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Title: The regulation of starch biosynthesis in developing wheat endosperms
Author: Riffkin, Harry Lionel
ISNI:       0000 0001 3518 5509
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1987
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The purpose of this work is to identify some of the mechanisms regulating the conversion of assimilates to starch during endosperm development in wheat. Since environmental conditions affect the rate of grain development, a standard system for ageing developing caryopses was devised. The system was based on the morphological changes accompanying caryopsis development, under field conditions, from anthesis to harvest-ripeness. Accordingly, because the wheat caryopsis passes through similar developmental changes which occur at approximately the same relative time regardless of the time scale, the tissues of the caryopsis, e.g. the endosperm, could be standardised according to this system. This ensured that tissues at the same stage of development could be compared, even when grown under different environmental conditions, e.g. under glass. Sucrose is the principal transported sugar in wheat and was found to be the most abundant sugar in developing wheat endosperms. Glucose and fructose were present in lower amounts and in different relative quantities, with glucose declining throughout endosperm development. The higher quantities of fructose were consistent with low levels of endosperm invertase activity and much higher activities of endosperm sucrose synthase. Endosperm sucrose synthase activity reached an apparent maximum catalytic rate during the period of rapid dry weight accumulation. The levels of UDP in developing endosperms also reached a maximum at this time. The curve of UDP-dependent sucrose synthase initial reaction velocities was sigmoidal, thus endosperm UDP levels may regulate the catabolism of sucrose. Levels of ADP in developing wheat endosperms were higher than UDP but UDP-glucose was present in amounts approximating to twice those of ADP-glucose. This implies that ADP-dependent sucrose synthase activity was not predominant in the catabolism of sucrose in developing wheat endosperms. Hexose sugars require to be in the form of hexose phosphates prior to further metabolism by either glycolytic enzymes or ADP- and/or UDP-glucose pyrophosphorylases. G6P was present in consistently higher amounts than either G1P or F6P suggesting rapid phosphorylation of glucose by hexokinase. Levels of G1P rose to a maximum during endosperm dry weight accumulation and were quite adequate to account for the measured rates of maximum velocity for endosperm ADP-glucose pyrophosphorylase. G1P may have been formed by the catabolism of UDP-glucose by the PPi-dependent UDP-glucose pyrophosphorylase reaction. This enzyme activity was found to be 5-7 times higher in developing wheat endosperms than either sucrose synthase or ADP-glucose pyrophosphorylase. Differences were observed in the properties of ADP-glucose pyrophosphorylases from endosperm and leaf tissues suggesting that the control mechanisms differed between tissues. Both enzymes were partially purified by ammonium sulphate fractionation and the precipitates stored in 85 per cent ammonium sulphate. The endosperm enzyme was stable in the unfractionated extract and in the stored precipitate but, on subsequent dialysis of the stored precipitate, rapidly lost activity, with a half-life of about 4h. The dialysed activity was dependent on MgCl^ and was partially stabilised by Pi but was not activated by 3-PGA. The leaf enzyme was stable to fractionation by ammonium sulphate and to storage and dialysis but both the crude tissue and partially purified activities were dependent on the presence of 3-PGA. PPi was found to be an efficient inhibitor of endosperm ADP-glucose pyrophosphorylase while Pi was not, implying that PPi must be removed from the site of ADP-glucose synthesis or hydrolysed. If the generation of G1P for ADP-glucose synthesis is a result of PPi-dependent catabolism of UDP-glucose then PPi/Pi metabolism may regulate starch biosynthesis in developing wheat endosperms.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Endosperm development in wheat