Thus, generation of the glucan primer seems more variable than formation of the starch granules regulated via SS4. Unfortunately, for both ptst2 and ptst3 such metabolic data are currently missing.
However, the increase of the starch granule number when PTST2 is overexpressed in wild type but not when overexpressed in the ss4 mutant is also in agreement with this assumption. Likewise, the observed increase of the starch granule number per chloroplast in ss4 by expression of the glycogen synthase is conforming. A Schematic presentation of starch granule synthesis.
For starch granule initiation maltodextrins from the chloroplastidial maltodextrin pool are used by protein targeting to starch 2 PTST2 and starch synthase 4 SS4 to produce a granule initiation complex. A lack of the related enzymes massively influences the starch granule number. Enzymes related to initiation reveal protein-protein interaction as depicted by red links. The chloroplastidial maltodextrin pool is affected by enzymes acting on glucans, efflux of carbohydrates to cytosol, and influx of carbohydrates via starch degradation.
For other enzymes acting on glucans labeled with? Dashed arrow represents multiple enzymatic reactions. B Starch granule number per chloroplast in different genotypes. Starch isolated from mutants with reduced number of starch granules per chloroplast reveal distinct alterations in the starch granule morphology. Starch granules from Arabidopsis leaves are thin and discoid. It was reported that the N-terminal domain of SS4 influences the morphology of starch granules Lu et al.
It seems that the generation of a reduced number of starch granules number per chloroplasts is connected with formation of spherical rather than thin and flattened starch granules. In the last years we got more insights into starch granule biosynthesis. The appearance of starch granules is a rather complex mechanism influenced by various factors. According to this, it is very important to distinguish between the possibility to initiate starch and the formation of starch granules.
It is quite clear that SSs, or more precisely SS4, is important in the formation of the typical number of starch granules per chloroplast in Arabidopsis , Crumpton-Taylor et al. However, the lack of SS4 is not limiting the formation of starch granules. The formation of starch granules occur independent of SS4 in several mutants. Starch isolated from these mutants analyzed so far did not reveal large inner structural alterations, although the starch morphology reveals considerable differences.
Thus, following the starch initiation, the process of starch granule formation is rather similar in these mutants and point to the overlapping functions of the further SSs. Interestingly, the majority of mutants affected in the starch granule number display mostly one starch granule per chloroplast Figure 2. The initiation of starch seems to depend on a chloroplastidial maltodextrin metabolism.
As this maltodextrin metabolism can be influenced by the ongoing starch turn-over and distinct enzymes of the starch metabolism e. In yeast cells, starch-like synthesis was recreated by introduction of Arabidopsis SSs without plant enzymes related to synthesis of a primer, thus the intrinsic primers of yeast seems to be sufficient for the formation of starch like polyglucans.
Furthermore, the experiments again showed that SS4 can be substituted by other SSs, as starch-like polyglucans were observed not exclusively when SS4 was introduced Pfister et al. All authors listed have made a substantial, direct, and intellectual contribution to the work, and approved it for publication. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Ball, S. The evolution of glycogen and starch metabolism in eukaryotes gives molecular clues to understanding the establishment of plastid endosymbiosis. Brust, H. Analysis of the functional interaction of Arabidopsis starch synthase and branching enzyme isoforms reveals that the cooperative action of SSI and BEs results in glucans with polymodal chain lenght distribution similar to amylopectin.
PLoS One 9:e Starch synthesizing reactions and paths: in vitro and in vivo studies. Chia, T. A cytosolic glucosyltransferase is required for conversion of starch in Arabidopsis leaves at night. Plant J. Colleoni, C. Phylogenetic and biochemical evidence supports the recruitment of an ADP-glucose translocator for the export of photosynthate during plastid endosymbiosis. Comparot-Moss, S. A putative phosphatase LSF1 is required for normal starch turnover in Arabidopsis leaves.
Plant Physiol. Crumpton-Taylor, M. Control of starch granule numbers in Arabidopsis chloroplasts. Starch synthase 4 is essential for coordination of starch granule formation with chloroplast division during Arabidopsis leaf expansion. New Phytol. Plastidial phosphorylase is required for normal starch synthesis in Chlamydomonas reinhardtii. Soluble starch synthase I: a major determinant for the synthesis of amylpectin in Arabidopsis thaliana leaves. Google Scholar.
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Hejazi, M. Hwang, S. Rice endosperm starch phosphorylase Pho1 assembles with disproportionating enzyme Dpe1 to form a protein complex that enhances synthesis of malto-oligosaccharides.
Cellulose and starch are polysaccharide or complex carbohydrates that contain multiple saccharides or simple carbohydrates. All carbohydrates contain carbon, hydrogen and oxygen, usually in a ratio of , or one carbon to two hydrogens to one oxygen. Carbohydrate chains, however, contain multiple connected simple carbohydrates. Starch contains multiple connected glucose molecules arranged as amylose in linear chains or amylopectin in highly branched chains.
The orientation of these chains determines whether the glucose chains form starch for storage or cellulose for plant growth and structural support, says the Polymer Science Learning Center. A glucose chain with an oxygen and hydrogen pair pointing downward is called an "alpha glucose," while a glucose chain with an oxygen and hydrogen pair pointing outward is called a "beta glucose.
When the plant needs energy, a glucose molecule is taken from the starch molecule to be broken down for the stored energy. Cellulose, on the other hand, forms when beta glucose molecules connect into long chains that fit together like blocks to create the structure of the plant's cell walls. These cellulose chains fit so tightly together that water molecules can't fit between them, making cellulose essentially waterproof.
While some glucose remains stored in the leaf as starch grains where it formed, much of the glucose combines with fructose to form sucrose before moving through the plant to provide energy for metabolic processes in cells that don't have access to sunlight. The sucrose moves due to differences in sugar and water concentrations in the cells, states Georgia State University.
Special cells found in the phloem carry the sugar from the leaves to the other, nonphotosynthesizing parts of the plant, including flowers, fruit, seeds, branches and roots. Once the sucrose reaches these new locations, some of the sucrose is used immediately while the excess is converted to starch or cellulose.
Some plants, like sugar cane Saccharum officinarum, U. Department of Agriculture plant hardiness zones 8 through 12; and sugar beets Beta vulgaris, zones 4 through 8 , store higher concentrations of sucrose than do other plants. Sugar cane, for example, contains about 14 percent sucrose, while sugar beets contain about 19 percent sucrose.
The polymerization and storage process in plants is performed by special cell parts—the amyloplasts. These non-pigmented organelles take glucose, turn it into starch and move it to another part of the cell, called the stroma.
The stroma is the colorless, spongy cell matrix that supports the plant cell itself. In tubers, rhizomes and other starch-storing plant organs, it also acts as a place to store food for later use. When the plant needs the energy in the starch, it converts the starch grains back into glucose. Palmer is a freelance writer and illustrator living in Milwaukee, Wis. She has been producing print and Web content for various organizations since and has been freelancing full-time since
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