Expression analysis of antioxidant genes in response to drought stress in the fl ag leaf of two Indonesian rice cultivars

The objective of this study was to analysis the expression of antioxidant genes in response to drought stress in Indonesian rice. The malondialdehyde (MDA) content and the expression of Cu-ZnSod1, cCu-ZnSod2, MnSod1, cApxa, cApxb, chl-sApx, Cat1, Cat2, Cat3, Gr1, Gr2, and Gr3 genes were assayed in the rice fl ag leaf of Ciherang and Situ Bagendit cultivars subjected to control, mild and severe drought during the grain fi lling phase. Increase in MDA content of Ciherang treated to mild and severe drought was almost two-fold and three-fold respectively, while MDA content in Situ Bagendit subjected to mild and severe drought increased approximately one-fold and two-fold as compared to the control. The semi quantitative reverse transcription polymerase chain reaction (sqRT-PCR) analysis showed that the expression of cCu-ZnSod1, MnSod1, Cat2, Gr3 genes of Ciherang, and cCu-ZnSod2, MnSod1, cApxa, cApxb, chl-sAPX, Cat2 and Gr1 genes of Situ Bagendit increased in fl ag leaf of plant treated to drought. Expressions of cApxb, chl-sApx, Cat3 of Ciherang and Cu-ZnSod1 and Gr2 genes of Situ Bagendit were not changed signifi cantly by drought stress. Decreased expression was shown by cCu-ZnSod2, cApxa, Cat1, Gr1 and Gr2 genes of Ciherang, and Cat1, Cat3 and Gr3 genes of Situ Bagendit. The results indicated that the activity of oxidative defense was regulated by four genes; cCu-ZnSod1, MnSod1, Cat2, Gr3 in Ciherang, and eight genes; cCu-ZnSod1, cCu-ZnSod2, MnSod1, cApxa, cApxb, chl-sApx, Cat2 and Gr1 in Situ Bagendit. Therefore, differences in the number of antioxidant genes controlling oxidative defense system might determine the difference of the oxidative defense capacity between both cultivars in response to drought stress during grain fi lling.


Introduction
Drought stimulated the formation of reactive oxygen species (ROS) such as superoxide (O 2 *-), hydrogen peroxide (H 2 O 2 ) and hydroxyl radical (OH*).Being highly reactive, the presence of excessive ROS could damage cellular components such as lipid, protein, nucleic acid and enzyme (Ahmad et al., 2008), which was called oxidative stress.The level of oxidative stress in plant could be determined by quantifying the content of melanoaldehyde (MDA) as indicator of lipid peroxidation.Plant, therefore, protected their cells from oxidative stress by generating oxidative self-defense systems consisting of antioxidant molecules such as ascorbic acid (AsA), alpha tocopherol (α-Toc), and glutathione as well as antioxidant enzymes e.g.superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), and glutathione reductase (GR) (Apel and Hirt, 2004).The SOD enzyme catalyzed the conversion of superdioxide radical (O 2 *-) into H 2 O 2 , while the APX and CAT enzymes catalyzed the alteration of H 2 O 2 into O 2 and H 2 O.The GR catalyzed the production of reduced glutathione (GSH) that was required as a substrate for AsA synthesis in the ascorbate-glutathione cycle.Both AsA and GSH could directly detoxify O 2 *-radical and thus contribute to non-enzymatic ROS scavenging.
Rice (Oryza sativa L) as a staple food for more than half of the world's population and providing 50-80% of their daily calorie intake (Khush, 2005) was adversely affected by drought.Fukai et al. (1999) stated that drought was the primary cause of rice yield lost among the abiotic stresses.The loss of rice yield due to drought was 13-35% (Khunthasuvon et al., 1998;Fukai et al., 1999).Selote and Khana-Copra (2004) found that seed formation in the drought stressed plants was significantly reduced as compared to the unstressed rice plants of the N22 and N188 cultivars.The failure of microsporogenesis caused by droughtinduced oxidative stress has also been reported in the rice of N31 cultivars (Nguyen and Sutton, 2009).Therefore, the yield loss under drought stress seemed to be associated with reducing the effi ciency of self-oxidative defense to eliminate ROS activity (Selote and Khana-Copra, 2004).
Reduction in the oxidative defense system appears to be regulated by the expression level of antioxidant genes.This has been studied extensively using transgenic plants subjected to various abiotic stresses.Transgenic species transformed with Sod genes showed an increase in the expression of genes associated with tolerance to the abiotic stresses (Murgia et al., 2004;Tang et al., 2006;Prashanth et al., 2008).Other studies reported that transgenic rice containing Cu-ZnSod 1 gene isolated from other species showed better tolerance to drought, salinity or methyl viologen induced-oxidative stress than untransgenic control plants (Wang et al., 2010, Sarangi et al., 2011).Moreover, enhanced oxidative defense in rice and tobacco plants applied to abiotic stress could be achieved through Apx overexpression (Sun et al., 2010;Sato et al., 2011).Overexpression of Cat gene also induced increasing oxidative defense in cabbage plants to reduce ROS damage stimulated by salinity stress (Tseng et al., 2007).The higher tolerance to oxidative stress was observed in tobacco plant transformed with Gr gene isolated from E. coli (Aono et al,. 1991).
The studies of the oxidative defense mechanism in plant including rice have been reported extensively and it has been suggested that different expression of an antioxidant gene(s) and the activity of an antioxidant enzyme(s) determined the capability of plant to cope with oxidative stress.However, most of those studies only emphasized on the defense system in the pre-anthesis phase.The oxidative defense mechanism that involves antioxidant genes on the fl ag leaf of Indonesian rice cultivars during postanthesis i.e. grain filling phase remained poorly understood.To address this issue, a study was performed on the expression of antioxidant genes in the Indonesian rice fl ag leaf of Ciherang (susceptible to drought) and Situ Bagendit (tolerant to drought) cultivars subjected to different drought stress based on the fraction of the transpirable soil water.The objective of the present study was to analyze the effects of drought stress on lipid peroxidation and the expression of antioxidant genes in the fl ag leaf of Ciherang and Situ Bagendit cultivars.

Plant material and Growth condition
Seeds of indica rice of Ciherang and Situ Bagendit cultivars were sterilized with 10% sodium hypochlorite for a minute and washed extensively using distilled water.Then seeds were imbibed in distilled water for 12 hours at dark condition.The imbibed seeds were germinated in petri dishes containing two sheets of the moistened fi lter paper at 30 o C in the dark room for 5 days.Uniformly germinated seeds were selected and planted in a plastic box (40 cm in length x 25 cm in width x 15 cm in height) containing a mixture of clay and sand (2:1,v/v).The germinated seeds were grown under natural light at 30 o C day and 25 o C night in a glasshouse.Twenty-days old seedlings were selected and cultivated in the polyethylene polybag fi lled with 7 kg of a clay-sand-compost mixture (2:1:1 v/v).Seedlings were grown in a natural controlled glasshouse.

Application of drought treatment
Drought treatments were based on the fraction of transpirable soil water (FTSW) (Sinclair and Ludlow, 1986) with three replications.Drought stress was imposed by initiating a soil dry-down protocol starting three days before anthesis.The dry-down process continued until the pot reached the target of FTSW.The drought treatments were started on the seventh day and ended on the twentieth day after the initial anthesis The drought treatments consisted of FTSW 1.0 (control or well-watered), FTSW 0.5 (mild drought) and FTSW 0.2 (severe drought) (He et al., 2009;Sharoni et al., 2012).The FTSW was maintained gravimetrically in all of the polybag during the treatment.

Estimation of lipid peroxidation
Lipid peroxidation was estimated as malonyaldehyde (MDA) content.Two hundred and fi fty mg of the frozen fl ag leaf was crushed to powder and homogenized in 5 ml 0.1% (w/v) trichchloroacetic acid solution.The sample was then centrifuged at 15,000 rpm, 4 o C for 20 min.One ml of the supernatant was pipetted into the test tube and added 4 ml 0.5% thiobarbituric acid in 20% trichchloroacetic acid.The test tube was heated at a temperature of 95 o C in the waterbath for 30 minutes, and then cooled in an ice block.The mixture was centrifuged at 15,000 rpm, 4 o C for 20 minutes.The supernatant was removed and absorption value of each supernatant was read at 450, 532 and 600 nm.Blank solution was 0.5% thiobarbituric acid in 20% trichchloroacetic acid solution.The MDA content was calculated using equation developed by Gao (2000).

RNA extraction and cDNA synthesis
Rice fl ag leaf at grain fi lling stage was harvested at seven days after application of drought treatment.The leaf was immediately frozen at -80°C, prior to the isolation of RNA.Total RNA was isolated using the RNeasy ® Plant Mini Kit (Qiagen, Germany) according to the manufacturer's instructions.The isolated RNA was quantifi ed by Nano Drop Spectrophotometer.The fi rst-strand cDNA was synthesized using SuperScript II fi rststrand synthesis system (Invitrogen, CA, USA) and oligo-dT 18 (Invitrogen, CA, USA) as per manufacturer's instruction then stored at -20 O C for further used.

Semi-quantitative RT PCR assay
To determine the expression profi le of the rice antioxidant genes, semi-quantitative r e v e r s e t r a n s c r i p t i o n -p o l y m e r a s e chain reaction (sq-RT-PCR) assay was conducted using the published specific primer sequences (Kim et al., 2004;Hong et al., 2009;Chou et al., 2012).The primers used in this research were:
For each sample, 5 μL PCR products were added with 1 mL loading dye (Promega), electrophoresed in a 1% syberseen (Promega)agarose gels buffered with 1X Tris-Boric-EDTA (1X TBE), and visualized under UV light.The images were recorded using a digital camera (Cannon).ImageJ analyzer software was used to quantify the PCR band intensities for evaluating the gene expression level.

Lipid peroxidation
Reactive oxygen species generated under drought or other stress leads to the oxidation of unsaturated fatty acids in cell membranes to yield MDA (Behnamnia et al., 2009).The content of MDA as indicator for the occurrence of oxidative stress was also determined within experiment to evaluate the negative impact of the drought-induced oxidative stress in the rice fl ag leaf of Ciherang and Situ Bagendit cultivars.Both cultivars showed a similar trend in MDA content in their fl ag leaf in response to drought stress, however an increase in the MDA content of Ciherang fl ag leaf was higher than that of Situ Bagendit fl ag leaf (Figure 1).Mild and severe drought stress increased almost two-fold and three-fold of the MDA content of the Ciherang fl ag leaf, and one-fold and two-fold in that of Situ Bagendit fl ag leaf as compared to their control.The different increase in MDA content of the plants caused by abiotic stress have been reported by previous studies (Basu, 2010;Damanik et al., 2010;Wu, et al., 2012).The difference of the MDA content between Ciherang and Situ Bagendit cultivars induced by drought stress indicated that there was difference in the intensity of cell impairment caused by ROS activity.Drought stress trigger the formation

Ciherang
Situ Bagendit of the O 2 -* and H 2 O 2 , radicals which could directly attack the membrane lipid (Mittler et al., 2004).Increase in H 2 O 2 levels in rice treated to drougth and chilling caused lipid peroxidation folowed by enhacement of MDA level (Nguyen et al., 2009;Sato et al., 2011).Increase in lipid peroxidation (MDA) ascribe to decline in the antioxidant system enzyme activities (Fu et al., 2011).
This result indicated that the fl ag leaf of Ciherang cultivar possessed a less capacity of the antioxidative defense mechanism as compared to that of Situ Bagendit.Therefore, it was suggested that the level of oxidative damages by ROS activity relied on the antioxidative defense system of each cultivar during the stress.The regulation of antioxidant gene expression seems to be a key of regulation of the defense mechanism of the fl ag leaf of both cultivars.

Antioxidant gene expression
Gene expression in response to abiotic stress is usually studied at the level of steady-state mRNA abundance because it gives a more precise estimation of antioxidant gene activation than enzyme activity (Hong et al., 2009).As revealed in that study, drought stress stimulated the alteration in antioxidant gene expression.The present study was attempted to explore more deeply the oxidative defense mechanism through gene regulation in the fl ag leaf of Ciherang and Situ Bagendit cultivars in response to drought stress

Sod genes
Both Ciherang and Situ Bagendit cultivars exhibited a similar pattern of Cu-ZnSod 1 gene expression in their fl ag leaf in response to drought stress (Figure 2 and 3A).The expression of Cu-ZnSod 1 gene in the fl ag leaf of Ciherang and Situ Bagendit increased to 33.3 and 39.7 in mild drought, and 12.1 and 6.3% in severe drought, respectively, when compared with the control.Unlike cCu-ZnSod 1 , the cCu-ZnSod 2 in the fl ag leaf of both cultivars showed a different expression  in response to an increase in drought stress (Figure 3B).The cCu-ZnSod 2 gene expression in the fl ag leaf of Ciherang decreased in mild (9.7%) and severe (33.3%) as compared to the control.Whereas the cCu-ZnSod 2 gene expression in the fl ag leaf of Situ Bagendit increased to 18.2 and 25.4% in mild and severe drought, respectively, as compared to the control.Figure 3C shows that the expression of MnSod 1 gene was increased in the fl ag leaf of both cultivars with increase in drought stress.The fl ag leaf of Ciherang and Situ Bagendit had 21.4 and 13.3% higher MnSod 1 expression, respectively, under mild drought than their control.The MnSod 1 expression in the flag leaf was higher in Ciherang (35%) and Situ Bagendit (16.7%) under severe drought as compared to that of their control.The result indicated that the highest involvement of the cCu-ZnSod 1 gene for supporting the oxidative defense of the fl ag leaf of both cultivars only took place in mild stress.Decrease in cCu-ZnSod 2 gene expression in the fl ag leaf of Ciherang indicated that the gene might not involve in controlling the activity of SOD enzyme for scavenging O 2 * radical during the drought stress.The present study indicated that cCu-ZnSod 2 gene involved in the regulation of the oxidative defense mechanism in the fl ag leaf of Situ Bagendit only.The defense mechanism in the fl ag leaf of both cultivars involved the regulation of MnSod 1 gene.The MnSod 1 gene played a crucial role for controlling the free radical destruction via increasing the catalytic activity of SOD enzyme in the fl ag leaf of Situ Bagendit.It could be inferred that the capacity of oxidative defense in scavenging superoxide radical was less effectively in the fl ag leaf of Ciherang than Situ Bagendit.The different capacity could also affect the level of the plant tolerance to drought stress.It is associated with the change of SOD enzyme activity to protect cells against oxidative stress caused by the deleterious activity of O 2 *-radical (Fu et al., 2010).Tayal et al. (2004) stated that the gene expression in response to oxidative stress was regulated by the transcription factor(s).Transcription factors e.g.DREB, HDG 11 and NAC became a regulator of expression genes involving in drought tolerance of rice (Oh et al., 2005;Hu et al., 2006;Yu et al.,2008).It seemed that transcription factors could involve the oxidative defense system via the regulation of the antioxidant gene expression in response to drought.

Apx genes
Drought stress stimulated a different expression of Apx gene in the flag leaf of Ciherang and Situ Bagendit cultivars (Figure 4).The Apx a expression in the fl ag leaf of Ciherang was down regulated but that in the flag leaf of Situ Bagendit was up-regulated in response to an increase in the level of drought stress (Figure 5A).The cApx a expression in the fl ag leaf of Ciherang decreased to 13.9 and 47.7% in mild and severe drought, respectively, as compared to the control.In contrast, the expression level of cApx a gene in the fl ag leaf of Situ Bagendit increased to 94.1% in mild drought and 155.9% in severe drought as compared with the control.Moreover, the expression of cApx b in the fl ag leaf of Ciherang enhanced in the mild drought (7.7%), but did not change signifi cantly in severe drought (1.1%) as compared with the expression level of that gene in control.The increase of cApx b expression level occured in the fl ag leaf of Situ Bagendit concomitant with increase in the level of drought stress (Figure 5B).The cApx b expression level increased by 23.2% in mild drought and 46.3% in severe drought compared to the control.Figure 5C shows that the higher expression of chl-sApx in the fl ag leaf of Ciherang under mild (58%) and severed (16.6%) drought than that of the control.The expression of chl-sApx in the fl ag leaf of Situ Bagendit enhanced to 400 and 480% in both mild and severe drought as compared to the control.
T h e r e s u l t s i n d i c a t e d t h a t t h e degradation of H 2 O 2 into H 2 O and O 2 by APX enzyme in the flag leaf of Ciherang under mild drought involved the regulation of cApx b and chl-sApx genes.But a slight decrease in function of both genes to regulate H 2 O 2 scavenging enzyme exhibited in the fl ag leaf of Ciherang treated to severed drought.The function of the APX enzyme in fl ag leaf of Situ the was regulated by three Apx genes.The previous studies found that the activity of APX enzyme in the leaf of rice plant treated to abiotic stress may be associated with the expression of Apx genes (Sato et al., 2011;Chou et al., 2012).Therefore, the Apx genes played a major role in the defense system of Situ Bagendit were chl-sApx, cApx b then cApx a .
The variation of relative Apx expression in the flag leaf indicated the differences of the oxidative defense system between both cultivars.These findings were well supported by previous studies on pea (Mittler and Zilinskas, 1994), Arabidopsis (Conklin and Last, 1995), rice (Morita et al., 1999), spinach (Yoshimura et al., 2000), and potato (Kawakami et al., 2002).Eight Apx genes in rice showed the different level of their expression in response to abiotic stress (Teixeira et al., 2006).The expression of cApx a was lower than that of cApx b in the leaf of rice seedlings in response to salinity stress (Menezes-Benavente et al., 2004).The overexpression of cApx b increased the tolerance of transgenic Arabidopsis plants against salinity stress (Lu et al., 2007).Taken together, these present study suggested that the role of each Apx gene in the oxidative defense of plants under abiotic stress depend on the kind of cultivar, organ, growth phase and stress.The study also suggested that the function of oxidative defense for scavenging H 2 O 2 radical in the fl ag leaf of Situ Bagendit was more effectively than that of Ciherang in accordance with an elevation of the expression of the Apx genes.

Cat genes
Together with APX, CAT enzyme plays an important role in the scavenging of hydrogen peroxide within cells.The activity of CAT enzyme was controlled by the Cat  genes.The Cat expression was shown in Figure 6.The Cat 1 expression profi le in the fl ag leaf of the Ciherang and Situ Bagendit cultivars had a similar pattern in response to drought stress (Figure 7A).The Cat 1 expression levels in the flag leaf of both cultivars either treated to mild or severe drought was lower than that of the control The decrease of the Cat 1 expression in the fl ag leaf of Ciherang and Situ Bagendit cultivars were 48.7 and 41.8% under mild drought, and 52.9 and 23.1% under severe drought, respectively, as compared to their controls.
The lower expression of the Cat 2 gene was detected in the flag leaf of the control of Ciherang and Situ Bagendit as compared to the both cultivars treated to either mild or severe drought stress (Figure 7B).However, there was no a substantial difference in the Cat 2 expression between cultivars when the plants were subjected to either mild or severe drought.The Cat 2 expression in the fl ag leaf of Ciherang enhanced to 92 and 76% under mild and severe drought compared to the control.In the leaf of Situ Bagendit the Cat 2 expression showed an increase by 33.3 and 27.8% under mild and severe drought in comparison with the control.
Figure 7C shows that the different drought level could not induce signifi cantly the distinction of Cat 3 expression in Ciherang plants.The Cat 3 expression level was reduced by either mild or severe drought stress as detected in the fl ag leaf of Situ Bagendit as compared to the control.The Cat 3 expression in the flag leaf of Ciherang was 10.5 and 5.3% higher under mild and severe drought, respectively, than the control.Whereas in the flag leaf of Situ Bagendit under mild and severe drought the Cat 3 gene showed a 21.9% decrease in its expression as compared to that of the control.The present study indicated that the H 2 O 2 degradation in the fl ag leaf of both cultivars only involved the regulation of Cat 2 gene.A previous study observed that expression of Cat 1 , Cat 2 and Cat 3 genes increased in leaves of the ABAtreated maize seedling (Scandalios, 2005).Study on Arabidopsis found that ABA induced an increase in Cat 1 expression (Xing et al., 2008).The expression of Cat 2 and Cat 3 also increased under salinity (Yamane et al., 2010).Therefore, the number of Cat genes for controlling oxidative defense system relied on the type of species and the level of abiotic stress.

Gr genes
The Gr 1 expression showed a similar pattern in the flag leaf of both Ciherang and Situ Bagendit cultivars in response to drought (Figure 8).The Gr 1 expression was higher in the fl ag leaf of Ciherang than that of Situ Bagendit when both cultivars were subjected to control and mild stress.But under severe drought the Gr 1 expression of Ciherang was lower that of Situ Bagendit.The highest expression of Gr 1 in both cultivars was noticed in mild drought stress (9A).The Gr 1 expression in the fl ag leaf of Ciherang was slightly higher (8%) in mild drought, but lower (40%) in severe drought than that in the control.Meanwhile, in the fl ag leaf of Situ Bagendit, Gr 1 expression increased to 70 and 47% in mild and severe drought, respectively, as compared to the control.The Gr 2 expression in the fl ag leaf of Ciherang also enhanced to 6% in mild drought stress but reduced 56.7% in severe drought stress when compared to its control.There was not any substantial change in the Gr 2 expression of Situ Bagendit fl ag leaf in response to increase in drought level (Figure 9B).Change in Gr 3 expression level in the fl ag leaf exhibited a contradictory pattern between both cultivars in response to the drought stress (Figure 9C).In the fl ag leaf of Ciherang, the Gr 3 expression was increased by 50 and 98.3% in mild and severe drought, respectively, as compared to the control.On the other hand, in the fl ag leaf of Situ Bagendit, the Gr 3 expression was increased by 16% in mild drought, and 40.3% in severe drought, when compared to the control.The results indicated that the role of Gr 1 for supporting the activity of the oxidative defense mechanism in the flag leaf of Ciherang acted maximally in mild drought stress only.However, the role of Gr 1 declined in severe drought stress.The Gr 1 role in the oxidative defense of the Situ Bagendit fl ag leaf enhanced with increase in drought stress.The study indicated that the function of Gr 2 in the fl ag leaf of Ciherang was more effectively in mild drought stress but not in severe drought as compared with the control.Whereas, the Gr 2 function in the fl ag leaf of Situ Bagendit remained stable even though the plants were subjected to mild and severe drought.This study suggested that the Gr 1 and Gr 2 genes did not engage the regulation of the oxidative defense system in the fl ag leaf of Ciherang under severe drought.The Gr 3 gene appeared to be a regulator of the oxidative defense system via accelerating the catalytic activity of glutathione reductase enzyme in the Ciherang leaf under drought stress.This enzyme catalyzes the production of ascorbate acid through a glutathione-ascorbate acid cycle and protects cells against oxidative damage maintaining a high GSH/GSSG ratio (Foyer and Noctor, 2005;Chao et al., 2010).Because of the decrease in Gr 3 expressions, it seems that this gene did not play a main role in the stress defense of the Situ Bagendit fl ag leaf.The activity of the GR enzyme in the defense system of Situ Bagendit was slightly and dominantly controlled by Gr 2 and Gr 1 gene, respectively.The different involvement of the Gr genes in the oxidative defense between both cultivars supported a previous study.Hong et al. (2009) found that the expression of Gr 1 decreased but Gr 2 and Gr 3 increased in rice seedling treated to salinity and ABA.The different expression of Gr genes in plant under abiotic stress most likely due to the differential sensitivity to such stress.In addition, the different expression patterns in response to abiotic stress suggests that these genes have different physiological roles depend upon cultivars (Tsai et al., 2005).

Conclusion
Ciherang cultivar was more suffered under drought induced oxidative stress than Situ Bagendit cultivars based on the difference of lipid peroxidation (MDA levels) occurred within their fl ag leaf.Antioxidant gene expression in the fl ag leaf of Ciherang differed to that of Situ Bagendit in response to drought stress.The results showed that the activity of oxidative defense was regulated by four genes (cCu-ZnSod 1 MnSod 1 , Cat 2 , Gr 3 ) in the fl ag leaf of Ciherang, and eight genes (cCu-ZnSod 1 , cCu-ZnSod 2 , MnSod 1 , cApx a , cApx b , chl-sApx, Cat 2 and Gr 1 ) in the fl ag leaf of Situ Bagendit of the twelve antioxidant genes that were observed.The degradation of O 2 *-radical into H 2 O 2 catalized by SOD enzyme in the fl ag leaf under drought seemed to be controlled by Cu-ZnSod 1 , MnSod 1 in Ciherang and cCu-ZnSod 1 , cCu-ZnSod 2 , MnSod 1 in Situ Bagendit.Moreover, the elimination of destructive activity of H 2 O 2 involved Cat 2 in the fl ag leaf of Ciherang, and cApx a , cApx b , chl-sApx, Cat 2 in the fl ag leaf of Situ Bagendit.The ascorbate-glutathione cycle of the oxidative system in the fl ag leaf under severe drought involved Gr 3 in Ciherang and Gr 1 in Situ Bagendit.The data suggested that differences in the number of antioxidant genes controlling oxidative defense system might determine the difference of the oxidative defense capacity between both cultivars in response to drought stress during grain fi lling.

Figure 1 .
Figure 1.The effect of drought stress on the content of malonyaldehyde (MDA) in the fl ag leaf of Ciherang and Situ Bagendit cultivars.Error bars indicate the SD (n=3).

Figure 2 .
Figure 2. Expression profi le of three Sod genes and OsActin in the fl ag leaf of Ciherang and Situ Bagendit cultivars treated to drought stress.

Figure 3 .
Figure 3.The effect of drought stress on the relative expression of Sod genes in the fl ag leaf of Ciherang and Situ Bagendit cultivars.A. cCu-ZnSod 1 , B. cCu-ZnSod 2 , C. MnSod 1 .Error bars indicate the SD (n=3)

Figure 4 .
Figure 4. Expression profi le of three Apx genes and OsActin in the fl ag leaf of Ciherang and Situ Bagendit cultivars treated to drought stress.

Figure 5 .
Figure 5.The effect of drought stress on the relative expression of Apx genes in the fl ag leaf of Ciherang and Situ Bagendit cultivars.A. cApx a , B. cApx b , C. chl-sApx.Error bars indicate the SD (n=3).

Figure 6 .
Figure 6.Expression profi le of three Cat genes and OsActin in the fl ag leaf of Ciherang and Situ Bagendit cultivars treated to drought stress.

Figure 7 .
Figure 7.The effect of drought stress on the relative expression of Cat genes in the fl ag leaf of Ciherang and Situ Bagendit cultivars.A. Cat 1 , B. Cat 2 , C. Cat 3 .Error bars indicate the SD (n=3)

Figure 8 .
Figure 8. Expression profi les of three Gr genes and OsActin in the fl ag leaf of Ciherang and Situ Bagendit cultivars treated to drought stress.

Figure 9 .
Figure 9.The effect of drought stress on the relative expression of Gr genes in the fl ag leaf of Ciherang and Situ Bagendit cultivars.A. Gr 1 , B. Gr 2 , C. Gr 3 .Error bars indicate the SD (n=3)