Remember as stated in the web notes for lecture 7, the utilization of triose phosphate generated by photosynthesis in starch, sucrose and/or fructan synthesis is tightly controlled (see Chapt. 9 particularly sections 9.1 – 9.3 of the Heldt text). Since 5/6 of triose phosphate synthesized in photosynthesis is needed for regeneration of RuBP, a maximum of 1/6 of triose phosphate formed is available for synthesis of fructose and other molecules in the cytoplasm. In C₃ plants only ~ 1/8 of triose phosphate is available for export from chloroplasts due to photorespiration. If more than 1/6 to 1/8 of the triose phosphate were removed from the Calvin-Benson cycle, RuBP could no longer be regenerated and the cycle would collapse. However, CO₂ fixation reactions of photosynthesis can only proceed if the triose phosphate formed is utilized in synthesis of molecules such as sucrose, starch or fructans. Thus formation of storage and transport carbohydrates can only but must proceed when sufficient photosynthesis occurs. Also remember the 3PG and Pi concentrations of the chloroplast stroma are held constant by triose P-Pi counter-exchange. When sucrose synthesis in the cytoplasm decreases, less Pi is released in the cytosol leading to Pi deficiency (>3PG/Pi) in chloroplasts which limits photosynthesis.

s

Sucrose phosphate synthase catalyses the following reaction in the cytosol:

UDP-glucose + fructose 6-P à sucrose 6-P (see Fig. 9.14 of text)

Both triose phosphate and glucose export from plastids contribute to the cytosolic glucose and fructose for sucrose synthesis and Pi for photosynthetic reactions (see also the answer to the question posted 2/11/99).

Thus it would be expected that increased sucrose phosphate synthase would increase photosynthesis up to the point that triose phosphate formation becomes limiting. This has been born out by some studies such as in the two manuscripts by Galtier et al. published in 1995 and abstracted below:

AUTHORS

Signora,-L.; Galtier,-N.; Skot,-L.; Lucas,-H.; Foyer,-C.H.

TITLE

Over-expression of sucrose phosphate synthase in Arabidopsis thaliana results in

increased foliar sucrose/starch ratios and favours decreased foliar carbohydrate

accumulation in plants after prolonged growth with CO2 enrichment.

SOURCE

J-exp-bot.Oxford : Oxford University Press. Apr 1998. v. 49 (321) p. 669-680..

CALL NUMBER

DNAL 450-J8224

LANGUAGE

English

ABSTRACT

Arabidopsis thaliana ecotype Columbia was transformed with a maize sucrose phosphate

synthase (SPS) cDNA under the control of the promoter for the small subunit of

ribulose-1,5-bisphosphate carboxylase from tobacco (rbcS). The effects of SPS

over-expression were compared in plants of the T2 and T3 generations grown either in air

or with CO2 enrichment (700 microliter l -1) for either 4 or 10 weeks. Maximal extractable

foliar SPS activities were three times those of the untransformed controls in the highest

rbcS-SPS expressing line. In untransformed Arabidopsis leaves SPS activity was not

subject to light/dark regulation, but was modified by incubation with either the inhibitor,

orthophosphate, or the activator, mannose. Photosynthesis (Amax) values were similar in all

lines grown in air. After 10 weeks of CO2 enrichment a decrease in Amax in the

untransformed controls, but not in the high SPS expressors, was observed. There was a

strong correlation between the sucrose-to-starch ratio of the leaves and their SPS activity in

both growth conditions. The total foliar carbohydrate contents of 4-week-old plants was

similar in all lines whether plants were grown in air or with CO2 enrichment. After 10

weeks growth the leaves of the high rbcS-SPS expressors accumulated much less total

carbohydrate than untransformed control leaves in both growth conditions. It was concluded

that SPS overexpression causes increased foliar sucrose/starch ratios in Arabidopsis leaves

and favours decreased foliar carbohydrate contents when plants are grown for long periods

with CO2 enrichment.

AUTHORS

Laporte,-M.M.; Galagan,-J.A.; Shapiro,-J.A.; Boersig,-M.R.; Shewmaker,-C.K.;

Sharkey,-T.D.

TITLE

Sucrose-phosphate synthase activity and yield analysis of tomato plants transformed with

maize sucrose-phosphate synthase.

SOURCE

Planta.Berlin ; New York : Springer-Verlag, 1925-. Oct 1997. v. 203 (2) p. 253-259..

CALL NUMBER

DNAL 450-P693

LANGUAGE

English

ABSTRACT

Sucrose synthesis is a major element of the interactions between photosynthesis and plant

growth and development. Tomato (Lycopersicon esculentum Mill. cv. UC82B) plants

transformed with maize sucrose-phosphate synthase (SPS; EC 2.3.1.14) expressed from

either a ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) small subunit

promoter (SSU) or the cauliflower mosaic virus 35S promoter (35S) were used to study

effects of increased sucrose synthesis rates on plant growth. The plants were grown in

growth chambers, field plots, and open-top chambers. The 35S plants had a 2 to 3-fold

increase in young-leaf SPS activity, a 10 to 20-fold increase in young-root SPS activity and

no increase in young-fruit SPS activity. The leaf SPS activity in one of the 35S lines fell to

control levels by two months of age. The SSU plants had a 4 to 5-fold increase in leaf SPS

activity and no significant increase in root or young-fruit SPS activity. One 35S line, which

maintained high leaf SPS activity throughout development, yielded 70-80% more than

controls at both normal and elevated CO2 in open-top chambers in the field and 20-30%

more than controls in two additional field trials. The other 35S line and the two SSU lines

either yielded less or did not differ from controls under several growth conditions. Since

only one of four transformed lines showed an increase in yield, we can not yet conclude that

increased leaf SPS activity leads to increased yield. However, increased leaf SPS activity

appears to result in increased fruit sugar content since all three lines with increased leaf

SPS usually also had increased fruit sugars. .

AUTHORS

Galtier,-N.; Foyer,-C.H.; Murchie,-E.; Alred,-R.; Quick,-P.; Voelker,-T.A.; Thepenier,-C.;

Lasceve,-G.; Betsche,-T.

TITLE

Effects of light and atmospheric carbon dioxide enrichment on photosynthesis and carbon

partitioning in the leaves of tomato (Lycopersicon esculentum L.) plants over-expressing

sucrose phosphate synthase.

SOURCE

J-exp-bot.Oxford : Oxford University Press. Sept 1995. v. 46 (special issue) p. 1335-1344..

CALL NUMBER

DNAL 450-J8224

LANGUAGE

English

ABSTRACT

Photosynthetic carbon assimilation, carbon partitioning and foliar carbon budgets were

measured in the leaves of transformed tomato plants expressing a maize sucrose-phosphate

synthase (SPS) gene in addition to the native enzyme, and in untransformed controls. The

maize SPS gene was expressed under control of either the promoter of the small subunit of

ribulose 1,5-bisphosphate carboxylase (rbcS promoter; lines 2, 9 and 18) or the 35S

promoter from cauliflower mosaic virus (CaMV promoter; line 13). The rate of sucrose

synthesis was increased relative to that of starch and sucrose/starch ratios were higher

throughout the photoperiod in the leaves of all plants expressing high SPS activity. The leaf

carbon budget over the day/night cycle in air at low irradiance (180 micromole photon m-2

s-1) was similar in all plants. Net photosynthesis measured in air and at elevated CO2

(800-1500 microliter l-1) on whole plants grown in air at 400 micromoles m-2 s-1

irradiance was significantly increased in the high SPS expressors compared to the

untransformed controls and was highest where SPS activity was greatest. At high CO2 the

stimulation of photosynthesis was more pronounced. We conclude that SPS activity is a

major point of control of photosynthesis particularly under saturating light and CO2. .

AUTHOR

Galtier,-N.; Foyer,-C.H.; Huber,-J.; Voelker,-T.A.; Huber,-S.C.

TITLE

Effects of elevated sucrose-phosphate synthase activity on photosynthesis, assimilate

partitioning, and growth in tomato (Lycopersicon esculentum var UC82B).

SOURCE

Plant-physiol.Rockville, MD : American Society of Plant Physiologists, 1926-. Feb 1993.

v. 101 (2) p. 535-543..

CALL NUMBER

DNAL 450-P692

LANGUAGE

English

ABSTRACT

The expression of a sucrose-phosphate synthase (SPS) gene from maize (Zea mays, a

monocotyledon) in tomato (Lycopersicon esculentum, a dicotyledon) resulted in marked

increases in extractable SPS activity in the light and the dark. Diurnal modulation of the

native tomato SPS activity was found. However, when the maize enzyme was present the

tomato leaf cells were unable to regulate its activation state. No detrimental effects were

observed and total dry matter production was unchanged. However, carbon allocation

within the plants was modified such that in shoots it increased, whereas in roots it

decreased. There was, therefore, a change in the shoot:root dry weight ratio favoring the

shoot. This was positively correlated with increased SPS activity in leaves. SPS was a

major determinant of the amount of starch in leaves as well as sucrose. There was a strong

positive correlation between the ratio of sucrose to starch and SPS activity in leaves.

Therefore, SPS activity is a major determinant of the partitioning of photosynthetically fixed

carbon in the leaf and in the whole plant. The photosynthetic rate in air was not significantly

increased as a result of elevated leaf SPS activity. However, the light- and CO2-saturated

rate of photosynthesis was increased by about 20% in leaves expressing high SPS. In

addition, the temporary enhancement of the photosynthetic rate following brief exposures to

low light was increased in the high SPS plants relative to controls. We conclude that the

level of SPS in the leaves plays a pivotal role in carbon partitioning. Furthermore, high SPS

levels have the potential to boost photosynthetic rates under favorable conditions. .

see also Micallef et al. (1995) Planta 196: 327-334.