Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA.
J Exp Bot. 2011 May;62(9):3109-18. doi: 10.1093/jxb/err035. Epub 2011 Mar 23.
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.
基本上，所有植物白天在叶子中储存淀粉，然后在接下来的晚上分解淀粉。当蔗糖合成能力有限时，这种短暂的淀粉积累充当了一种溢出机制，短暂的淀粉也充当了碳库，在夜间提供糖分。短暂的淀粉分解可以通过两种途径之一发生；在理解 C(3)植物中的这些途径方面已经取得了重大进展。生成麦芽糖的水解（淀粉水解）途径似乎是夜间从 C(3)叶绿体中输出糖分的主要来源，而磷酸化途径则为叶绿体反应提供碳，特别是在光照下。在景天酸代谢（CAM）植物中，当植物以 C(3)模式运行时，水解途径占主导地位，但当它们以 CAM 模式运行时，磷酸化途径占主导地位。由于显微镜和蛋白质组学研究的结合，现在已经获得了 C(4)植物中短暂淀粉代谢的信息。在不成熟的玉米叶片组织中，所有细胞类型都积累淀粉，但在成熟的叶片组织中，除了阻止叶片中糖分输出的情况外，叶肉细胞中不再积累淀粉。对进入淀粉的碳量、淀粉分解途径以及淀粉积累位置的适当调节可以帮助确保 C(4)代谢的工程与高效光合作用所需的下游反应相协调。
