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The relocation of starch metabolism to chloroplasts: when, why and how

2008; Elsevier BV; Volume: 13; Issue: 11 Linguagem: Inglês

10.1016/j.tplants.2008.08.009

1878-4372

Philippe Deschamps, Ilka Haferkamp, Christophe d’Hulst, H. Ekkehard Neuhaus, Steven Ball,

Cassava research and cyanide

Plastid endosymbiosis was accompanied by the appearance of a novel type of semi-cristalline storage polysaccharide (starch). Interestingly, starch is found in the cytoplasm of Rhodophyceae and Glaucophyta but is localized to the chloroplast stroma of Chloroplastida. The pathway is presumed to have been cytosolic in the common ancestor of the three Archaeplastida lineages. The means by which in green plants and algae an entire suite of nuclear-encoded starch-metabolism genes could have had their protein products rewired simultaneously to plastids are unclear. This opinion article reviews the timing and the possible reasons underlying this rewiring and proposes a hypothesis that explains its mechanism. The consequences of this mechanism on the complexity of starch metabolism in Chloroplastida are discussed. Plastid endosymbiosis was accompanied by the appearance of a novel type of semi-cristalline storage polysaccharide (starch). Interestingly, starch is found in the cytoplasm of Rhodophyceae and Glaucophyta but is localized to the chloroplast stroma of Chloroplastida. The pathway is presumed to have been cytosolic in the common ancestor of the three Archaeplastida lineages. The means by which in green plants and algae an entire suite of nuclear-encoded starch-metabolism genes could have had their protein products rewired simultaneously to plastids are unclear. This opinion article reviews the timing and the possible reasons underlying this rewiring and proposes a hypothesis that explains its mechanism. The consequences of this mechanism on the complexity of starch metabolism in Chloroplastida are discussed. soluble starch synthases that require ADP-glucose as a substrate and that are involved in amylopectin synthesis. These enzymes are of cyanobacterial phylogeny. groups all eukaryotic lines whose plastids descend directly from endosymbiotic cyanobacteria. Usually photosynthetic, they all contain double-membrane plastids and store starch. The Archaeplastida include three major lineages: the Glaucophyta, Rhodophyceae and Chloroplastida [3]. They are thought to descend from a single common ancestor and are thus considered monophyletic. single-cell and multicellular organisms containing plastids called 'chloroplasts' with chlorophyll-b- and LHC-containing PSI and PSII antennae. Sexual cycles are reported and starch is stored in the plastid. They are often referred to as green lineage or chlorophytes. The chloroplastida contain groups usually referred to as green algae and land plants. is an α-1,4 glucanotransferase. This enzyme transfers mostly maltosyl residues from a donor MOS of at least three glucose residues to an acceptor MOS or glucose. It thus disproportionates a homogenous population of MOSs of defined size into a heterogeneous population of longer and shorter sizes. D-enzyme is needed to complete degradation of MOS by phosphorylase, which would otherwise stop at the malto-tetraose level. expressed sequence tag. is the major protein bound to starch and one of the very few enzymes working within the polysaccharide granule itself. GBSS is responsible for amylose synthesis. GBSS is phylogenetically related to ADPGSS. single-cell organisms containing plastids called cyanelles with peptidoglycan and phycobilisomes (no LHC reported). Glaucophyta are currently thought to have diverged first from the common ancestor of Archaeplastida. No sexual cycle is reported and starch is stored in the cytoplasm [3]. They are often referred to as glaucophytes or glaucocystophytes. is responsible for the phophorylation of the starch at the C6 position of glucose residues. Starch phosphorylation loosens the structure, which therefore becomes accessible to hydrolases. GWD is an essential enzyme of starch breakdown. three distinct isoamylases are known in Chloroplastida. Isa1 and Isa2 are standard isoamylases involved in amylopectin maturation. However, Isa3 is more effective on WSPs that have been first recessed by β-amylase or phosphorylase and is selectively involved in starch degradation. is a direct debranching enzyme. These enzymes directly hydrolyze the α-1,6 branch. isoamylases can be distinguished from pullulanases by the fact that they can hydrolyze loosely branched WSPs such as glycogen but are less or not effective towards densely branched WSPs, such as pullulan or amylopectin clusters. light-harvesting complex. maltose export protein. This plastidial transporter is known to transport maltose out of the plastid to the cytosol. MEX1 proteins are Chloroplastida-specific and are not found in other lineages. linear (unbranched) α-1,4 linked malto-oligosaccharides, typically less than twenty glucose residues long. Maltose (two residues) maltotriose (three residues) and maltotetraose (four residues) define the shortest of these oligosaccharides. degrades the outer chain glucose of MOS or WSP, provided the latter contains at least four residues. Degradation is mediated through orthophosphate and results in the production of glucose-1-P. In Chloroplastida, distinct starch phosphorylases have been identified in the cytosol (cPho) and plastids (pPho). is a direct debranching enzyme. Pullulanases can be distinguished from isoamylases by the fact that they can hydrolyze densely branched WSPs, such as pullulan, but are less or not effective towards glycogen. This enzyme, which is related to GWD, will phosphorylate starch at the C3 position, provided the latter has already been phosphorylated by GWD. PWD is also involved in starch degradation. single-cell and multicellular organisms containing plastids called 'rhodoplasts' with phycobilisomes associated with PSII and LHC-containing PSI antennae. Sexual cycles are reported and starch is stored in the cytoplasm. They are often referred to as red lineage, rhodophytes or red algae. soluble starch synthases that require UDP-glucose as a substrate and that are involved in amylopectin synthesis. These enzymes are of eukaryotic host origin and are related to corresponding glycogen synthases found in Parabasalids, Amoebas and Ciliates. in this context these are medium- to high-mass hydrosoluble α-1,4 linked α-1,6 branched polymers. Glycogen can thus be defined as a WSP. are endo-type enzymes that attack inner α-1,4 linkages of amylopectin, amylose or WSPs. The depolymerization products are called α-dextrins. From unbranched chains, α-amylase will predominantly generate α-maltose as the ultimate degradation product. are exo-type enzymes that attack the penultimate α-1,4 linkages of amylopectin, amylose or WSP's outer chains. This mode of attack generates β-maltose.