An Arabidopsis thaliana knock-out mutant of the chloroplast triose phosphate/phosphate translocator is severely compromised only when starch synthesis, but not starch mobilisation is abolished.
The Arabidopsis thaliana tpt-1 mutant which is defective in the chloroplast triose phosphate/phosphate translocator (TPT) was isolated by reverse genetics. It contains a T-DNA insertion 24 bp upstream of the start ATG of the TPT gene. The mutant lacks TPT transcripts and triose phosphate (TP)-specific transport activities are reduced to below 5% of the wild type. Analyses of diurnal variations in the contents of starch, soluble sugars and phosphorylated intermediates combined with 14CO2 labelling studies showed, that the lack of TP export for cytosolic sucrose biosynthesis was almost fully compensated by both continuous accelerated starch turnover and export of neutral sugars from the stroma throughout the day. The utilisation of glucose 6-phosphate (generated from exported glucose) rather than TP for sucrose biosynthesis in the light bypasses the key regulatory step catalysed by cytosolic fructose 1,6-bisphosphatase. Despite its regulatory role in the feed-forward control of sucrose biosynthesis, variations in the fructose 2,6-bisphosphate content upon illumination were similar in the mutant and the wild type. Crosses of tpt-1 with mutants unable to mobilise starch (sex1) or to synthesise starch (adg1-1) revealed that growth and photosynthesis of the double mutants was severely impaired only when starch biosynthesis, but not its mobilisation, was affected. For tpt-1/sex1 combining a lack in the TPT with a deficiency in starch mobilisation, an additional compensatory mechanism emerged, i.e. the formation and (most likely) fast turnover of high molecular weight polysaccharides. Steady-state RNA levels and transport activities of other phosphate translocators capable of transporting TP remained unaffected in the mutants.
Schneider
Anja
A
Botanisches Institut der Universität zu Köln, Gyrhofstrasse 15, Germany. anja.schneider@uni-koeln.de
Häusler
Rainer E
RE
Kolukisaoglu
Uner
U
Kunze
Reinhard
R
van der Graaff
Eric
E
Schwacke
Rainer
R
Catoni
Elisabetta
E
Desimone
Marcelo
M
Flügge
Ulf-Ingo
UI
D017360:Arabidopsis / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics; D001692:Biological Transport; D002245:Carbon Dioxide / Q000378:metabolism; D060365:Chloroplast Proteins; D002736:Chloroplasts / Q000235:genetics / Q000378:metabolism*; D005635:Fructosediphosphates / Q000378:metabolism; D017353:Gene Deletion; D018506:Gene Expression Regulation, Plant; D008027:Light; D026901:Membrane Transport Proteins / Q000235:genetics* / Q000378:metabolism*; D010507:Periodicity; D010641:Phenotype; D010788:Photosynthesis; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism*; D013213:Starch / Q000096:biosynthesis / Q000378:metabolism*
D029681:Arabidopsis Proteins; D060365:Chloroplast Proteins; D005635:Fructosediphosphates; D026901:Membrane Transport Proteins; D010940:Plant Proteins; D002245:Carbon Dioxide; C027652:fructose 2,6-diphosphate; D013213:Starch