The expression of a sucrose-phosphate synthase (SPS) gene from maize (Zea mays, a monocotyledon) in tomato (Lycopersicon esculenfum, a dicotyledon) resulted i n marked increases in extractable SPS activity in the light and the dark. Diurna1 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 shootroot 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 i n 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 C02-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 leve1 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. ~ ~~~~~~~~ SPS catalyzes the penultimate step of sucrose biosynthesis in leaves (Pontis, 1977). This rate-limiting, allosteric enzyme forms a major control point subject to regulation by metabolites and by covalent modification involving protein phosphorylation (Harbron et al., 1981; Huber, 1983;Stitt et al., 1987;Huber et al., 1989;Stitt and Quick, 1989). The activity of this enzyme can be a limiting factor for de novo SUC synthesis and also photosynthesis (Pontis, 1977; Huber, 1983;Stitt et al., 1987). The amount of extractable SPS activity is known to vary in response to light/dark transitions (Huber et al., 1987) and to the metabolic status of the leaf tissue in terms of substrate concentrations (Harbron et al., 1981; Huber, 1983; Kalt-Torres and Huber, 1987;Kerr and Huber, 1987;Stitt et al., 1987).* Corresponding author; fax 33-30-83-30-96. Multiple forms of SPS have been observed in leaves of plants such as maize (Kalt-Torres et al., 1987). The multiplicity of enzyme forms appears to arise from posttranslational modifications (Huber et al., 1989; Huber and Huber, 1991) to the enzyme protein because the cDNA isolated from maize appears to have only one predominant species of transcript (Worrell et al., 1991). The maize leaf SPS has been shown to be subject to light/dark regulation (Kalt-Torres and Huber, 1987), a process mediated by phosphorylation ...

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 (180μmol photon m−2 s−1) was similar in all plants. Net photosynthesis measured in air and at elevated CO2 (800–1500 μl l−1) on whole plants grown in air at 400μmol 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.

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The effects of cadmium (Cd) on cellular proteolytic responses were investigated in the roots and leaves of tomato (Solanum lycopersicum L., var Ibiza) plants. Three-week-old plants were grown for 3 and 10 days in the presence of 0.3-300 microM Cd and compared to control plants grown in the absence of Cd. Roots of Cd treated plants accumulated four to fivefold Cd as much as mature leaves. Although 10 days of culture at high Cd concentrations inhibited plant growth, tomato plants recovered and were still able to grow again after Cd removal. Tomato roots and leaves are not modified in their proteolytic response with low Cd concentrations (< or =3 microM) in the incubation medium. At higher Cd concentration, protein oxidation state and protease activities are modified in roots and leaves although in different ways. The soluble protein content of leaves decreased and protein carbonylation level increased indicative of an oxidative stress. Conversely, protein content of roots increased from 30 to 50%, but the amount of oxidized proteins decreased by two to threefold. Proteolysis responded earlier in leaves than in root to Cd stress. Additionally, whereas cysteine- and metallo-endopeptidase activities, as well as proteasome chymotrypsin activity and subunit expression level, increased in roots and leaves, serine-endopeptidase activities increased only in leaves. This contrasted response between roots and leaves may reflect differences in Cd compartmentation and/or complexation, antioxidant responses and metabolic sensitivity to Cd between plant tissues. The up-regulation of the 20S proteasome gene expression and proteolytic activity argues in favor of the involvement of the 20S proteasome in the degradation of oxidized proteins in plants.

Leaf sucrose, starch, hexose and maximum extractable soluble acid invertase activity were compared throughout the day in source leaves of 13 plant species chosen for their putative phloem-loading type (apoplastic or symplastic). Four species which represent the different phloem-loading types (tomato, barley, maize and Fuchsia) were studied in detail. Using this information we wished to determine whether a positive correlation between foliar carbohydrates and acid invertase activity exists in leaves from different species and, furthermore, whether this relationship is determined by phloem-loading type. Acid invertase activity was relatively constant throughout the day in all species. The extent of sucrose, hexose and starch accumulation and the sucrose : starch ratio measured at a given time were species-dependent. No correlations were found between foliar soluble acid invertase activity and the hexose, sucrose or starch content of the leaves in any of the species, regardless of phloem-loading type. The species examined could be divided into three distinct groups : (1) high sucrose, low invertase ; (2) low sucrose, low invertase ; and (3) low sucrose, high invertase. The absence of an inverse relationship between leaf sucrose, hexose or starch contents and endogenous soluble acid invertase suggests that this enzyme is not directly involved in carbon partitioning in leaves but serves an auxiliary function.