RNAi knockdown of rice SE5 gene is sensitive to the herbicide methyl viologen by the down-regulation of antioxidant defense

Carbon monoxide/heme oxygenase system in plant: Roles in abiotic stress response and crosstalk with other signals molecules

Carbon monoxide (CO) has been recognized as a crucial gasotransmitter mainly produced by heme oxygenase (HO)-catalyzed heme degradation in plant. Recent studies have shown that CO plays an important role in regulating growth and development of plant, as well as and responding to a variety of abiotic stresses. Meanwhile, many studies have reported on CO working in combination with other signal molecules to mitigate abiotic stress. Here, we presented a comprehensive overview of recent developments in which CO reduces plant damage caused by abiotic stresses. The regulation of antioxidant system, photosynthetic system, ion balance and transport are the main mechanisms of CO-alleviated abiotic stress. We also proposed and discussed the relationship between CO and other signal molecules, including nitric oxide (NO), hydrogen sulfide (H2S), hydrogen gas (H2), abscisic acid (ABA), indole 3-acetic acid (IAA), gibberellin (GA), cytokine (CTK), salicylic acid (SA), jasmonic acid (JA), hydrogen peroxide (H2O2) and calcium ion (Ca2+). Furthermore, the important role of HO genes in alleviating abiotic stress was also discussed. We proposed promising and new research directions for the study of plant CO, which can provide further insights on the role of CO in plant growth and development under abiotic stress.

Rice (Oryza sativa L.) cytochrome P450 protein 716A subfamily CYP716A16 regulates disease resistance

BACKGROUND

The sustainable development of rice production is facing severe threats by a variety of pathogens, such as necrotrophic Rhizoctonia solani and hemibiotrophic Xanthomonas oryzae pv. oryzae (Xoo). Mining and applying resistance genes to increase the durable resistance of rice is an effective method that can be used to control these diseases.

RESULTS

In this research, we isolated and characterized CYP716A16, which is a positive regulator of rice to R. solani AG1-IA and Xoo, and belongs to the cytochrome P450 (CYP450) protein 716A subfamily. Overexpression (OE) of CYP716A16 resulted in enhanced resistance to R. solani AG1-IA and Xoo, while RNA interference (RNAi) of CYP716A16 resulted in increased susceptibility compared with wild-type (WT) plants. Additionally, jasmonic acid (JA)-dependent defense responses and reactive oxygen species (ROS) were activated in the CYP716A16-OE lines after R. solani AG1-IA inoculation. The comparative transcriptomic and metabolomics analysis of CYP716A16-OE and the WT lines showed that OE of CYP716A16 activated the biosynthesis of flavonoids and increased the amounts of narcissoside, methylophiopogonanone A, oroxin A, and amentoflavone in plants.

CONCLUSION

Based on these results, we suggest that JA-dependent response, ROS level, multiple resistance-related proteins, and flavonoid contents play an important role in CYP716A16-regulated R. solani AG1-IA and Xoo resistance. Our results broaden our knowledge regarding the function of a P450 protein 716A subfamily in disease resistance and provide new insight into the molecular mechanism of rice immune response.

Double mutation of two homologous genes YL1 and YL2 results in a leaf yellowing phenotype in soybean [Glycine max (L.) Merr]

KEY MESSAGE

Two homologous, chloroplast located CAAX proteases were identified to be functional redundancy in determining soybean leaf color, and they probably play essential roles in regulating light harvesting and absorption during photosynthesis process. Leaf color mutants are ideal materials for studying photosynthesis and chlorophyll metabolism. The soybean [Glycine max (L.) Merr.] yellowing leaf (yl) variation is a recombinant mutant characterized by yellow foliage, which derived from the specific cross between green seed-coated and yellow seed-coated soybean varieties. Molecular cloning and subsequent gene silencing revealed that the yellow leaf trait of yl was controlled by two recessive nuclear genes, glyma11g04660 and glyma01g40650, named as YL1 and YL2 respectively, and the latter was confirmed to be same as the earlier reported green seed-coat gene G. Both YL1 and YL2 belonged to chloroplast-located proteases possessing Abi domain, and these genes were expressed in various tissues, especially in young leaves. In yl, the expression of YL1 and YL2 were suppressed in most tissues, and the young leaf of yl presented an increased maximal photochemical efficiency (Fv/Fm) as well as enhanced net photosynthesis activity (Pn), indicating that YL1 and YL2 are involved in light absorption regulation during photosynthesis process. Collectively, the identification and description of YL1 and YL2 in our study provides insights for the regulatory mechanism of photosynthesis process, and these findings will further assist to clarify the close relationship between photosynthesis and chlorophyll metabolism.

Identification of two recessive etiolation genes (py1, py2) in pakchoi (Brassica rapa L. ssp. chinensis)

BACKGROUND

Leaf color is a major agronomic trait, which has a strong influence on crop yields. Isolating leaf color mutants can represent valuable materials for research in chlorophyll (Chl) biosynthesis and metabolism regulation.

RESULTS

In this study, we identified a stably inherited yellow leaf mutant derived from 'Huaguan' pakchoi variety via isolated microspore culture and designated as pylm. This mutant displayed yellow leaves after germination. Its etiolated phenotype was nonlethal and stable during the whole growth period. Its growth was weak and its hypocotyls were markedly elongated. Genetic analysis revealed that two recessive nuclear genes, named py1 and py2, are responsible for the etiolation phenotype. Bulked segregant RNA sequencing (BSR-Seq) showed that py1 and py2 were mapped on chromosomes A09 and A07, respectively. The genes were single Mendelian factors in F_3:4 populations based on a 3:1 phenotypic segregation ratio. The py1 was localized to a 258.3-kb interval on a 34-gene genome. The differentially expressed gene BraA09004189 was detected in the py1 mapping region and regulated heme catabolism. One single-nucleotide polymorphism (SNP) of BraA09004189 occurred in pylm. A candidate gene-specific SNP marker in 1520 F_3:4 yellow-colored individuals co-segregated with py1. For py2, 1860 recessive homozygous F_3:4 individuals were investigated and localized py2 to a 4.4-kb interval. Of the five genes in this region, BraA07001774 was predicted as a candidate for py2. It encoded an embryo defective 1187 and a phosphotransferase related to chlorophyll deficiency and hypocotyl elongation. One SNP of BraA07001774 occurred in pylm. It caused a single amino acid mutation from Asp to Asn. According to quantitative real-time polymerase chain reaction (qRT-PCR), BraA07001774 was downregulated in pylm.

CONCLUSIONS

Our study identified a Chl deficiency mutant pylm in pakchoi. Two recessive nuclear genes named py1 and py2 had a significant effect on etiolation. Candidate genes regulating etiolation were identified as BraA09004189 and BraA07001774, respectively. These findings will elucidate chlorophyll metabolism and the molecular mechanisms of the gene interactions controlling pakchoi etiolation.

Characterization and Expression Analysis of Heme Oxygenase Genes from Sorghum bicolor

Heme oxygenases (HOs) have a major role in phytochrome chromophore biosynthesis, and chromophores in turn have anti-oxidant properties. Plant heme oxygenases are divided into the HO1 sub-family comprising HO1, HO3, and HO4, and the HO2 sub-family, which consists of 1 member, HO2. This study identified and characterized 4 heme oxygenase members from Sorghum bicolor. Multiple sequence alignments showed that the heme oxygenase signature motif (QAFICHFYNI/V) is conserved across all SbHO proteins and that they share above 90% sequence identity with other cereals. Quantitative real-time polymerase chain reaction revealed that SbHO genes were expressed in leaves, stems, and roots, but most importantly their transcript level was induced by osmotic stress, indicating that they might play a role in stress responses. These findings will strengthen our understanding of the role of heme oxygenases in plant stress responses and may contribute to the development of stress tolerant crops.

PE-1, Encoding Heme Oxygenase 1, Impacts Heading Date and Chloroplast Development in Rice ( Oryza sativa L.)

The duration of the rice growth phase has always been an important target trait. The identification of mutations in rice that alter these processes and result in a shorter growth phase could have potential benefits for crop production. In this study, we isolated an early aging rice mutant, pe-1, with light green leaves, using γ-mutated indica rice cultivar and subsequent screening methods, which is known as the phytochrome synthesis factor Se5 that controls rice flowering. The pe-1 plant is accompanied by a decreased chlorophyll content, an enhanced photosynthesis, and a decreased pollen fertility. PE-1, a close homologue of HY1, is localized in the chloroplast. Expression pattern analysis indicated that PE-1 was mainly expressed in roots, stems, leaves, leaf sheaths, and young panicles. The knockout of PE-1 using the CRISPR/Cas9 system decreased the chlorophyll content and downregulated the expression of PE-1-related genes. Furthermore, the chloroplasts of pe-1 were filled with many large-sized starch grains, and the number of osmiophilic granules (a chloroplast lipid reservoir) was significantly decreased. Altogether, our findings suggest that PE-1 functions as a master regulator to mediate in chlorophyll biosynthesis and photosynthetic pathways.

Trithorax-group protein ATX5 mediates the glucose response via impacting the HY1-ABI4 signaling module

Trithorax-group Protein ARABIDOPSIS TRITHORAX5 modulates the glucose response. Glucose is an evolutionarily conserved modulator from unicellular microorganisms to multicellular animals and plants. Extensive studies have shown that the Trithorax-group proteins (TrxGs) play essential roles in different biological processes by affecting histone modifications and chromatin structures. However, whether TrxGs function in the glucose response and how they achieve the control of target genes in response to glucose signaling in plants remain unknown. Here, we show that the Trithorax-group Protein ARABIDOPSIS TRITHORAX5 (ATX5) affects the glucose response and signaling. atx5 loss-of-function mutants display glucose-oversensitive phenotypes compared to the wild-type (WT). Genome-wide RNA-sequencing analyses have revealed that ATX5 impacts the expression of a subset of glucose signaling responsive genes. Intriguingly, we have established that ATX5 directly controls the expression of HY1 by trimethylating H3 lysine 4 of the Arabidopsis Heme Oxygenase1 (HY1) locus. Glucose signaling causes the suppression of ATX5 activity and subsequently reduces the H3K4me3 levels at the HY1 locus, thereby leading to the increased expression of ABSCISIC ACID-INSENSITIVE4 (ABI4). This result suggests that an important ATX5-HY1-ABI4 regulatory module governs the glucose response. This idea is further supported by genetic evidence showing that an atx5 hy1-100 abi4 triple mutant showed a similar glucose-insensitive phenotype as compared to that of the abi4 single mutant. Our findings show that a novel ATX5-HY1-ABI4 module controls the glucose response in Arabidopsis thaliana.

l-cysteine desulfhydrase-related H2 S production is involved in OsSE5-promoted ammonium tolerance in roots of Oryza sativa

Previous studies revealed that rice heme oxygenase PHOTOPERIOD SENSITIVITY 5 (OsSE5) is involved in the regulation of tolerance to excess ammonium by enhancing antioxidant defence. In this study, the relationship between OsSE5 and hydrogen sulfide (H₂ S), a well-known signalling molecule, was investigated. Results showed that NH₄ Cl triggered the induction of l-cysteine desulfhydrase (l-DES)-related H₂ S production in rice seedling roots. A H₂ S donor not only alleviated the excess ammonium-triggered inhibition of root growth but also reduced endogenous ammonium, both of which were aggravated by hypotaurine (HT, a H₂ S scavenger) or dl-propargylglycine (PAG, a l-DES inhibitor). Nitrogen metabolism-related enzymes were activated by H₂ S, thus resulting in the induction of amino acid synthesis and total nitrogen content. Interestingly, the activity of l-DES, as well as the enzymes involved in nitrogen metabolism, was significantly increased in the OsSE5-overexpression line (35S:OsSE5), whereas it impaired in the OsSE5-knockdown mutant (OsSE5-RNAi). The application of the HT/PAG or H₂ S donor could differentially block or rescue NH₄ Cl-hyposensitivity or hypersensitivity phenotypes in 35S:OsSE5-1 or OsSE5-RNAi-1 plants, with a concomitant modulation of nitrogen assimilation. Taken together, these results illustrated that H₂ S function as an indispensable positive regulator participated in OsSE5-promoted ammonium tolerance, in which nitrogen metabolism was facilitated.

Difference in oxidative stress tolerance between rice cultivars estimated with chlorophyll fluorescence analysis

BACKGROUND

Oxidative stress is considered to be involved in growth retardation of plants when they are exposed to a variety of biotic and abiotic stresses. Despite its potential importance in improving crop production, comparative studies on oxidative stress tolerance between rice (Oryza sativa L.) cultivars are limited. This work describes the difference in term of oxidative stress tolerance between 72 rice cultivars.

METHODS

72 rice cultivars grown under naturally lit greenhouse were used in this study. Excised leaf discs were subjected to a low concentration of methyl viologen (paraquat), a chemical reagent known to generate reactive oxygen species in chloroplast. Chlorophyll fluorescence analysis using a two-dimensional fluorescence meter, ion leakage analysis as well as the measurement of chlorophyll contents were used to evaluate the oxidative stress tolerance of leaf discs. Furthermore, fluorescence intensities were finely analyzed based on new fluorescence theories that we have optimized.

RESULTS

Treatment of leaf discs with methyl viologen caused differential decrease of maximum quantum yield of photosystem II (Fv/Fm) between cultivars. Decrease of Fv/Fm was also closely correlated with increase of ion leakage and decrease of chlorophyll a/b ratio. Fv/Fm was factorized into photochemical and non-photochemical parameters to classify rice cultivars into sensitive and tolerant ones. Among the 72 compared rice cultivars, the traditional cultivar Co13 was identified as the most tolerant to oxidative stress. Koshihikari, a dominant modern Japonica cultivar in Japan as well as IR58, one of the modern Indica breeding lines exhibited a strong tolerance to oxidative stress.

CONCLUSIONS

Close correlation between Fv/Fm and chlorophyll a/b ratio provides a simple method to estimate oxidative stress tolerance, without measurement of chlorophyll fluorescence with special equipment. The fact that modern cultivars, especially major cultivars possessed tolerance to oxidative stress suggests that oxidative stress tolerance is one of the agricultural traits prerequisite for improvement of modern rice cultivars. Data presented in this study would enable breeding of rice cultivars having strong tolerance to oxidative stress.

Heme oxygenase 1 defects lead to reduced chlorophyll in Brassica napus

We previously described a Brassica napus chlorophyll-deficient mutant (ygl) with yellow-green seedling leaves and mapped the related gene, BnaC.YGL, to a 0.35 cM region. However, the molecular mechanisms involved in this chlorophyll defect are still unknown. In this study, the BnaC07.HO1 gene (equivalent to BnaC.YGL) was isolated by the candidate gene approach, and its function was confirmed by genetic complementation. Comparative sequencing analysis suggested that BnaC07.HO1 was lost in the mutant, while a long noncoding-RNA was inserted into the promoter of the homologous gene BnaA07.HO1. This insert was widely present in B. napus cultivars and down-regulated BnaA07.HO1 expression. BnaC07.HO1 was highly expressed in the seedling leaves and encoded heme oxygenase 1, which was localized in the chloroplast. Biochemical analysis showed that BnaC07.HO1 can catalyze heme conversion to form biliverdin IXα. RNA-seq analysis revealed that the loss of BnaC07.HO1 impaired tetrapyrrole metabolism, especially chlorophyll biosynthesis. According, the levels of chlorophyll intermediates were reduced in the ygl mutant. In addition, gene expression in multiple pathways was affected in ygl. These findings provide molecular evidences for the basis of the yellow-green leaf phenotype and further insights into the crucial role of HO1 in B. napus.

Ectopic over-expression of BoHO1, a cabbage heme oxygenase gene, improved salt tolerance in Arabidopsis: A case study on proteomic analysis

Plant heme oxygenase (HO) catalyzes the oxygenation of heme to biliverdin, carbon monoxide, and free iron, and is regarded as a stress-responsive protein. Here, a cabbage HO1 gene (named as BoHO1) was isolated and characterized. BoHO1 shares a high degree homology with Arabidopsis AtHO1, and could locate in Arabidopsis chloroplast. BoHO1 mRNA was ubiquitously expressed in cabbage tissues, and was responsive to several stimuli and chemicals. Genetic evidence illustrated that over-expression of BoHO1 in transgenic Arabidopsis plants (35S:BoHO1-1 and 35S:BoHO1-2) significantly alleviated salinity stress-inhibited seedling growth, which were accompanied with the re-establishment of reactive oxygen species and ion homeostasis. Comparative proteomic analysis was subsequently performed. Results revealed that protein abundance related to light reactions was greatly suppressed by NaCl stress in wild-type, whereas was partially recovered in 35S:BoHO1-1. Salinity stress also strongly activated stress-related metabolic processes in wild-type, i.e. carbon and energy metabolism, ammonium detoxification, and protein turnover, and these induced tendencies were more intensive in 35S:BoHO1-1. Particularly, proteins related to glutathione metabolism and ion homeostasis were specifically enriched in NaCl-stressed 35S:BoHO1-1. On the basis of above results, we propose that BoHO1 could activate multiple stress-responsive pathways to help Arabidopsis regain cellular homeostasis, thus presenting enhanced adaptation to salinity stress.

Overexpression of a harpin-encoding gene popW from Ralstonia solanacearum primed antioxidant defenses with enhanced drought tolerance in tobacco plants

KEY MESSAGE

The tobacco plants transformed with popW gene showed enhanced drought tolerance, and the mechanism was found with primed antioxidant defenses and reduced drought stress damages in the transgenic lines. Harpin proteins are elicitors produced by several gram-negative plant pathogenic bacteria, triggering multiple beneficial responses in plants, such as induction of defense response against diverse pathogens and insects, growth promotion, and drought tolerance. In this study, the harpin-encoding gene popW derived from Ralstonia solanacearum ZJ3721 was transferred to tobacco. We examined the tolerance of transgenic tobacco plants toward drought stress under greenhouse conditions and analyzed the molecular mechanisms underlying the enhanced drought tolerance. The results revealed that the transgenic lines primed antioxidant defenses and reduced drought stress damages. In addition, they displayed lower malondialdehyde and relative electrical conductivity, while higher relative water content and recovery intension than the tobacco plants transformed with empty vector pBI121 and the wild-type (WT) plants under drought stress. Furthermore, the transgenic lines displayed a significant increase in peroxidase, superoxide dismutase, catalase activities, and ascorbic acid content compared with control plants under drought stress, and these levels were up to 1.95, 1.68, 1.34, and 1.43 times higher than those of WT plants, respectively. Overexpression of popW in tobacco also significantly enhanced the relative transcript levels of oxidative stress-responsive genes NtAPX, NtCAT1, NtGST, and NtCu/Zn-SOD under drought stress. The relative transcript levels of these genes in the transgenic line PW12 were up to 1.94, 2.36, 5.24, and 3.62 times higher than those of WT plants, respectively. These results confirmed that the popW gene, which was transformed into tobacco primed antioxidant responses, increased tolerance to drought stress in tobacco plants.

Ascribing Functions to Genes: Journey Towards Genetic Improvement of Rice Via Functional Genomics

Rice, one of the most important cereal crops for mankind, feeds more than half the world population. Rice has been heralded as a model cereal owing to its small genome size, amenability to easy transformation, high synteny to other cereal crops and availability of complete genome sequence. Moreover, sequence wealth in rice is getting more refined and precise due to resequencing efforts. This humungous resource of sequence data has confronted research fraternity with a herculean challenge as well as an excellent opportunity to functionally validate expressed as well as regulatory portions of the genome. This will not only help us in understanding the genetic basis of plant architecture and physiology but would also steer us towards developing improved cultivars. No single technique can achieve such a mammoth task. Functional genomics through its diverse tools viz. loss and gain of function mutants, multifarious omics strategies like transcriptomics, proteomics, metabolomics and phenomics provide us with the necessary handle. A paradigm shift in technological advances in functional genomics strategies has been instrumental in generating considerable amount of information w.r.t functionality of rice genome. We now have several databases and online resources for functionally validated genes but despite that we are far from reaching the desired milestone of functionally characterizing each and every rice gene. There is an urgent need for a common platform, for information already available in rice, and collaborative efforts between researchers in a concerted manner as well as healthy public-private partnership, for genetic improvement of rice crop better able to handle the pressures of climate change and exponentially increasing population.

Heme-heme oxygenase 1 system is involved in ammonium tolerance by regulating antioxidant defence in Oryza sativa

Despite substantial evidence showing the ammonium-altered redox homeostasis in plants, the involvement and molecular mechanism of heme-heme oxygenase 1 (heme-HO1), a novel antioxidant system, in the regulation of ammonium tolerance remain elusive. To fill in these gaps, the biological function of rice HO1 (OsSE5) was investigated. Results showed that NH4 Cl up-regulated rice OsSE5 expression. Oxidative stress and subsequent growth inhibition induced by excess NH4 Cl was partly mitigated by pretreatment with carbon monoxide (CO, a by-product of HO1 activity) or intensified by zinc protoporphyrin (ZnPP, a potent inhibitor of HO1 activity). Pretreatment with HO1 inducer hemin, not only up-regulated OsSE5 expression and HO activity, but also rescued the down-regulation of antioxidant transcripts, total and related isozymatic activities, thus significantly counteracting the excess NH4 Cl-triggered reactive oxygen species overproduction, lipid peroxidation and growth inhibition. OsSE5 RNAi transgenic rice plants revealed NH4 Cl-hypersensitive phenotype with impaired antioxidant defence, both of which could be rescued by CO but not hemin. Transgenic Arabidopsis plants over-expressing OsSE5 also exhibited enhanced tolerance to NH4 Cl, which might be attributed to the up-regulation of several antioxidant transcripts. Altogether, these results illustrated the involvement of heme-HO1 system in ammonium tolerance by enhancing antioxidant defence, which may improve plant tolerance to excess ammonium fertilizer.

Cobalt alleviates GA-induced programmed cell death in wheat aleurone layers via the regulation of H2O2 production and heme oxygenase-1 expression

Heme oxygenase-1 (HO-1) and hydrogen peroxide (H2O2) are key signaling molecules that are produced in response to various environmental stimuli. Here, we demonstrate that cobalt is able to delay gibberellic acid (GA)-induced programmed cell death (PCD) in wheat aleurone layers. A similar response was observed when samples were pretreated with carbon monoxide (CO) or bilirubin (BR), two end-products of HO catalysis. We further observed that increased HO-1 expression played a role in the cobalt-induced alleviation of PCD. The application of HO-1-specific inhibitor, zinc protoporphyrin-IX (ZnPPIX), substantially prevented the increases of HO-1 activity and the alleviation of PCD triggered by cobalt. The stimulation of HO-1 expression, and alleviation of PCD might be caused by the initial H2O2 production induced by cobalt. qRT-PCR and enzymatic assays revealed that cobalt-induced gene expression and the corresponding activities of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX), three enzymes that metabolize reactive oxygen species, were consistent with the H2O2 accumulation during GA treatment. These cobalt responses were differentially blocked by co-treatment with ZnPPIX. We therefore suggest that HO-1 functions in the cobalt-triggered alleviation of PCD in wheat aleurone layers, which is also dependent on the enhancement of the activities of antioxidant enzymes.

Cobalt alleviates GA-induced programmed cell death in wheat aleurone layers via the regulation of H2O2 production and heme oxygenase-1 expression

Heme oxygenase-1 (HO-1) and hydrogen peroxide (H₂O₂) are key signaling molecules that are produced in response to various environmental stimuli. Here, we demonstrate that cobalt is able to delay gibberellic acid (GA)-induced programmed cell death (PCD) in wheat aleurone layers. A similar response was observed when samples were pretreated with carbon monoxide (CO) or bilirubin (BR), two end-products of HO catalysis. We further observed that increased HO-1 expression played a role in the cobalt-induced alleviation of PCD. The application of HO-1-specific inhibitor, zinc protoporphyrin-IX (ZnPPIX), substantially prevented the increases of HO-1 activity and the alleviation of PCD triggered by cobalt. The stimulation of HO-1 expression, and alleviation of PCD might be caused by the initial H₂O₂ production induced by cobalt. qRT-PCR and enzymatic assays revealed that cobalt-induced gene expression and the corresponding activities of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX), three enzymes that metabolize reactive oxygen species, were consistent with the H₂O₂ accumulation during GA treatment. These cobalt responses were differentially blocked by co-treatment with ZnPPIX. We therefore suggest that HO-1 functions in the cobalt-triggered alleviation of PCD in wheat aleurone layers, which is also dependent on the enhancement of the activities of antioxidant enzymes.

Carbon monoxide regulates the expression of the wound-inducible gene ipomoelin through antioxidation and MAPK phosphorylation in sweet potato

Carbon monoxide (CO), one of the haem oxygenase (HO) products, plays important roles in plant development and stress adaptation. However, the function of CO involved in wounding responses is seldom studied. A wound-inducible gene, ipomoelin (IPO), of sweet potato (Ipomoea batatas cv. Tainung 57) was used as a target to study the regulation of CO in wounding responses. After wounding for 1h, the endogenous CO content and IbHO expression level were significantly reduced in leaves. IPO expression upon wounding was prohibited by the HO activator hemin, whereas the HO inhibitor zinc protoporphyrin IX elevated IPO expression. The IPO expression induced by wounding, H2O2, or methyl jasmonate was inhibited by CO. CO also affected the activities of ascorbate peroxidase, catalase, and peroxidase, and largely decreased H2O2 content in leaves. CO inhibited the extracellular signal-regulated kinase (ERK) phosphorylation induced by wounding. IbMAPK, the ERK of sweet potato, was identified by immunoblotting, and the interaction with its upstream activator, IbMEK1, was further confirmed by bimolecular fluorescence complementation and co-immunoprecipitation. Conclusively, wounding in leaves repressed IbHO expression and CO production, induced H2O2 generation and ERK phosphorylation, and then stimulated IPO expression.

Gene expression profiles of Arabidopsis under the stress of methyl viologen: a microarray analysis

Methyl viologen (MV) is the main ingredient of Paraquat. It is little known about how plants respond to this compound. To understand the mode of MV action and molecular mechanism of plant response, we performed experiments of microarray on Arabidopsis. In MV treated seedling, approximately 6% genes were altered at mRNA levels, including 818 genes increased, whereas 1,440 genes decreased. Studies of these genes expression patterns provided some new information on the reaction process of plant after the treatment with MV. These included signaling molecules for MV response and reactive oxygen species formation, enzymes required for secondary metabolism and, cell wall maintenance and strategy of photostasis balance. The expression kinetics of the genes induced by MV will provides useful information for the abiotic stress defense mechanism in plants.

Hydrogen-rich water confers plant tolerance to mercury toxicity in alfalfa seedlings

In this report, the effect of hydrogen-rich water (HRW), which was used to investigate the physiological roles of hydrogen gas (H2) in plants recently, on the regulation of plant adaptation to mercury (Hg) toxicity was studied. Firstly, we observed that the exposure of alfalfa seedlings to HgCl2 triggered production of reactive oxygen species (ROS), growth stunt and increased lipid peroxidation. However, such effects could be obviously blocked by HRW. Meanwhile, significant decreases in the relative ion leakage and Hg accumulation were observed. Hg-induced increases in total and isozymatic activities of superoxide dismutase (SOD) were significantly reversed by HRW. Further results suggested that HRW-induced the activities of guaiacol peroxidase (POD) and ascorbate peroxidase (APX), two hydrogen peroxide-scavenging enzymes, was at transcriptional levels. Meanwhile, obvious increases of the ratios of reduced/oxidized glutathione (GSH), homoglutathione (hGSH), and ascorbic acid (AsA) and corresponding gene expression were consistent with the decreased oxidative damage in seedling roots. In summary, the results of this investigation indicated that HRW was able to alleviate Hg toxicity in alfalfa seedlings by (i) alleviating growth stunt and reducing Hg accumulation, and (ii) avoidance of oxidative stress and reestablishment of redox homeostasis.

The lectin from Musa paradisiaca binds with the capsid protein of tobacco mosaic virus and prevents viral infection

It has been demonstrated that the lectin from Musa paradisiaca (BanLec-1) could inhibit the cellular entry of human immunodeficiency virus (HIV). In order to evaluate its effects on tobacco mosaic virus (TMV), the banlec-1 gene was cloned and transformed into Escherichia coli and tobacco, respectively. Recombinant BanLec-1 showed metal ions dependence, and higher thermal and pH stability. Overexpression of banlec-1 in tobacco resulted in decreased leaf size, and higher resistance to TMV infection, which includes reduced TMV cellular entry, more stable chlorophyll contents, and enhanced antioxidant enzymes. BanLec-1 was found to bind directly to the TMV capsid protein in vitro, and to inhibit TMV infection in a dose-dependent manner. In contrast to limited prevention in vivo, purified rBanLec-1 exhibited more significant effects on TMV infection in vitro. Taken together, our study indicated that BanLec-1 could prevent TMV infection in tobacco, probably through the interaction between BanLec-1 and TMV capsid protein.

Interaction between HY1 and H2O2 in auxin-induced lateral root formation in Arabidopsis

Haem oxygenase-1 (HO-1) and hydrogen peroxide (H2O2) are two key downstream signals of auxin, a well-known phytohormone regulating plant growth and development. However, the inter-relationship between HO-1 and H2O2 in auxin-mediated lateral root (LR) formation is poorly understood. Herein, we revealed that exogenous auxin, 1-naphthylacetic acid (NAA), could simultaneously stimulate Arabidopsis HO-1 (HY1) gene expression and H2O2 generation. Subsequently, LR formation was induced. NAA-induced HY1 expression is dependent on H2O2. This conclusion was supported by analyzing the removal of H2O2 with ascorbic acid (AsA) and dimethylthiourea (DMTU), both of which could block NAA-induced HY1 expression and LR formation. H2O2-induced LR formation was inhibited by an HO-1 inhibitor zinc protoporphyrin IX (Znpp) in wild-type and severely impaired in HY1 mutant hy1-100. Simultaneously, HY1 is required for NAA-mediated H2O2 generation, since Znpp inhibition of HY1 blocked the NAA-induced H2O2 production and LR formation. Genetic data demonstrated that hy1-100 was significantly impaired in H2O2 production and LR formation in response to NAA, compared with wild-type plants. The addition of carbon monoxide-releasing molecule-2 (CORM-2), the carbon monoxide (CO) donor, induced H2O2 production and LR formation, both of which were decreased by DMTU. Moreover, H2O2 and CORM-2 mimicked the NAA responses in the regulation of cell cycle genes expression, all of which were blocked by Znpp or DMTU, respectively, confirming that both H2O2 and CO were important in the early LR initiation. In summary, our pharmacological, genetic and molecular evidence demonstrated a close inter-relationship between HY1 and H2O2 existing in auxin-induced LR formation in Arabidopsis.

Hydrogen-rich water regulates cucumber adventitious root development in a heme oxygenase-1/carbon monoxide-dependent manner

Hydrogen gas (H2) is an endogenous gaseous molecule in plants. Although its reputation is as a "biologically inert gas", recent results suggested that H2 has therapeutic antioxidant properties in animals and plays fundamental roles in plant responses to environmental stresses. However, whether H2 regulates root morphological patterns is largely unknown. In this report, hydrogen-rich water (HRW) was used to characterize H2 physiological roles and possible signaling transduction pathways in the promotion of adventitious root (AR) formation in cucumber explants. Our results showed that a 50% concentration of HRW was able to mimic the effect of hemin, an inducer of a carbon monoxide (CO) synthetic enzyme, and heme oxygenase-1 (HO-1), in restoring AR formation in comparison with the inhibition effect conferred by auxin-depletion treatment alone. It was further shown that the inducible effect of HRW could be further blocked by the co-treatment with N-1-naphthylphtalamic acid (NPA; an auxin transport inhibitor). The HRW-induced response, at least partially, was HO-1-dependent. This conclusion was supported by the fact that the exposure of cucumber explants to HRW up-regulates cucumber HO-1 gene expression and its protein levels. HRW-mediated induction of representative target genes related to auxin signaling and AR formation, such as CsDNAJ-1, CsCDPK1/5, CsCDC6, CsAUX22B-like, and CsAUX22D-like, and thereafter AR formation (particularly in the AR length) was differentially sensitive to the HO-1 inhibitor zinc protoporphyrin IX (ZnPP). Above blocking actions were clearly reversed by CO, further confirming that the above response was HO-1/CO-specific. However, the addition of a well-known antioxidant, ascorbic acid (AsA), failed to influence AR formation triggered by HRW, thus ruling out the involvement of redox homeostasis in this process. Together, these results indicated that HRW-induced adventitious rooting is, at least partially, correlated with the HO-1/CO-mediated responses. We also suggested that exogenous HRW treatment on plants might be a good option to induce root organogenesis.

Hydrogen sulfide delays GA-triggered programmed cell death in wheat aleurone layers by the modulation of glutathione homeostasis and heme oxygenase-1 expression

Hydrogen sulfide (H2S) is considered as a cellular signaling intermediate in higher plants, but corresponding molecular mechanisms and signal transduction pathways in plant biology are still limited. In the present study, a combination of pharmacological and biochemical approaches was used to study the effect of H2S on the alleviation of GA-induced programmed cell death (PCD) in wheat aleurone cells. The results showed that in contrast with the responses of ABA, GA brought about a gradual decrease of l-cysteine desulfhydrase (LCD) activity and H2S production, and thereafter PCD occurred. Exogenous H2S donor sodium hydrosulfide (NaHS) not only effectively blocked the decrease of endogenous H2S release, but also alleviated GA-triggered PCD in wheat aleurone cells. These responses were sensitive to hypotaurine (HT), a H2S scavenger, suggesting that this effect of NaHS was in an H2S-dependent fashion. Further experiment confirmed that H2S, rather than other sodium- or sulphur-containing compounds derived from the decomposing of NaHS, was attributed to the rescuing response. Importantly, the reversing effect was associated with glutathione (GSH) because the NaHS triggered increases of endogenous GSH content and the ratio of GSH/oxidized GSH (GSSG) in GA-treated layers, and the NaHS-mediated alleviation of PCD was markedly eliminated by l-buthionine-sulfoximine (BSO, a selective inhibitor of GSH biosynthesis). The inducible effect of NaHS was also ascribed to the modulation of heme oxygenase-1 (HO-1), because the specific inhibitor of HO-1 zinc protoporphyrin IX (ZnPP) significantly suppressed the NaHS-related responses. By contrast, the above inhibitory effects were reversed partially when carbon monoxide (CO) aqueous solution or bilirubin (BR), two of the by-products of HO-1, was added, respectively. NaHS-triggered HO-1 gene expression in GA-treated layers was also confirmed. Together, the above results clearly suggested that the H2S-delayed PCD in GA-treated wheat aleurone cells was associated with the modulation of GSH homeostasis and HO-1 gene expression.

Alleviation of cadmium toxicity in Medicago sativa by hydrogen-rich water

Hydrogen gas (H₂) induces plant tolerance to several abiotic stresses, including salinity and paraquat exposure. However, the role of H₂ in cadmium (Cd)-induced stress amelioration is largely unknown. Here, pretreatment with hydrogen-rich water (HRW) was used to characterize physiological roles and molecular mechanisms of H₂ in the alleviation of Cd toxicity in alfalfa plants. Our results showed that the addition of HRW at 10% saturation significantly decreased contents of thiobarbituric acid reactive substances (TBARS) caused by Cd, and inhibited the appearance of Cd toxicity symptoms, including the improvement of root elongation and seedling growth. These responses were related to a significant increase in the total or isozymatic activities of representative antioxidant enzymes, or their corresponding transcripts. In vivo imaging of reactive oxygen species (ROS), and the detection of lipid peroxidation and the loss of plasma membrane integrity provided further evidence for the ability of HRW to improve Cd tolerance significantly, which was consistent with a significant enhancement of the ratio of reduced/oxidized (homo)glutathione ((h)GSH). Additionally, plants pretreated with HRW accumulated less amounts of Cd. Together, this study suggested that the usage of HRW could be an effective approach for Cd detoxification and could be explored in agricultural production systems.

Roles of NIA/NR/NOA1-dependent nitric oxide production and HY1 expression in the modulation of Arabidopsis salt tolerance

Despite substantial evidence on the separate roles of Arabidopsis nitric oxide-associated 1 (NOA1)-associated nitric oxide (NO) production and haem oxygenase 1 (HY1) expression in salt tolerance, their integrative signalling pathway remains largely unknown. To fill this knowledge gap, the interaction network among nitrate reductase (NIA/NR)- and NOA1-dependent NO production and HY1 expression was studied at the genetic and molecular levels. Upon salinity stress, the majority of NO production was attributed to NIA/NR/NOA1. Further evidence confirmed that HY1 mutant hy1-100, nia1/2/noa1, and nia1/2/noa1/hy1-100 mutants exhibited progressive salt hypersensitivity, all of which were significantly rescued by three NO-releasing compounds. The salinity-tolerant phenotype and the stronger NO production in gain-of-function mutant of HY1 were also blocked by the NO synthetic inhibitor and scavenger. Although NO- or HY1-deficient mutants showed a compensatory mode of upregulation of HY1 or slightly increased NO production, respectively, during 2 d of salt treatment, downregulation of ZAT10/12-mediated antioxidant gene expression (cAPX1/2 and FSD1) was observed after 7 d of treatment. The hypersensitive phenotypes and stress-related genes expression profiles were differentially rescued or blocked by the application of NO- (in particular) or carbon monoxide (CO)-releasing compounds, showing a synergistic mode. Similar reciprocal responses were observed in the nia1/2/noa1/hy1-100 quadruple mutant, with the NO-releasing compounds exhibit the maximal rescuing responses. Overall, the findings present the combination of compensatory and synergistic modes, linking NIA/NR/NOA1-dependent NO production and HY1 expression in the modulation of plant salt tolerance.

Hydrogen gas acts as a novel bioactive molecule in enhancing plant tolerance to paraquat-induced oxidative stress via the modulation of heme oxygenase-1 signalling system

Hydrogen gas (H2) was recently proposed as a novel antioxidant and signalling molecule in animals. However, the physiological roles of H2 in plants are less clear. Here, we showed that exposure of alfalfa seedlings to paraquat stress increased endogenous H2 production. When supplied with exogenous H2 or the heme oxygenase-1 (HO-1)-inducer hemin, alfalfa plants displayed enhanced tolerance to oxidative stress induced by paraquat. This was evidenced by alleviation of the inhibition of root growth, reduced lipid peroxidation and the decreased hydrogen peroxide and superoxide anion radical levels. The activities and transcripts of representative antioxidant enzymes were induced after exposure to either H2 or hemin. Further results showed that H2 pretreatment could dramatically increase levels of the MsHO-1 transcript, levels of the protein it encodes and HO-1 activity. The previously mentioned H2-mediated responses were specific for HO-1, given that the potent HO-1-inhibitor counteracted the effects of H2. The effects of H2 were reversed after the addition of an aqueous solution of 50% carbon monoxide (CO). We also discovered enhanced tolerance of multiple environmental stresses after plants were pretreated with H2 . Together, these results suggested that H2 might function as an important gaseous molecule that alleviates oxidative stress via HO-1 signalling.

Heme oxygenase-1 is involved in nitric oxide- and cGMP-induced α-Amy2/54 gene expression in GA-treated wheat aleurone layers

Here, α-Amy2/54 gene expression was used as a molecular probe to investigate the interrelationship among nitric oxide (NO), cyclic GMP (cGMP), and heme oxygenase-1 (HO-1) in GA-treated wheat aleurone layers. The inducible expressions of α-Amy2/54 and α-amylase activity were respectively amplified by two NO-releasing compounds, sodium nitroprusside (SNP) and spermine NONOate, in a GA-dependent fashion. Similar responses were observed when an inducer of HO-1, hemin-or one of its catalytic products, carbon monoxide (CO) in aqueous solution-was respectively added. The SNP-induced responses, mimicked by 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP), a cGMP derivative, were NO-dependent. This conclusion was supported by the fact that endogenous NO overproduction was rapidly induced by SNP, and thereafter induction of α-Amy2/54 gene expression and increased α-amylase activity were sensitive to the NO scavenger. We further observed that the above induction triggered by SNP and 8-Br-cGMP was partially prevented by zinc protoporphyrin IX (ZnPPIX), an inhibitor of HO-1. These blocking effects were clearly reversed by CO, confirming that the above response was HO-1-specific. Further analyses showed that both SNP and 8-Br-cGMP rapidly up-regulated HO-1 gene expression and increased HO activity, and SNP responses were sensitive to cPTIO and the guanylate cyclase inhibitor 6-anilino-5,8-quinolinedione (LY83583). Molecular evidence confirmed that GA-induced GAMYB and ABA-triggered PKABA1 transcripts were up-regulated or down-regulated by SNP, 8-Br-cGMP or CO cotreated with GA. Contrasting changes were observed when cPTIO, LY83583, or ZnPPIX was added. Together, our results suggested that HO-1 is involved in NO- and cGMP-induced α-Amy2/54 gene expression in GA-treated aleurone layers.