Starch Accumulation in the Bundle Sheaths of C3 Plants: A Possible Pre-Condition for C4 Photosynthesis

C-4 plants have evolved > 60 times from their C-3 ancestors. C-4 photosynthesis requires a set of closely co-ordinated anatomical and biochemical characteristics. However, it is now recognized that the evolution of C-4 plants requires fewer changes than had ever been considered, because of the genetic, biochemical and anatomical pre-conditions of C-3 ancestors that were recruited into C-4 photosynthesis. Therefore, the pre-conditions in C-3 plants are now being actively investigated to clarify the evolutionary trajectory from C-3 to C-4 plants and to engineer C-4 traits efficiently into C-3 crops. In the present mini review, the anatomical characteristics of C-3 and C-4 plants are briefly reviewed and the importance of the bundle sheath for the evolution of C-4 photosynthesis is described. For example, while the bundle sheath of C-3 rice plants accumulates large amounts of starch in the developing leaf blade and at the lamina joint of the mature leaf, the starch sheath function is also observed during leaf development in starch accumulator grasses regardless of photosynthetic type. The starch sheath function of C-3 plants is therefore also implicated as a possible pre-condition for the evolution of C-4 photosynthesis. The phylogenetic relationships between the types of storage carbohydrates and of photosynthesis need to be clarified in the future.