Starch in the Arabidopsis plant

The genetic and genomic resources available for the model plant Arabidopsis have allowed rapid progress in understanding the pathways of leaf starch synthesis and degradation. The pathway of starch synthesis is generally similar to that in other plants, but Arabidopsis research has permitted new insights into the mechanism of granule initiation. The pathway of starch degradation is very different from the textbook version, largely derived from research on germinating cereal grains. The starch granule is attacked by beta- rather than a-amylases, and this process is strongly dependent on a cycle of phosphorylation and dephosphorylation of the granule surface. The major product of granule degradation is maltose, which is exported to the cytosol where it is metabolised to hexose phosphate and then to sucrose. Cytosolic maltose metabolism requires a glucanotransferase and a glucosyl acceptor which is believed to be a complex heteroglycan. Plant productivity and yield are highly dependent on leaf starch turnover, and starch metabolism is coordinated with other factors and processes that determine growth rate. Research in Arabidopsis provides an important knowledge base for the study of starch metabolism in other, less tractable species.