AOBPreview originally published online on November 25, 2005
Annals of Botany 2006 97(1):133-140; doi:10.1093/aob/mcj010
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Isolation and Characterization of Four Ascorbate Peroxidase cDNAs Responsive to Water Deficit in Cowpea Leaves
1 Laboratoire d'Ecophysiologie Moléculaire, UMR 137 Biosol, Université Paris 12, 61 avenue du Général de Gaulle, 94010 Créteil Cedex, France and 2 Laboratoire de Géochimie des Eaux Université Paris 7 and IPGP, 2 place Jussieu case 7052 75251 Paris cedex 05, France
* For correspondence. E-mail lameta{at}univ-paris12.fr
Received: 4 July 2005 Returned for revision: 18 August 2005 Accepted: 30 September 2005 Published electronically: 25 November 2005
| ABSTRACT |
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Background and Aims Abiotic stresses stimulate formation of active oxygen species in plant tissues. Among antioxidant mechanisms, H2O2 detoxication by ascorbate peroxidases (APX) plays an important role. Several APX isoforms exist in plant cells, and they have rarely been studied separately. The aim of this work was to study changes in cytosolic, peroxisomal, stromatic and thylakoid APX gene expression in response to progressive drought, rapid desiccation and application of exogenous abscisic acid in the leaves of cowpea (Vigna unguiculata) plants.
Methods Two cowpea (V. unguiculata) cultivars, EPACE-1 which is drought-tolerant and 1183which is drought-sensitive, were submitted to drought stress by withholding irrigation. Detached leaves were air-dried or treated with exogenous abscisic acid. APX cDNAs were isolated by PCR and cloned in plasmid vectors. Changes in gene expression were studied using reverse-transcription PCR.
Key Results Four new V. unguiculata cDNAs encoding putative cytosolic, peroxisomal and chloroplastic (stromatic and thylakoidal) APX were isolated and characterized. In response to the different treatments, higher increases in steady-state transcript levels of the cytoplasmic and peroxisomal APX genes were observed in 1183 compared with EPACE-1. On the other hand, the expression of the chloroplastic APX genes was stimulated earlier in the tolerant cultivar when submitted to progressive drought.
Conclusions Water deficit induced differences in transcript accumulation of APX genes between the two cultivars that were related to their respective tolerance to drought. Chloroplastic APX genes responded early to progressive water deficit in the tolerant plant, suggesting a capacity to efficiently detoxify active oxygen species at their production site. The more sensitive 1183 was also able to respond to drought by activating its whole set of APX genes.
Key words: Active oxygen species, drought tolerance, ascorbate peroxidase, abscissic acid, gene expression, Vigna unguiculata
| INTRODUCTION |
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Water deficit (drought and desiccation) is known to generate active oxygen species (AOS). Among these, H2O2 is produced mainly in the chloroplasts and mitochondria of stressed cells and is the source of important cell damage (Foyer et al., 1994
To understand better the role of the different forms of APX under water deficit, comparative studies were carried out using two Vigna unguiculata (Vu) cultivars: EPACE-1 which is drought-tolerant and 1183 which is drought-sensitive. When submitted to drought, the two cultivars showed different physiological responses: EPACE-1 closed its stomata at lower leaf water potentials and therefore maintained photosynthetic activity longer than 1183 did (Cruz de Carvalho et al., 1998
). At the cellular level, EPACE-1 could maintain cellular homeostasis at lower leaf water potentials compared with 1183 (Vasquez-Tello et al., 1990
). This resulted from a lesser stimulation of lipolytic and proteolytic activities (Roy-Macauley et al., 1992
; Sahsah et al., 1998
) and from lower expression of the corresponding genes (El Maarouf et al., 1999
; Cruz de Carvalho et al., 2001
; Matos et al., 2001
). Also, EPACE-1 was less prone to photooxidative damage (Ferrari-Iliou et al., 1994
) and could accumulate more protective molecules (d'Arcy-Lameta et al., 1996
).
In this study, first APX activities were measured in the leaves of cowpea plants submitted to drought, then four cowpea APX cDNAs corresponding to Vu1183 cytosolic (cAPX), peroxisomal (pAPX) and chloroplastic (thylakoid and stromatic, tAPX and sAPX, respectively) leaf compartments were isolated and characterized. The APX gene expression was studied in response to progressive drought and air desiccation. Exogenous abscisic acid (ABA) treatments were also carried out, as ABA is known to be involved in drought-induced signalling pathways leading to adaptive antioxidant processes (Zhu, 2002
).
| MATERIALS AND METHODS |
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Plant material and stress treatments
Two Vigna unguiculata (L.) Walp. (Vu) cultivars were used: EPACE-1 which is drought-tolerant originated from semi-arid north-eastern Brazil and 1183 which is drought-sensitive originated from humid areas of China. Plants were grown in pots (9 cm diameter), in a mixture of peat and vermiculite (50 : 50, v/v), in a greenhouse under conditions described previously (El Maarouf et al., 1999
Different harmful conditions known to cause oxidative stress were applied to cowpea EPACE-1 and 1183 plants to characterize expression of the VuAPX cDNA isolated here: air desiccation, exogenous ABA treatment and progressive drought. Desiccated leaves were air-dried at 24 °C under dim light for 30 min, 2 h and 5 h. Exogenous ABA was applied by immersing the petioles of excised leaves in a 0.1 mM aqueous solution of mixed ABA isomers (Sigma, St Quentin Fallavier, France) for 30 min, 2 h and 24 h under ambient conditions (24 °C and 250 µmol s1 m2 light intensity). Progressive drought was applied by withholding irrigation from treated plants for 215 d. Leaf water potentials (
w) were measured using a pressure bomb (PMS instrument, Corvallis, USA) (Scholander et al., 1964
). In control leaves, water potentials were 0·3 to 0·5 MPa. Treated leaves were harvested at
w = 1·0 ± 0·1 MPa (S1),
w = 1·5 ± 0·2 MPa (S2) and
w = 2·0 ± 0·2 MPa (S3).
Ascorbate peroxidase activity
To prepare APX extracts, 1 g of fresh leaf tissue was homogenized in 10 ml of 50 mM phosphate buffer pH 7·8, containing 2 mM EDTA, 0·5 mM phenylmethylsulfonyl fluoride and 2 mM ascorbate, in the presence of 1·2 % insoluble PVP and 0·1 % Triton X-100. Leaf homogenates were centrifuged at 30 000 g for 20 min at 4 °C, APX activity was measured in the supernatant as the decrease in A290 per mg of protein and per minute (Nakano and Asada, 1981
). Protein contents were determined according to Bradford (1976)
with the Bio-Rad protein assay reagent, using bovine serum albumin as a standard.
RNA extraction and cDNA identification
Total RNA was isolated from leaf samples using RNeasy plant midi or maxi Kit (Qiagen, Courtaboeuf, France). Poly (A)+ RNA was isolated from total RNA using Oligotex columns (Qiagen, France), following manufacturer's instructions. Total and mRNA were quantified using a Nanodrop ND-1000 spectrophotometer (Starlab, Paris, France) at 260 nm.
Previously isolated APX sequences were aligned using the CLUSTALW program (Thompson et al., 1994
). Degenerated oligonucleotides were designed from conserved regions and used as primers. Reverse-transcription PCR (RTPCR) was performed from 100-ng 1183 mRNA template, using One Step RTPCR kit (Qiagen, France). cDNA fragments were PCR amplified in 35 cycle reactions at 50 °C annealing temperature for cAPX and 60 °C for pAPX, tAPX and sAPX. Amplified APX cDNA fragments were purified by Wizard PCR Prep (Promega, Charbonnières, France) after electrophoretic separation on 1·2 % (w/v) agarose gels and cloned in pGEM-T Easy plasmid, according to the manufacturer's instructions (Promega, France). DNA sequencing was performed on both strands (ESGS, Lille, France).
The four putatively full-length APX cDNA (cAPX, pAPX and chloroplastic t and sAPX) were obtained, using 3'- and 5'-RACE method (5'/3' RACE kit, Roche Diagnostics, Meylan, France), following the manufacturer's protocols. 1183 purified poly (A)+ mRNA was used as a template.
RTPCR and gene expression
APX gene expression analysis was carried out by RTPCR reactions, using One-Step RTPCR Kit (Qiagen, France). Fifty or 100 ng of total leaf RNA were used as templates. Vu APX specific primers were used to amplify cDNA fragments of 790 bp (cAPX), 387 bp (pAPX), 396 bp (tAPX) and 551 bp (sAPX). To evaluate the amount of template RNA in each RTPCR reaction, a region of Vu 18S gene encoding a ribosomal RNA with near constitutive expression under water deficit conditions was amplified. All PCR reactions included an initial denaturation step at 94 °C, followed by 3035 cycles depending on the template with a denaturation step (40 s at 94 °C), an annealing step (40 s at 60 °C) and an extension step (40 s at 72 °C). A final extension was carried out for 7 min at 72 °C. Amplification products were visualized on 1·2 % (w/v) agarose gels, using a UV light transilluminator (Snapshot, Syngene, Ozyme, Saint Quentin en Yvelines, France). Densitometric evaluation of DNA bands were performed with the Imager 1D/2D software (Appligene, Strasbourg, France). Preliminary experiments (not shown) with various PCR cycle numbers indicated that in the RTPCR conditions in this study (50 ng RNA, 30 cycles for cAPX and pAPX, 250 ng RNA and 35 cycles for chloroplastic APX), amplifications were not in the plateau phase, and therefore allowed for semi-quantitative estimations of transcript levels.
| RESULTS AND DISCUSSION |
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Effect of water withholding on APX activity
APX activity was 60 % higher in control EPACE-1 plants than in control 1183 plants (Fig. 1). In response to drought stress (S1, S2 and S3 plants), variations in APX activity were not significant in EPACE-1. Conversely, drought conditions induced significant changes in the levels of APX activity in 1183 since S2 plants showed a 78 % increase compared with control plants. In S3 plants, however, APX activity decreased back to control level (Fig. 1). Changes in APX activities in higher plants subjected to environmental stresses have been extensively studied (for a review see, Shigeoka et al., 2002
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Isolation and characterization of four APX cDNAs
Regarding cytosolic APX, a 1117-bp-long cDNA denoted VucAPX (Genbank, accession number U61379 [GenBank] ) was obtained. The open reading frame (ORF) started with an ATG codon at position 89 and was interrupted by a stop codon (TAA) at position 839. It was flanked by an 89-bp 5' untranslated region (UTR) and a 278-bp 3' UTR. A putative polyadenylation signal AAATAA was found at position 984. The deduced protein had 250 amino acid residues and a calculated mol. wt of 27 kDa. At the amino acid level, it shared 92 % and 80·8 % identity with pea (X62077 [GenBank] ; Mittler and Zilinskas, 1991
Regarding peroxisomal APX, a full-length cDNA of 1169 bp was obtained and denoted VupAPX (AY466858 [GenBank] ). Its ORF started with an ATG codon at position 130 and terminated with a stop codon (TAA) at position 1094. It was flanked by 172 additional nucleotides on the 3' end. A putative polyadenylation signal AAATAA was found at position 1109. The ORF encoded a deduced polypeptide of 320 amino acid residues (calculated mol. wt 31·7 kDa) that shared 84·7 % and 75·7 % sequence identity with Cucurbita (AB070626 [GenBank] ) and Hordeum (AB063117 [GenBank] ) pAPX, respectively.
Regarding chloroplastic APX, two cDNA were obtained. The longest thylakoidal cDNA sequence, designated VutAPX (AY484492
[GenBank]
), was 1354 bp long. The ORF started with an ATG codon at position 61 and terminated with a TAG stop codon at position 1299. A putative polyadenylation signal ATAAT was found at position 1329. The ORF encoded a deduced protein of 413 amino acid residues (calculated mol. wt 45·2 kDa) that shared 77·5 % and 79·3 % sequence identity with tAPX from spinach (D77997
[GenBank]
) and pumpkin (D83656
[GenBank]
), respectively. The longest stromatic cDNA sequence, denoted VusAPX (AY484493), was 1505 bp long. Its ORF started with an ATG codon at position 61 and terminated with a TAA stop codon at position 1153, the ORF encoded a deduced protein of 364 amino acid residues identical to that of VutAPX (calculated mol. wt 39·8 kDa). It was followed by 350 additional nucleotides. Among these, a 151-nucleotide sequence corresponded to an intron (Ishikawa et al., 1996
; Mano et al., 1997
) and a 195-bp span was identical to the end of the 3'-end region of the VutAPX sequence (Fig. 2).
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The four cowpea APX deduced proteins have been aligned and compared with a pea cytosolic APX (Mittler and Zilinskas, 1991
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The thylakoidal and stromal APX sequences described herein were identical, except for the presence of a 49 amino acid region at the C-terminal end of the thylakoidal sequence with a very hydrophobic profile which could correspond to a membrane-spanning region (Shigeoka et al., 2002
Compared with VucAPX, VupAPX presented an additional C-terminal region of 39 amino acid residues that shared high sequence similarity with Cucurbita and arabidopsis pAPX C-terminal regions. In these putative membrane-spanning regions, the YEVKRXK-COOH motif seemed characteristic of peroxisomal (microbody) proteins (Mullen and Trelease, 2000
). According to Bunkelman and Trelease (1996) the pAPX is bound to the external side of the membrane of peroxisomes.
Effect of stress conditions on APX gene expression
In EPACE-1 air-desiccated leaves, no significant changes were observed in steady-state levels of VucAPX and VupAPX transcripts in response to rapid water loss (Fig. 4) and exogenous ABA treatment (Fig. 5). In the case of 1183, important increases in steady-state transcript levels of cAPX and pAPX were observed after 2 h of ABA treatment and after 30 min of desiccation. It is interesting to note that VupAPX, which codes for a protein facing the cytosolic side of peroxisomes, showed an expression profile similar to that of VucAPX. The present results concerning EPACE-1 are equivalent to those obtained by Yoshimura et al. (2000)
on spinach. However, results on the sensitive cowpea cultivar are different and indicate an up-regulation of APX gene expression by rapid water loss and ABA. ABA is implicated in signalling pathways induced by water stress (Shinozaki and Yamaguchi-Shinozaki, 1997
; Chinnusamy et al., 2004
) and APX gene expression was stimulated by ABA (Jiang and Zhang, 2002
). However, since APX gene expression induced by ABA occurred later than drought-induced transcript accumulation a role for ABA in drought-induced APX transcript accumulation is not supported. The role of other mechanisms, such as changes to photosynthesis, have to be considered (Chang et al., 2004
).
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In response to progressive drought (Fig. 6), as in the case of rapid water loss, variations in VucAPX and VupAPX transcript levels were more important in the drought-sensitive cultivar than in the drought-tolerant one. Concerning chloroplastic APX, two signals were visualized on the agarose gel, since Vu chloroplastic APX primers were chosen so as to include the 151-bp insertion found at the 3' end of the stromatic isoform (Fig. 2). In EPACE-1, slight water stress strongly stimulated both chloroplastic APX gene expression (Fig. 6), in 1183, stimulation of the stromal isoform occurred much later, at severe water deficits. Although chloroAPX gene expression was weak compared with those of VucAPX and VupAPX, their role may be important since they detoxify AOS at their production site. In transgenic tobacco plants, Yabuta et al. (2002)
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Regarding protective molecules against AOS, lipophilic extracts from EPACE-1 resisted photoperoxidation better than those from 1183 (Ferrari-Iliou et al., 1994
| CONCLUSIONS |
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When comparing gene expression patterns (Fig. 6) and enzymatic activity (Fig. 1), the present results suggest that subtle changes in the intracellular distribution of protective enzymes and different sensitivities to AOS exhibited by APX isozyme forms might be more important for protection than an overall increase in total enzyme activity. Adjustments of isozyme form affinities for their substrates may be the main strategy to increase the efficiency and the fine-tuning of the antioxidant system (Foyer et al., 1994
| ACKNOWLEDGEMENTS |
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The authors thank Dr Anne Repellin for language correction and constructive comments on the manuscript.
| FOOTNOTES |
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The nucleotide sequence data are registered in Genbank under the accession numbers: cAPX, U61379 [GenBank] ; tAPX, AY484492 [GenBank] ; sAPX, AY484493 [GenBank] ; pAPX, AY466858 [GenBank] .
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) at position 1153 bp. In tAPX, a stop codon TAG (





