The role of transitory starch in C3, CAM, and C4 metabolism and opportunities for engineering leaf starch accumulation

Essentially all plants store starch in their leaves during the day and break it down the following night. This transitory starch accumulation acts as an overflow mechanism when the sucrose synthesis capacity is limiting, and transitory starch also acts as a carbon store to provide sugar at night. Transitory starch breakdown can occur by either of two pathways; significant progress has been made in understanding these pathways in C-3 plants. The hydrolytic (amylolytic) pathway generating maltose appears to be the primary source of sugar for export from C-3 chloroplasts at night, whereas the phosphorolytic pathway supplies carbon for chloroplast reactions, in particular in the light. In crassulacean acid metabolism (CAM) plants, the hydrolytic pathway predominates when plants operate in C-3 mode, but the phosphorolytic pathway predominates when they operate in CAM mode. Information on transitory starch metabolism in C-4 plants has now become available as a result of combined microscopy and proteome studies. Starch accumulates in all cell types in immature maize leaf tissue, but in mature leaf tissues starch accumulation ceases in mesophyll cells except when sugar export from leaves is blocked. Proper regulation of the amount of carbon that goes into starch, the pathway of starch breakdown, and the location of starch accumulation could help ensure that engineering of C-4 metabolism is coordinated with the downstream reactions required for efficient photosynthesis.