AR242 PP56 04 ARI 28 March 2005 15 47 Starch Degradation Annu Rev Plant Biol 2005 56 73 98 Downloaded from arjournals annualreviews org by University of Georgia on 04 04 06 For personal use only Alison M Smith 1 Samuel C Zeeman 2 and Steven M Smith3 4 1 Department of Metabolic Biology John Innes Centre Norwich NR4 7UH United Kingdom email alison smith bbsrc ac uk 2 Institute of Plant Sciences University of Bern CH 3013 Bern Switzerland email sam zeeman ips unibe ch 3 Institute of Cell and Molecular Biology University of Edinburgh Edinburgh EH9 3JH United Kingdom email s smith ed ac uk 4 School of Biomedical Biomolecular and Chemical Sciences University of Western Australia Crawley WA 6009 Australia email ssmith cyllene uwa edu au Annu Rev Plant Biol 2005 56 73 98 doi 10 1146 annurev arplant 56 032604 144257 c 2005 by Copyright Annual Reviews All rights reserved First published online as a Review in Advance on January 13 2005 1543 5008 05 06020073 20 00 Key Words amylase Arabidopsis cereal endosperm chloroplast maltose Abstract Recent research reveals that starch degradation in Arabidopsis leaves at night is signi cantly different from the textbook version of this process Although parts of the pathway are now understood other parts remain to be discovered Glucans derived from starch granules are hydrolyzed via amylase to maltose which is exported from the chloroplast In the cytosol maltose is the substrate for a transglucosylation reaction producing glucose and a glucosylated acceptor molecule The enzyme that attacks the starch granule to release glucans is not known nor is the nature of the cytosolic acceptor molecule An Arabidopsis type pathway may operate in leaves of other species and in nonphotosynthetic organs that accumulate starch transiently However in starchstoring organs such as cereal endosperms and legume seeds the process differs from that in Arabidopsis and may more closely resemble the textbook pathway We discuss the differences in relation to the biology of each system 73 AR242 PP56 04 ARI 28 March 2005 15 47 Contents Annu Rev Plant Biol 2005 56 73 98 Downloaded from arjournals annualreviews org by University of Georgia on 04 04 06 For personal use only INTRODUCTION THE PATHWAY OF STARCH DEGRADATION IN ARABIDOPSIS LEAVES The Attack on the Granule Surface The Importance of Starch Phosphorylating Enzymes Debranching Metabolism of Soluble Linear Glucans Fate of Maltose and Maltotriose The Metabolism of Maltose in the Cytosol Further Characterization of the Pathway CONTROL OF FLUX THROUGH THE PATHWAY IN ARABIDOPSIS LEAVES Regulation at the Level of Gene Expression Regulation of Enzyme Activity THE PATHWAY IN LEAVES OF OTHER SPECIES The Production of Soluble Linear Glucans Hydrolytic versus Phosphorolytic Degradation Maltose Metabolism THE PATHWAY IN OTHER PLANT ORGANS The Attack on the Granule in Storage Organs Degradation of Soluble Glucans in Storage Organs The Pathway in Vegetative Tissues CONCLUSION 74 74 75 77 77 78 79 79 80 81 81 82 84 84 85 85 86 86 89 90 90 INTRODUCTION Our aim in this article is to discuss new information about the pathway of starch degradation in Arabidopsis leaves at night and to use it to reassess our understanding of starch degradation in other plant organs Although starch 74 Smith Zeeman Smith degradation has been extensively studied in germinating cereal endosperm 8 29 76 the nature and regulation of the process in this and other plant organs is poorly understood There is good a priori reason to think that the process in endosperms differs from that in other organs because the mature endosperm is not a living tissue whereas starch degradation in all other plant organs occurs within living cells Biochemical analyses show that many plant organs possess a wealth of isoforms of several different types of enzymes capable of degrading starch and related glucans However discovering the roles and importance of each of these forms in catalyzing starch degradation in vivo has been hampered by a lack of tools for this purpose In the last ve years the genetic and genomic resources available in Arabidopsis have facilitated new approaches to the pathway in leaves We present below the picture that has emerged for Arabidopsis leaves then discuss to what extent this is applicable to leaves of other species and to other plant organs THE PATHWAY OF STARCH DEGRADATION IN ARABIDOPSIS LEAVES During the day starch and sucrose are synthesized together as the products of photosynthetic carbon assimilation in Arabidopsis leaves Sucrose is exported to nonphotosynthetic parts of the plant and starch accumulates in the chloroplasts The ratio of starch to sucrose synthesis varies with environmental conditions but in our standard growth conditions 12 h light 20 C and about 180 mol quanta of photosynthetically active radiation m 2 s 1 about half of the newly assimilated carbon is partitioned into starch and the content at the end of the day is 10 15 mg g 1 fresh weight 14 117 During the subsequent night the starch is degraded to provide substrates for sucrose synthesis to allow continued export to nonphotosynthetic parts of the plant and to provide carbon skeletons energy and reductant within the leaf cell The supply of carbohydrate provided by nighttime starch degradation is essential for the normal Annu Rev Plant Biol 2005 56 73 98 Downloaded from arjournals annualreviews org by University of Georgia on 04 04 06 For personal use only AR242 PP56 04 ARI 28 March 2005 15 47 Figure 1 Proposed pathway of starch degradation in Arabidopsis leaves at night Steps about which uncertainty remains are represented as stippled or dashed arrows and with question marks GWD is glucan water dikinase and PWD is phosphoglucan water dikinase Further details of the enzymes and reactions involved are in Table 1 growth of the plant Mutants that synthesize less starch during the day or have a reduced capacity to degrade it at night have reduced growth rates under many environmental conditions 13 85 In the following sections we discuss the steps involved in converting granular starch in the chloroplast into hexose phosphate in the cytosol The scheme we describe is presented in Figure 1 and further details on the proteins involved are in Table 1 The Attack on the Granule Surface Starch in chloroplasts like starch in other parts of plants is in the form of granules composed of branched polymers of glucose Most of these polymers are amylopectin an 1 4 1 6 linked polymer with a distinctive