Involvement of hydrogen peroxide in heat shock- and cadmium-induced expression of ascorbate peroxidase and glutathione reductase in leaves of rice seedlings

Hydrogen peroxide (H2O2) is considered a signal molecule inducing cellular stress. Both heat shock (HS) and Cd can increase H2O2 content. We investigated the involvement of H2O2 in HS- and Cd-mediated changes in the expression of ascorbate peroxidase (APX) and glutathione reductase (GR) in leaves of rice seedlings. HS treatment increased the content of H2O2 before it increased activities of APX and GR in rice leaves. Moreover, HS-induced H2O2 production and APX and GR activities could be counteracted by the NADPH oxidase inhibitors dipehenylene iodonium (DPI) and imidazole (IMD). HS-induced OsAPX2 gene expression was associated with HS-induced APX activity but was not regulated by H2O2. Cd-increased H2O2 content and APX and GR activities were lower with than without HS. Cd did not increase the expression of OsAPX and OsGR without HS treatment. Cd increased H2O2 content by Cd before it increased APX and GR activities without HS. Treatment with DPI and IMD effectively inhibited Cd-induced H2O2 production and APX and GR activities. Moreover, the effects of DPI and IMD could be rescued with H2O2 treatment. H2O2 may be involved in the regulation of HS- and Cd-increased APX and GR activities in leaves of rice seedlings.

The rolB gene suppresses reactive oxygen species in transformed plant cells through the sustained activation of antioxidant defense

The rolB (for rooting locus of Agrobacterium rhizogenes) oncogene has previously been identified as a key player in the formation of hairy roots during the plant-A. rhizogenes interaction. In this study, using single-cell assays based on confocal microscopy, we demonstrated reduced levels of reactive oxygen species (ROS) in rolB-expressing Rubia cordifolia, Panax ginseng, and Arabidopsis (Arabidopsis thaliana) cells. The expression of rolB was sufficient to inhibit excessive elevations of ROS induced by paraquat, menadione, and light stress and prevent cell death induced by chronic oxidative stress. In rolB-expressing cells, we detected the enhanced expression of antioxidant genes encoding cytosolic ascorbate peroxidase, catalase, and superoxide dismutase. We conclude that, similar to pathogenic determinants in other pathogenic bacteria, rolB suppresses ROS and plays a role not only in cell differentiation but also in ROS metabolism.

Overexpression of Arabidopsis dehydration- responsive element-binding protein 2C confers tolerance to oxidative stress

Dehydration-responsive element-binding proteins (DREBs)regulate plant responses to environmental stresses. In the current study, transcription of DREB2C, a class 2 Arabidopsis DREB, was induced by a superoxide anion propagator, methyl viologen (MV). The oxidative stress tolerance of DREB2C-overexpressing transgenic plants was significantly greater than that of wild-type plants, as measured by ion leakage and chlorophyll fluorescence under light conditions. The transcriptional activity of several ascorbate peroxidase (APX) genes as well as APX protein activity was induced in DREB2C overexpressors. Additionally, the level of H2O2 in the overexpressors was lower than in wt plants under similar oxidative stress conditions. An electrophoretic mobility shift assay and transient activator reporter assay showed that APX2 expression was regulated by heat shock factor A3 (HsfA3) and that HsfA3 is regulated at the transcriptional level by DREB2C. These results suggest that DREB2C plays an important role in promoting oxidative stress tolerance in Arabidopsis.

Efficient acclimation of the chloroplast antioxidant defence of Arabidopsis thaliana leaves in response to a 10- or 100-fold light increment and the possible involvement of retrograde signals

Chloroplasts are equipped with a nuclear-encoded antioxidant defence system the components of which are usually expressed at high transcript and activity levels. To significantly challenge the chloroplast antioxidant system, Arabidopsis thaliana plants, acclimated to extremely low light slightly above the light compensation point or to normal growth chamber light, were moved to high light corresponding to a 100- and 10-fold light jump, for 6 h and 24 h in order to observe the responses of the water-water cycle at the transcript, protein, enzyme activity, and metabolite levels. The plants coped efficiently with the high light regime and the photoinhibition was fully reversible. Reactive oxygen species (ROS), glutathione and ascorbate levels as well as redox states, respectively, revealed no particular oxidative stress in low-light-acclimated plants transferred to 100-fold excess light. Strong regulation of the water-water cycle enzymes at the transcript level was only partly reflected at the protein and activity levels. In general, low light plants had higher stromal (sAPX) and thylakoid ascorbate peroxidase (tAPX), dehydroascorbate reductase (DHAR), and CuZn superoxide dismutase (CuZnSOD) protein contents than normal light-grown plants. Mutants defective in components relevant for retrograde signalling, namely stn7, ex1, tpt1, and a mutant expressing E .coli catalase in the chloroplast showed unaltered transcriptional responses of water-water cycle enzymes. These findings, together with the response of marker transcripts, indicate that abscisic acid is not involved and that the plastoquinone redox state and reactive oxygen species do not play a major role in regulating the transcriptional response at t=6 h, while other marker transcripts suggest a major role for reductive power, metabolites, and lipids as signals for the response of the water-water cycle.

Brittle leaf disease induces an oxidative stress and decreases the expression of manganese-related genes in date palm (Phoenix dactylifera L.)

In Tunisia, date orchards are being decimated by a disease called brittle leaf disease of unknown origin. Previous studies reported that affected soils, roots and leaves were manganese deficient. In this study, we investigated the biochemical and molecular response of MFC-affected date palms to the oxidative stress generated by manganese deficiency. Both the malondialdehyde (MDA) content which is indicative of lipid peroxidation and the activities of antioxidant enzyme were measured in affected leaves and roots. The expression profiles of oxidative stress-related genes encoding superoxide dismutases and peroxidases were also investigated. The data show that the MDA concentration increased but not significantly in affected leaves. However, such MDA increase was significant in roots of MFC-affected plants. The total superoxide dismutase (SOD) activity increased in affected leaves and roots, while RT-PCR experiments showed that MnSOD RNA decreased in affected leaves and roots unlike FeSOD and Cu/Zn-SOD RNA expression increased in these organs. In addition ascorbate peroxidase (APx) and glutathione peroxidase (GPx) RNA expression increased in diseased leaves and roots.

[Effect of water deficit on gene expression of enzymes related with hydrogen peroxide detoxification system in Scutellaria baicalensis]

OBJECTIVE

To analysis the effects of water deficit on the transcript level of SOD, APX, DHAR and MDHAR genes in Scutellaria baicalensis.

METHOD

Three-month-old S. baicalensis was in glasshouse under water deficit stress, and the transcript level of SOD, APX, DHAR and MDHAR genes were analysis utilized semi-quantitative RT-PCR.

RESULT

Compared with the control group, a significant decline of the transcriptional level of APX gene was observed at 70 days after water deficit. The transcript level of DHAR gene was reduced at 30 and 50 days after water deficit. And MDHARI gene was significant declined at 50 days.

CONCLUSION

AsA which is an important antioxidant plays a major role in hydrogen peroxide clear system under water deficit, and maybe have an antagonistic effect to the accumulation of baicalein.

A rice calcium-dependent protein kinase OsCPK12 oppositely modulates salt-stress tolerance and blast disease resistance

Calcium-dependent protein kinases (CDPKs) regulate the downstream components in calcium signaling pathways. We investigated the effects of overexpression and disruption of an Oryza sativa (rice) CDPK (OsCPK12) on the plant's response to abiotic and biotic stresses. OsCPK12-overexpressing (OsCPK12-OX) plants exhibited increased tolerance to salt stress. The accumulation of hydrogen peroxide (H(2) O(2) ) in the leaves was less in OsCPK12-OX plants than in wild-type (WT) plants. Genes encoding reactive oxygen species (ROS) scavenging enzymes (OsAPx2 and OsAPx8) were more highly expressed in OsCPK12-OX plants than in WT plants, whereas the expression of the NADPH oxidase gene, OsrbohI, was decreased in OsCPK12-OX plants compared with WT plants. Conversely, a retrotransposon (Tos17) insertion mutant, oscpk12, and plants transformed with an OsCPK12 RNA interference (RNAi) construct were more sensitive to high salinity than were WT plants. The level of H(2) O(2) accumulation was greater in oscpk12 and OsCPK12 RNAi plants than in the WT. These results suggest that OsCPK12 promotes tolerance to salt stress by reducing the accumulation of ROS. We also observed that OsCPK12-OX seedlings had increased sensitivity to abscisic acid (ABA) and increased susceptibility to blast fungus, probably resulting from the repression of ROS production and/or the involvement of OsCPK12 in the ABA signaling pathway. Collectively, our results suggest that OsCPK12 functions in multiple signaling pathways, positively regulating salt tolerance and negatively modulating blast resistance.

Induction of defense-related enzymes in soybean leaves by class IId bacteriocins (thuricin 17 and bacthuricin F4) purified from Bacillus strains

We have recently discovered a new class of bacteriocin (class IId) which stimulates plant growth in a way similar to Nod factors. Nod factors have been shown to provoke aspects of plant disease resistance. We investigated the effects of bacteriocins [thuricin 17 (T17) and bacthuricin F4 (BF4)] on the activities of phenylalanine ammonia lyase (PAL), guaiacol peroxidase (POD), ascorbate peroxidase (APX), superoxide dismutase (SOD), and polyphenol oxidase (PPO). Bacteriocin solutions were fed into the cut stems of soybean (Glycine max L. Merr. cv. OAC Bayfield) seedlings at the first trifoliate stage. PAL activity in T17 treated leaves was the highest at 72h after treatment and was 75.5% greater than the control at that time. At 72h after treatment POD activities in T17 and BF4 treated leaves increased by 72.7 and 91.3%, respectively, as compared with the control treatment. APX activity was 52.3 and 49.6% respectively, greater than the control in T17 and BF4 treated leaves at 72h after treatment. SOD activity in T17 treated leaves was the highest at 72h after treatment and was 26.0% greater than the control at that time. SOD activity was 70.5 and 60.2% greater, respectively, than the control in T17 and BF4 treated leaves, at 72h. Using PAGE we found that one APX isozyme (28kDa isoform) showed the strongest induction in all bacteriocin treated leaves at 72h. Activity of the seven SOD isozymes was increased by both bacteriocins, relative to the control treatment. The 33kDa PPO isozyme was induced strongly by both bacteriocins, relative to the control treatment. These results indicate that class IId bacteriocins can act as an inducer of plant disease defense-related enzymes and may be acting through mechanisms similar to Nod factors.

Ascorbate peroxidase-related (APx-R) is not a duplicable gene

Phylogenetic, genomic and functional analyses have allowed the identification of a new class of putative heme peroxidases, so called APx-R (APx-Related). These new class, mainly present in the green lineage (including green algae and land plants), can also be detected in other unicellular chloroplastic organisms. Except for recent polyploid organisms, only single-copy of APx-R gene was detected in each genome, suggesting that the majority of the APx-R extra-copies were lost after chromosomal or segmental duplications. In a similar way, most APx-R co-expressed genes in Arabidopsis genome do not have conserved extra-copies after chromosomal duplications and are predicted to be localized in organelles, as are the APx-R. The member of this gene network can be considered as unique gene, well conserved through the evolution due to a strong negative selection pressure and a low evolution rate.

Overexpression of SsCHLAPXs confers protection against oxidative stress induced by high light in transgenic Arabidopsis thaliana

To evaluate the physiological importance of chloroplastic ascorbate peroxidase (CHLAPX) in the reactive oxygen species (ROS)-scavenging system of a euhalophyte, we cloned the CHLAPX of Suaeda salsa (SsCHLAPX) encoding stromal APX (sAPX) and thylakoid-bound APX. The stromal APX of S. salsa (Ss.sAPX) cDNA consists of 1726 nucleotides including an 1137-bp open reading frame (ORF) and encodes 378 amino acids. The thylakoid-bound APX of S. salsa (Ss.tAPX) cDNA consists of 1561 nucleotides, including a 1284-bp ORF, and encodes 427 amino acids. The N-terminal 378 amino acids of Ss.sAPX are identical with those of Ss.tAPX, whereas the C-terminal 49 amino acids differ. Arabidopsis thaliana lines overexpressing Ss.sAPX and Ss.tAPX were constructed using Agrobacterium tumefaciens transformation methods. Under high light (1000 µmol m⁻² s⁻¹), malondialdehyde (MDA) content was lower in transgenic plants than in the wild type. Under high light, Fv/Fm and chlorophyll contents of both overexpressing lines and the wild type declined but were significantly higher in the overexpressing lines than in the wild type. The activities of APX (EC 1.11.1.11), catalase (CAT 1.11.1.6) and superoxide dismutase (SOD EC 1.15.1.1) were higher in the overexpressing lines than in the wild type. The transgenic plants showed increased tolerance to oxidative stress caused by high light. These results suggest that SsCHLAPX plays an important role in scavenging ROS in chloroplasts under stress conditions such as high light.

Evidence for a SAL1-PAP chloroplast retrograde pathway that functions in drought and high light signaling in Arabidopsis

Compartmentation of the eukaryotic cell requires a complex set of subcellular messages, including multiple retrograde signals from the chloroplast and mitochondria to the nucleus, to regulate gene expression. Here, we propose that one such signal is a phosphonucleotide (3'-phosphoadenosine 5'-phosphate [PAP]), which accumulates in Arabidopsis thaliana in response to drought and high light (HL) stress and that the enzyme SAL1 regulates its levels by dephosphorylating PAP to AMP. SAL1 accumulates in chloroplasts and mitochondria but not in the cytosol. sal1 mutants accumulate 20-fold more PAP without a marked change in inositol phosphate levels, demonstrating that PAP is a primary in vivo substrate. Significantly, transgenic targeting of SAL1 to either the nucleus or chloroplast of sal1 mutants lowers the total PAP levels and expression of the HL-inducible ASCORBATE PEROXIDASE2 gene. This indicates that PAP must be able to move between cellular compartments. The mode of action for PAP could be inhibition of 5' to 3' exoribonucleases (XRNs), as SAL1 and the nuclear XRNs modulate the expression of a similar subset of HL and drought-inducible genes, sal1 mutants accumulate XRN substrates, and PAP can inhibit yeast (Saccharomyces cerevisiae) XRNs. We propose a SAL1-PAP retrograde pathway that can alter nuclear gene expression during HL and drought stress.

Role of peroxidases in the compensation of cytosolic ascorbate peroxidase knockdown in rice plants under abiotic stress

Current studies, particularly in Arabidopsis, have demonstrated that mutants deficient in cytosolic ascorbate peroxidases (APXs) are susceptible to the oxidative damage induced by abiotic stress. In contrast, we demonstrate here that rice mutants double silenced for cytosolic APXs (APx1/2s) up-regulated other peroxidases, making the mutants able to cope with abiotic stress, such as salt, heat, high light and methyl viologen, similar to non-transformed (NT) plants. The APx1/2s mutants exhibited an altered redox homeostasis, as indicated by increased levels of H₂O₂ and ascorbate and glutathione redox states. Both mutant and NT plants exhibited similar photosynthesis (CO₂) assimilation and photochemical efficiency) under both normal and stress conditions. Overall, the antioxidative compensatory mechanism displayed by the mutants was associated with increased expression of OsGpx genes, which resulted in higher glutathione peroxidase (GPX) activity in the cytosolic and chloroplastic fractions. The transcript levels of OsCatA and OsCatB and the activities of catalase (CAT) and guaiacol peroxidase (GPOD; type III peroxidases) were also up-regulated. None of the six studied isoforms of OsApx were up-regulated under normal growth conditions. Therefore, the deficiency in cytosolic APXs was effectively compensated for by up-regulation of other peroxidases. We propose that signalling mechanisms triggered in rice mutants could be distinct from those proposed for Arabidopsis.

Differential antioxidative responses to dehydration-induced oxidative stress in core set of foxtail millet cultivars [Setaria italica (L.)]

Foxtail millet (Setaria italica L.) known as a relatively drought-tolerant crop across the world is grown in arid and semi-arid regions. To the best of our knowledge, no systematic study on drought tolerance screening of foxtail millet germplasm being a drought-tolerant crop has been reported so far. To explore genetic diversity of drought-induced oxidative stress tolerance in foxtail millet, we employed lipid peroxidation measure to assess membrane integrity under stress as biochemical marker to screen 107 cultivars and classified the genotypes as highly tolerant, tolerant, sensitive, and highly sensitive. From this comprehensive screening, four cultivars showing differential response to dehydration tolerance were selected to understand the physiological and biochemical basis of tolerance mechanisms. The dehydration-tolerant cultivars (IC-403579 and Prasad) showed considerably lower levels of lipid peroxidation and electrolyte leakage as compared with dehydration-sensitive cultivars (IC-480117 and Lepakshi), indicating better cell membrane integrity in tolerant cultivars. Correspondingly, tolerant genotypes maintained higher activity of catalase (EC 1.11.1.6), ascorbate peroxidase (APX; EC 1.11.1.11), and glutathione reductase (GR; EC 1.6.4.2) across different time-course period of polyethylene glycol (PEG) treatments in comparison to sensitive ones. The above biochemical results were further validated through quantitative real-time PCR analysis of APX and GR, whose transcripts were substantially induced by PEG treatments in tolerant cultivars. These results suggest that tolerant cultivars possess wider array of antioxidant machinery with efficient ascorbate-glutathione pathway to cope with drought-induced oxidative stress.

Nitric oxide activates superoxide dismutase and ascorbate peroxidase to repress the cell death induced by wounding

Wounding caused by rain, wind, and pathogen may lead plants to onset defense response. Previous studies indicated that mechanical wounding stimulates plants to generate nitric oxide (NO) and hydrogen peroxide (H(2)O(2)). In this study, the functions of NO and H(2)O(2) after wounding in sweet potato (Ipomoea batatas cv. Tainung 57) was further analyzed. Mechanical wounding damaged cells and resulted in necrosis, but the presence of NO donors or NO scavenger might reduce or enhance the cell death caused by wounding, respectively. The amount of H(2)O(2) induced by wounding was also decreased or increased when plants were incubated with NO donors or NO scavenger, individually. These results indicate that NO may regulate H(2)O(2) generation to affect cell death. NO-induced proteins isolated from two-dimensional electrophoresis were identified to be Copper/Zinc superoxide dismutases (CuZnSODs). The activities of CuZnSODs and ascorbate peroxidase (APX) could be enhanced by NO. In addition, the expression of CuZnSOD and APX was induced by wounding via NO, and their expression was further stimulated by NO through the generation of cGMP. The influx of calcium ions and the activity of NADPH oxidase were also involved in the NO signal transduction pathway inducing APX expression. Collectively, the generation of H(2)O(2) in wounded plants might trigger cell death. Meanwhile, the production of NO induced by wounding stimulated signal transducers including cGMP, calcium ions, and H(2)O(2) to activate CuZnSOD and APX, which further decreased H(2)O(2) level and reduced the cell death caused by wounding.

Hydrogen peroxide formation in cacao tissues infected by the hemibiotrophic fungus Moniliophthora perniciosa

In plant-pathogen interaction, the hydrogen peroxide (H₂O₂) may play a dual role: its accumulation inhibits the growth of biotrophic pathogens, while it could help the infection/colonization process of plant by necrotrophic pathogens. One of the possible pathways of H₂O production involves oxalic acid (Oxa) degradation by apoplastic oxalate oxidase. Here, we analyzed the production of H₂O₂, the presence of calcium oxalate (CaOx) crystals and the content of Oxa and ascorbic acid (Asa)--the main precursor of Oxa in plants--in susceptible and resistant cacao (Theobroma cacao L.) infected by the hemibiotrophic fungus Moniliophthora perniciosa. We also quantified the transcript level of ascorbate peroxidase (Apx), germin-like oxalate oxidase (Glp) and dehydroascorbate reductase (Dhar) by RT-qPCR. We report that the CaOx crystal amount and the H₂O₂ levels in the two varieties present distinct temporal and genotype-dependent patterns. Susceptible variety accumulated more CaOx crystals than the resistant one, and the dissolution of these crystals occurred in the early infection steps and in the final stage of the disease in the resistant and the susceptible variety, respectively. High expression of the Glp and accumulation of Oxa were observed in the resistant variety. The content of Asa increased in the inoculated susceptible variety, but remained constant in the resistant one. The susceptible variety presented reduced Dhar expression. The role of H₂O₂ and its formation from Oxa via Apx and Glp in resistant and susceptible variety infected by M. perniciosa were discussed.

Distinct mechanisms for aerenchyma formation in leaf sheaths of rice genotypes displaying a quiescence or escape strategy for flooding tolerance

BACKGROUND AND AIMS

Rice is one of the few crops able to withstand periods of partial or even complete submergence. One of the adaptive traits of rice is the constitutive presence and further development of aerenchyma which enables oxygen to be transported to submerged organs. The development of lysigenous aerenchyma is promoted by ethylene accumulating within the submerged plant tissues, although other signalling mechanisms may also co-exist. In this study, aerenchyma development was analysed in two rice (Oryza sativa) varieties, 'FR13A' and 'Arborio Precoce', which show opposite traits in flooding response in terms of internode elongation and survival.

METHODS

The growth and survival of rice varieties under submergence was investigated in the leaf sheath of 'FR13A' and 'Arborio Precoce'. The possible involvement of ethylene and reactive oxygen species (ROS) was evaluated in relation to aerenchyma formation. Cell viability and DNA fragmentation were determined by FDA/FM4-64 staining and TUNEL assay, respectively. Ethylene production was monitored by gas chromatography and by analysing ACO gene expression. ROS production was measured by using Amplex Red assay kit and the fluorescent dye DCFH(2)-DA. The expression of APX1 was also evaluated. AVG and DPI solutions were used to test the effect of inhibiting ethylene biosynthesis and ROS production, respectively.

KEY RESULTS

Both the varieties displayed constitutive lysigenous aerenchyma formation, which was further enhanced when submerged. 'Arborio Precoce', which is characterized by fast elongation when submerged, showed active ethylene biosynthetic machinery associated with increased aerenchymatous areas. 'FR13A', which harbours the Sub1A gene that limits growth during oxygen deprivation, did not show any increase in ethylene production after submersion but still displayed increased aerenchyma. Hydrogen peroxide levels increased in 'FR13A' but not in 'Arborio Precoce'.

CONCLUSIONS

While ethylene controls aerenchyma formation in the fast-elongating 'Arborio Precoce' variety, in 'FR13A' ROS accumulation plays an important role.

Identification of copper-induced genes in the marine alga Ulva compressa (Chlorophyta)

In order to identify genes/proteins involved in copper tolerance, the marine alga Ulva compressa was cultivated with 10 μM copper for 3 days. The activities of antioxidant enzymes ascorbate peroxidase (AP), peroxiredoxin (PRX), thioredoxin (TRX), and glutathione-S-transferase (GST) and the level of lipoperoxides were determined in the alga cultivated with and without copper addition. Antioxidant enzyme activities and lipoperoxides level increased in response to copper excess, indicating that the alga was under oxidative stress. A cDNA library was prepared using U. compressa cultivated with 10 μM copper for 3 days. A total of 3 × 10(4) clones were isolated and 480 clones were sequenced, resulting in 235 non-redundant ESTs, of which 104 encode proteins with known functions. Among them, we identified proteins involved in (1) antioxidant metabolism such as AP, PRX, TRX, GST, and metalothionein (MET), (2) signal transduction, such as calmodulin (CAM), (3) calcium-dependent protein kinase (CDPK) and nucleoside diphosphate kinase (NDK), (4) gene expression, (5) protein synthesis and degradation, and (6) chloroplast and mitochondria electron transport chains. Half of the identified proteins are potentially localized in organelles. The relative level of 18 genes, including those coding for AP, PRX, TRX, GST, MET, CAM, CDPK, and NDK were determined by quantitative RT-PCR in the alga cultivated with 10 μM copper for 0 to 7 days. Transcript levels increased in response to copper stress and most of them reached a maximum at days 3 and 5. Thus, the selected genes are induced by copper stress and they are probably involved in copper acclimation and tolerance.

Distinct mechanisms for aerenchyma formation in leaf sheaths of rice genotypes displaying a quiescence or escape strategy for flooding tolerance

BACKGROUND AND AIMS

Rice is one of the few crops able to withstand periods of partial or even complete submergence. One of the adaptive traits of rice is the constitutive presence and further development of aerenchyma which enables oxygen to be transported to submerged organs. The development of lysigenous aerenchyma is promoted by ethylene accumulating within the submerged plant tissues, although other signalling mechanisms may also co-exist. In this study, aerenchyma development was analysed in two rice (Oryza sativa) varieties, 'FR13A' and 'Arborio Precoce', which show opposite traits in flooding response in terms of internode elongation and survival.

METHODS

The growth and survival of rice varieties under submergence was investigated in the leaf sheath of 'FR13A' and 'Arborio Precoce'. The possible involvement of ethylene and reactive oxygen species (ROS) was evaluated in relation to aerenchyma formation. Cell viability and DNA fragmentation were determined by FDA/FM4-64 staining and TUNEL assay, respectively. Ethylene production was monitored by gas chromatography and by analysing ACO gene expression. ROS production was measured by using Amplex Red assay kit and the fluorescent dye DCFH(2)-DA. The expression of APX1 was also evaluated. AVG and DPI solutions were used to test the effect of inhibiting ethylene biosynthesis and ROS production, respectively.

KEY RESULTS

Both the varieties displayed constitutive lysigenous aerenchyma formation, which was further enhanced when submerged. 'Arborio Precoce', which is characterized by fast elongation when submerged, showed active ethylene biosynthetic machinery associated with increased aerenchymatous areas. 'FR13A', which harbours the Sub1A gene that limits growth during oxygen deprivation, did not show any increase in ethylene production after submersion but still displayed increased aerenchyma. Hydrogen peroxide levels increased in 'FR13A' but not in 'Arborio Precoce'.

CONCLUSIONS

While ethylene controls aerenchyma formation in the fast-elongating 'Arborio Precoce' variety, in 'FR13A' ROS accumulation plays an important role.

Heme oxygenase-1 is involved in the cytokinin-induced alleviation of senescence in detached wheat leaves during dark incubation

This study tested whether an inducible isoform of heme oxygenase (HO, EC 1.14.99.3), HO-1, is involved in the cytokinin (CTK)-induced alleviation of senescence in detached wheat (Triticum aestivum L.) leaves during dark incubation. We discovered that exogenous supplement of 6-benzylaminopurine (6-BA) at 10 μM for 48 h not only delayed the dark-induced loss of chlorophyll and protein contents in detached wheat leaves, but also significantly increased HO activity in a time-dependent manner. This induction reached a maximum within 3h of 6-BA supply, which was further confirmed by using semi-quantitative RT-PCR and protein gel blot analysis. Furthermore, the decreases in intracellular thiobarbituric acid reactive substances (TBARS) content, and the increases in the transcript level, total and isozymatic activities of some important antioxidant enzymes, such as catalase (CAT, EC 1.11.1.6), peroxidase (POD, EC 1.11.1.7), superoxide dismutase (SOD, EC 1.15.1.1), and ascorbate peroxidase (APX, EC 1.11.1.11), were observed. Reversed responses of chlorophyll, protein and TBARS contents, HO activity, and the expression of above antioxidant enzymes were observed when zinc protoporphyrin-IX (ZnPPIX), a potent HO-1 inhibitor, was added together with 6-BA. In contrast, HO-1 inducer hemin could partially mimic the effects of 6-BA. Together, the results suggest that HO-1 might be involved in the CTK-induced alleviation of senescence and lipid peroxidation in detached wheat leaves.