Planta

, 182:445 | Cite as

Control analysis of photosynthate partitioning

Impact of reduced activity of ADP-glucose pyrophosphorylase or plastid phosphoglucomutase on the fluxes to starch and sucrose inArabidopsis thaliana (L.) Heynh.
  • H. Ekkehard Neuhaus
  • Mark Stitt
Article

Abstract

Experiments were carried out to investigate the contribution of ADP-glucose pyrophosphorylase and the plastid phosphoglucosemutase to the control of starch synthesis. Mutants ofArabidopsis thaliana (L.) Heyhn. were constructed with 50% and 7% of the wild-type adenosine 5′-diphosphoglucose pyrophosphorylase (ADPGlc-PPase), or 50% and null plastid phosphoglucomutase (PGM). The changes in the steady-state rates of sucrose synthesis, starch synthesis and CO2 fixation were measured in saturating CO2 in low (75 μmol·m−2·s−1) and high (600 μmol·m−2·s−1) irradiance. In low irradiance, a 50% decrease of PGM had no significant effect on fluxes, while a 50% and 93% decrease of ADPGlc-PPase led to a 23% and 74% inhibition of starch synthesis. Decreased ADPGlc-PPase led to an increase of hexose phosphates, triose phosphates and fructose-1,6-bisphosphate. Fixation of CO2 was not inhibited because the inhibition of starch synthesis was matched by a stimulation of sucrose synthesis. In high irradiance, a 50% decrease of PGM led to a 20% inhibition of starch synthesis. A 50% and 93% decrease of ADPGlc-PPase led to a 39% and 90% inhibition of starch synthesis. Sucrose synthesis was also inhibited, and the rate of photosynthesis was decreased. Decreased ADPGlc-PPase led to an increase of hexose phosphates but triose phosphates and fructose-1,6-bisphosphate did not increase. These results are used to estimate flux-control coefficients for these enzymes for starch synthesis. Firstly, the flux to starch is only controlled by ADPGlc-PPase in low irradiance, but control is redistributed to other enzymes in the pathway when a rapid flux is imposed, e.g. in high irradiance and CO2. Secondly, reducing the rate of starch synthesis by decreasing the activity of enzymes in this pathway does not always lead to a compensating increase in the rate of sucrose synthesis. Thirdly, decreasing the activity of an enzyme by a factor of two compared to the remainder of the pathway often leads to it exerting very considerable control. Fourthly, each enzyme starts to exert considerable control when only a fraction of its Vmax activity is being utilised in vivo, for example the maximum flux at ADPGlc-PPase never exceeded 20% of the Vmax activity. The summation theory is also applied to check whether additional major control sites are required. In low irradiance, the efficiency of light harvesting will exert considerable control over the rate of starch synthesis.

Key words

ADP-glucose pyrophosphorylase Arabidopsis (starch synthesis) Phosphoglucomutase Flux control coefficient Photosynthate partitioning Starch synthesis Sucrose synthesis 

Abbreviations

ADPGlc-PPase

adenosine 5′-diphosphate-glucose pyrophosphorylase

Fru1,6bisP

fructose-1,6-bisphosphate

Fru2,6bisP

fructose-2,6-bisphosphate

Fru6P

fructose-6-phosphate

Glc1P

glucose-1-phosphate

Glc6P

glucose-6-phosphate

PGA

glycerate-3-phosphate

PGI

phosphoglucose isomerase

PGM

phosphoglucomutase

Pi

inorganic phosphate

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Copyright information

© Springer-Verlag 1990

Authors and Affiliations

  • H. Ekkehard Neuhaus
    • 1
  • Mark Stitt
    • 1
  1. 1.Lehrstuhl für PflanzenphysiologieUniversität BayreuthBayreuthFederal Republic of Germany

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