Regulation of photosynthesis during Arabidopsis leaf development in continuous light

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 (CO2)-C-14 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.