Transgenic tobacco plants that express a chi- The inhibition of photosynthesis that can occur when excess excitation energy reaches the reaction center is commonly referred to as photoinhibition. High light intensity, especially at extreme temperatures or water deficit, can cause increased electron flow to 02, resulting in greater production ofO2 and H202. Although oxygen radicals appear to be involved in photoinhibition (9)(10)(11), the role of SOD in limiting the oxidative damage associated with photoinhibition has not been directly demonstrated (12, 13).To investigate the possible protective functions of SOD in plant chloroplasts, we have developed transgenic tobacco plants that overexpress chloroplast-localized Cu/Zn SOD. These plants were analyzed for photosynthetic rate when exposed to light and temperature conditions that inhibit photosynthesis and for their ability to recover photosynthetic capacity after stress. Our results indicate that these transgenic plants have improved photosynthetic function at chilling temperatures and moderate light intensity, and they recover more effectively from severe stress than control plants. These changes correlate with increased resistance to oxidative damage caused by the herbicide methyl viologen (MV).

The aim of this study was to determine the response of photosynthetic carbon metabolism in spinach and bean to low temperature. (a) Exposure of warm-grown spinach and bean plants to 10°C for 10 days resulted in increases in the total activities of a number of enzymes, including ribulose 1,5-bisphosphate carboxylase (Rubisco), stromal fructose 1,6 bisphosphatase (Fru 1,6-P2ase), sedoheptulose 1,7-bisphosphatase (Sed 1,7-P2ase), and the cytosolic Fru 1,6-P2ase. In spinach, but not bean, there was an increase in the total activity of sucrose-phosphate synthase. (b) The C02-saturated rates of photosynthesis for the coldacclimated spinach plants were 68% greater at 100C than those for warm-acclimated plants, whereas in bean, rates of photosynthesis at 10°C were very low after exposure to low temperature. (c) When spinach leaf discs were transferred from 27 to 100C, the stromal Fru 1,6-P2ase and NADP-malate dehydrogenase were almost fully activated within 8 minutes, and Rubisco reached 90% of full activation within 15 minutes of transfer. An initial restriction of Calvin cycle fluxes was evident as an increase in the amounts of ribulose 1,5-bisphosphate, glycerate-3-phosphate, Fru 1,6-P2, and Sed 1,7-P2. In bean, activation of stromal Fru 1,6-P2ase was weak, whereas the activation state of Rubisco decreased during the first few minutes after transfer to low temperature. However, NADP-malate dehydrogenase became almost fully activated, showing that no loss of the capacity for reductive activation occurred. (d) Temperature compensation in spinach evidently involves increases in the capacities of a range of enzymes, achieved in the short term by an increase in activation state, whereas long-term acclimation is achieved by an increase in the maximum activities of enzymes. The inability of bean to activate fully certain Calvin cycle enzymes and sucrose-phosphate synthase, or to increase nonphotochemical quenching of chlorophyll fluorescence at 100C, may be factors contributing to its poor performance at low temperature. complete in evergreen woody species that are subject to large seasonal variations in temperature, such as Eucalyptus species and the desert evergreen, Nerium oleander. For such plants acclimated to low temperature, temperature response curves for photosynthesis indicate an increased photosynthetic capacity over a wide range of temperatures (2, 7). The increases in photosynthetic capacity that result from acclimation to a lower growth temperature could be the result of a number of factors, as plants acclimating to low temperature show increases in, for example, soluble protein, the rate of electron transport, and in the activities of enzymes such as Rubisco and the stromal Fru 1,6-P2ase,2 which parallel the increase in photosynthetic capacity (2, 3).There are a number of other reports of increases in Rubisco at lower temperatures, for example, in the arctic-alpine species Oxyria digyna (5), in the C4 plant Atriplex lentiformis (24), and in the grass Dactylis glomerata (30). Gas-exchange studies also...

Photosynthesis of leaf discs from transgenic tobacco plants (Nicotiana tabacum) that express a chimeric gene that encodes chloroplast-localized Cu/Zn superoxide dismutase (SOD+) was protected from oxidative stress caused by exposure to high light intensity and low temperature. Under the same conditions, leaf discs of plants that did not express the pea SOD isoform (SOD-) had substantially lower photosynthetic rates. Young plants of both genotypes were more sensitive to oxidative stress than mature plants, but SOD+ plants retained higher photosynthetic rates than SOD- plants at all developmental stages tested. Not surprisingly, SOD+ plants had approximately 3-fold higher SOD specific activity than SOD- plants. However, SOD+ plants also exhibited a 3- to 4-fold increase in ascorbate peroxidase (APX) specific activity and had a corresponding increase in levels of APX mRNA. Dehydroascorbate reductase and glutathione reductase specific activities were the same in both SOD+ and SOD- plants. These results indicate that transgenic tobacco plants that overexpress pea Cu/Zn SOD II can compensate for the increased levels of SOD with increased expression of the H2O2-scavenging enzyme APX. Therefore, the enhancement of the active oxygen-scavenging system that leads to increased oxidative stress protection in SOD+ plants could result not only from increased SOD levels but from the combined increases in SOD and APX activity.

Drought and salinity are two major limiting factors in crop productivity. One way to reduce crop loss caused by drought and salinity is to increase the solute concentration in the vacuoles of plant cells. The accumulation of sodium ions inside the vacuoles provides a 2-fold advantage: (i) reducing the toxic levels of sodium in cytosol; and (ii) increasing the vacuolar osmotic potential with the concomitant generation of a more negative water potential that favors water uptake by the cell and better tissue water retention under high soil salinity. The success of this approach was demonstrated in several plants, where the overexpression of the Arabidopsis gene AtNHX1 that encodes a vacuolar sodium/proton antiporter resulted in higher plant salt tolerance. Overexpression of AtNHX1 increases sodium uptake in vacuoles, which leads to increased vacuolar solute concentration and therefore higher salt tolerance in transgenic plants. In an effort to engineer cotton for higher drought and salt tolerance, we created transgenic cotton plants expressing AtNHX1. These AtNHX1-expressing cotton plants generated more biomass and produced more fibers when grown in the presence of 200 mM NaCl in greenhouse conditions. The increased fiber yield was probably due to better photosynthetic performance and higher nitrogen assimilation rates observed in the AtNHX1-expressing cotton plants as compared with wild-type cotton plants under saline conditions. Furthermore, the field-grown AtNHX1-expressing cotton plants produced more fibers with better quality, indicating that AtNHX1 can indeed be used for improving salt stress tolerance in cotton.

It is still unclear what parameter(s), other than grain yield, might be a suitable indicator in a wheat (Triticum aestivum L.) breeding program for drought resistance. In this study, the leaf relative water content (RWC) and gas‐exchange parameters were compared between a drought‐resistant winter wheat genotype (cv. TAM W‐101) and a drought‐susceptible genotype (cv. Sturdy) to determine if these physiological parameters contribute to drought resistance in TAM W‐101. Plants were grown under well‐watered conditions in growth chambers until drought stress was imposed by limited watering of plants at anthesis or during vegetative growth. In both growth stages, TAM W‐101 maintained a higher RWC and apparent photosynthesis (A) than Sturdy under moderate to severe drought stress. TAM W‐101 plants also maintained a higher photosynthetic capacity (higher A at a given intercellular CO2 concentration [Ci]) under stress than did Sturdy in both growth stages. Photosynthetic water use efficiency (pWUE = A/stomatal conductance) generally increased with stress severity until very severe stress levels were attained. Thus, genotypic pWUE comparisons using stressed plants should be evaluated on a water‐status basis (e.g., RWC) to avoid the confounding effect of stress severity on pWUE. TAM W‐101 tended to have higher pWUE (RWC basis) than Sturdy under moderate to severe stress conditions, but not under well‐watered conditions. High leaf RWC, A, and photosynthetic capacity are traits that may contribute to drought resistance in TAM W‐101.

This study examined the effect of increasing chloroplastic superoxide dismutase (SOD), ascorbate peroxidase (APX), or glutathione reductase (GR) activity via plant transformation of cotton on the initial recovery of photosynthesis following exposures to 10 degrees C and high photon flux density (PFD). Growing wild-type or non-expressing segregate plants (controls) and transformants at two PFDs (600 micromol m(-2) s(-1) and full sun) resulted in a range of total antioxidant enzyme activities. Total SOD activities above that for control leaves grown in full sun did not substantially improve the recoveries of CO(2)-saturated photosynthesis, especially for stress treatments lasting more than 1 h, while elevated APX or GR activity did improve recoveries after 1-3 h of the chilling treatment. No synergistic effects were noted when the activities of more than one antioxidant enzyme were elevated in transgenic hybrids. Although these results suggest that the protection of photosynthesis can be realized by reducing either superoxide or H(2)O(2) levels, thereby reducing the possibility of hydroxyl radical formation, the situation is complicated, since elevated APX or GR activity can improve recoveries even when additional SOD activity has no effect. In conclusion, to enhance the protection of photosynthesis using stroma-targeted antioxidant enzymes, enhancing metabolism associated with H(2)O(2) is more effective than enhancing the capacity for superoxide scavenging. Although small, the improvement in the protection of photosynthetic capacity may be sufficient to improve cotton yield in temperate regions with large diurnal temperature fluctuations.

Ascorbate peroxidase (APX) exists as several isoforms that are found in various compartments in plant cells. The cytosolic and chloroplast APXs appear to play important roles in antioxidation metabolism in plant cells, yet the function of peroxisomal APX is not well studied. In this study, the localization of a putative peroxisomal membrane-bound ascorbate peroxidase, APX3 from Arabidopsis, was confirmed by studying the green fluorescent protein (GFP)-APX3 fusion protein in transgenic plants. GFP-APX3 was found to co-localize with a reporter protein that was targeted to peroxisomes by the peroxisomal targeting signal 1. The function of APX3 in Arabidopsis was investigated by analysing an APX3 knockout mutant under normal and several stress conditions. It was found that loss of function in APX3 does not affect Arabidopsis growth and development, suggesting that APX3 may not be an important antioxidant enzyme in Arabidopsis, at least under the conditions that were tested, or the function of APX3 could be compensated by other antioxidant enzymes in plant cells.

The enzymatic component of the antioxidant system is discussed as one of the defensive mechanisms providing protection against excessive light absorption in plants. We present an analysis of attempts to improve stress tolerance by means of the creation of transgenic plants with elevated antioxidant enzyme activities and conclude that the effect of such transgenic manipulation strongly depends on the manner in which the stress is imposed. The following factors may diminish the differences in photosynthetic performance between transgenic plants and wild type under field conditions: effective functioning of the thermal dissipation mechanisms providing a primary line of defense against excessive light, long-term adjustments of the antioxidant system and other photoprotective mechanisms, the relatively low level of control over electron transport exerted by the Water-Water cycle, especially under warm conditions, and a decrease in the content of the transgenic product during leaf aging.