Tissue-specific changes in apoplastic proteins and cell wall structure during cold acclimation of winter wheat crowns

The wheat (Triticum aestivum L.) crown is the critical organ of low temperature stress survival over winter. In cold-acclimated crowns, ice formation in the apoplast causes severe tissue disruption as it grows at the expense of intracellular water. While previous crown studies have shown the vascular transition zone (VTZ) to have a higher freezing sensitivity than the shoot apical meristem (SAM), the mechanism behind the differential freezing response is not fully understood. Cooling cold-acclimated crowns to –10 °C resulted in an absence of VTZ tetrazolium chloride staining, whereas the temperatures at which 50% of the SAM stained positive and 50% of plants recovered (LT50) were similar after cold acclimation for 21 (–16 °C) and 42 d (–20 °C) at 4 °C. Proteomic analysis of the apoplastic fluids identified dehydrins, vernalization-responsive proteins, and cold shock proteins preferentially accumulated in the SAM. In contrast, modifications to the VTZ centered on increases in pathogenesis-related proteins, anti-freeze proteins, and sugar hydrolyzing enzymes. Fourier transform infrared spectroscopy focal plane array analysis identified the biochemical modification of the cell wall to enhance methyl-esterified cross-linking of glucuronoarabinoxylans in the VTZ. These findings indicate that the SAM and VTZ express two distinct tissue-specific apoplastic responses during cold acclimation.

普通小麦（Triticum aestivum L.）的冠部是其越冬低温胁迫存活的关键器官。在冷驯化后的冠部中，质外体（apoplast）内的冰晶形成会以消耗细胞内水分为代价扩张，进而造成严重的组织损伤。过往针对冠部的研究已证实，维管转移区（vascular transition zone, VTZ）的冻害敏感性高于茎尖分生组织（shoot apical meristem, SAM），但二者冻害响应差异背后的调控机制尚未完全阐明。将经冷驯化的冠部降温至-10℃时，维管转移区未出现氯化三苯基四氮唑（tetrazolium chloride）染色反应；而在4℃下分别冷驯化21天（半致死温度LT50为-16℃）与42天（LT50为-20℃）后，茎尖分生组织50%样本染色呈阳性、50%植株存活的半致死温度（LT50）较为相近。针对质外体体液的蛋白质组学分析显示，脱水素（dehydrins）、春化响应蛋白（vernalization-responsive proteins）与冷休克蛋白（cold shock proteins）在茎尖分生组织中优先富集。与之相对，维管转移区的蛋白修饰则以病程相关蛋白（pathogenesis-related proteins）、抗冻蛋白（anti-freeze proteins）与糖水解酶（sugar hydrolyzing enzymes）的表达上调为主要特征。傅里叶变换红外光谱（Fourier transform infrared spectroscopy, FTIR）焦平面阵列（focal plane array, FPA）分析进一步发现，维管转移区的细胞壁发生了生化修饰，增强了葡萄糖醛酸阿拉伯木聚糖（glucuronoarabinoxylans）的甲基酯化交联。上述研究结果表明，茎尖分生组织与维管转移区在冷驯化过程中，分别呈现出两种截然不同的组织特异性质外体响应模式。