药和胚珠优先保持未冻状态是花芽中的一种新的冷冻行为。本文讨论了其物理化学和生物学机制/意义。
