Introduction of polyphosphate as a novel phosphate pool in the chloroplastof transgenic potato plants modifies carbohydrate partitioning

Potato plants (Solanum tuberosum L., cv. Desiree) were transformed with the polyphosphate kinase gene from Escherichia coil fused to the leader sequen ce of the ferredoxin oxidoreductase gene (FNR) from Spinacea oleracea under the control of the leaf specific St-LS1 promoter to introduce a novel phos phate pool in the chloroplasts of green tissues. Transgenic plants (cpPPK) in tissue culture developed necrotic lesions in older leaves and showed ear lier leaf senescence while greenhouse plants showed no noticeable phenotype . Leaves of cpPPK plants contained less starch but higher concentrations of soluble sugars. The presence of polyphosphate in cpPPK leaves was demonstr ated by toluidine blue staining and unambiguously verified and quantified b y in vitro P-31-NMR of extracts. Polyphosphate accumulated during leaf deve lopment from 0.06 in juvenile leaves to 0.83 mg P g(-1) DW in old leaves an d had an average chain length of 18 residues in mature leaves. In situ P-31 -NMR on small leaf pieces perfused with well-oxygenated medium showed only 0.036 mg P g(-1) DW polyphosphate that was, however, greatly increased upon treatment with 50 mM ammonium sulfate at pH 7.3. This phenomenon along wit h a yield of 0.47 mg P g(-1) DW polyphosphate from an extract of the same l eaf material suggests that 93% of the polyphosphate pool is immobile. This conclusion is substantiated by the observation that no differences in polyp hosphate pool sizes could be discerned between darkened and illuminated lea ves: leaves treated with methylviologen or anaerobis and control leaves, tr eatments causing a change in the pool of ATP available for polyP(i) synthes is. Results are discussed in the context of the chelating properties of pol yphosphates for cations and its consequences for the partitioning of photoa ssimilate between starch and soluble sugars. (C) 2000 Elsevier Science B.V. All rights reserved.