Starch biosynthesis pathway

By using mutants of maize and other species, progress has been made in understanding the pathways and enzymes involved in starch biosynthesis and the fine structure of starch polysaccharides. However, starch biosynthesis (Chapter 4) and granule formation are still not completely understood. Thus, integration of the information on polysaccharide biosynthesis (Section 3.6) with that on mutant effects (Section 3.7), is necessary to fully understand polysaccharide biosynthesis and to delineate the limits of this knowledge. 

Obviously, our understanding of starch biosynthesis is still incomplete, since mutants occur for which the primary metabolic effect has not been determined. Continued evaluation of isozymes and effector compounds, and studies of the in vivo pattern and rate of 14C labeling of intermediates of starch biosynthesis in normal, mutants and mutant combinations should aid in clarifying the nature of the mutations and the pathways of starch biosynthesis. Other aspects of starch formation also remain to be explained. For example, how are starch granules formed as the... 

In spite of these limitations to our complete knowledge of starch biosynthesis, information about the pathway of starch biosynthesis gained from studies of maize endosperm mutants can probably be generalized to other plant species because related mutants have occurred in peas, sorghum, barley, rice and Chlamydomonas, and because the same enzymes are found in starch-synthesizing tissues in other plant species. Variation in the number of isozymes and their developmental expression, and variations in cellular compartmentation, however, could result in a range of pathways with significant differences. 

Genes encoding enzymes involved in starch biosynthesis or other relevant pathways (e.g., synthesis of sucrose) can be used for the overexpression of enzyme activity, as described in this chapter for ADPGlc PPase. Another approach is the use of antisense (complementary) DNA or RNA to decrease gene expression, a good way to assess the role of an enzyme and whether it limits the rate of the overall pathway. 

Keeling, P. L., Banisadr, R., Barone, L., Wasserman, B. P., and Singletary, G. W. 1994. Effects of temperature on enzymes in the pathway of starch biosynthesis in developing wheat and maize grain. Aust. J. Plant Physiol 21, 807-827. 

Keeling, P. L., Wood, J. R., Tyson, R. H., and Bridges, I. G. 1988. Starch biosynthesis in developing wheat grain Evidence against the direct involvement of triose phosphates in the metabolic pathway. Plant PhysioL 87,311-319. 

Villand, P., and Kleczkowski, L. A. 1994. Is there an alternative pathway for starch biosynthesis in cereal seeds Z. Naturforsch 49c, 215-219. 

In all studies thus far made on starch synthetase, the incorporation of D-glucose from a D-glucosyl ester of a nucleotide into an acceptor molecule has been made by using a radioactively labeled D-glucosyl group in the nucleotide ester, and so the results are unambiguous. However, the extent of the incorporation of D-glucose into the acceptor was very low in the early experiments, and the view has been expressed that starch synthetase is not the major pathway for metabolism of starch. This conclusion seems very reasonable starch biosynthesis is probably a multi-pathway process. Of interest in this connection is a comparison that has been made of starch synthetase activity in non-waxy and waxy maize and rice. ... 

Some of the cell wall mutants, such as the mur mutants in Arabidopsis, affect dovmstream enzymes, which supply substrates for the glycosyl transferases involved in cell wall synthesis (Williamson et al. 2002). These types of mutations usually lead to a significant overall reduction in the rate of cellulose synthesis. Sucrose synthase (SuSy) (FC2.4.1.13) catalyzes the reversible conversion of sucrose and UDP to UDP-glucose and fructose thereby channeling sucrose into numerous pathways, including cell wall and starch biosynthesis. 

It has, thus, been demonstrated that redirecting the poly(3HB) biosynthetic pathway from the cytoplasm to the plastid resulted in an approximate 100-fold increase in poly(3HB) production [24]. However, it must be kept in mind that the rate of poly(3HB) biosynthesis in A thaliana leaves was relatively low, since poly(3HB) accumulated progressively over 40-60 days to reach 10-14% of the dry weight, whereas synthesis of starch can reach 17% dry weight for a 12 h photoperiod and seed storage lipids can reach 8% dry weight per day. 

Poly(3HB) synthesis in various subcellular compartments could be used to study how plants adjust their metabolism and gene expression to accommodate the production of a new sink, and how carbon flux through one pathway can affect carbon flux through another. For example, one could study how modifying the flux of carbon to starch or lipid biosynthesis in the plastid affects the flux of carbon to acetyl-CoA and poly(3HB). Alternatively, one could study how plants adjust the activity of genes and proteins involved in isoprenoid and flavonoid biosynthesis to the creation of the poly(3HB) biosynthetic pathway in the cytoplasm, since these three pathways compete for the same building block, i. e., acetyl-CoA. 

The metabolic routes leading to polyglucan synthesis were elucidated after the discovery of nucleoside-diphosphate sugars by L. F. Leloir and co-workers in 1955. This finding led to the conclusion that biosynthesis and degradation of glycogen and starch occur by different pathways. 

Viola, R., Davies, H. V., and Chudeck, A. R. 1991. Pathways of starch and sucrose biosynthesis in developing tubers of potato (Solanum tuberosum L.) and seeds of faba bean (Vicia faba L.) Elucidation by 13C-nuclear magnetic-resonance spectroscopy. Planta 183, 202-208. 

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