Developmental Biochemistry of Cotton Seed Embryogenesis and Germination

Abstract

Tbe enzymic basis for the flow of .ltrgeu from arginie to e during the first 3 days of gIlnaon has bed In extracts from cotton (Goss_ypm hnm) cotyledons. Eviene that a synthetase reguates n accuma t (for tsrt to the axis) Is Further, eviene that the bWuk of the passed from one generatio to the next In dicots through a cycle Invdvi the fIn seq ee r arg storae protein -arginine -* asparaIe s discussed.Data on the pathway of ammonia assimilation in nodules of leguminous roots and green leaves have been accumulating during the past several years, two review articles appearing recently (19,20). In green leaves, ammonia from nitrate assimilation serves as a nitrogen source for amino acid synthesis, whereas in root nodules, ammonia produced by nitrogen fixation serves as a nitrogen source. In both cases, much ofthe carbon skeletons for amino acid synthesis presumably comes as a result of photosynthesis. The situation is likely to be different in dicot seeds, such as legumes and cotton, that do not depend on external nitrogen sources. The storage proteins and starch or lipid satisfy their nutritional needs at least in the early days of germination. In cotton, amino acids and carbon skeletons released by the hydrolysis of the stored material appear to go through active interconversions as indicated by the lack of correlation between the amino acid composition of the storage proteins (13) and that of the free amino acid content of germinating cotyledons (4). The predominant nitrogen storage unit in the cotton storage protein is arginine which contains at least 25% of the total stored nitrogen (13), whereas most of the nitrogen transported from cotyledons to the growing axis is by way of asparagine (4). It seems that a major pathway of nitrogen metabolism in germinating cotyledons lies in the transfer of nitrogen from ARG' to ASN. Furthermore, we have observed that cotyledons germinated in actinomycin D (Act D) do not accumulate ASN to the extent that untreated cotyledons do, but rather accumulate massive amounts of glutamine (4). This suggests that nitrogen for the build-up of ASN may pass through glutamine and that the enzyme(s) responsible for synthesis of ASN at the expense of glutamine may be derived from mRNA newly synthesized in germination.Against this background we have examined the levels during germination ofactivities in crude extracts ofcotyledons ofenzymes 'Abbreviations: Act D: actinomycin D; GDH: glutamate dehydrogenase; GS: glutamine synthetase; GOGAT: glutamate synthase; AS: asparagine synthetase; ASN: asparagine; ARG: arginine.likely to be involved in the transfer of nitrogen from ARG to ASN, and have examined the effects of Act D and cycloheximide on some of these activity levels.Arginase (EC 3.5.3.1; L-argimne amidinohydrolase; reaction 1) and urease (EC 3.5.1.5; urea amidohydrolase; reaction 2) activities were assayed to establish the possible pathway for the release of ammonia from arginine of the storage protein. purchased from Morrow Bros., Farmington, Georgia, were germinated in the dar...

Ectopic Overexpression of Asparagine Synthetase in Transgenic Tobacco

“…The rapid leaf development that occurs during vegetative growth imposes a strong demand for nutrient availability, and nitrogen is typically the most critical nutrient at this time due to the synthesis of new proteins in expanding and enlarging tissues. Nitrogen assimilated and accumulated at this time is subsequently recycled in the plant and deposited in seed reserves; as well as being a major long-distance transport amino acid, Asn also plays an important role in the formation of seed reserves (Dilworth and Dure, 1978;Sieciechowicz et al, 1988). The two 2127 lines were found to outgrow untransformed controls over a 6-week period up to the end of vegetative growth and conferred a 10 and 33% growth advantage.…”

Overexpression of the ASN1 Gene Enhances Nitrogen Status in Seeds of Arabidopsis

“…Because the efficiency of protein synthesis depends on the light/dark regulation of AS activities (Dembinski et al, 1996a), elevations of leaf AS activities and Asn levels have been used as parameters to screen for high grain protein cultivars in maize (Dembinski et al, 1995) and rye (Secale cereale; Dembinski and Bany, 1991). Moreover, Asn may also play a role in the formation of seed nitrogen reserves (Dilworth and Dure, 1978;Sieciechowicz et al, 1988). A current model on seed protein biosynthesis also suggests that free Asn in siliques may play a major role in supplying Asp and nitrogen resources to seed coats and cotyledons during seed development (Bewley et al, 2000).…”

Seeds

“…In species where the cotyledons are the storage organs, these are connected to the growing regions by a network of vascular bundles, and the products of hydrolysis must be translocated to the axis largely within the phloem. Loading of amino acids into the translocation stream might be aided in some species, e.g., broad bean (Viciafaba), by the presence of transfer cells that Dilworth and Dure (1978).…”