INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Plants are subjected to fluctuating environmental conditions, which result in changes in the supply of light and CO₂ for photosynthesis. Regulatory mechanisms exist in the chloroplasts, which match the rates of electron transport with carbon assimilation to ensure the coordination of the photosynthetic reactions. The PCR and photorespiratory cycles consume ATP and NAD(P)H produced by chloroplast electron transport and the availability of these compounds affects their functioning. Several photosynthetic enzymes are regulated by metabolites, either by their end products or intermediates occurring later in the PCR cycle (for review, see Stitt, 1996). Linear electron transport also leads to alkalization and an increase in the Mg²⁺ concentration in the stroma (Heldt, 1979; Portis, 1981). Many PCR cycle enzymes have a pH optimum around 8.0 and require Mg²⁺ as a cofactor, so these light-induced ionic movements promote their activation. In addition, many chloroplast enzymes are light regulated via the ferredoxin-thioredoxin pathway (Buchanan, 1984).

The activity of Rubisco also changes in vivo. The activation of Rubisco generally increases as light intensity increases (Mächler and Nösberger, 1980; von Caemmerer and Edmondson, 1986) and deactivation is sometimes observed when stromal inorganic phosphate is depleted (Sharkey, 1990). Rubisco is active only when a specific lysyl residue within its catalytic site is carbamylated (complexed with CO₂) and bound with Mg²⁺. This carbamylation process is considered to be enhanced by the light-dependent stromal alkalization and increase in Mg²⁺ concentration (Lorimer et al., 1976; Andrews and Lorimer, 1987). However, it is evident that, in vivo, the changes in carbamylation state are mediated by another stromal protein, Rubisco activase (Somerville et al., 1982; Salvucci et al., 1985).

Activase requires ATP hydrolysis to function and is inhibited by ADP, so presumably it is sensitive to the stromal ATP/ADP ratio (Robinson and Portis, 1988; Wang and Portis, 1992). This may not be the only mechanism regulating Rubisco activase. The transthylakoid proton gradient and electron transport may also be involved in mediating the response of its activity to light (Campbell and Ogren, 1990a, 1992). Additionally, the activity of Rubisco is affected by chloroplast metabolites. Phosphorylated compounds, including the substrate ribulose 1,5-bisphosphate (RuBP), can bind to uncarbamylated sites preventing activation (Portis, 1995). Rubisco activase facilitates the detachment of these compounds and thus promotes carbamylation and catalysis. The relationship between Rubisco and RuBP concentration is complicated because under some conditions, the carbamylation may be promoted by the presence of RuBP (Mate et al., 1996).

In this study we used two different transgenic tobacco (Nicotiana tabacum) types to investigate the interactions between chloroplast light reactions and carbon metabolism. Plants with reduced amounts of the chloroplast cytochrome (Cyt) b/f complex (anti-b/f plants) have a decreased electron transport capacity, which should lead to a low transthylakoid pH gradient (pH) and a reduction in the ATP and NADPH synthesis (Price et al., 1995b, 1998). In contrast, plants with a reduction in the activity of glyceraldehyde 3-phosphate dehydrogenase (anti-GAPDH plants) have a decreased capacity to use ATP and NADPH in carbon assimilation and have a high pH (Price et al., 1995a). We studied the energy status of these contrasting plant types by measuring the whole-leaf adenylate levels. In addition, we used the activation state of chloroplast NADP-dependent malate dehydrogenase (NADP-MDH) as an indicator of the stromal NADPH/NADP⁺ ratio. These measurements enabled us to evaluate how the electron transport rate, transthylakoid pH gradient, ATP/ADP ratio, and RuBP concentration interact in determining the degree of Rubisco activation and how the activity of Rubisco activase may be regulated in response to the light signal in chloroplasts.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Plant Material and Growth Conditions

The transformation of tobacco (Nicotiana tabacum L. cv W38) with an antisense construct directed against the Rieske FeS subunit of the chloroplast Cyt b/f complex (FeS) (referred to as anti-b/f plants) has been described previously (Price et al., 1995b). Antisense plants in this study were raised from selfed T₃ seeds of the line B6F-2.2-513-16, producing a range of phenotypes in respect to FeS protein content and CO₂ assimilation rates (Price et al., 1998). Both untransformed W38 and antisense plants were grown in 5-L pots in a growth cabinet and fertilized with Hewitt's complete nutrient solution (Hewitt and Smith, 1975) three times a week. The light intensity was 100 to 120 µmol quanta m² s¹, the photoperiod was 20 h, and the temperature was kept constant at 25°C. The low growth irradiance and long days were used to minimize the instability of the antisense phenotype (Price et al., 1995b). The anti-glyceraldehyde 3-phosphate dehydrogenase (anti-GAPDH) plants were grown from T₁ seeds of plant GAP-R (Price et al., 1995a), and had a variety of GAPDH activities. Anti-GAPDH and untransformed W38 tobacco plants were grown in an air-conditioned greenhouse where the peak photon flux density was 700 to 900 µmol m² s¹. The plants were used at 8 to 16 weeks after germination, and young fully expanded leaves were selected for the measurements.

Gas Exchange Measurements and Rapid Freeze-Clamping of Leaves

Leaf gas exchange was measured in a chamber attached to a rapid freeze-clamping apparatus (Badger et al., 1984) using a portable gas exchange system (LI 6400, LI-COR, Lincoln, NE) as described in Ruuska et al. (1998). CO₂ assimilation was measured at an irradiance of 1,000 µmol quanta m² s¹ at either 350 to 380 microbars (µbar) or 700 µbar CO₂ in air, and the leaf temperature was kept at 25°C. For the measurements conducted at 350 µbar, the leaf was stabilized for at least 40 min before it was rapidly freeze-clamped. For the 700-µbar CO₂ measurements a section of the same leaf was first kept at 350 µbar CO₂ in air, 1,000 µmol quanta m² s¹ until stomatal opening was nearly complete (about 20 min), after which the incoming CO₂ partial pressure was increased to 700 µbar. The gas exchange characteristics were recorded after 30 to 40 min, and a leaf disc was freeze-clamped. Additional leaf discs were collected for chlorophyll and Cyt f measurements and GAPDH activity assays.

Enzyme Assays

For Rubisco assays, one-half of the freeze-clamped leaf disc (2.7 cm²) was homogenized in 1.4 mL of ice-cold CO₂-free extraction buffer containing 50 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)-NaOH, pH 7.8, 1 mM Na-EDTA, 5 mM MgCl₂, 10 mM dithiothreitol (DTT), 1% (w/v) polyvinylpolypyrrolidone, and 1 mM phenylmethylsulfonyl fluoride (PMSF). The extract was centrifuged for 15 to 20 s (10,000g), and the initial activity of Rubisco was measured immediately at 25°C by adding a 10-µL aliquot of the supernatant into 262 µL of the reaction mixture containing 55 mM 4-(2-hydroxyethyl)-1-piperazinepropanesulfonic acid (HEPPS)-KOH, pH 8.0, 22 mM MgCl₂, 0.27 mM Na-EDTA, 13 mM NaH¹⁴CO₃ (specific radioactivity approximately 1,500 cpm/nmol), and 0.4 mM RuBP. Total activity was assayed after 10 µL of the supernatant was incubated at 25°C for 5 min in 55 mM HEPPS-KOH, pH 8.0, 22 mM MgCl₂, 0.27 mM Na-EDTA, and 14.5 mM NaH¹⁴CO_3. The assay was initiated by adding 0.4 mM RuBP. In both assays the reaction was terminated after 60 s with formic acid, samples were dried and acid-stable ¹⁴C was measured by liquid scintillation. The exact specific radioactivity of NaH¹⁴CO₃ was determined for each set of assays by measuring the amount of ¹⁴C rendered acid stable by allowing a reaction containing 10 nmol of RuBP (measured spectrophotometrically according to the method of He et al. [1997]) go to completion with excess Rubisco using the procedure described for total activity.

The Rubisco catalytic site concentration was determined by the stoichiometric binding of [¹⁴C]CPBP (an unresolved mixture of 2'-carboxy-D-arabinitol-1,5-bisphosphate and 2'-carboxy-D-ribitol-1,5-bisphosphate), and the carbamylation level was measured by exchanging loosely bound [¹⁴C]CPBP at non-carbamylated sites with an excess of [¹²C]CPBP (Butz and Sharkey, 1989) as described in Ruuska et al. (1998). The catalytic turnover rates were calculated by dividing the initial or total in vitro activities by the number of carbamylated or total Rubisco sites.

Total GAPDH activity was determined spectrophotometrically as described in Stitt et al. (1989) and Ruuska et al. (1998).

The activation state of chloroplast NADP-MDH was assayed according to the method of Scheibe and Stitt (1988) with minor modifications. One-half of a freeze-clamped leaf disc was extracted in 900 µL of ice-cold buffer (sparged with humidified nitrogen) containing 50 mM Na-acetate, pH 6.0, 1% (w/v) bovine serum albumin (BSA), 4 mM DTT, 0.1% (w/v) Triton X-100, 0.5 mM benzamidine, 0.5 mM -aminocaproic acid, and 0.5 mM PMSF. The crude extract was centrifuged at 10,000g for 5 min at 4°C, and the initial activity of NADP-MDH was assayed immediately in a total volume of 1.5 mL containing 100 mM Tris-HCl, pH 8.0 (sparged with humidified nitrogen), 1 mM Na-EDTA, 1 mM DTT, 0.2 mM NADPH, and 100 µL of supernatant.

The reaction was initiated by adding 2 mM oxaloacetic acid and the decline in A₃₄₀ was monitored. Reductive activation of NADP-MDH was achieved by incubating an aliquot of the supernatant in 250 mM Tris-HCl, pH 9.0 (sparged with nitrogen), and 125 mM DTT under nitrogen atmosphere at room temperature for 15 min. Time course experiments confirmed that 15 min was sufficient to fully activate tobacco NADP-MDH. The low DTT concentration in the extraction buffer (4 mM) did not cause activation of the enzyme. NADP-MDH activities were corrected for the side activity of NAD-MDH, which was assumed to be 0.2% of the total NAD-MDH activity (Scheibe and Stitt, 1988; Grace and Logan, 1996). The total NAD-MDH activity was measured in 1.5 mL in 100 mM Tris-HCl, pH 8.0, 10 mM MgCl₂, 0.2 mM NADH, and 10 µL of diluted supernatant. The reaction was initiated with 1 mM oxaloacetic acid (Scheibe and Stitt, 1988). With this procedure, activation levels below 5% were obtained from dark- or low-light-adapted tobacco.

Western-Blot Quantitation of Cyt f and Rubisco Activase Content

Anti-b/f plants have been characterized previously by measuring the amount of FeS polypeptide in leaves by western blotting. In this study an antibody directed against the Cyt f protein was used (Barkan et al., 1986) because of its higher specificity. A close correlation between the amounts of FeS and Cyt f protein has been demonstrated (Anderson et al., 1997; Price et al., 1998), indicating that the content of the whole Cyt b/f complex is reduced by the antisense construct. The western blotting and protein quantitation were performed as described previously (Price et al., 1995b). The amount of Rubisco activase in leaves was measured as in Mate et al. (1993).

Metabolite and Adenylate Assays

Extraction Procedure

Luminometric Determination of ATP and ADP

Luminometric Determination of 3-Phosphoglycerateolate (PGA) and RuBP

Recovery Tests

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

CO₂ Assimilation Rates

Individual tobacco plants with a variety of Cyt f contents were screened initially by measuring chlorophyll fluorescence (Price et al., 1995b). Plants with Cyt f contents ranging between 60% to 5% of the average wild-type level were obtained (Fig. 1, A and C). These plants had to be grown under low light to minimize the instability of the antisense phenotype (Price et al., 1995b). However, the extended daylength ensured that CO₂ assimilation rates of the growth-cabinet-grown wild-type plants were similar to the wild-type plants grown together with the anti-GAPDH plants in the greenhouse (Fig. 1, A and B). The CO₂ assimilation rate decreased when the amount of Cyt f was reduced (Fig. 1A). The most severely effected anti-b/f plants had assimilation rates below 3 µmol m² s¹, while the average rate for the wild-type plants was 12.8 µmol m² s¹. When the CO₂ assimilation rates were measured at 700 µbar CO₂, the relationship between leaf Cyt f content and assimilation rate at high light became strikingly linear (Fig. 1C), whereas at 350 µbar CO₂ the relationship was more curvilinear (Fig. 1A).

View larger version (24K): [in this window] [in a new window]   Figure 1.   CO2 assimilation rates of low-light-grown wild-type tobacco plants () and transgenic tobacco plants with a variety of Cyt b/f contents () (A and C), and greenhouse-grown wild-type () and transgenic tobacco plants with different activities of GAPDH () (B and D) were measured using the combined gas exchange and rapid-kill procedure described by Badger et al. (1984). Measurements were conducted at an irradiance of 1,000 µmol quanta m2 s1, a CO2 partial pressure of 350 µbar or 700 µbar, 21% (v/v) O2, and a leaf temperature of 25°C. Leaves were kept under these conditions for at least 40 min before the CO2 assimilation rate was recorded and a leaf disc was rapidly freeze-clamped.

Anti-GAPDH plants had a variety of GAPDH activities from about 50% to less than 5% of wild-type level and assimilation rates ranging to below 10% of the average wild-type rate when measured at 350 µbar CO₂ (Fig. 1, B and D). There was no effect on the CO₂ assimilation rate at 350 µbar CO₂ until GAPDH activity was reduced to below 30% of wild-type level. The relationship between the assimilation rate and GAPDH activity was similar at 700 and 350 µbar CO_2.

The Contents of Rubisco, 3-PGA, and RuBP

The average Rubisco contents of leaves are presented in Table I. Rubisco content was on average 25% lower in low-light-grown plants than in greenhouse-grown plants. The soluble protein content, on a leaf-area basis, was also lower in low-light-grown plants (data not shown). Consequently, the Rubisco content as a fraction of soluble proteins was similar at the two light intensities, being on average, 28% ± 2% for all low-light grown plants and 31% ± 3% for all greenhouse-grown plants. The decrease in either the Cyt b/f content or GAPDH activity did not cause significant changes in the content of Rubisco of the leaves compared with their respective control plants (Table I).

View this table: [in this window] [in a new window]   Table I.   Contents of Rubisco, RuBP, and 3-PGA in leaves of anti-b/f, anti-GAPDH, and the respective wild-type (WT) tobacco plants The measurements were made at 350 or 700 µbar CO2 at an irradiance of 1,000 µmol quanta m2 s1 and a leaf temperature of 25°C. The values are averages ± SE, with the number of replicates indicated in the parentheses.

The growth conditions had a strong impact on the RuBP content of wild-type leaves. When measured from leaves kept at 1,000 µmol quanta m² s¹, the low-light-grown wild-type plants had approximately 50% less RuBP than greenhouse-grown wild-type plants, regardless of the measuring CO₂ concentration (Table I). Because both RuBP and Rubisco contents were lower in low-light-grown wild-type plants, the ratio of RuBP to Rubisco sites was only slightly less than in the greenhouse-grown wild-type plants. The reduction in both the Cyt b/f content and GAPDH activity reduced the contents of RuBP in the transgenic plants. When measured at 350 µbar CO₂, the RuBP content declined to close to the Rubisco site concentration in both plant types, but not below it. The amount of 3-PGA was comparable to wild-type levels in both of the antisense plant types at 350 µbar CO₂. As a consequence, the RuBP/3-PGA ratio fell significantly below wild-type levels in both transgenic plant types.

When the leaves were exposed to 700 µbar CO₂, the RuBP content decreased in all plants (Table I). This was associated with an increase in 3-PGA pool sizes, so the RuBP/3-PGA ratio was lower than at 350 µbar CO₂ in all plants. Despite the decrease in RuBP content at elevated CO₂, the ratio between RuBP and Rubisco sites remained above 1 in most of the wild-type and anti-b/f plants. At 700 µbar CO₂, however, the RuBP content in almost all anti-GAPDH plants decreased below the Rubisco site concentration, and the average RuBP per site ratio was 0.72.

Whole-Leaf Adenylate Contents

The amounts of ATP and ADP in the leaves were measured to assess changes in the adenylate pools due to the shifted balance between ATP production and consumption in the chloroplasts (Fig. 2; Table II). The reduction in Cyt f content did not affect the whole-leaf adenylate content: both the total amounts of ATP and ADP and the ratio between the two remained similar to wild-type levels, even in the most severe antisense plants (Fig. 2, A, C, E, and G). On average, the low-light grown plants had 14.8 ± 2 µmol ATP and 10.9 ± 1 µmol ADP m² and the ratio of ATP/ADP was 1.43 ± 0.1. In contrast, when the activity of GAPDH was reduced below 50% of the wild-type level, the amount of ATP increased and, simultaneously, ADP content decreased. This led to an approximately 2-fold rise in the ATP/ADP ratio compared with wild-type plants (on average, 1.55 ± 0.34 in the wild type and up to 3.1 in the most severe anti-GAPDH plants), whereas the sum of ATP + ADP remained constant, being 44 ± 6 µmol m² in all greenhouse-grown plants (Fig. 2, B, D, F, and H).

View larger version (28K): [in this window] [in a new window]   Figure 2.   Whole-leaf ATP (A and B) and ADP (C and D) concentrations, the sum of ATP + ADP (E and F), and the ATP/ADP ratio (G and H) in leaves of wild-type, anti-b/f, and anti-GAPDH tobacco plants measured at 350 µbar CO2. Leaves were freeze-clamped after the gas-exchange measurements at 350 µbar CO2 shown in Figure 1. Symbols are as in Figure 1. These data are summarized in Table I and compared with similar measurements at 700 µbar CO2.

View this table: [in this window] [in a new window]   Table II.   Leaf adenylate contents in leaves of anti-b/f, anti-GAPDH, and the respective wild-type (WT) tobacco plants The measurements were made at 350 or 700 µbar CO2 at an irradiance of 1,000 µmol quanta m2 s1 and a leaf temperature of 25°C. The values are averages ± SE, with the number of replicates indicated in parentheses.

When the CO₂ concentration was increased to 700 µbar, it had little effect on the ATP/ADP ratios in the wild-type and anti-b/f tobacco plants compared with the ratios at 350 µbar (Table II). There was also no difference in the ATP/ADP ratio between the low-light-grown wild-type and anti-b/f plants at 700 µbar CO₂. At elevated CO₂, however, the average ATP/ADP ratio in the anti-GAPDH plants decreased and become similar to the wild-type ratio. It is worth noting that at 700 µbar CO₂ the ATP/ADP ratio was higher in the anti-GAPDH plants with the lowest GAPDH activity than in wild-type plants and transgenic plants with intermediate GAPDH activity (data not shown).

NADP-MDH Activation State

The activation state of the chloroplast NADP-MDH was measured as the ratio between the initial (non-activated) activity and the total activity (after 15 min of reductive activation, see Scheibe and Jacquot [1983]) from leaf samples collected from 350 µbar CO₂ and 1,000 µmol quanta m² s¹. The activation state decreased dramatically when the Cyt b/f content was reduced (Fig. 3A), indicating a strong decline in the NADPH/NADP⁺ ratio. At 1,000 µmol quanta m² s¹ the NADP-MDH activation in low-light-grown wild-type plants was 76% ± 4%, and decreased to 5% in the most severely affected antisense plants. The reduction in the activation level was due to a decrease in the initial activity of the enzyme (data not shown). In contrast, lowered GAPDH activities did not affect the NADP-MDH activation level compared with the wild-type plants (Fig. 3B). The average activation level in greenhouse-grown wild-type and anti-GAPDH plants was 53% (±4%) when measured at 1,000 µmol quanta m² s¹.

View larger version (14K): [in this window] [in a new window]   Figure 3.   NADP-MDH activation levels in the leaves of wild-type, anti-b/f, and anti-GAPDH tobacco plants measured at 350 µbar CO2. Symbols, measuring conditions, and sampling were as in Figures 1 and 2. The mean total activities were 11.6 ± 0.04 and 10.4 ± 0.15 µmol m2 s1, respectively, for anti-b/f and their wild-type plants and 15.1 ± 0.2 and 18.5 ± 0.3 µmol m2 s1, respectively, for anti-GAPDH and their wild-type plants.

Rubisco Carbamylation State and in Vitro Activities

At high light and 350 µbar CO₂, the carbamylation state of Rubisco was high (80%) in all wild-type and anti-GAPDH plants (Fig. 4B; Table III). However, as the amount of Cyt b/f content reduced, the Rubisco carbamylation status declined, being less than 50% in the most severe anti-b/f plants (Fig. 4A). In Figure 4, C to F, the catalytic turnover rates of Rubisco sites are presented as a proportion of the average rate in control plants, and the mean values are given in Table III. There was no significant difference in the turnover rate of the carbamylated sites (initial activity per carbamylated sites) in either of the transgenic plant types compared with control plants. The total turnover rates (total activity per total sites) in anti-GAPDH plants were identical to controls as well. Nevertheless, in anti-b/f plants there was a significant decrease in the total activity per total Rubisco sites as the Cyt f content declined, such that the most severe anti-b/f plants had about half the turnover rate of the control plants. Similar results were obtained when the leaves were measured at 700 µbar CO₂ (Table III).

View larger version (27K): [in this window] [in a new window]   Figure 4.   Carbamylation state of Rubisco (A and B), initial in vitro activity of Rubisco per carbamylated (carb.) Rubisco sites (C and D), and total in vitro activity of Rubisco per total Rubisco sites (E and F) in leaves of wild-type, anti-b/f, and anti-GAPDH tobacco plants. Leaves were freeze-clamped after the gas-exchange measurements at 350 µbar CO2 (shown in Fig. 1). The activity results (C-F) are expressed as a percentage of the average wild-type activities; mean values are given in Table II, which compares them with analog values measured at 700 µbar. The symbols are as in Figure 1.

View this table: [in this window] [in a new window]   Table III.   Rubisco carbamylation (carb.) and in vitro activities (act. of) anti-b/f, anti-GAPDH, and the respective wild-type tobacco plants The measurements were made at 350 or 700 µbar CO2 at an irradiance of 1,000 µmol quanta m2 s1 and a leaf temperature of 25°C. The values are averages ± SE, with the number of replicates indicated in parentheses. Init., Initial.

RuBP Content and Rubisco Turnover Rates in Vivo

Figure 5, A and B, illustrates the changing relationships between RuBP and Rubisco sites in both anti-b/f and anti-GAPDH plants. As summarized in Table I, the RuBP/Rubisco site ratio at 350 µbar CO₂ declined from wild-type-levels (between 4 and 7) down to around 1. As the RuBP content declines, the in vivo turnover rates of Rubisco decrease due to substrate limitation. Figure 5, C and D, present the in vivo turnover rates of carbamylated Rubisco sites (calculated from the gas exchange measurements and Rubisco assays) at 350 µbar CO₂. The average rates were 1.18 ± 0.08 s¹ and 0.97 ± 0.06 s¹ for the low-light and greenhouse-grown wild-type plants, respectively. In the most severe transgenic plants with the lowest RuBP content, the turnover rates were more than 50% lower than in the wild types. Figure 5, E and F, shows the in vivo turnover rates of carbamylated Rubisco sites as a function of RuBP content.

View larger version (30K): [in this window] [in a new window]   Figure 5.   Ratio between RuBP and Rubisco sites (A and B) and the gross CO2 assimilation rate per carbamylated (Carb.) Rubisco sites (C and D) and the relationship between gross CO2 assimilation rate per carbamylated Rubisco site and the ratio between RuBP and Rubisco sites (E and F) in leaves of wild-type, anti-b/f, and anti-GAPDH tobacco plants. The gross CO2 assimilation rates were calculated as A + Rd, where A is the CO2 assimilation at 350 µbar CO2 and Rd the dark respiration rate. Leaves were freeze-clamped after the gas-exchange measurements at 350 µbar CO2 shown in Figure 1. The symbols are as in Figure 1.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

CO₂ Assimilation Decreases in Anti-b/f and Anti-GAPDH Plants

The decrease in Cyt b/f content caused the CO₂ assimilation rate to decline (Fig. 1, A and C), as demonstrated earlier (Price et al., 1995b, 1998). At 350 µbar CO₂, the relationship between the Cyt b/f content and the CO₂ assimilation rate was curvilinear, but as the leaves were exposed to 700 µbar CO₂, the relationship became linear. This was expected, since at high CO₂ and high light CO₂ assimilation is limited solely by the electron transport capacity (Farquhar et al., 1980). The curvilinear relationship between the assimilation rate and the Cyt b/f content at 350 µbar CO₂ indicates that at ambient CO₂, wild-type plants are close to the transition from Rubisco carboxylation-limitation to RuBP-regeneration limitation (von Caemmerer and Farquhar, 1981). As reported previously (Price et al., 1995a), the activity of GAPDH could be reduced to about 30% of the average wild-type activity before the CO₂ assimilation rates at 350 µbar CO₂ were affected. The activity of GAPDH does not appear to be limiting for photosynthesis even at elevated CO₂ because, again, the CO₂ assimilation rates decreased only in plants with less than 30% of the wild-type GAPDH activity.

As reported previously (Price et al., 1995a, 1998), the decreased CO₂ assimilation rate in both anti-b/f and anti-GAPDH plants was associated with reduced RuBP concentration (Table I; Fig. 5, A and B). In anti-GAPDH plants, the reason for the lowered RuBP concentration was straightforward: the reduced amount of GAPDH forms a bottleneck in the regeneration pathway. In anti-b/f plants, however, there are two reasons. First, the electron transport rate has decreased in these plants, leading to decreased rate of ATP and NADPH synthesis. Second, there is a possibility that, in anti-b/f plants, the ferredoxin-thioredoxin-mediated activation of some of the PCR cycle enzymes has also decreased, further impairing the RuBP regeneration capacity. This assumption is supported by the large decrease in NADP-MDH activation state (Fig. 3A), which indicates a severe limitation of the NADPH synthesis and ferredoxin reduction, as discussed below.

At high CO₂ and high light, CO₂ assimilation in C₃ plants is determined by the RuBP regeneration rate. The transfer to elevated CO₂ decreased the RuBP pool sizes in all plants (Table I), but the RuBP content in most anti-b/f plants and all wild-type plants remained above the Rubisco site concentration. This is similar to previous studies on the response of chloroplast metabolites to changes in CO₂ showing that even at high CO₂, RuBP contents tend to exceed the Rubisco site concentration (Badger et al., 1984; Dietz and Heber, 1984; von Caemmerer and Edmondson, 1986; Seemann et al., 1987). The RuBP pool sizes were only reduced below the Rubisco site concentration in the anti-GAPDH plants when exposed to 700 µbar CO₂. However, it should be kept in mind that, in vivo, the conditions in the chloroplast might be such that the maximal carboxylation efficiency requires RuBP concentrations well above the Rubisco site concentrations. Assuming that the Rubisco site concentration in the chloroplasts is 2 mM and the K_m for RuBP is 0.2 mM (allowing for competition by other phosphorylated compounds, but ignoring chelation by Mg²⁺), the approximate fractional activity of Rubisco is 0.95 when the RuBP/site ratio equals 2, and decreases to 0.75 when this ratio reaches 1 (for details, see Ruuska et al. [1998]). Reduction of the free RuBP concentration by sequestration with divalent metals will reduce activity further.

Whole-Leaf ATP/ADP Does Not Change in Anti-b/f Plants, But the Activation of NADP-MDH Decreases

The balance between chloroplast electron transport and carbon assimilation, and therefore the production and consumption of ATP and NADPH, has been altered in opposite directions in anti-b/f and anti-GAPDH plants. The energy status of the leaves was studied by measuring whole-leaf adenylate levels. In addition, the activation state of the chloroplast NADP-MDH was measured and used as an indicator of the stromal NADPH/NADP⁺ ratio (Scheibe and Jacquot, 1983; Harbinson et al., 1990; Foyer, 1993).

The adenylate ratios in anti-b/f plants did not differ from wild-type plants irrespective of the CO₂ conditions (Table II). However, the severely decreased activation state of NADP-MDH in these plants (Fig. 3A) suggests that the rate of linear electron transport, and presumably also proton translocation, were reduced markedly. Chlorophyll fluorescence measurements have shown that the transthylakoid pH gradient is low in anti-b/f plants (Price et al., 1995b). Since only the whole-leaf adenylates were measured, it is possible that the cytosolic and mitochondrial pools may have masked the changes in the chloroplast ATP/ADP ratio. In a study conducted with protoplasts of wheat leaves, it was found that 47% of the total adenylates were located in the chloroplasts; 44% were located in the cytosol and 9% in the mitochondria (Stitt et al., 1982). The ATP/ADP ratio in the cytosol is higher than that in the chloroplasts, whether measured in the light or the dark, because ATP is transported there from chloroplasts and mitochondria (Stitt et al., 1982; Gardeström and Wigge, 1988; Heineke et al., 1991).

The observed reduction in the activation level of NADP-MDH in anti-b/f plants (Fig. 3A) resembles the results of Harbinson et al. (1990) and Krall et al. (1995). These authors measured NADP-MDH activation in leaves at different light intensities and noted a strong correlation between light intensity, electron transport rate, and NADP-MDH activation. The low activation state of NADP-MDH in anti-b/f plants indicates that the reduction state of the stromal NADP⁺ pool is significantly lowered. Decreased NADPH/NADP⁺ ratio has two main implications for carbon fixation. First, the lack of reducing equivalents limits the function of the PCR cycle and RuBP regeneration. Second, the low stromal reduction status reduces the activity of the chloroplast ferredoxin-thioredoxin pathway, and therefore the enzymes regulated by this system (for example Fru 1,6-bisphosphatase) are likely to be less active (Holfgrefe et al., 1997). The decreased activity of the PCR cycle enzymes may explain why the ATP/ADP ratio did not change in anti-b/f plants. The consumption of ATP by the PCR cycle may be severely restricted, thus compensating for the decreased rate of ATP synthesis. The same mechanism may also explain why light intensity generally has little effect on leaf adenylate levels (Dietz and Heber, 1984; Brooks et al., 1988a; Sage et al., 1990).

The activation level of NADP-MDH in the most severe anti-b/f plants at high light (Fig. 3A) resembles that seen in wild-type plants in darkness or very low light (data not shown), suggesting that b/f complex depletion impairs perception of the light signal by the thioredoxin pathway. When leaves were exposed to 1,000 µmol quanta m² s¹, NADP-MDH in the low-light-grown wild-type plants activated to over 80%. This activation level is high compared with, for example, 35% in pea leaves measured in air and 1,000 µmol quanta m² s¹ (Harbinson et al., 1990) or 53% in the greenhouse-grown wild-type and anti-GAPDH plants (Fig. 3B), which experienced growth light intensities close to measuring conditions. Greenhouse-grown plants had about 1.5 times greater total NADP-MDH activities (on an area basis) than low-light-grown plants (legend to Fig. 3), and this greater capacity may explain the different activation states observed in wild-type plants.

ATP/ADP Increases in Anti-GAPDH Plants But Not the Activation of NADP-MDH

The increase in the whole-leaf ATP/ADP ratio in anti-GAPDH plants at 350 µbar CO₂ (Fig. 2H) is comparable to the observations of other transgenic tobacco plants with impaired capacity for carbon assimilation, due to either the reduced amount of Rubisco (Quick et al., 1991) or of phosphoribulokinase (Paul et al., 1996). This effect of decreased carbon fixation capacity on the adenylate pools is analogous to the increase in ATP/ADP ratio when photosynthesis is limited by low CO₂ (Gardeström, 1987; Gilmore and Björkman, 1994), since the majority of ATP synthesized in chloroplasts is used in carbon metabolism. This was clearly the case in the anti-GAPDH plants, since increasing the CO₂ concentration caused the average ATP/ADP ratio to decline to close to wild-type levels (Table II).

Interestingly, the reduction state of the stromal NADP⁺ pool at 350 µbar CO₂ (estimated from the NADP-MDH activation level, Fig. 3B) in anti-GAPDH plants was similar to the wild-type plants. It might be expected to rise in the antisense plants with decreased capacity to use reducing equivalents in carbon fixation. These results are comparable to data obtained by Lauerer et al. (1993), who measured the activation state of NADP-MDH at growth irradiance in plants with reduced amounts of Rubisco. The maintenance of a wild-type-like NADPH/NADP⁺ ratio in anti-GAPDH plants indicates that PSII efficiency is well regulated in these plants, as had previously been demonstrated with measurements of chlorophyll fluorescence (Price et al., 1995a). Transport of reducing equivalents from chloroplasts to cytosol via the "malate shuttle" (Scheibe, 1987) may contribute to this regulation.

Reduction in the Electron Transport Capacity Affects Rubisco in Anti-b/f Plants

Carbamylation

Role of RuBP

In Vitro Activity

Phosphorylated sugars can bind tightly either to uncarbamylated Rubisco sites, preventing activation, or to carbamylated sites, inhibiting catalysis. In vivo, Rubisco activase facilitates the dissociation of these compounds from active sites. The presence of tightly binding inhibitors in Rubisco sites can be detected as a decrease in the catalytic rate measured under substrate saturation (Seemann et al., 1985; Keys et al., 1995; Parry et al., 1997). We measured the carbamylated and total Rubisco site concentrations, as well as initial and total activities, and calculated the catalytic turnover rates of Rubisco sites. The turnover rates of the carbamylated sites in both anti-b/f and anti-GAPDH plants were comparable to control values (Fig. 4, C and D), indicating that the carbamylated sites were free from inhibitory ligands after extraction. However, the total activity per total sites in the anti-b/f plants declined as the Cyt b/f content decreased (Fig. 4E). The reason for this could be that the uncarbamylated sites had tightly bound inhibitory compounds, which did not dissociate during the 5-min activating incubation. Nevertheless, during the carbamylation assays these inhibitory ligands were displaced by [¹⁴C]CPBP, which has very high affinity to Rubisco sites.

It is possible that the presence of inhibitors in Rubisco sites in anti-b/f plants is another consequence of the decreased activity of Rubisco activase, in addition to the lowered carbamylation state. Studies with anti-activase tobacco plants discovered a slight impairment in the in vitro turnover rates of both carbamylated and uncarbamylated Rubisco sites (He et al., 1997). A tight-binding inhibitor of Rubisco has recently been found in tobacco leaves in the light (Parry et al., 1997). However, endogenous inhibitors are not the only factors that can decrease Rubisco activities measured in vitro. Rubisco extracted from leaves may be partially degraded by endogenous proteases (Servaites, 1985) or may be inhibited by polyphenolics (Bahr et al., 1981) and polyphenol oxidase (Koivuniemi et al., 1980), all of which are abundant in tobacco leaves. It is therefore possible that the decrease in the total catalytic activity of Rubisco in anti-b/f plants is the result of direct inactivation. The difference in catalytic turnover rates between greenhouse-grown and growth-cabinet-grown wild-type plants may be attributable to the same phenomena (Table III).

In Vivo Catalysis

Studies of transgenic tobacco plants with severely reduced amounts of Rubisco activase show that not only carbamylation, but also in vivo turnover rates of carbamylated Rubisco sites, are impaired by the lack of activase (He et al., 1997). As both RuBP content and RuBP/3-PGA ratios are high in anti-activase plants, the low Rubisco turnover rate can be deduced as resulting from the lack of activase. Since the RuBP concentrations and the RuBP/3-PGA ratio are lowered in both anti-GAPDH and anti-b/f plants, we cannot judge if there is any additional decrease in the catalysis of Rubisco in the anti-b/f plants, other than that which would be expected from the substrate limitation by RuBP (Fig. 5).