Abstract
The flow of carbon in vivo from glucose to cellulose in developing cotton fibers which were actively engaged in the deposition of a cellulosic secondary cell wall was traced. Steady state levels of nucleotide sugars were analyzed from developing cotton fibers harvested between 13 and 36 days post-anthesis (DPA). The UDP-sugars were the predominant nucleotide sugars. UDP-glucose constituted over 75% of the UDP-sugar fraction, but UDP-galactose and traces of other UDP-sugars were also detected. The UDP-glucose levels increased from 0.2 ..mu..mol/boll at 13 DPA to over 2.1 ..mu..mol/boll by 24 DPA, just prior to the maximum rate of secondary wall cellulose synthesis; the levels dropped precipitously at the time when cellulose synthesis ceased. Fibers, cultured in vitro possess a very similar nucleotide-sugar composition to that of plant-grown fibers. When such fibers are pulse-labeled in vivo with (/sup 14/C)glucose, UDP-glucose is the predominant nucleotide sugar labeled. Pool size and rates of labeling of glucose, glucose-phosphate, and UDP-glucose pool data were analyzed using a computer simulation model, and it was determined that the rate of synthesis and turnover of UDP-glucose is more than sufficient to account for the combined rates of accumulation of sucrose, sterylglucosides, ..beta..-1,3-glucan, and cellulose. These data strongly indicate more » that UDP-glucose is a precursor to secondary wall cellulose in the cotton fiber. « less
Highlights
Since in uitm synthesis of cellulose has not been convincingly demonstrated in any higher plasnytstem, these studies were undertaken in order to trace the flow of carbon in vivo from glucose to cellulose in developing cotton fibers which were actively engaged in thedeposition of a cellulosic secondary cell wall
The results presented here offer strong supporftor the hypothesis thUatDP-glucose is on the pathway of secondary wall cellulose synthesis in cotton fibers
T h eprofiles shown are for extracts prepared from plant-grown fibers harvested 27 to 28 days post-anthesis (DPA), but similar profiies were observed for all ages of fibers examined
Summary
The specific activity of UDP-glucose was determined the radioactivity in UDP-glucose was determined by quantitation of the radioactivity found in UDP-['4C]glucuronic acid following the dehydrogenase assay For this purpose, the product was separated from the remaining UDP-sugars by high pressure liquid chromatography as described under Method 2 above. Identification of the UDP-Sugars quickly using plastic transferbaskets(3)into six successivewash solutions containing the incubation mediumwhich contained only 10 mM unlabeled glucose as carbonsource; this treatment (about[2] min) reduced theradioactivity in the medium tloess than 0.1%of the pulse medium. Ovules, with their associated fibers, were harvested a t var-. The anions were eluted with 10 ml of 0.5 M NH4HC03, and NH4HC0, thwenas removedby repeated evaporation
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