Yamazaki Tomokazu, Kawamura Yukio, Minami Anzu, Uemura Matsuo
The 21st Century Center of Excellence Program, Iwate University, Morioka, Iwate 020-8550, Japan.
Plant Cell. 2008 Dec;20(12):3389-404. doi: 10.1105/tpc.108.062679. Epub 2008 Dec 16.
Plant freezing tolerance involves the prevention of lethal freeze-induced damage to the plasma membrane. We hypothesized that plant freezing tolerance involves membrane resealing, which, in animal cells, is accomplished by calcium-dependent exocytosis following mechanical disruption of the plasma membrane. In Arabidopsis thaliana protoplasts, extracellular calcium enhanced not only freezing tolerance but also tolerance to electroporation, which typically punctures the plasma membrane. However, calcium did not enhance survival when protoplasts were exposed to osmotic stress that mimicked freeze-induced dehydration. Calcium-dependent freezing tolerance was also detected with leaf sections in which ice crystals intruded into tissues. Interestingly, calcium-dependent freezing tolerance was inhibited by extracellular addition of an antibody against the cytosolic region of SYT1, a homolog of synaptotagmin known to be a calcium sensor that initiates exocytosis. This inhibition indicates that the puncture allowing the antibody to flow into the cytoplasm occurs during freeze/thawing. Thus, we propose that calcium-dependent freezing tolerance results from resealing of the punctured site. Protoplasts or leaf sections isolated from Arabidopsis SYT1-RNA interference (RNAi) plants lost calcium-dependent freezing tolerance, and intact SYT1-RNAi plants had lower freezing tolerance than control plants. Taken together, these findings suggest that calcium-dependent freezing tolerance results from membrane resealing and that this mechanism involves SYT1 function.
植物的抗冻性涉及防止冻融诱导的对质膜的致命损伤。我们推测植物的抗冻性涉及膜修复，在动物细胞中，这是通过质膜机械破坏后依赖钙的胞吐作用来完成的。在拟南芥原生质体中，细胞外钙不仅增强了抗冻性，还增强了对电穿孔的耐受性，电穿孔通常会刺穿质膜。然而，当原生质体暴露于模拟冻融诱导脱水的渗透胁迫时，钙并没有提高其存活率。在冰晶侵入组织的叶片切片中也检测到了依赖钙的抗冻性。有趣的是，通过在细胞外添加针对SYT1胞质区域的抗体，依赖钙的抗冻性受到了抑制，SYT1是突触结合蛋白的同源物，已知是一种启动胞吐作用的钙传感器。这种抑制表明，在冻融过程中发生了使抗体流入细胞质的穿孔。因此，我们提出依赖钙的抗冻性是由穿孔部位的修复导致的。从拟南芥SYT1-RNA干扰（RNAi）植物中分离的原生质体或叶片切片失去了依赖钙的抗冻性，完整的SYT1-RNAi植物的抗冻性低于对照植物。综上所述，这些发现表明依赖钙的抗冻性是由膜修复导致的，并且这种机制涉及SYT1的功能。
