Planta

, Volume 185, Issue 1, pp 81–90

A “futile” cycle of sucrose synthesis and degradation is involved in regulating partitioning between sucrose, starch and respiration in cotyledons of germinating Ricinus communis L. seedlings when phloem transport is inhibited

  • Peter Geigenberger
  • Mark Stitt
Article

DOI: 10.1007/BF00194518

Cite this article as:
Geigenberger, P. & Stitt, M. Planta (1991) 185: 81. doi:10.1007/BF00194518
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Abstract

There was a dramatic alteration in the pattern of metabolism of [U14C]glucose by cotyledons of germinating Ricinus communis L. seedlings when phloem transport was inhibited by removing most of the hypocotyl and root. (i) Incorporation into sucrose was decreased two- to threefold, incorporation into starch was stimulated three- to sixfold, and there was a small increase of respiration, (ii) Pulse-chase experiments using 14C and measurements of the total sucrose content revealed a rapid cycle of sucrose synthesis and degradation. When export is inhibited there is a two- to threefold inhibition of unidirectional sucrose synthesis and a three-fold stimulation of unidirectional sucrose degradation. As a result, the net flux switches from rapid net synthesis to slow net mobilisation of sucrose, (iii) The cotyledons contained adequate activities of sucrose synthase, acid and alkaline invertase and sucrose-phosphate synthase to catalyse the observed rate of sucrose breakdown and synthesis, respectively. The extracted activities of the degradative enzymes did not change after inhibiting phloem transport. The maximum activity of sucrose-phosphate synthase was also unaltered, but the activity measured in the presence of limiting substrates and phosphate was decreased twofold, indicating that sucrose-phosphate synthase has been deactivated by a mechanism analogous to that occurring in spinach leaves. (iv) The switch from sucrose export to starch synthesis when phloem transport was prevented was accompanied by only a small (20–50%) increase of the sucrose concentration in the cotyledons, no change of hexose-phosphates, an increase (16–70%) of fructose-1,6-bisphosphate and triosephosphate, and a small decrease (15–30%) of glycerate-3-phosphate, glycerate-2-phosphate and phosphoenolpyruvate. Fructose-2,6-bisphosphate and pyrophosphate doubled when 10 mM phosphate was included in the medium bathing the cotyledons, but not when phosphate was omitted (v) It is concluded that a futile cycle involving simultaneous synthesis and degradation of sucrose allows sucrose metabolism to respond in an extremely sensitive manner when phloem export is inhibited. There is a dramatic switch of flux through the sucrose pool, even though there are only marginal changes in the concentrations of sucrose and metabolites, or in the rate of respiration.

Key words

Futile cycleRicinusSucrose (metabolism, transport)

Abbreviations

Fru1,6bisP

fructose-1,6-bisphosphate

Fru1,6Pase

fructose-1,6-bisphosphatase

Fru2,6bisP

fructose-2,6-bisphosphate

Fru6P

fructose-6-phosphate

FW

fresh weight

Glc1P

glucose-1-phosphate

Glc6P

glucose-6-phosphate

PFP

pyrophosphate:fructose-6-phosphate phosphotransferase

PEP

phosphoenolpyruvate

3PGA

glycerate-3-phosphate

2PGA

glycerate-2-phosphate

Pi

inorganic phosphate

PPi

inorganic pyrophosphate

SPS

sucrose-phosphate synthase

UDPGIc

undine 5′-diphosphoglucose

Copyright information

© Springer-Verlag 1991

Authors and Affiliations

  • Peter Geigenberger
    • 1
  • Mark Stitt
    • 1
  1. 1.Lehrstuhl für Pflanzenphysiologie, Universität BayreuthBayreuthFederal Republic of Germany