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

C4 plants have evolved >60 times from their C3 ancestors. C4 photosynthesis requires a set of closely co-ordinated anatomical and biochemical characteristics. However, it is now recognized that the evolution of C4 plants requires fewer changes than had ever been considered, because of the genetic, biochemical and anatomical pre-conditions of C3 ancestors that were recruited into C4 photosynthesis. Therefore, the pre-conditions in C3 plants are now being actively investigated to clarify the evolutionary trajectory from C3 to C4 plants and to engineer C4 traits efficiently into C3 crops. In the present mini review, the anatomical characteristics of C3 and C4 plants are briefly reviewed and the importance of the bundle sheath for the evolution of C4 photosynthesis is described. For example, while the bundle sheath of C3 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 C3 plants is therefore also implicated as a possible pre-condition for the evolution of C4 photosynthesis. The phylogenetic relationships between the types of storage carbohydrates and of photosynthesis need to be clarified in the future.