Enhanced CO2 alters the relationship between photosynthesis and defence in cyanogenic Eucalyptus cladocalyx F. Muell.
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The effect of elevated CO2 and different levels of nitrogen on the partitioning of nitrogen between photosynthesis and a constitutive nitrogen-based secondary metabolite (the cyanogenic glycoside prunasin) was examined in Eucalyptus cladocalyx. Our hypothesis was that the expected increase in photosynthetic nitrogen-use efficiency of plants grown at elevated CO2 concentrations would lead to an effective reallocation of available nitrogen from photosynthesis to prunasin. Seedlings were grown at two concentrations of CO2 and nitrogen, and the proportion of leaf nitrogen allocated to photosynthesis, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), protein and prunasin compared. Up to 20% of leaf nitrogen was allocated to the cyanogenic glycoside, although this proportion varied with leaf age, position and growth conditions. Leaf prunasin concentration was strongly affected by nitrogen supply, but did not increase, on a dry weight basis, in the leaves from the elevated CO2 treatments. However, the proportion of nitrogen allocated to prunasin increased significantly, in spite of a decreasing pool of leaf nitrogen, in the plants grown at elevated concentrations of CO2. There was less protein in leaves of plants grown at elevated CO2 in both nitrogen treatments, while the concentration of active sites of Rubisco only decreased in plants from the low-nitrogen treatment. These changes in leaf chemistry may have significant implications in terms of the palatability of foliage and defence against herbivores.Keywords:
Palatability
The role of Rubisco activase in steady-state and non-steady-state photosynthesis was analyzed in wild-type (Oryza sativa) and transgenic rice that expressed different amounts of Rubisco activase. Below 25°C, the Rubisco activation state and steady-state photosynthesis were only affected when Rubisco activase was reduced by more than 70%. However, at 40°C, smaller reductions in Rubisco activase content were linked to a reduced Rubisco activation state and steady-state photosynthesis. As a result, overexpression of maize Rubisco activase in rice did not lead to an increase of the Rubisco activation state, nor to an increase in photosynthetic rate below 25°C, but had a small stimulatory effect at 40°C. On the other hand, the rate at which photosynthesis approached the steady state following an increase in light intensity was rapid in Rubisco activase-overexpressing plants, intermediate in the wild-type, and slowest in antisense plants at any leaf temperature. In Rubisco activase-overexpressing plants, Rubisco activation state at low light was maintained at higher levels than in the wild-type. Thus, rapid regulation by Rubisco activase following an increase in light intensity and/or maintenance of a high Rubisco activation state at low light would result in a rapid increase in Rubisco activation state and photosynthetic rate following an increase in light intensity. It is concluded that Rubisco activase plays an important role in the regulation of non-steady-state photosynthesis at any leaf temperature and, to a lesser extent, of steady-state photosynthesis at high temperature.
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Ribulose–1,5–bisphosphate carboxylase/oxygenase (Rubisco) is the rate–limiting enzyme for photosynthesis. Rubisco activase (RCA) can regulate the Rubisco activation state, influencing Rubisco activity and photosynthetic rate. We obtained transgenic maize plants that overproduced rice RCA (OsRCAOE) and evaluated photosynthesis in these plants by measuring gas exchange, energy conversion efficiencies in photosystem (PS) I and PSII, and Rubisco activity and activation state. The OsRCAOE lines showed significantly higher initial Rubisco activity and activation state, net photosynthetic rate, and PSII photochemical quantum yield than wild–type plants. These results suggest that OsRCA overexpression can promote maize photosynthesis by increasing the Rubisco activation state.
C4 Photosynthesis
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The biochemical lesion that causes impaired chloroplast metabolism (and, hence, photosynthetic capacity) in plants exposed to water deficits is still a subject of controversy. In this study we used tobacco (Nicotiana tabacum L.) transformed with "antisense" ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) DNA sequences to evaluate whether Rubisco or some other enzymic step in the photosynthetic carbon reduction cycle pathway rate limits photosynthesis at low leaf water potential ([psi]w). These transformants, along with the wild-type material, provided a novel model system allowing for an evaluation of photosynthetic response to water stress in near-isogenic plants with widely varying levels of functional Rubisco. It was determined that impaired chloroplast metabolism (rather than decreased leaf conductance to CO2) was the major cause of photosynthetic inhibition as leaf [psi]w declined. Significantly, the extent of photosynthetic inhibition at low [psi]w was identical in wild-type and transformed plants. Decreasing Rubisco activity by 68% did not sensitize photosynthetic capacity to water stress. It was hypothesized that, if water stress effects on Rubisco caused photosynthetic inhibition under stress, an increase in the steady-state level of the substrate for this enzyme, ribulose 1,5-bisphosphate (RuBP), would be associated with stress-induced photosynthetic inhibition. Steady-state levels of RuBP were reduced as leaf [psi]w declined, even in transformed plants with low levels of Rubisco. Based on the similarity in photosynthetic response to water stress in wild-type and transformed plants, the reduction in RuBP as stress developed, and studies that demonstrated that ATP supply did not rate limit photosynthesis under stress, we concluded that stress effects on an enzymic step involved in RuBP regeneration caused impaired chloroplast metabolism and photosynthetic inhibition in plants exposed to water deficits.
Ribulose
Ribulose 1,5-bisphosphate
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Cucumis
Carboxylation
Ribulose
Photosynthetic capacity
Carbon fixation
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Carboxylation
Photorespiration
Ribulose
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Ribulose‐1,5‐bisphosphate carboxylase (Rubisco) efficiency for CO2‐saturated photosynthesis was examined in leaves of rice (Oryza sativa L.). The amount of Rubisco in a leaf was calculated to be 30–55% in excess for the light‐saturated rate of photosynthesis at 100 Pa CO2. Long‐term exposure to CO2 enrichment decreased the amount of Rubisco protein. However, N was not reallocated from decreased Rubisco to other components limiting photosynthesis, and the decrease in Rubisco was simply due to a decrease in total leaf‐N content by CO2 enrichment. Thus, rice plants did not optimize N allocation into Rubisco at elevated CO2. Transgenic rice plants with decreased Rubisco were obtained by transformation with the rbcS antisense gene. The transformant with 65% wild‐type Rubisco was selected as a plant with optimal Rubisco content for CO2‐saturated photosynthesis at the level of a single leaf. This selected transgenic plant had 20% lower rates of photosynthesis at normal CO2 (36 Pa), but 5–15% higher rates of photosynthesis at elevated CO2 (100 Pa) for a given leaf N content. However, such transgenic plants did not necessarily show greater production of biomass even under conditions of CO2enrichment. Although they had a higher N‐use efficiency for plant growth under such conditions during the middle stage of growth, the growth rate was lower during the early stage of growth. Thus, improvement of N‐use efficiency by a single leaf did not necessarily lead to greater production of biomass by the whole plant.
Genetically modified rice
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The present study provides a synthesis of the in vitro and in vivo temperature responses of Rubisco Michaelis–Menten constants for CO2 (Kc) and O2 (Ko), specificity factor (Sc,o) and maximum carboxylase turnover rate (kcatc) for 49 species from all the main photosynthetic kingdoms of life. Novel correction routines were developed for in vitro data to remove the effects of study-to-study differences in Rubisco assays. The compilation revealed differences in the energy of activation (∆Ha) of Rubisco kinetics between higher plants and other photosynthetic groups, although photosynthetic bacteria and algae were under-represented and very few species have been investigated so far. Within plants, the variation in Rubisco temperature responses was related to species' climate and photosynthetic mechanism, with differences in ∆Ha for kcatc among C3 plants from cool and warm environments, and in ∆Ha for kcatc and Kc among C3 and C4 plants. A negative correlation was observed among ∆Ha for Sc/o and species' growth temperature for all data pooled, supporting the convergent adjustment of the temperature sensitivity of Rubisco kinetics to species' thermal history. Simulations of the influence of varying temperature dependences of Rubisco kinetics on Rubisco-limited photosynthesis suggested improved photosynthetic performance of C3 plants from cool habitats at lower temperatures, and C3 plants from warm habitats at higher temperatures, especially at higher CO2 concentration. Thus, variation in Rubisco kinetics for different groups of photosynthetic organisms might need consideration to improve prediction of photosynthesis in future climates. Comparisons between in vitro and in vivo data revealed common trends, but also highlighted a large variability among both types of Rubisco kinetics currently used to simulate photosynthesis, emphasizing the need for more experimental work to fill in the gaps in Rubisco datasets and improve scaling from enzyme kinetics to realized photosynthesis.
Carbon fixation
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The effects of different irrigations on photosynthesis rate and rubisco of radish leaves were studied in this paper. The results showed that the photosynthesis rate of the radish leaves with drip irrigation treatment was higher than that with flooding irrigation treatment during the growth of the radish. The photosynthesis rate was very significantly correlative to initial Rubisco activity, total Rubisco activity and Rubisco content. There was an obvious difference (F=0.400416, p0.05) on Rubisco of the radish leaves with different treatment. The Rubisco content was not the only influencing factors but together with the Rubisco activity, to the photosynthesis rate.
Raphanus
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Plant Physiology
Photosynthetic capacity
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The activation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which catalyses CO2 fixation in photosynthesis, requires the assistance of the regulatory protein Rubisco activase. Rubisco activase promotes carbamylation of Rubisco by releasing inhibitory sugar phosphates bound to the catalytic site of Rubisco in the light. To clarify the effects of Rubisco activase contents on the photosynthesis of rice, we investigated the steady-state photosynthesis and light-induction of photosynthesis in transgenic rice plants, in which leaf Rubisco activase levels were reduced. The reduction in Rubisco activase did not affect steady-state photosynthesis under high light intensity until the Rubisco activase was about 15% of that in control plants. However, light-induction of photosynthesis, namely, increase in photosynthetic rate following a transition from a low to high light intensity, was considerably low in transgenic rice plants with 20−25% Rubisco activase, which was sufficient to support the steady-state photosynthesis. In addition, the Rubisco activase content was highly correlated with the initial rate of Rubisco activation after the increase in light intensity. These results suggest that Rubisco activase in rice leaves largely limits the light-induction of photosynthesis, but not steady-state photosynthesis.
Carbon fixation
Light intensity
Genetically modified rice
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