Sucrose biosynthesis in C-4 plants

Sucrose has a central role in the metabolism of all higher plants. Here we discuss the location of sucrose synthesis in the leaves of C-4 plants, the control of flux through the pathway and its response to environmental condi tions, and finally some of the prospects for the genetic manipulation of su crose metabolism in C-4 plants, Much of our knowledge about sucrose synthes is in plants comes from C-3 species; it is evident, from those studies of C -4 species that are available, that there are many similarities between suc rose synthesis in C-3 and C-4 plants. However, some of the basic regulatory mechanisms common to both C-3 and C-4 plants seem to have been adapted for the specialized photosynthetic metabolism of C-4 plants. There are also so me other important differences: for example, some C-4 species possess photo synthetic cells with little or no capacity for sucrose synthesis. For these reasons it is not always appropriate to extrapolate directly from C-3 to C -4 plants. Even where data are available from one C-4 species, usually maiz e, it should not be assumed that all C-4 plants are necessarily the same in every respect. Although C-4 plants constitute a small fraction of the plan t kingdom as a whole, they include a disproportionate number of agricultura lly important species. C-4 plants also have some advantages over C-3 plants for studies of the regulation of cell-specific gene expression and the con trol of metabolism. For these reasons we suggest that they are worthy of mo re attention by researchers.