Stessman Dan, Miller Adam, Spalding Martin, Rodermel Steven
Department of Botany and Interdepartmental Plant Physiology Major, Iowa State University, 353 Bessey Hall, Ames, IA, 50011, USA,
Photosynth Res. 2002;72(1):27-37. doi: 10.1023/A:1016043003839.
Previous investigations in our laboratory have shown that leaf developmental programming in tobacco is regulated by source strength. One hypothesis to explain how source strength is perceived is that hexokinase acts as a sensor of carbohydrate flux to regulate the expression of photosynthetic genes, possibly as a result of sucrose cycling through acid invertase and hexokinase. We have turned to Arabidopsis as a model system to study leaf development and have examined various photosynthetic parameters during the ontogeny of a single leaf on the Arabidopsis rosette grown in continuous light. We found that photosynthetic rates, photosynthetic gene expression, pigment contents and total protein amounts attain peak levels early in the expansion phase of development, then decline progressively as development proceeds. In contrast, the flux of (14)CO(2) into hexoses increases modestly until full expansion is attained, then falls in the fully expanded leaf. Partitioning of carbon into hexoses versus sucrose increases until full expansion is attained, then falls. The in vitro activities of hexokinase, vacuolar acid invertase, and cell wall acid invertase do not change until the late stages of senescence, when they increase markedly. At this time there are also dramatic increases in hexose pool sizes and in senescence-associated gene (SAG) expression. Taken together, our results suggest that invertase and hexokinase activities do not control the partitioning of label into hexoses during development. We conclude that our data are not readily compatible with a simple model of leaf development, whereby alterations in photosynthetic rates are mediated directly by hexose flux or by hexose pool sizes. Yet, these factors might contribute to the control of gene expression.
我们实验室之前的研究表明，烟草叶片的发育程序受源强调控。一种解释源强如何被感知的假说是，己糖激酶作为碳水化合物通量的传感器来调节光合基因的表达，这可能是蔗糖通过酸性转化酶和己糖激酶循环的结果。我们选用拟南芥作为研究叶片发育的模式系统，并在持续光照下生长的拟南芥莲座叶单叶个体发育过程中检测了各种光合参数。我们发现，光合速率、光合基因表达、色素含量和总蛋白量在发育的扩展阶段早期达到峰值水平，然后随着发育进程逐渐下降。相比之下，(14)CO(2)进入己糖的通量在达到完全扩展之前适度增加，然后在完全展开的叶片中下降。碳分配到己糖与蔗糖的比例在达到完全扩展之前增加，然后下降。己糖激酶、液泡酸性转化酶和细胞壁酸性转化酶的体外活性直到衰老后期才发生变化，此时它们显著增加。此时己糖库大小和衰老相关基因（SAG）表达也有显著增加。综合来看，我们的结果表明，在发育过程中，转化酶和己糖激酶活性并不控制标记物向己糖的分配。我们得出结论，我们的数据与简单的叶片发育模型不太相符，在该模型中，光合速率的改变直接由己糖通量或己糖库大小介导。然而，这些因素可能有助于基因表达的调控。
