Contribution of adenosine 5 '-diphosphoglucose pyrophosphorylase to the control of starch synthesis is decreased by water stress in growing potato tubers

Water stress stimulates sucrose synthesis and inhibits starch and cell-wall synthesis in tissue slices of growing potato (Solanum tuberosum L. cv. Des iree) tubers. Based on the analysis of fluxes and metabolites, Geigenberger et al, (1997, Planta. 201: 502-518) proposed that water deficits up to -0. 72 MPa stimulate sucrose synthesis, leading to decreased starch synthesis a s a result of the resulting decline of phosphorylated metabolite levels, wh ereas more-severe water deffcits directly inhibit the use of ADP-glucose. P otato plants with decreased expression of adenosine 5'-diphosphoglucose pyr ophosphorylase (AGPase) have been used to test the prediction that the cont ribution of AGPase to the control of starch synthesis should decrease in se verely water-stressed tuber material. Freshly cut slices from wild-type and antisense tubers were incubated at a range of mannitol concentrations (20, 300 and 500 mM) and the metabolism of [C-14]glucose was analysed, A 86-97% reduction of ACPase activity led to a major but non-stoichiometric inhibit ion of starch accumulation in intact growing tubers attached to the plant ( 40-85%), and an inhibition of starch synthesis in non-stressed tuber slices incubated in 20 mM mannitol (60-80%). The inhibition of starch synthesis w as accompanied by a 2- to 8-fold increase in the levels of sugars in intact tubers and a 2- to 3-fold stimulation of sucrose synthesis in tuber slices , whereas respiration and cell-wall synthesis were not significantly affect ed. The strong impact of ACPase on carbon partitioning in non-stressed tube rs and tuber slices was retained in slices subjected to moderate water defi cit (300 mM mannitol, corresponding to -0.72 MPa). In discs incubated in 50 0 mM mannitol (corresponding to -1.2 MPa) this response was modified. A 80- 97% reduction of AGPase resulted in only a 0-40% inhibition of starch synth esis, Further, the water stress-induced stimulation of sucrose synthesis wa s abolished in the transformants. The results provide direct evidence that the contribution of AGPase to the control of starch synthesis can be modifi ed by environmental factors, leading to a lower degree of control during se vere water deficits. There was also a dramatic decrease in the labelling of cell-wall components in wildtype tuber slices incubated with 300 or 500 mM mannitol. The water stress-induced inhibition of cell-wall synthesis occur red independently of AGPase expression and the accompanying changes in star ch and sucrose metabolism, indicating a direct inhibition of cell-wall synt hesis in response to water stress.