Maltose Biochemistry and Transport in Plant Leaves

Transitory starch builds up in photosynthesizing leaves during the day and is then degraded each night. It has recently been shown through mutant analysis and genetically engineered plants that transitory starch is required for maximal rates of photosynthesis, especially in high carbon dioxide atmospheres. Understanding the pathway by which carbon is exported from chloroplasts at night is limited. Previous work has shown that starch conversion to sucrose at night involves maltose export from chloroplasts. This pathway for carbon export from chloroplasts normally does not occur during the day. The regulation of maltose formation, the transporters that allow export of maltose from the chloroplast, and the cytosolic reactions by which maltose is converted to sucrose remain obscure. Genomic and biochemical approaches are proposed here to address maltose metabolism and transport. The research will cover three areas (1) how are hydrolytic and phosphorylytic starch degradation regulated, (2) how is maltose exported from chloroplasts, and (3) how is maltose converted to sucrose in the cytosol? It is expected that this research will lead to new insights about photosynthesis at high carbon dioxide levels, how carbon is partitioned in plants, and how partitioning can be altered to adapt plants to human needs.