Abstract
Photosynthetic carbon metabolism was investigated in antisense Arabidopsis lines with decreased expression of sucrose phosphate synthase (SPS) and cytosolic fructose-1,6-bisphosphatase (cFBPase). In the light, triose phosphates are exported from the chloroplast and converted to sucrose via cFBPase and SPS. At night, starch is degraded to glucose, exported and converted to sucrose via SPS. cFBPase therefore lies upstream and SPS downstream of the point at which the pathways for sucrose synthesis in the day and night converge. Decreased cFBPase expression led to inhibition of sucrose synthesis; accumulation of phosphorylated intermediates; Pi-limitation of photosynthesis; and stimulation of starch synthesis. The starch was degraded to maintain higher levels of sugars and a higher rate of sucrose export during the night. This resembles the response in other species when expression of enzymes in the upper part of the sucrose biosynthesis pathway is reduced. Decreased expression of SPS inhibited sucrose synthesis, but phosphorylated intermediates did not accumulate and carbon partitioning was not redirected towards starch. Sugar levels and sucrose export was decreased during the night as well as during the day. Although ribulose-1,5-bisphosphate regeneration and photosynthesis were inhibited, the PGA/triose-P ratio remained low and the ATP/ADP ratio high, showing that photosynthesis was not limited by the rate at which Pi was recycled during end-product synthesis. Two novel responses counteracted the decrease in SPS expression and explain why phosphorylated intermediates did not accumulate, and why allocation was not altered in the antisense SPS lines. Firstly, a threefold decrease of PPi and a shift of the UDP-glucose/hexose phosphate ratio favoured sucrose synthesis and prevented the accumulation of phosphorylated intermediates. Secondly, there was no increase of AGPase activity relative to cFBPase activity, which would prevent a shift in carbon allocation towards starch synthesis. These responses are presumably triggered when sucrose synthesis is decreased in the night, as well as by day.
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Claims 14
Claim · model-written summary
Reduced cFBPase expression inhibited sucrose synthesis.
- Taxon
- Arabidopsis thaliana
Arabidopsis thalianat0 - Treatment
- Decreased cFBPase expressione0
- Gene
- cFBPasee0
- Outcome
- sucrose synthesise0
- Direction
- inhibitione0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Decreased cFBPase expression led to inhibition of sucrose synthesis”
t0“Arabidopsis thaliana”
Claim · model-written summary
Reduced cFBPase expression caused phosphorylated intermediates to accumulate.
- Taxon
- Arabidopsis thalianaArabidopsis thalianat0
- Treatment
- Decreased cFBPase expressione0
- Gene
- cFBPasee0
- Outcome
- phosphorylated intermediatese0
- Direction
- accumulatione0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Decreased cFBPase expression led to inhibition of sucrose synthesis; accumulation of phosphorylated intermediates”
t0“Arabidopsis thaliana”
Claim · model-written summary
Reduced cFBPase expression led to Pi limitation of photosynthesis.
- Taxon
- Arabidopsis thalianaArabidopsis thalianat0
- Outcome
- photosynthesise0
- Direction
- Pi-limitatione0
Not reportedTreatmentComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Pi-limitation of photosynthesis”
t0“Arabidopsis thaliana”
Claim · model-written summary
Reduced cFBPase expression stimulated starch synthesis.
- Taxon
- Arabidopsis thalianaArabidopsis thalianat0
- Outcome
- starch synthesise0
- Direction
- stimulatione0
Not reportedTreatmentComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“stimulation of starch synthesis”
t0“Arabidopsis thaliana”
Claim · model-written summary
Starch degradation maintained higher nighttime sugar levels and sucrose export.
- Taxon
- Arabidopsis thalianaArabidopsis thalianat0
- Outcome
- higher levels of sugars and a higher rate of sucrose exporte0
- Direction
- highere0
- Qualifier
- The starch was degradede0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“The starch was degraded to maintain higher levels of sugars and a higher rate of sucrose export during the night”
t0“Arabidopsis thaliana”
Claim · model-written summary
Reduced SPS expression inhibited sucrose synthesis.
- Taxon
- Arabidopsis thalianaArabidopsis thalianat0
- Treatment
- Decreased expression of SPSe0
- Gene
- SPSe0
- Outcome
- sucrose synthesise0
- Direction
- inhibitede0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Decreased expression of SPS inhibited sucrose synthesis”
t0“Arabidopsis thaliana”
Claim · model-written summary
Phosphorylated intermediates did not accumulate with reduced SPS expression.
- Taxon
- Arabidopsis thalianaArabidopsis thalianat0
- Outcome
- phosphorylated intermediatese0
- Direction
- did not accumulatee0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“but phosphorylated intermediates did not accumulate”
t0“Arabidopsis thaliana”
Claim · model-written summary
Carbon partitioning was not redirected toward starch with reduced SPS expression.
- Taxon
- Arabidopsis thalianaArabidopsis thalianat0
- Outcome
- carbon partitioninge0
- Direction
- was not redirectede0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“carbon partitioning was not redirected towards starch”
t0“Arabidopsis thaliana”
Claim · model-written summary
Sugar levels and sucrose export decreased during day and night with reduced SPS expression.
- Taxon
- Arabidopsis thalianaArabidopsis thalianat0
- Outcome
- Sugar levels and sucrose exporte0
- Direction
- decreasede0
Not reportedTreatmentComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Sugar levels and sucrose export was decreased during the night as well as during the day”
t0“Arabidopsis thaliana”
Claim · model-written summary
Ribulose-1,5-bisphosphate regeneration and photosynthesis were inhibited.
- Taxon
- Arabidopsis thalianaArabidopsis thalianat0
- Outcome
- ribulose-1,5-bisphosphate regeneration and photosynthesise0
- Direction
- inhibitede0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“ribulose-1,5-bisphosphate regeneration and photosynthesis were inhibited”
t0“Arabidopsis thaliana”
Claim · model-written summary
PGA/triose-P ratio remained low and ATP/ADP ratio high, indicating photosynthesis was not Pi recycling limited.
- Taxon
- Arabidopsis thalianaArabidopsis thalianat0
- Outcome
- photosynthesise0
- Direction
- was not limitede0
- Qualifier
- the rate at which Pi was recycled during end-product synthesise0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the PGA/triose-P ratio remained low and the ATP/ADP ratio high, showing that photosynthesis was not limited by the rate at which Pi was recycled during end-product synthesis”
t0“Arabidopsis thaliana”
Claim · model-written summary
PPi decreased threefold in SPS antisense lines.
- Taxon
- Arabidopsis thalianaArabidopsis thalianat0
- Outcome
- PPie0
- Direction
- decreasee0
- Value
- threefolde0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“a threefold decrease of PPi”
t0“Arabidopsis thaliana”
Claim · model-written summary
A shifted UDP-glucose/hexose phosphate ratio favoured sucrose synthesis and prevented intermediate accumulation.
- Taxon
- Arabidopsis thalianaArabidopsis thalianat0
- Outcome
- sucrose synthesise0
- Direction
- favourede0
- Qualifier
- a shift of the UDP-glucose/hexose phosphate ratioe0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“a shift of the UDP-glucose/hexose phosphate ratio favoured sucrose synthesis and prevented the accumulation of phosphorylated intermediates”
t0“Arabidopsis thaliana”
Claim · model-written summary
AGPase activity did not increase relative to cFBPase activity.
- Taxon
- Arabidopsis thalianaArabidopsis thalianat0
- Gene
- AGPasee0
- Outcome
- AGPase activitye0
- Direction
- no increasee0
- Comparator
- cFBPase activitye0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“there was no increase of AGPase activity relative to cFBPase activity”
t0“Arabidopsis thaliana”
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