When sucrose-phosphate synthase (SPS; EC 2.4.1.14) is expressed in tomato (Lycopersicon esculentum Mill.) from a ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) small subunit (rbcS) promoter, yields are often unchanged but when SPS is expressed from a Cauliflower Mosaic Virus 35S promoter, yield is enhanced up to 80%. Two explanations for this phenomenon are (i) that expression of SPS in tissues other than leaves accounts for the increased yield or (ii) that the lower level of expression directed by the 35S promoter is more beneficial than the high level of expression directed by the rbcS promoter. To test the first hypothesis, we conducted a reciprocal graft experiment, which showed that root SPS activity did not substantially affect growth. To test the second hypothesis, we conducted a field trial using a backcrossed, segregating, population of SPS-transformed plants derived from 35S and rbcS lines. The optimal dose of SPS activity for growth was approximately twice that of the wild type regardless of which promoter was used. The effect of SPS on growth was the result of a shift in partitioning of carbon among starch, sucrose, and ionic compounds (primarily amino acids), rather than of an increase in net photosynthesis. Excessive SPS activity resulted in a decreased rate of amino acid synthesis, which could explain the non-linear response of plant growth to the level of SPS expression.
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 carboxylaseoxygenase (Rubisco) small subunit promoter (SSU) or the cauli¯ower mosaic virus 35S promoter (35S) were used to study eects of increased sucrose synthesis rates on plant growth. The plants were grown in growth chambers, ®eld 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 signi®cant 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 CO 2 in open-top chambers in the ®eld and 20±30% more than controls in two additional ®eld trials. The other 35S line and the two SSU lines either yielded less or did not dier 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.Key words: Cauli¯ower mosaic virus 35S promoter ± Growth chamber (open topped) Lycopersicon ± Open top chambers ± Ribulose-1,5-bisphosphate carboxylase-oxygenase small subunit promoter ± Sucrose-phosphate synthase ± Yield (transformed tomato) Abbreviations: DAS = days after seeding; SPS = sucrosephosphate synthase; SSU = small subunit of Rubisco Correspondence to: M. Laporte;
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