ap Rees T, Burrell M M, Entwistle T G, Hammond J B, Kirk D, Kruger N J
Botany School, University of Cambridge, UK.
Symp Soc Exp Biol. 1988;42:377-93.
The effects of lowering the temperature from 25 degrees C to 2-8 degrees C on carbohydrate metabolism by plant cells are considered. Particular emphasis is placed on the mechanism of cold-induced sweetening in tubers of potato (Solanum tuberosum). Temperatures between 0 and 10 degrees C were shown to cause a marked reduction in the rate of respiration of a wide range of plant tissues. At these temperatures the ability of suspension cultures of soybean (Glycine max), and callus cultures and tubers of potato to metabolize [14C]glucose was appreciably diminished. The detailed distribution of 14C showed that lowering the temperature decreased the proportion of the metabolized [14C]glucose that entered the respiratory pathways and increased the proportion converted to sucrose. Pulse and chase experiments, in which [14C]glucose was supplied to potato tubers at 2 and 25 degrees C, showed that lowering the temperature led to accumulation of label in hexose 6-phosphates, which were subsequently converted to sucrose. The patterns of 14CO2 production from specifically labelled [14C]glucose supplied to soybean suspension cultures and disks of potato tuber suggested that lowering the temperature reduced the activity of glycolysis more than that of the oxidative pentose phosphate pathway. It is argued that the above experiments demonstrate that lowering the temperature not only reduces the rate of carbohydrate metabolism but also alters the relative activities of the different pathways involved. A disproportionate reduction in glycolysis at the lower temperatures is suggested. Mature tubers of many varieties of potato accumulate sucrose and hexose when stored between 2 and 10 degrees C. Starch is the source of carbon for this synthesis of sugar. We could not detect cytosolic fructose-1,6-bisphosphatase in potato tubers and suggest that carbon for sugar synthesis in the cold leaves the amyloplast, not as triose phosphate, but probably as a six-carbon compound. Evidence is presented that phosphofructokinase (EC 2.7.1.11) plays a major role in regulating the entry of hexose 6-phosphates into glycolysis in potato tubers. Phosphofructokinase was purified from potato tubers and shown to consist of four forms. Three of these forms were shown to have higher Q10 values over the range 2-6 degrees C than over the range 12-16 degrees C and are regarded as being cold-labile. No such cold-lability was detected for the key enzymes involved in sucrose synthesis and the oxidative pentose phosphate pathway.(ABSTRACT TRUNCATED AT 250 WORDS)
本文考虑了将温度从25摄氏度降至2 - 8摄氏度对植物细胞碳水化合物代谢的影响。特别强调了马铃薯（Solanum tuberosum）块茎冷诱导变甜的机制。已表明0至10摄氏度之间的温度会导致多种植物组织的呼吸速率显著降低。在这些温度下，大豆（Glycine max）悬浮培养物、马铃薯愈伤组织培养物和块茎代谢[14C]葡萄糖的能力明显下降。14C的详细分布表明，降低温度会减少进入呼吸途径的代谢[14C]葡萄糖的比例，并增加转化为蔗糖的比例。脉冲追踪实验表明，在2摄氏度和25摄氏度下将[14C]葡萄糖供应给马铃薯块茎，降低温度会导致己糖6 - 磷酸中标记物的积累，随后这些己糖6 - 磷酸会转化为蔗糖。向大豆悬浮培养物和马铃薯块茎切片供应特定标记的[14C]葡萄糖后产生14CO2的模式表明，降低温度对糖酵解活性的降低幅度大于对氧化戊糖磷酸途径的降低幅度。有人认为，上述实验表明降低温度不仅会降低碳水化合物代谢速率，还会改变所涉及的不同途径的相对活性。建议在较低温度下糖酵解会有不成比例的降低。许多品种的成熟马铃薯块茎在2至10摄氏度储存时会积累蔗糖和己糖。淀粉是这种糖合成的碳源。我们在马铃薯块茎中未检测到胞质果糖 - 1,6 - 二磷酸酶，并认为低温下用于糖合成的碳离开造粉体时，不是以磷酸丙糖的形式，而是可能以六碳化合物的形式。有证据表明磷酸果糖激酶（EC 2.7.1.11）在调节马铃薯块茎中己糖6 - 磷酸进入糖酵解过程中起主要作用。从马铃薯块茎中纯化出磷酸果糖激酶，并表明它由四种形式组成。其中三种形式在2至6摄氏度范围内的Q10值高于12至16摄氏度范围内的Q10值，被认为是冷不稳定的。参与蔗糖合成和氧化戊糖磷酸途径的关键酶未检测到这种冷不稳定性。（摘要截取自250字）
