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MadhuraBabu Kunta and Eliezer Louzada*

Ascorbate Peroxidase (APX) is a heme-containing, non-glycosylated enzyme that destroys harmful hydrogen peroxide via the ascorbate glutathione pathway in plants. This enzyme is considered to be an indispensable part of the electron-scavenging pathway and is involved in preventing oxidative damage in plants. Using differential display RT-PCR and 5' RACE a full length c-DNA clone was isolated, from citrus, with very high similarity at the nucleotide and amino acid level, to ascorbate peroxidases from several plant species. It is well known that APXs have highly conserved motifs like the Arg-38, Ars-71, Glu-65 and Asp-208 residues around the distal Hist-42 and proximal His-163. These residues are essential for binding the ligand heme. Additionally, Trp-179 is conserved in most APXs and is the third participant in hydrogen bonding network, together with His-163 and Asp-208. All these conserved motifs were present in the putative APX from citrus in addition of the presence of the peroxidase active site motif residues (APITLRLAWHSA) and the peroxidase heme-ligand motif (DIVVLSGGHTL). Expression analysis in E. Coli reviewed a recombinant protein of 27 Kda.

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Yueju Wang, Michael Wisniewski, Richard Meilan, Minggang Cui, Robert Webb, and Leslie Fuchigami

Ascorbate peroxidase (APX) plays an important role in the metabolism of hydrogen peroxide in higher plants, affording them protection against oxidative stress. We studied the effect of overexpressing a cytosolic ascorbate peroxidase (cAPX) gene—derived from pea (Pisum sativum L.)—in transgenic tomato plants (Lycopersicon esculentum L.). Transformants were selected in vitro using kanamycin resistance and confirmed by polymerase chain reaction (PCR) and northern analyses. An APX native-gel assay indicated that, in the absence of stress, APX activity in transgenic plants was several times greater than that measured in wild-type (WT) plants. Several independently transformed lines were propagated and evaluated for resistance to chilling and salt stress. After placing seeds at 9 °C for 5 weeks, percent germination was greater for seeds obtained from transgenic lines (26% to 37%) compared to the WT (3%). Plants from transgenic lines also had lower electrolyte leakage (20% to 23%) than WT (44%) after exposure to 4 °C. Visual assessment of transgenic and WT lines exposed to salinity stress (200 or 250 mm) confirmed that overexpression of APX minimized leaf damage. Moreover, APX activity was nearly 25- and 10-fold higher in the leaves of transgenic plants in response to chilling and salt stresses, respectively. Our results substantiate that increased levels of APX activity brought about by overexpression of a cytosolic APX gene may play an important role in ameliorating oxidative injury induced by chilling and salt stress.

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Madhurababu Kunta, H. Sonia del Rio, and Eliezer Louzada

Reactive oxygen species (ROS) are continuously produced during the normal aerobic metabolism and also under environmental stress conditions. They are the major damaging factors to the photosynthetic machinery under stress conditions and need to be scavenged for the normal growth of the plant. Ascorbate peroxidase (APX) is the key enzyme in detoxifying H2O2, one of ROS from chloroplast and cytosol. A cDNA encoding a putative APXcit was isolated from mature `Dancy' tangerine (Citrus reticulata Blanco) juice vesicles using differential display reverse transcription-polymerase chain reaction (RT-PCR). Subsequently, full-length APXcit cDNA clone and genomic clone were obtained and sequenced. The full-length APXcit sequence is composed by 1082-bp nucleotides, including an open reading frame (ORF) of 753 bp, encoding a protein of 250 amino acids (27 kDa). The 5' un-translated region (UTR) of the APXcit gene consisted of 91 nucleotides and the 3' UTR consisted of 238 nucleotides. Homology search for APXcit at GenBank database showed high similarity to APX from several plant species.

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Lailiang Cheng and Fengwang Ma

Xanthophyll cycle conversion and the antioxidant system in the peel of apple fruit (Malus ×domestica Borkh. `Liberty') were monitored in the field over a diurnal course at about 3 months after full bloom. Compared with leaves, sun-exposed peel of apple fruit had much lower photosystem II operating efficiency at any given photon flux density (PFD) and a larger xanthophyll cycle pool size on a chlorophyll basis. Zeaxanthin (Z) level increased with rising PFD in the morning, reached the highest level during midday, and then decreased with falling PFD for the rest of the day. At noon, Z accounted for >90% of the xanthophyll cycle pool in the fruit peel compared with only 53% in leaves. Efficiency of excitation transfer to PSII reaction centers (F v′/F m′) was negatively related to the level of Z in fruit peel and leaves throughout the day. In fruit peel, the antioxidant enzymes in the ascorbate-glutathione cycle, ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) showed a diurnal pattern similar to that of incident PFD. In contrast, the activities of APX and GR in leaves did not change significantly during the day although activities of both MDAR and DHAR were higher in the afternoon than in the morning. In both fruit peel and leaves, superoxide dismutase activity did not change significantly during the day; catalase activity showed a diurnal pattern opposite to that of PFD with much lower activity in fruit peel than in leaves. The total ascorbate pool was much smaller in fruit peel than in leaves on an area basis, but the ratio of reduced ascorbate to oxidized ascorbate reached a maximum in the early afternoon in both fruit peel and leaves. The total glutathione pool, reduced glutathione and the ratio of reduced glutathione to oxidized glutathione in both fruit peel and leaves also peaked in the early afternoon. We conclude that the antioxidant system as well as the xanthophyll cycle responds to changing PFD over the course of a day to protect fruit peel from photooxidative damage.

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Priscila L. Gratão, Carolina C. Monteiro, Lázaro E.P. Peres, and Ricardo Antunes Azevedo

have concentrated our attention on some of the key antioxidant enzymes such as catalase (CAT), guaiacol peroxidase (GPOX), ascorbate peroxidase (APX), glutathione reductase (GR), and superoxide dismutase (SOD). The Micro-Tom cultivar was kindly

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Na Zhang, Lu Han, Lixin Xu, and Xunzhong Zhang

). Ascorbate peroxidase is a H 2 O 2 -scavenging enzyme that is unique to plants and is indispensable to protect chloroplasts and other cellular constituents from damage caused by H 2 O 2 and hydroxyl radicals (•HO) produced from it ( Asada, 1992 ). Better

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Yong In Kuk and Ji San Shin

. Ascorbate peroxidase activity in leaf 4 was generally higher than that of older leaves up to 72 h after chilling treatment ( Fig. 5C ). Similar to SOD, APX activity in all leaf ages was generally induced as a result of chilling stress, but induction of APX

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Shanshan Sun, Mengying An, Liebao Han, and Shuxia Yin

effect on POD activity ( Fig. 5 ). Fig. 5. Effects of 24-epibrassinolide (EBR) on superoxide dismutase (SOD) ( A ), ascorbate peroxidase (APX) ( B ), catalase (CAT) ( C ), and peroxidase (POD) ( D ) activities of perennial ryegrass under salt stress

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Abu Shamim Mohammad Nahiyan and Yoh-ichi Matsubara

-AMF-inoculated plants; AMF = Glomus sp. R10. Bars represent se s (n = 10). *Significant difference between non-AMF and AMF plants ( t test, P ≤ 0.05). Fig. 2. Superoxide dismutase (SOD), ascorbate peroxidase (APX), 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical

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Ju Ding, Kai Shi, Yan-Hong Zhou, and Jing-Quan Yu

for antioxidant enzyme analysis, including superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), and catalase (CAT). All operations were carried out at 0 to 4 °C. An aliquot of the extract was used to determine its protein