Due to the pioneering work of Jack Preiss elucidating the regulation of AGPase , Thomas ap Rees unravelling the metabolic regulation of sucrose metabolism , Mark Stitt deciphering the regulation of carbon partitioning , Chris Sommerville introducing Arabidopsis mutants to unravel pathways of starch and their colleagues, a complex picture of the regulation of starch is arising.
ISBN 13: 9780444537300
The most complete picture of starch metabolism has been obtained for the model plant Arabidopsis thaliana. Arabidopsis stores a substantial amount of photoassimilates in the form of transitory starch. Interestingly, the amount of starch accumulated during the day is more or less completely consumed during the night. This implies that Arabidopsis is able to predict at the end of the day, the amount of starch it will need to support metabolic processes during the following night.
This opens the question of how Arabidopsis is able to sense the amount of starch produced and to balance the rate of synthesis and degradation. In C3 plants, triose phosphates are produced as the primary product of photosynthesis. While most of the triose phosphates remain within the cycle, the excess is partitioned between sucrose and starch synthesis. It is assumed that chloroplastic triose phosphates are exchanged against cytosolic phosphate via the triose phosphate translocator. Cytosolic triose phosphates are converted to sucrose via six consecutive biochemical steps.
Finally, sucrose is exported to sink tissues through the phloem system. When the rate of synthesis exceeds demand in sink tissues, it is believed that sucrose accumulates in mesophyll cells. High sucrose levels serve as a signal to stimulate starch synthesis and adjust photosynthetic activity to the overall needs of the plant. Hence, the level of cytosolic triose phosphates increases leading to the inhibition of triose phosphate export from chloroplasts. This classical view explains the metabolic balance between starch and sucrose metabolism and suggests that transitory starch serves as an overflow valve for excess photoassimilates.
However, it fails to explain how plants can adapt their starch content according to future needs. Thus, mechanisms must exist to sense the total amount of starch granule size or weight? Work in several research areas over the last years has provided first insights into the complexity of the regulation of starch metabolism, but the current picture is like a jigsaw puzzle with many of its pieces missing.
It has long been known that expression of starch biosynthetic genes is regulated by sugars and the circadian clock. Sugar sensing and signal transduction pathways have been intensively studied.
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Unlike hormone and light signalling, sugar signalling pathways are only vaguely understood. This may be due to the dual role of sugars, being signals and metabolites and also due to extensive crosstalk with other signalling pathways. Based mainly on studies in Arabidopsis thaliana, a tentative model for sucrose sensing has been developed. This is supported by data obtained from transgenic Arabidopsis plants with elevated T6P content, which showed an increased starch level.
Thus, in this case, SnRK1 would be required for anabolic rather than catabolic reactions. Looking into different plant species, the story becomes even more complicated. This includes genes involved in redox regulation, nutrient acquisition and central carbon metabolism. This suggests crosstalk between clock and sugar signals as highlighted in Stitt and Zeeman However, in tubers, oscillation of GBSSI transcripts was linked to diurnal changes in sucrose supply rather than to a direct circadian control. Despite this, the protein amount of most starch enzymes does not directly follow the circadian regulation of the transcripts.
In cereal, the situation is different.
Based on this, it is tempting to speculate that protein phosphorylation plays also an important role in Arabidopsis and it will be interesting to see how mutations in plastidic protein kinases and phosphatase will influence starch metabolism. In rsr1, knockout mutants increased amylose content and an altered amylopectin structure could be observed Fu and Xue, So far, there is only one example in which altered expression of a potential regulator of starch synthesis resulted in modified starch quality in rice endosperm see above, Fu and Xue, It should be more active in plant tissues and consequently lead to increases in starch, if it represents the limiting factor of the pathway.
More promising results have been obtained when nucleotide turnover, de novo synthesis or transport were manipulated. Whether these approaches also lead to increases in starch in embryo or endosperm tissue has not yet been reported. These results seem to be questionable although, as the same tubers showed no differences, or often decreases, in dry matter contents.
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For these reasons, the reported results have to be interpreted cautiously and further experimentation is needed as tuber density determinations were not undertaken, and the procedure by which dry matter content was determined was not described in the study. Many mutants have been described in Arabidopsis that display a starch excess phenotype and are mostly affected in enzymes involved in starch mobilization Stitt and Zeeman, This technology has consequently been successfully applied to fodder crops to increase the content of easily digestible carbohydrates.
Therefore, fold changes might significantly depend on the time point of harvest. Sex4 was first described in Arabidopsis and encodes a phosphoglucan phosphatase, which is essential for starch mobilization in Arabidopsis. As amylose and amylopectin are quite contrasting molecules in their physical nature, the major target in changing starch composition is to generate crops that preferably contain only one or the other. A secondary approach is to generate amylopectin with altered chain length distribution and, in addition, containing modified levels of covalently attached phosphate groups.
Even though many mutants have been described in various species that contain starches that are composed solely of amylopectin, or that contain starches with a very high amylose content Kossmann and Lloyd, , it seemed desirable to introduce those traits into cassava and potato, because these are species which are generally difficult to mutate because of their ploidy levels and their genetic complexity.