Susan Dunford, University of Cincinnati

The use of mutants and transgenic plants enables us to ask a new set of questions about allocation. For example, what happens when one of the competing processes, sucrose or starch synthesis, is inhibited or enhanced? Starch-deficient tobacco mutants synthesize only trace amounts of starch but are able to compensate for a lack of stored carbon by doubling the rate of sucrose synthesis and export during the day and by switching most of their growth to the day (Geiger et al. 1995). Mutants with low FBPase or plants with antisense repression of FPBase expression generally have an increased starch accumulation, sometimes accompanied by a decreased sucrose content. However, some species (and sometimes the same species!) maintain the wild-type sucrose content. It is thought that some other enzyme activity compensates for the low FBPase activity in these plants. Photosynthesis and growth are often impaired in the low FBPase plants (Serrato et al. 2009).

The activity of the phosphate translocator, which exchanges triose phosphate from the chloroplast for P_i from the cytoplasm during the day, also affects the balance between starch and sucrose synthesis. Potato plants accumulate four times more starch during the daytime when the phosphate translocator activity (see textbook Figure 10.22) is decreased 30% due to antisense repression of its RNA. Under these conditions, fixed carbon cannot exit the chloroplast as well as in the control, and more starch is synthesized. These plants respond to increased starch accumulation by exporting more of their fixed carbon during the night, when different transporters are operating between the chloroplast and the cytoplasm (see textbook Figure 8.14).

Like the potato plants just described, tobacco plants with an 80% reduction of phosphate-translocator activity also accumulate more starch in the chloroplasts and utilize the nighttime translocator to transport fixed carbon out of the chloroplast. However, they use the starch reserves to synthesize translocated sugar during the day, rather than during the night. Plants in which the phosphate translocator activity is inhibited generally show no obvious changes in growth. They simply bypass the phosphate translocator and utilize alternate means to move carbon out of the chloroplast. These and other results reveal the amazing flexibility of plants (Flugge et al. 2003).