Regulation of sucrose and starch metabolism in potato tubers in response to short-term water deficit |
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Authors: | Peter Geigenberger Ralph Reimholz Michael Geiger Lucia Merlo Vittoria Canale Mark Stitt |
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Institution: | (1) Botanisches Institut, Im Neuenheimer Feld 360, D-69120 Heidelberg, Germany, DE |
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Abstract: | To investigate the effect of water stress on carbon metabolism in growing potato tubers (Solanum tuberosum L.), freshly cut and washed discs were incubated in a range of mannitol concentrations corresponding to external water potential
between 0 and −1.2 MPa. (i) Incorporation of 14C]glucose into starch was inhibited in water-stressed discs, and labeling of sucrose was increased. High glucose overrode
the changes at low water stress (up to −0.5 MPa) but not at high water stress. (ii) Although 14C]sucrose uptake increased in water-stressed discs, less of the absorbed 14C]sucrose was metabolised. (iii) Analysis of the sucrose content of the discs confirmed that increasing water deficit leads
to a switch, from net sucrose degradation to net sucrose synthesis. (iv) In parallel incubations containing identical concentrations
of sugars but differing in which sugar was labeled, degradation of 14C]sucrose and labeling of sucrose from 14C]glucose and fructose was found at each mannitol concentration. This shows that there is a cycle of sucrose degradation and
resynthesis in these tuber discs. Increasing the extent of water stress changed the relation between sucrose breakdown and
sucrose synthesis, in favour of synthesis. (v) Analysis of metabolites showed a biphasic response to increasing water deficit.
Moderate water stress (0–200 mM mannitol) led to a decrease of the phosphorylated intermediates, especially 3-phosphoglycerate
(3PGA). The decrease of metabolites at moderate water stress was not seen when high concentrations of glucose were supplied
to the discs. More extreme water stress (300–500 mM mannitol) was accompanied by an accumulation of metabolites at low and
high glucose. (vi) Moderate water stress led to an activation of sucrose phosphate synthase (SPS) in discs, and in intact
tubers. The stimulation involved a change in the kinetic properties of SPS, and was blocked␣by protein phosphatase inhibitors.
(vii) The amount of ADP-glucose (ADPGlc) decreased when discs were incubated on 100 or 200 mM mannitol. There was a strong
correlation between the in vivo levels of ADPGlc and 3PGA when discs were subjected to moderate water stress, and when the
sugar supply was varied. (viii) The level of ADPGlc increased and starch synthesis was further inhibited when discs were incubated
in 300–500 mM mannitol. (ix) It is proposed that moderate water stress leads to an activation of SPS and stimulates sucrose
synthesis. The resulting decline of 3PGA leads to a partial inhibition of ADP-glucose pyrophosphorylase and starch synthesis.
More-extreme water stress leads to a further alteration of partitioning, because it inhibits the activities of one or more
of the enzymes involved in the terminal reactions of starch synthesis.
Received: 26 August 1996 / Accepted: 5 November 1996 |
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Keywords: | Key words: Adenosine 5′ -diphosphoglucose Starch synthesis Solanum (carbon metabolism tuber) Sucrose-phosphate synthase Sucrose metabolism (futile cycle) Water stress |
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