Research and Thesis Projects

How is starch made?

Although composed of the simple monosaccharide, glucose, starch itself is remarkable in terms of its structural complexity. It takes the form of massive insoluble granules, with an internal lamellar structure, the nature of which is only poorly understood. The major constituent, amylopectin, is a branched polymer. Chains of α-1,4-linked glucose units are conected to each other by α-1,6-linkages (branch points) forming a tree-like or 'racemose' structure. During amylopectin biosynthesis, a suite of enzymes work together to create this structure, producing a specific distribution of chain lengths and branch points at defined positions and frequencies. This architecture results in groups of neighbouring, unbranched segments of chains (clusters). These clusters assume a semi-crystalline structure by forming arrays of adjacent double helices.

Although many of the enzymes of amylopectin synthesis are known, it is still not understood how they work together to produce the specific, crystallisation-competent structure described above. We are studying a key enzyme involved in this process, called debranching enzyme. Although this enzyme removed branches, its action is necessary for normal starch production. We are elucidating the molecular mechanisms that underlie this requirement.

Our work in this area

Streb S., Delatte T., Umhang M., Eicke S., Schorderet M., Reinhardt D., and Zeeman S.C. (2008) Starch Granule Biosynthesis in Arabidopsis Is Abolished by Removal of All Debranching Enzymes but Restored by the Subsequent Removal of an Endoamylase. The Plant Cell, 20, 3448-3466. external pageDOI

Zeeman, S.C., Smith, S.M. and Smith, A.M. (2007) The diurnal metabolism of leaf starch. Biochem. J., 401, 13-28. external pageDOI

Delatte, T., Treviasan, M., Parker, M.L., and Zeeman S.C. (2005) Arabidopsis mutants Atisa1 and Atisa2 have identical phenotypes and lack the same multimeric isoamylase, which influences the branch point distribution of amylopectin during starch synthesis. The Plant Journal 41, 815-830. external pageDOI

Full list of publications

Other useful references

Ball, S.G. and Morell, M.K. (2003) From bacterial glycogen to starch: Understanding the biogenesis of the plant starch granule. Ann Rev Plant Biol., 54, 207-233. external pagePubMed

Tetlow, I.J., Morell, M.K., Emes and M.J. (2004) Recent developments in understanding the regulation of starch metabolism in higher plants. J Exp Bot., 55, 2131-2145. external pagePubMed

How is starch broken down?

The remobilisation of starch fuels respiration and plant growth and development when photosynthesis is not possible. This is necessary during the night when it is dark, during re-growth from roots or tubers in spring or after herbivory and during the germination of seeds. Although starch breakdown has been studied for many years in germinating cereal seeds, the pathway and regulation in most plant tissues was not well understood until recently. We have developed Arabidopsis as an important tool for the study of starch breakdown in leaves. Through the characterisation of starch-excess (sex) mutants and through targeted gene disruption, we have helped establish a new model for starch breakdown in leaves. This provides the basis for rigorous testing and elaboration, and for exploring the regulation of the pathway.

Studies in the past decade have demonstrated the importance of starch-bound phosphate for starch degradation. Mutants lacking enzymes that phosphorylate crystalline amylopectin (glucan, water dikinases) or dephosphorylate it (phosphoglucan phosphatase) cannot break down their starch properly. This provides important new insight, and current models propose that a cycle of transient phosphorylation effectively solubilises the surface of the semi-crystalline strarch.

References

Fulton, D.C., Stettler, M., Mettler, T., Vaughan, C.K., Li, J., Francisco, P., Gil, M., Reinhold, H., Eicke, S., Messerli, G., Dorken, G., Halliday, K., Smith, A.M., Smith, St.M. and Zeeman S.C. (2008) Beta-AMYLASE4 a noncatalytic protein required for starch breackdown, acts upstream of three acive beta-amylases in arabidopsis chloroplasts. Plant Cell, 20, 1040-1058. external pageDOI

Zeeman, S.C., Delatte, T., Messerli, G., Umhang, M., Stettler, M., Mettler, T., Streb, S., Reinhold, H. and Kötting, O. (2007) Starch breakdown: recent discoveries suggest distinct pathways and novel mechanisms. Functional Plant Biology, 34, 465-473. external pageDOI

Delatte, T., Umhang, M., Trevisan, M., Eicke, S., Thorneycroft, T., Smith, S.M. and Zeeman, S.C. (2006) Evidence for distinct mechanisms of starch granule breakdown in plants. J Biol Chem, 281, 12050-12059. external pageDOI

Niittylä, T., Comparot-Moss, S., Lue, W.-L., Messerlie, G., Trevisan, M., Seymour, M.D.J., Gatehouse, J.A., Villadsen, D., Smith, S.M., Chen, J., Zeeman, S.C. and Smith, A.M. (2006) Similar protein phosphatases control starch metabolism in plants and glycogen metabolism in mammals. J Biol Chem, 281, 11815-11818. external pageDOI

Niittylä, T., Messerli, G., Trevisan, M., Chen, J., Smith, A.M. and S.C. Zeeman (2004) A novel maltose transporter essential for starch degradation in leaves. Science 303, 87-89. external pageDOI

Full list of references

other useful references

Hejazi, M., Fettke, J., Haebel, S., Edner, C., Paris, O., Frohberg, C., Steup, M. and Ritte, G. (2008) Glucan, water dikinase phosphorylates crystalline maltodextrins and thereby initiates solubilization. Plant J., 55, 323-34. external pagePubMed

Edner, C., Li, J., Albrecht, T., Mahlow, S., Hejazi, M., Hussain, H., Kaplan, F., Guy, C., Smith, S.M., Steup, M. and Ritte, G. (2007) Glucan, water dikinase activity stimulates breakdown of starch granules by plastidial beta-amylases. Plant Physiol., 145, 17-28. external pagePubMed

Kötting, O., Pusch, K., Tiessen, A., Geigenberger, P., Steup, M., and Ritte, G. (2005) Identification of a Novel Enzyme Required for Starch Metabolism in Arabidopsis Leaves. The Phosphoglucan, Water Dikinase. Plant Physiol., 137, 242-252. external pagePubMed

Ritte, G., Lloyd, J.R., Eckermann, N., Rottmann, A., Kossmann, J. and Steup, M. (2002) The starch-related R1 protein is an alpha-glucan, water dikinase. Proc. Natl. Acad. Sci. USA, 99, 7166-7171. external pagePubMed

Lorberth, R., Ritte, G., Willmitzer, L. and Kossmann, J. (1998) Inhibition of a starch-granule-bound protein leads to modified starch and repression of cold sweetening. Nat Biotechnol., 16, 473-477. external pagePubMed