Synthesis and mobilization of carbohydrate in xylem parenchyma cells of cotton during water deficiency
Roper, Becky Jo
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The aim of this study was to determine whether water deficiency affected the carbohydrate content of cotton xylem parenchyma cells and the total extractable activity of key enzymes associated with starch synthesis and mobilization and sucrose synthesis. Wild-type cotton (Gossypium hirsutum L. cv Coker 312) were subjected to well-watered, water-deficit, and re-watered treatments along with SPS+ cotton to determine whether the total sucrose phosphate synthase (SPS) activity and sucrose content of xylem parenchyma cells increased when the spinach gene for SPS is overexpressing in cotton. Cotton plants were grown to certain periods of development (seedling, flowering, or boll developing) and subjected to a gradual water deficit. Once these stages were determined and showed significant effect by water deficiency, the process was repeated on flowering cotton in the summer and autumn, and with boll developing cotton in the autumn. In these single stage experiments, half of the water deficient treated cotton were left to be re-watered for four days and sampled to determine the extent of the recovery of sink metabolism and photosynthate production. Leaf CO2-exchange measurements were conducted the day before sampling with pre-dawn leaf water status taken the day of sampling. Nodal xylem tissue with the bark removed were sampled, frozen, and stored in -85C until extraction analysis could be preformed. Activities of SPS, ADP-glucose pyrophosphorylase (AGPase), sucrose synthase (SucSyn), and amylase (fall boll only) from xylem tissue samples were determined and compared with the contents of hexose, sucrose, and starch. The results indicated that stored starch in the xylem parenchyma cells of cotton is catabolized (â€œmobilizedâ€) to hexose, glucose and fructose, for sucrose synthesis or other uses during water deficit stress. Water deficit stress in both types of cotton in all major stages of growth caused a decline in SucSyn and an increase in beta-amylase. This supports the hypothesis that sink carbohydrate metabolism can be downregulated and carbohydrate metabolism using stored reserves as a source of soluble sugars can be upregulated in xylem parenchyma cells during environmental stress. The information gained here will provide further insight on the possibility that starch reserves in xylem parenchyma cells can be modified to serve as sources of soluble carbohydrate for osmotic adjustment and growth of strong sinks (e.g., bolls) during water deficit stress.