Ascorbate (AsA) is a major antioxidant and free-radical scavenger in plants. Monodehydroascorbate reductase (MDAR; EC 1.6.5.4) is crucial for AsA regeneration and essential for maintaining a reduced pool of AsA. To examine whether an overexpressed level of MDAR could minimize the deleterious effects of environmental stresses, we developed transgenic tobacco plants overexpressing Arabidopsis thaliana MDAR gene (AtMDAR1) in the cytosol. Incorporation of the transgene in the genome of tobacco plants was confirmed by PCR and Southern-blot analysis and its expression was confirmed by Northern- and Western-blot analyses. These transgenic plants exhibited up to 2.1-fold higher MDAR activity and 2.2-fold higher level of reduced AsA compared to non-transformed control plants. The transgenic plants showed enhanced stress tolerance in term of significantly higher net photosynthesis rates under ozone, salt and polyethylene glycol (PEG) stresses and greater PSII effective quantum yield under ozone and salt stresses. Furthermore, these transgenic plants exhibited significantly lower hydrogen peroxide level when tested under salt stress. These results demonstrate that an overexpressed level of MDAR properly confers enhanced tolerance against ozone, salt and PEG stress.

Ascorbate (vitamin C) is a potent antioxidant protecting plants against oxidative damage imposed by environmental stresses such as ozone and drought. Dehydroascorbate reductase (DHAR; EC 1.8.5.1) is one of the two important enzymes functioning in the regeneration of ascorbate (AsA). To examine the protective role of DHAR against oxidative stress, we developed transgenic tobacco plants overexpressing cytosolic DHAR gene from Arabidopsis thaliana. Incorporation of the transgene in the genome of tobacco plants was confirmed by polymerase chain reaction and Southern blot analysis, and its expression was confirmed by Northern and Western blot analyses. These transgenic plants exhibited 2.3–3.1 folds higher DHAR activity and 1.9–2.1 folds higher level of reduced AsA compared with non‐transformed control plants. The transgenic plants showed maintained redox status of AsA and exhibited an enhanced tolerance to ozone, drought, salt, and polyethylene glycol stresses in terms of higher net photosynthesis. In this study, we report for the first time that the elevation of AsA level by targeting DHAR overexpression in cytosol properly provides a significantly enhanced oxidative stress tolerance imposed by drought and salt.

Aluminum (Al) inhibits plant growth partly by causing oxidative damage that is promoted by reactive oxygen species and can be prevented by improving antioxidant capacity. Ascorbic acid (AsA), the most abundant antioxidant in plants, is regenerated by the action of monodehydroascorbate reductase (MDAR) and dehydroascorbate reductase (DHAR). We investigated the role of MDAR and DHAR in AsA regeneration during Al stress using transgenic tobacco (Nicotiana tabacum) plants overexpressing Arabidopsis cytosolic MDAR (MDAR-OX) or DHAR (DHAR-OX). DHAR-OX plants showed better root growth than wild-type (SR-1) plants after exposure to Al for 2 weeks, but MDAR-OX plants did not. There was no difference in Al distribution and accumulation in the root tips among SR-1, DHAR-OX, and MDAR-OX plants after Al treatment for 24 h. However, DHAR-OX plants showed lower hydrogen peroxide content, less lipid peroxidation and lower level of oxidative DNA damage than SR-1 plants, whereas MDAR-OX plants showed the same extent of damage as SR-1 plants. Compared with SR-1 plants, DHAR-OX plants consistently maintained a higher AsA level both with and without Al exposure, while MDAR-OX plants maintained a higher AsA level only without Al exposure. Also, DHAR-OX plants maintained higher APX activity under Al stress. The higher AsA level and APX activity in DHAR-OX plants contributed to their higher antioxidant capacity and higher tolerance to Al stress. These findings show that the overexpression of DHAR, but not of MDAR, confers Al tolerance, and that maintenance of a high AsA level is important to Al tolerance.

Few genes are available to develop drought-tolerant bread wheat (Triticum aestivum L.) cultivars. One way to enhance bread wheat’s genetic diversity would be to take advantage of the diversity of wild species by creating synthetic hexaploid wheat (SW) with the genomic constitution of bread wheat. In this study, we compared the expression of traits encoded at different ploidy levels and evaluated the applicability of Aegilops tauschii drought-related traits using 33 Ae. tauschii accessions along with their corresponding SW lines under well-watered and drought conditions. We found wide variation in Ae. tauschii, and even wider variation in the SW lines. Some SW lines were more drought-tolerant than the standard cultivar Cham 6. Aegilops tauschii from some regions gave better performing SW lines. The traits of Ae. tauschii were not significantly correlated with their corresponding SW lines, indicating that the traits expressed in wild diploid relatives of wheat may not predict the traits that will be expressed in SW lines derived from them. We suggest that, regardless of the adaptability and performance of the Ae. tauschii under drought, production of SW could probably result in genotypes with enhanced trait expression due to gene interactions, and that the traits of the synthetic should be evaluated in hexaploid level.

Aegilops tauschii Coss. is the D-genome donor to hexaploid bread wheat (Triticum aestivum) and is the most promising wild species as a genetic resource for wheat breeding. To study the population structure and diversity of 81 Ae. tauschii accessions collected from various regions of its geographical distribution, the genomic representation of these lines were used to develop a diversity array technology (DArT) marker array. This Ae. tauschii array and a previously developed DArT wheat array were used to scan the genomes of the 81 accessions. Out of 7500 markers (5500 wheat and 2000 Ae. tauschii), 4449 were polymorphic (3776 wheat and 673 Ae. tauschii). Phylogenetic and population structure studies revealed that the accessions could be divided into three groups. The two Ae. tauschii subspecies could also be separately clustered, suggesting that the current taxonomy might be valid. DArT markers are effective to detect very small polymorphisms. The information obtained about Ae. tauschii in the current study could be useful for wheat breeding. In addition, the new DArT array from this Ae. tauschii population is expected to be an effective tool for hexaploid wheat studies.

Chromosome elimination occurs frequently in interspecific hybrids between distantly related species in Poaceae. However, chromosomes from both parents behave stably in a hybrid of female oat (Avena sativa L.) pollinated by pearl millet (Pennisetum glaucum L.). To analyze the chromosome behavior in this hybrid, we cloned the centromere-specific histone H3 (CENH3) genes of oat and pearl millet and produced a pearl millet-specific anti-CENH3 antibody. Application of this antibody together with a grass species common anti-CENH3 antibody revealed the dynamic CENH3 composition of the hybrid cells before and after fertilization. Despite co-expression of CENH3 genes encoded by oat and pearl millet, only an oat-type CENH3 was incorporated into the centromeres of both species in the hybrid embryo. Oat CENH3 enables a functional centromere in pearl millet chromosomes in an oat genetic background. Comparison of CENH3 genes among Poaceae species that show chromosome elimination in interspecific hybrids revealed that the loop 1 regions of oat and pearl millet CENH3 exhibit exceptionally high similarity.

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Ascorbate (vitamin C) is a powerful antioxidant and scavenger of free radicals that protects plants against oxidative damage caused by adverse environmental conditions such as drought, salt stress, and herbicide use. Dehydroascorbate reductase (DHAR; EC 1.8.5.1) and monodehydroascorbate reductase (MDAR; EC 1.6.5.4) are crucial for ascorbate regeneration and the maintenance of a pool of the reduced form. In this study, we report the development of transgenic potato (Solanum tuberosum L.) that overexpresses the Arabidopsis thaliana DHAR gene (AtDHAR1) in the cytosol. The transgenic plants exhibited up to 4.5 times the DHAR activity and up to 2.8 times the level of reduced ascorbate found in the wild-type plants. When subjected to methylviologen treatment, the transgenic plants exhibited enhanced tolerance in terms of less ion leakage, greater chlorophyll contents, less accumulation of hydrogen peroxide, and less severe visual injury symptoms. Moreover, the transgenic plants exhibited faster growth under drought and salt stress. Our results demonstrate that elevating ascorbate contents by overproducing DHAR represents a viable approach for the development of herbicide-tolerant potato.Key Words: ascorbate, potato, vitamin C, dehydroascorbate, methylviologen. IntroductionAdverse environmental conditions such as drought, salt stress, and use of herbicide promote the overproduction of reactive oxygen species (ROS) in plant cells. ROS such as singlet oxygen (O 2 1 ), superoxide radicals (O 2 ⋅), hydrogen peroxide (H 2 O 2 ), and hydroxyl radicals (OH⋅) are believed to be the major factor responsible for rapid cellular damage due to their high reactivity with membrane lipids, proteins, and DNA (Asada and Takahashi 1987, Hung et al. 2005, Mittler 2002). Scavenging and reducing the deleterious effects of ROS is performed by several enzymatic and nonenzymatic systems in plant cells. A network of low-molecularweight antioxidants, such as ascorbate (AsA), glutathione (GSH), and α-tocopherol, and antioxidative enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT) are crucial for preventing and controlling the dangerous affects of ROS (Noctor and Foyer 1998). Specifically, enzymes involved in catalyzing redox reactions and in maintaining pools of reduced AsA and GSH in the AsA-GSH pathway, mainly monodehydroascorbate reductase (MDAR; EC 1.6.5.4), dehydroascorbate reductase (DHAR; EC 1.8.5.1), and glutathione reductase, are considered of paramount importance in plant antioxidant defense mechanisms.The antioxidant AsA can directly scavenge free radicals (Halliwell and Gutteridge 2000), and is particularly important as an electron donor for detoxification of H 2 O 2 via ascorbate peroxidase in plant cells (Noctor and Foyer 1998). Moreover, AsA is a major redox buffer in plants , a cofactor of many enzymes (Smirnoff and Wheeler 2000), a regulator of cell division and growth (Kerk and Feldman 1995), and a molecule used in signal transduction in plants (Noctor et al. 2000). AsA is synthesized in t...