BnHO1, a haem oxygenase-1 gene from Brassica napus, is required for salinity and osmotic stress-induced lateral root formation

Exogenous Hemin enhances the antioxidant defense system of rice by regulating the AsA-GSH cycle under NaCl stress

Abiotic stress caused by soil salinization remains a major global challenge that threatens and severely impacts crop growth, causing yield reduction worldwide. In this study, we aim to investigate the damage of salt stress on the leaf physiology of two varieties of rice (Huanghuazhan, HHZ, and Xiangliangyou900, XLY900) and the regulatory mechanism of Hemin to maintain seedling growth under the imposed stress. Rice leaves were sprayed with 5.0 μmol·L-1 Hemin or 25.0 μmol·L-1 ZnPP (Zinc protoporphyrin IX) at the three leaf and one heart stage, followed by an imposed salt stress treatment regime (50.0 mmol·L-1 sodium chloride (NaCl)). The findings revealed that NaCl stress increased antioxidant enzymes activities and decreased the content of nonenzymatic antioxidants such as ascorbate (AsA) and glutathione (GSH). Furthermore, the content of osmoregulatory substances like soluble proteins and proline was raised. Moreover, salt stress increased reactive oxygen species (ROS) content in the leaves of the two varieties. However, spraying with Hemin increased the activities of antioxidants such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) and accelerated AsA-GSH cycling to remove excess ROS. In summary, Hemin reduced the effect of salt stress on the physiological characteristics of rice leaves due to improved antioxidant defense mechanisms that impeded lipid peroxidation. Thus, Hemin was demonstrated to lessen the damage caused by salt stress.

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.

Salt Stress-Induced Modulation of Porphyrin Biosynthesis, Photoprotection, and Antioxidant Properties in Rice Plants (Oryza sativa)

Salt stress disrupts cellular ion homeostasis and adversely impacts plant growth and productivity. We examined the regulatory mechanisms of porphyrin biosynthesis, photoprotection, and antioxidant properties in salt-stressed rice seedlings. In response to 150 mM NaCl, the rice seedlings exhibited dehydration, reduced relative water content, and increased levels of conductivity, malondialdehyde, and H2O2. The expression levels of the salt-stress-responsive genes NHX1, SOS1, and MYB drastically increased after NaCl treatment. The seedlings grown under NaCl stress displayed declines in Fv/Fm, ΦPSII, rETRmax, and photochemical quenching but increases in nonphotochemical quenching (NPQ) and the expression of genes involved in zeaxanthin formation, BCH, and VDE. Under salt stress conditions, levels of chlorophyll precursors significantly decreased compared to controls, matching the downregulation of CHLD, CHLH, CHLI, and PORB. By contrast, NaCl treatment led to increased heme content at 24 h of treatment and significant upregulations of FC2, HO1, and HO2 compared to controls. Salt-stressed seedlings also increased their expression of CATs (catalases) and APXs (ascorbate peroxidases) as well as the activities of superoxide dismutase, CAT, APX, and peroxidase. Our results indicate that chlorophyll and heme biosynthesis involve the protective strategies for salt stress alleviation through photoprotection by the scavenging of chlorophyll precursors and NPQ as well as activating antioxidant enzymes.

Heme Oxygenase/Carbon Monoxide Participates in the Regulation of Ganoderma lucidum Heat-Stress Response, Ganoderic Acid Biosynthesis, and Cell-Wall Integrity

Carbon monoxide (CO), a product of organic oxidation processes, arises in vivo principally from the enzymatic reaction of heme oxygenase (HO, transcription gene named HMX1). HO/CO has been found to exert many salutary effects in multiple biological processes, including the stress response. However, whether HO/CO is involved in the regulation of the heat-stress (HS) response of Ganoderma lucidum (G. lucidum) is still poorly understood. In this paper, we reported that under heat stress, the HMX1 transcription level, HO enzyme activity, and CO content increased by 5.2-fold, 6.5-fold and 2-fold, respectively. HMX1 silenced strains showed a 12% increase in ganoderic acid (GA) content under HS as analyzed by HPLC. Furthermore, according to Western blot analysis of the protein phosphorylation levels, HMX1 attenuated the increase in phosphorylation levels of slt2, but the phosphorylation levels were prolonged over a 3 h HS time period. The chitin and glucan content in HMX1 silenced strains increased by 108% and 75%, respectively. In summary, these findings showed that the HO/CO system responds to heat stress and then regulates the HS-induced GA biosynthesis and the cell-wall integrity mediated by the Slt-MAPK phosphorylation level in G. lucidum.

Heme oxygenase-nitric oxide crosstalk-mediated iron homeostasis in plants under oxidative stress

Plant growth under abiotic stress conditions significantly enhances intracellular generation of reactive oxygen species (ROS). Oxidative status of plant cells is directly affected by the modulation of iron homeostasis. Among mammals and plants, heme oxygenase-1 (HO-1) is a well-known antioxidant enzyme. It catalyzes oxygenation of heme, thereby producing Fe²⁺, CO and biliverdin as byproducts. The antioxidant potential of HO-1 is primarily due to its catalytic reaction byproducts. Biliverdin and bilirubin possess conjugated π-electrons which escalate the ability of these biomolecules to scavenge free radicals. CO also enhances the ROS scavenging ability of plants cells by upregulating catalase and peroxidase activity. Enhanced expression of HO-1 in plants under oxidative stress accompanies sequestration of iron in specialized iron storage proteins localized in plastids and mitochondria, namely ferritin for Fe³⁺ storage and frataxin for storage of Fe-S clusters, respectively. Nitric oxide (NO) crosstalks with HO-1 at multiple levels, more so in plants under oxidative stress, in order to maintain intracellular iron status. Formation of dinitrosyl-iron complexes (DNICs) significantly prevents Fenton reaction during oxidative stress. DNICs also release NO upon dissociation in target cells over long distance in plants. They also function as antioxidants against superoxide anions and lipidic free radicals. A number of NO-modulated transcription factors also facilitate iron homeostasis in plant cells. Plants facing oxidative stress exhibit modulation of lateral root formation by HO-1 through NO and auxin-dependent pathways. The present review provides an in-depth analysis of the structure-function relationship of HO-1 in plants and mammals, correlating them with their adaptive mechanisms of survival under stress.

Hemin-decreased cadmium uptake in pak choi (Brassica chinensis L.) seedlings is heme oxygenase-1 dependent and relies on its by-products ferrous iron and carbon monoxide

Cadmium (Cd) is a major pollutant in farmland, which not only greatly restricts crop production, but also brings a serious threat to human health through entering the food chain. Our previous study showed that hemin treatment could reduce the accumulation of Cd in pak choi seedlings. However, the underlying mechanism remains unclear. In this study, we used non-invasive micro-test technology (NMT) to detect the real-time Cd²⁺ flux from pak choi roots and demonstrated that hemin treatment decreased Cd uptake rather than its translocation within plants. Moreover, through comparing the responses of different chemical treatments in pak choi seedlings and Arabidopsis wild-type and heme oxygenase-1 (HO-1) mutant, we provided evidence that hemin-decreased Cd uptake was HO-1 dependent. Furthermore, analyses of hemin degradation products suggested that the hemin-derived suppression of Cd uptake suppression was probably relying on its degradation by-products, ferrous iron (Fe²⁺) and carbon monoxide (CO), via repressing the expression of a Fe²⁺/Cd²⁺ transporter BcIRT1 in pak choi roots.

Effect of the heme oxygenase gene on mycelial growth and polysaccharide synthesis in Ganoderma lucidum

The heme oxygenase gene has antioxidant and cytoprotective effects in organisms, but no related research has been conducted in Ganoderma lucidum. For the first time, we cloned the HMX1 gene in G. lucidum. The CDS is 1092 bp in length and encodes 363 amino acids. The HMX1 protein was prokaryotically expressed and purified, and the enzyme activity of the purified protein was measured. The value of K_m was 0.699 μM, and V_m was 81.9 nmol BV h^-1  nmol^-1 protein. By constructing the silencing vector pAN7-dual-HMX1i, the transformants HMX1i1 and HMX1i2 were obtained. Compared with the wild-type (WT), the average growth rate of HMX1i1 and HMX1i2 decreased by 31% and 23%, respectively, and the mycelium biomass decreased by 53% and 48%, respectively. Compared with the WT, the extracellular polysaccharide content of HMX1i1 and HMX1i2 increased by 59% and 51%, and the intracellular polysaccharide content increased by 24% and 22%, respectively. These results indicate that the HMX1 gene affects mycelial growth and polysaccharide synthesis in G. lucidum.

Auxin-Induced SaARF4 Downregulates SaACO4 to Inhibit Lateral Root Formation in Sedum alfredii Hance

Lateral root (LR) formation promotes plant resistance, whereas high-level ethylene induced by abiotic stress will inhibit LR emergence. Considering that local auxin accumulation is a precondition for LR generation, auxin-induced genes inhibiting ethylene synthesis may thus be important for LR development. Here, we found that auxin response factor 4 (SaARF4) in Sedum alfredii Hance could be induced by auxin. The overexpression of SaARF4 decreased the LR number and reduced the vessel diameters. Meanwhile, the auxin distribution mode was altered in the root tips and PIN expression was also decreased in the overexpressed lines compared with the wild-type (WT) plants. The overexpression of SaARF4 could reduce ethylene synthesis, and thus, the repression of ethylene production decreased the LR number of WT and reduced PIN expression in the roots. Furthermore, the quantitative real-time PCR, chromatin immunoprecipitation sequencing, yeast one-hybrid, and dual-luciferase assay results showed that SaARF4 could bind the promoter of 1-aminocyclopropane-1-carboxylate oxidase 4 (SaACO4), associated with ethylene biosynthesis, and could downregulate its expression. Therefore, we concluded that SaARF4 induced by auxin can inhibit ethylene biosynthesis by repressing SaACO4 expression, and this process may affect auxin transport to delay LR development.

AUREA maintains the balance between chlorophyll synthesis and adventitious root formation in tomato

Flooding tolerance is an important trait for tomato breeding. In this study, we obtained a recessive mutant exhibiting highly enhanced submergence resistance. Phenotypical analyses showed that this resistant to flooding (rf) mutant displays slightly chlorotic leaves and spontaneous initiation of adventitious roots (ARs) on stems. The mutation was mapped to the phytochromobilin synthase gene AUREA (AU), in which a single amino acid substitution from asparagine to tyrosine occurred. In addition to the classic function of AU in phytochrome and chlorophyll biogenesis in leaves, we uncovered its novel role in mediating AR formation on stems. We further observed temporal coincidence of the two phenotypes in the rf mutant: chlorosis and spontaneous AR formation and revealed that AU functions by maintaining heme homeostasis. Interestingly, our grafting results suggest that heme might play roles in AR initiation via long-distance transport from leaves to stems. Our results present genetic evidence for the involvement of the AU-heme oxygenase-1-heme pathway in AR initiation in tomato. As fruit production and yield in the rf mutant are minimally impacted, the mutation identified in this study may provide a target for biotechnological renovation of tomato germplasm in future breeding.

Phytoremediation of Cadmium Contaminated Soil Using Brassica juncea: Influence on PSII Activity, Leaf Gaseous Exchange, Carbohydrate Metabolism, Redox and Elemental Status

Phytoremediation is an ecologically and economically feasible technique to remove heavy metal from soil. The aim of the study was to examine cadmium (Cd) toxicity and phytoremediation aptitude of Brassica juncea. In the present study, plants survived when exposed to different levels of Cd (0, 25, 50 and 100 mg/kg soil) and accumulated a large amount of Cd in its root and shoot. Translocation factor (TF) of Cd from root to shoot was > 1 at both 45 and 60-day stage of growth suggesting that B. juncea is a hyperaccumulator and strong candidate for phytoextraction of Cd. Alongside, Cd impaired photolysis of water, PSII activity, nutrient uptake, photosynthesis and sugar accumulation in the plant. Cd-generated oxidative stress restricts the growth of B. juncea. The toxic effect of Cd was more pronounced at 45-day stage of growth signifying the drifting of plant towards acquirement of exclusion strategy.

EsHO 1 mediated mitigation of NaCl induced oxidative stress and correlation between ROS, antioxidants and HO 1 in seedlings of Eruca sativa: underutilized oil yielding crop of arid region

Study have focused on NaCl induced HO 1 production and its co-relation to ROS and antioxidant regulation in Eruca sativa. Seedlings were subjected to NaCl stress ranges from 10 to 150 mM. After 96 h of treatment, plants samples were harvested to evaluate the cellular equilibrium and salt tolerance mechanisms through morphological, stress parameters, non enzymatic and antioxidant enzymes. The HO 1 activity was found to be highest at 75 mM NaCl in leaves and roots which were 2.49 and 2.02 folds respectively. The expression of EsHO 1 was also observed and the higher expression was recorded in roots than leaves. The overall activity of other antioxidants (APX and proline) was also found to be higher at 75 mM concentration. The highest HO 1 activity with other antioxidants indicates the decline in LPX and ROS at 75 mM NaCl. The present study concluded that HO 1 helps in amelioration of NaCl stress by working within a group of antioxidants that create the defense machinery in seedlings of E. sativa by manipulating various physiological processes of plants. These findings for the first time suggest the protective role of HO 1 in scavenging ROS in E. sativa under salinity stress.

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.

The inhibition of heme oxygenase-1 enhances the chemosensitivity and suppresses the proliferation of pancreatic cancer cells through the SHH signaling pathway

Pancreatic cancer (PC) is a type of cancer associated with a high fatality rate due to a poor prognosis and resistance to treatment. Heme oxygenase-1 (HO-1) is significantly overexpressed in a number of types of cancer and seems to play an important role in cancer progression. In this study, we examined the potential effects of HO-1 on PC cell proliferation and sensivity to gemcitabine (Gem). Furthermore, the role of the sonic hedgehog (SHH) signaling pathway in the regulatory effects of HO-1 on PC progression were examined. For this purpose, the expression of HO-1 was examined in cultured PC cells by real-time PCR, western blot analysis and immunofluorescence. Transfection with small interfering RNA against HO-1 or an overexpression plasmid were used to regulate the expression of HO-1 in the MIA PaCa-2 and PANC-1 cell lines. Cell proliferation was examined by MTT assays in response to the different treatments. The results revealed that HO-1 expression differed significantly in the different PC cells. The overexpression of HO-1 induced PC cell proliferation and the inhibition of HO-1 decreased the cell proliferative ability. Furthermore, HO-1 activated the SHH signaling pathway in the PC cells. In addition, the SHH signaling pathway was found to play a role in HO-1-induced PC cell proliferation. The inhibition of HO-1 enhanced the responsiveness of PC cells to Gem and Gem was found to regulate the expression of HO-1 and the activation of the SHH pathway. On the whole, our findings indicate that HO-1 overexpression in PC cells may be responsible for the increased cell proliferation and the resistance to anticancer therapy. Furthermore, the SHH signaling pathway, the activation of which was initiated by HO-1, may be one of the endogenous mechanisms in this process. Our data shed light into the association between HO-1 and SHH in PC cells, and may aid in the development of novel therapeutic targets for the treatment of patients with PC.

Haem oxygenase: A functionally diverse enzyme of photosynthetic organisms and its role in phytochrome chromophore biosynthesis, cellular signalling and defence mechanisms

Haem oxygenase (HO) is a universal enzyme that catalyses stereospecific cleavage of haem to BV IX α and liberates Fe⁺² ion and CO as by-product. Beside haem degradation, it has important functions in plants that include cellular defence, stomatal regulation, iron mobilization, phytochrome chromophore synthesis, and lateral root formation. Phytochromes are an extended family of photoreceptors with a molecular mass of 250 kDa and occur as a dimer made up of 2 equivalent subunits of 125 kDa each. Each subunit is made of two components: the chromophore, a light-capturing pigment molecule and the apoprotein. Biosynthesis of phytochrome (phy) chromophore includes the oxidative splitting of haem to biliverdin IX by an enzyme HO, which is the decisive step in the biosynthesis. In photosynthetic organisms, BVα is reduced to 3Z PΦB by a ferredoxin-dependent PΦB synthase that finally isomerised to PΦB. The synthesized PΦB assembles with the phytochrome apoprotein in the cytoplasm to generate holophytochrome. Thus, necessary for photomorphogenesis in plants, which has confirmed from the genetic studies, conducted on Arabidopsis thaliana and pea. Besides the phytochrome chromophore synthesis, the review also emphasises on the current advances conducted in plant HO implying its developmental and defensive role.

Evaluation of heme oxygenase 1 (HO 1) in Cd and Ni induced cytotoxicity and crosstalk with ROS quenching enzymes in two to four leaf stage seedlings of Vigna radiata

Research on heme oxygenase in plants has received consideration in recent years due to its several roles in development, defense, and metabolism during various environmental stresses. In the current investigation, the role of heme oxygenase (HO) 1 was evaluated in reducing heavy metal (Cd and Ni) uptake and alleviating Cd and Ni toxicity effects in the hydroponically grown seedlings of Vigna radiata var. PDM 54. Seedlings were subjected to Cd- and Ni-induced oxidative stress independently at different concentrations ranging from 10 to 100 μM. After 96 h (fourth day) of treatment, the stressed plants were harvested to study the cellular homeostasis and detoxification mechanism by examining the growth, stress parameters (LPX, H₂O₂ content), and non-enzymatic and enzymatic parameters (ascorbate peroxidase (APX), guaicol peroxidase (GPX), and catalase (CAT)) including HO 1. At 50 μM CdCl₂ and 60 μM NiSO₄, HO 1 activity was found to be highest in leaves which were 1.39 and 1.16-fold, respectively. The greatest HO 1 activity was reflected from the reduction of H₂O₂ content at these metal concentrations (50 μM CdCl₂ and 60 μM NiSO₄) which is correlated with the increasing activity of other antioxidant enzymes (CAT, APX). Thus, HO 1 works within a group that generates the defense machinery for the plant's survival by scavenging ROS which is confirmed by a time-dependent study. Hence, it is concluded that seedlings of V. radiata were more tolerant towards metal-induced oxidative stress in which HO 1 is localized in its residential area (plastids).

Salt stress-induced FERROCHELATASE 1 improves resistance to salt stress by limiting sodium accumulation in Arabidopsis thaliana

Ferrochelatase-1 as a terminal enzyme of heme biosynthesis regulates many essential metabolic and physiological processes. Whether FC1 is involved in plant response to salt stress has not been described. This study shows that Arabidopsis overexpressing AtFC1 displays resistance to high salinity, whereas a T-DNA insertion knock-down mutant fc1 was more sensitive to salt stress than wild-type plants. AtFC1 conferred plant salt resistance by reducing Na⁺ concentration, enhancing K⁺ accumulation and preventing lysis of the cell membrane. Such observations were associated with the upregulation of SOS1, which encodes a plasma membrane Na⁺/H⁺ antiporter. AtFC1 overexpression led to a reduced expression of several well known salt stress-responsive genes such as NHX1 and AVP1, suggesting that AtFC1-regulated low concentration of Na⁺ in plants might not be through the mechanism for Na⁺ sequestration. To investigate the mechanism leading to the role of AtFC1 in mediating salt stress response in plants, a transcriptome of fc1 mutant plants under salt stress was profiled. Our data show that mutation of AtFC1 led to 490 specific genes up-regulated and 380 specific genes down-regulated in fc1 mutants under salt stress. Some of the genes are involved in salt-induced oxidative stress response, monovalent cation-proton (Na⁺/H⁺) exchange, and Na⁺ detoxification.

A cadmium stress-responsive gene AtFC1 confers plant tolerance to cadmium toxicity

BACKGROUND

Non-essential trance metal such as cadmium (Cd) is toxic to plants. Although some plants have developed elaborate strategies to deal with absorbed Cd through multiple pathways, the regulatory mechanisms behind the Cd tolerance are not fully understood. Ferrochelatase-1 (FC1, EC4.99.1.1) is the terminal enzyme of heme biosynthesis, catalyzing insertion of ferrous ion into protoporphyrin IX. Recent studies have shown that FC1 is involved in several physiological processes. However, its biological function associated with plant abiotic stress response is poorly understood.

RESULTS

In this study, we showed that AtFC1 was transcriptionally activated by Cd exposure. AtFC1 overexpression (35S::FC1) lines accumulated more Cd and non-protein thiol compounds than wild-type, and conferred plant tolerance to Cd stress, with improved primary root elongation, biomass and chlorophyll (Chl) content, and low degree of oxidation associated with reduced H₂O₂, O^·2- and peroxides. In contrast, the AtFC1 loss of functional mutant fc1 showed sensitivity to Cd stress. Exogenous provision of heme, the product of AtFC1, partially rescued the Cd-induced toxic phenotype of fc1 mutants by improving the growth of seedlings, generation of glutathione (GSH) and phytochelatins (PCs), and GSH/PCs-synthesized gene expression (e.g. GSH1, GSH2, PCS1, and PCS2). To investigate the mechanism leading to the AtFC1 regulating Cd stress response in Arabidopsis, a transcriptome of fc1 mutant plants under Cd stress was profiled. Our data showed that disfunction of AtFC1 led to 913 genes specifically up-regulated and 522 genes down-regulated in fc1 mutants exposed to Cd. Some of the genes are involved in metal transporters, Cd-induced oxidative stress response, and detoxification.

CONCLUSION

These results indicate that AtFC1 would act as a positive regulator of plant tolerance to Cd stress. Our study will broaden our understanding of the role of FC1 in mediating plant response to Cd stress and provide a basis for further exploration of its downstream genes.

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.

Involvement of glutathione in β-cyclodextrin-hemin complex-induced lateral root formation in tomato seedlings

β-cyclodextrin-hemin complex (β-CDH) was shown to induce lateral root (LR) formation in tomato. However, the molecular mechanism is still elusive. In this report, the role of reduced glutathione (GSH) in the induction of lateral root triggered by β-CDH was investigated. Similar to the responses of β-CDH, exogenously applied with 0.1 mΜ GSH not only increased endogenous GSH content determined by spectrophotography and the monochlorobimane (MCB)-dependent fluorescent analysis, but also induced, thereafter, LR formation. Meanwhile, both β-CDH- and GSH-induced lateral root primordia (LRP) exhibited a similar accelerated anatomic structure. Above inducible responses were blocked significantly when the _L-buthionine-(S,R)-sulfoximine (BSO), a potent and specific inhibitor of the enzyme catalyzing the first step of GSH biosynthesis, was separately applied. Upon β-CDH treatment, the changes of endogenous GSH content determined by spectrophotography and fluorescent analysis were consistent with the transcripts of two GSH synthetic genes, GSH1 and GSH2 encoding γ-glutamyl cysteine synthetase and glutathione synthetase, respectively. Exogenously applied with β-CDH could rescue N-1-naphthylphthalamic acid (NPA; IAA depletion)-triggered inhibition of LR formation. Further molecular evidence revealed that both β-CDH and GSH modulated gene expression of cell cycle regulatory genes (CYCA2;1, CYCA3;1, CYCD3;1, and CDKA1) and auxin signaling genes (ARF7 and RSI-1), six marker genes responsible for LR formation. By contrast, above changes were sensitive to the co-treatment with BSO. All together, these results suggest a role for GSH in the regulation of tomato LR development triggered by β-CDH.

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.

Heme Oxygenase-1 Delays Gibberellin-Induced Programmed Cell Death of Rice Aleurone Layers Subjected to Drought Stress by Interacting with Nitric Oxide

Cereal aleurone layers undergo a gibberellin (GA)-regulated process of programmed cell death (PCD) following germination. Heme oxygenase-1 (HO-1) is known as a rate-liming enzyme in the degradation of heme to biliverdin IXα, carbon monoxide (CO), and free iron ions (Fe(2+)). It is a critical component in plant development and adaptation to environment stresses. Our previous studies confirmed that HO-1 inducer hematin (Ht) promotes the germination of rice seeds in drought (20% polyethylene glycol-6000, PEG) conditions, but the corresponding effects of HO-1 on the alleviation of germination-triggered PCD in GA-treated rice aleurone layers remain unknown. The present study has determined that GA co-treated with PEG results in lower HO-1 transcript levels and HO activity, which in turn results in the development of vacuoles in aleurone cells, followed by PCD. The pharmacology approach illustrated that up- or down-regulated HO-1 gene expression and HO activity delayed or accelerated GA-induced PCD. Furthermore, the application of the HO-1 inducer Ht and nitric oxide (NO) donor sodium nitroprusside (SNP) not only activated HO-1 gene expression, HO activity, and endogenous NO content, but also blocked GA-induced rapid vacuolation and accelerated aleurone layers PCD under drought stress. However, both HO-1 inhibitor zinc protoporphyrin IX (ZnPPIX) and NO scavenger 2-(4-carboxyphenyl0-4, 4,5,5-tetramethylimidazoline-l-oxyl-3-oxide potassium salt (cPTIO) reserved the effects of Ht and SNP on rice aleurone layer PCD under drought stress by down-regulating endogenous HO-1 and NO, respectively. The inducible effects of Ht and SNP on HO-1 gene expression, HO activity, and NO content were blocked by cPTIO. Together, these results clearly suggest that HO-1 is involved in the alleviation of GA-induced PCD of drought-triggered rice aleurone layers by associating with NO.

The roles of tetrapyrroles in plastid retrograde signaling and tolerance to environmental stresses

This review provides new insights that tetrapyrrole signals play important roles in nuclear gene expression, chloroplast development and plant's resistance to environmental stresses. Higher plants contain many tetrapyrroles, including chlorophyll (Chl), heme, siroheme, phytochromobilin and some of their precursors, all of which have important biological functions. Genetic and physiological studies indicated that tetrapyrrole (mainly Mg-protoporphyrin IX) retrograde signals control photosynthesis-associated nuclear gene (PhANG) expression. Recent studies have shown that tetrapyrrole-derived signals may correlate with plant resistance to environmental stresses such as drought, high-light stress, water stress, osmotic stress, salinity and heavy metals. Signaling and physiological roles of Mg-protoIX-binding proteins (such as PAPP5, CRD and HSP90) and heme-binding proteins (such as HO and TSPO) and tetrapyrrole-signaling components (such as GUN1, ABI4 and CBFA) are summarized. Some of them positively regulate plant development and response to environmental stresses. The intermediate signaling components (such as PTM, HSP70-HSP90-HAP1 complex and PAPP5) between the nucleus and the plastid also positively regulate plant resistance to environmental stresses. This review provides new insights that genetically modified plants with enhanced tetrapyrrole levels have improved resistance to environmental stresses.

Methane-rich water induces cucumber adventitious rooting through heme oxygenase1/carbon monoxide and Ca(2+) pathways

Methane-rich water triggered adventitious rooting by regulating heme oxygenase1/carbon monoxide and calcium pathways in cucumber explants. Heme oxygenase1/carbon monoxide (HO1/CO) and calcium (Ca(2+)) were reported as the downstream signals in auxin-induced cucumber adventitious root (AR) formation. Here, we observed that application of methane-rich water (MRW; 80% saturation) obviously induced AR formation in IAA-depleted cucumber explants. To address the universality, we checked adventitious rooting in soybean and mung bean explants, and found that MRW (50 and 10% saturation, respectively) exhibited the similar inducing results. To further determine if the HO1/CO system participated in MRW-induced adventitious rooting, MRW, HO1 inducer hemin, its activity inhibitor zinc protoporphyrin IX (ZnPP), and its catalytic by-products CO, bilirubin, and Fe(2+) were used to detect their effects on cucumber adventitious rooting in IAA-depleted explants. Subsequent results showed that MRW-induced adventitious rooting was blocked by ZnPP and further reversed by 20% saturation CO aqueous solution. However, the other two by-products of HO1, bilirubin and Fe(2+), failed to induce AR formation. Above responses were consistent with the MRW-induced increases of HO1 transcript and corresponding protein level. Further molecular evidence indicted that expression of marker genes, including auxin signaling-related genes and cell cycle regulatory genes, were modulated by MRW alone but blocked by the cotreatment with ZnPP, the latter of which could be significantly rescued by the addition of CO. By using the Ca(2+)-channel blocker and Ca(2+) chelator, the involvement of Ca(2+) pathway in MRW-induced adventitious rooting was also suggested. Together, our results indicate that MRW might serve as a stimulator of adventitious rooting, which was partially mediated by HO1/CO and Ca(2+) pathways.

β-Cyclodextrin-hemin complex-induced lateral root formation in tomato: involvement of nitric oxide and heme oxygenase 1

β-Cyclodextrin-hemin complex-induced tomato lateral root formation was associated with nitric oxide and heme oxygenase 1 by modulating cell cycle regulatory genes. β-Cyclodextrin-hemin complex (β-CDH), a complex by combining β-cyclodextrin (β-CD) with hemin, a heme oxygenase 1 (HO1) inducer, was a trigger of cucumber adventitious root formation by enhancing HO1 gene expression. In this report, our results identified the previously unknown function of β-CDH in plants: the inducer of tomato lateral root (LR) formation. β-CDH-triggered LR formation is hemin-specific, since β-CD failed to induce LR development. Because nitric oxide (NO) is involved in LR formation, the correlation of β-CDH with NO and HO1 was investigated. Our analysis suggested that β-CDH induced an increase in endogenous NO production, followed by up-regulation of tomato HO1 gene and LR formation, all of which were mimicked by hemin and two NO-releasing compounds (SNP and GSNO). The induction of HO1 gene expression and LR formation triggered by β-CDH or hemin were significantly blocked by an inhibitor of HO1. Further results revealed that both β-CDH- and SNP-stimulated HO1 gene expression and thereafter LR formation were sensitive to the removal of NO with a potent NO scavenger, and the responses of SNP were significantly blocked by an inhibitor of HO1. Molecular evidence illustrated that representative cell cycle regulatory genes, including SlCDKA1, SlCYCA3;1, SlCYCA2;1, and SlCYCD3;1, were significantly up-regulated by β-CDH and SNP, but obviously blocked when seedlings were co-treated with the scavenger of NO or the inhibitor of HO1. In summary, our physiological and molecular evidence demonstrated that both NO and HO1 were involved in the β-CDH-induced LR formation with, at least partially, HO1 acting downstream of NO signaling.

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.

Heme oxygenase-1 is involved in sodium hydrosulfide-induced lateral root formation in tomato seedlings

By using pharmacological and molecular approaches, we discovered the involvement of HO-1 in NaHS-induced lateral root formation in tomato seedlings. Heme oxygenase-1 (HO-1) and hydrogen sulfide (H2S) regulate various responses to abiotic stress and root development, but their involvement in the simultaneous regulation of plant lateral root (LR) formation is poorly understood. In this report, we observed that the exogenously applied H2S donor sodium hydrosulfide (NaHS) and the HO-1 inducer hemin induce LR formation in tomato seedlings by triggering intracellular signaling events involving the induction of tomato HO-1 (SlHO-1), and the modulation of cell cycle regulatory genes, including the up-regulation of SlCDKA;1 and SlCYCA2;1, and simultaneous down-regulation of SlKRP2. The response of NaHS in the induction of LR formation was impaired by the potent inhibition of HO-1, which was further blocked when 50 % saturation of carbon monoxide (CO) aqueous solution, one of the catalytic by-products of HO-1, was added. Further molecular evidence revealed that the NaHS-modulated gene expression of cell cycle regulatory genes was sensitive to the inhibition of HO-1 and reversed by cotreatment with CO. The impairment of LR density and length as well as lateral root primordia number, the decreased tomato HO-1 gene expression and HO activity caused by an H2S scavenger hypotaurine were partially rescued by the addition of NaHS, hemin and CO (in particular). Together, these results revealed that at least in our experimental conditions, HO-1 might be involved in NaHS-induced tomato LR formation. Additionally, the use of NaHS and hemin compounds in crop root organogenesis should be explored.

Hydrogen sulfide is a novel gasotransmitter with pivotal role in regulating lateral root formation in plants

Hydrogen sulfide (H 2S), the third gasotransmitter after nitric oxide (NO) and carbon monoxide (CO), is a critical neuromodulator in the pathogenesis of various diseases from neurodegenerative diseases to diabetes or heart failure. The crosstalk between NO and H 2S has been well established in mammalian physiology. In planta, NO is demonstrated to regulate lateral root formation by acting downstream of auxin. The recent reports revealed that H 2S is a novel inducer of lateral root (LR) formation by stimulating the expression of cell cycle regulatory genes (CCRGs), acting similarly with NO, CO, and IAA. Interestingly, during the initiation of lateral root primordia, IAA is a potent inducer of endogenous H 2S and CO, which is produced by L-cysteine desulfhydrase (LCD) and heme oxygenase-1 (HO-1), respectively. The increasing evidences suggest that H 2S-promoted LR growth is dependent on the endogenous production of CO. In addition, our results indicate that the H 2S signaling in the regulation of LR formation can be associated to NO and Ca 2+. In this addendum, we advanced a proposed schematic model for H 2S-mediated signaling pathway of plant LR development.

Hydrogen sulfide is a novel gasotransmitter with pivotal role in regulating lateral root formation in plants

Hydrogen sulfide (H ₂S), the third gasotransmitter after nitric oxide (NO) and carbon monoxide (CO), is a critical neuromodulator in the pathogenesis of various diseases from neurodegenerative diseases to diabetes or heart failure. The crosstalk between NO and H ₂S has been well established in mammalian physiology. In planta, NO is demonstrated to regulate lateral root formation by acting downstream of auxin. The recent reports revealed that H ₂S is a novel inducer of lateral root (LR) formation by stimulating the expression of cell cycle regulatory genes (CCRGs), acting similarly with NO, CO, and IAA. Interestingly, during the initiation of lateral root primordia, IAA is a potent inducer of endogenous H ₂S and CO, which is produced by _L-cysteine desulfhydrase (LCD) and heme oxygenase-1 (HO-1), respectively. The increasing evidences suggest that H ₂S-promoted LR growth is dependent on the endogenous production of CO. In addition, our results indicate that the H ₂S signaling in the regulation of LR formation can be associated to NO and Ca ²⁺. In this addendum, we advanced a proposed schematic model for H ₂S-mediated signaling pathway of plant LR development.

In vitro evaluation of mitochondrial-chloroplast subcellular localization of heme oxygenase1 (HO1) in Glycine max

Heme oxygenase1 (HO1) catalyzes the degradation of heme in to biliverdin, carbon monoxide, and ferrous ions. Its role in higher plants has been found as an antioxidant and precursor of phytochrome synthesis. The present study focuses on subcellular localization of HO1 in leaves of soybean has been investigated. Most activity appeared to be located within chloroplast due to its role in phytochrome synthesis but mitochondria also share its localization. Mitochondrial location of HO1 might be on its inner membranous space due to its role in the synthesis of electron donor species which facilitates HO1 catalyzed reaction. Study reports the co-localization of HO1 in both chloroplast and mitochondria.

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.

Chlorophyll deficiency in the maize elongated mesocotyl2 mutant is caused by a defective heme oxygenase and delaying grana stacking

BACKGROUND

Etiolated seedlings initiate grana stacking and chlorophyll biosynthesis in parallel with the first exposure to light, during which phytochromes play an important role. Functional phytochromes are biosynthesized separately for two components. One phytochrome is biosynthesized for apoprotein and the other is biosynthesized for the chromophore that includes heme oxygenase (HO).

METHODOLOGY/PRINCIPAL FINDING

We isolated a ho1 homolog by map-based cloning of a maize elongated mesocotyl2 (elm2) mutant. cDNA sequencing of the ho1 homolog in elm2 revealed a 31 bp deletion. De-etiolation responses to red and far-red light were disrupted in elm2 seedlings, with a pronounced elongation of the mesocotyl. The endogenous HO activity in the elm2 mutant decreased remarkably. Transgenic complementation further confirmed the dysfunction in the maize ho1 gene. Moreover, non-appressed thylakoids were specifically stacked at the seedling stage in the elm2 mutant.

CONCLUSION

The 31 bp deletion in the ho1 gene resulted in a decrease in endogenous HO activity and disrupted the de-etiolation responses to red and far-red light. The specific stacking of non-appressed thylakoids suggested that the chlorophyll biosynthesis regulated by HO1 is achieved by coordinating the heme level with the regulation of grana stacking.

Transcriptome/degradome-wide identification of R. glutinosa miRNAs and their targets: the role of miRNA activity in the replanting disease

Rehmannia glutinosa, a traditional Chinese medicine herb, is unable to grow normally in a soil where the same species has recently been cultivated. The biological basis of this so called "replanting disease" is unknown, but it may involve the action of microRNAs (miRNAs), which are known to be important regulators of plant growth and development. High throughput Solexa/Illumina sequencing was used to generate a transcript library of the R. glutinosa transcriptome and degradome in order to identify possible miRNAs and their targets implicated in the replanting disease. A total of 87,665 unigenes and 589 miRNA families (17 of which have not been identified in plants to date) was identified from the libraries made from a first year (FP) and a second year (SP) crop. A comparison between the FP and SP miRNAs showed that the abundance of eight of the novel and 295 of the known miRNA families differed between the FP and SP plants. Sequencing of the degradome sampled from FP and SP plants led to the identification of 165 transcript targets of 85 of the differentially abundant miRNA families. The interaction of some of these miRNAs with their target(s) is likely to form an important part of the molecular basis of the replanting disease of R. glutinosa.

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.

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

Plant heme oxygenase (HO) catalyzes the oxygenation of heme to biliverdin, carbon monoxide (CO), and free iron (Fe(2+))-and Arabidopsis and rice (Oryza sativa) HOs are involved in light signaling. Here, we identified that the rice PHOTOPERIOD SENSITIVITY 5 (SE5) gene, which encoded a putative HO with high similarity to HO-1 from Arabidopsis (HY1), exhibited HO activity, and localized in the chloroplasts. Rice RNAi mutants silenced for SE5 were generated and displayed early flowering under long-day conditions, consistent with phenotypes of the null mutation in SE5 gene reported previously (se5 and s73). The herbicide methyl viologen (MV), which produces reactive oxygen species (ROS), was applied to determine whether SE5 regulates oxidative stress response. Compared with wild-type, SE5 RNAi transgenic plants aggravated seedling growth inhibition, chlorophyll loss and ROS overproduction, and decreased the transcripts of some representative antioxidative genes. By contrast, administration of exogenous CO partially rescued corresponding MV hypersensitivity in the SE5 RNAi plants. Alleviation of seed germination inhibition, chlorophyll loss and ROS overproduction, as well as the induction of antioxidant defense were further observed when SE5 or HY1 was overexpressed in transgenic Arabidopsis plants, indicating that SE5 may be useful for molecular breeding designed to improve plant tolerance to oxidative stress.

Haem oxygenase-1 is involved in salicylic acid-induced alleviation of oxidative stress due to cadmium stress in Medicago sativa

This work examines the involvement of haem oxygenase-1 (HO-1) in salicylic acid (SA)-induced alleviation of oxidative stress as a result of cadmium (Cd) stress in alfalfa (Medicago sativa L.) seedling roots. CdCl(2) exposure caused severe growth inhibition and Cd accumulation, which were potentiated by pre-treatment with zinc protoporphyrin (ZnPPIX), a potent HO-1 inhibitor. Pre-treatment of plants with the HO-1 inducer haemin or SA, both of which could induce MsHO1 gene expression, significantly reduced the inhibition of growth and Cd accumulation. The alleviation effects were also evidenced by a decreased content of thiobarbituric acid-reactive substances (TBARS). The antioxidant behaviour was confirmed by histochemical staining for the detection of lipid peroxidation and the loss of plasma membrane integrity. Furthermore, haemin and SA pre-treatment modulated the activities of ascorbate peroxidase (APX), superoxide dismutase (SOD), and guaiacol peroxidase (POD), or their corresponding transcripts. Significant enhancement of the ratios of reduced/oxidized homoglutathione (hGSH), ascorbic acid (ASA)/dehydroascorbate (DHA), and NAD(P)H/NAD(P)(+), and expression of their metabolism genes was observed, consistent with a decreased reactive oxygen species (ROS) distribution in the root tips. These effects are specific for HO-1, since ZnPPIX blocked the above actions, and the aggravated effects triggered by SA plus ZnPPIX were differentially reversed when carbon monoxide (CO) or bilirubin (BR), two catalytic by-products of HO-1, was added. Together, the results suggest that HO-1 is involved in the SA-induced alleviation of Cd-triggered oxidative stress by re-establishing redox homeostasis.

Involvement of heme oxygenase-1 in β-cyclodextrin-hemin complex-induced cucumber adventitious rooting process

Our previous results showed that β-cyclodextrin-hemin complex (CDH) exhibited a vital protective role against cadmium-induced oxidative damage and toxicity in alfalfa seedling roots by the regulation of heme oxygenase-1 (HO-1) gene expression. In this report, we further test whether CDH exhibited the hormonal-like response. The application of CDH and an inducer of HO-1, hemin, were able to induce the up-regulation of cucumber HO-1 gene (CsHO1) expression and thereafter the promotion of adventitious rooting in cucumber explants. The effect is specific for HO-1 since the potent HO-1 inhibitor zinc protoporphyrin IX (ZnPP) blocked the above responses triggered by CDH, and the inhibitory effects were reversed further when 30% saturation of CO aqueous solution was added together. Further, molecular evidence showed that CDH triggered the increases of the HO-1-mediated target genes responsible for adventitious rooting, including one DnaJ-like gene (CsDNAJ-1) and two calcium-dependent protein kinase (CDPK) genes (CsCDPK1 and CsCDPK5), and were inhibited by ZnPP and reversed by CO. The calcium (Ca2+) chelator ethylene glycol-bis (2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) and the Ca2+ channel blocker lanthanum chloride (LaCl3) not only compromised the induction of adventitious rooting induced by CDH but also decreased the transcripts of above three target genes. However, the application of ascorbic acid (AsA), a well-known antioxidant in plants, failed to exhibit similar inducible effect on adventitious root formation. In short, above results illustrated that the response of CDH in the induction of cucumber adventitious rooting might be through HO-1-dependent mechanism and calcium signaling.

KEY MESSAGE

Physiological, pharmacological and molecular evidence showed that β-cyclodextrin-hemin complex (CDH) was able to induce cucumber adventitious rooting through heme oxygenase-1 (HO-1)-dependent mechanism and calcium signaling.

Nitric oxide is involved in hemin-induced cucumber adventitious rooting process

Hemin, a heme oxygenase-1 (HO-1) inducer, was shown to exert numerous beneficial physiological functions in animals. Our previous study suggests that HO-1/carbon monoxide (CO) acts as a novel downstream signal system in the auxin-induced adventitious rooting. The objective of this study was to test whether nitric oxide (NO) is involved in hemin-induced cucumber adventitious rooting. Applications of hemin or CO aqueous solution to auxin-depleted cucumber explant induced up-regulation of cucumber HO-1 transcripts (CsHO1), NO production, and thereafter adventitious root formation, and some above responses were blocked by the combination treatment with two nitric oxide synthase (NOS)-like enzyme inhibitors N(G)-nitro-L-arginine methylester hydrochloride and N(G)-nitro-L-arginine, a HO-1 specific inhibitor zinc protoporphyrin IX, and a specific NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt. However, these blocking responses were not observed using tungstate, an inhibitor of nitrate reductase, another NO producing enzyme in plants. Furthermore, the guanylate cyclase inhibitors 1H-(1,2,4)-oxadiazole[4,3-a]quinoxalin-1-one and 6-anilino-5,8-quinolinedione reduced root development induced by hemin, whereas the cell-permeable cyclic guanosine monophosphate (cGMP) derivative 8-Br-cGMP reversed this effect. Together, our results indicated that at least in our experimental conditions, NO might operate downstream of hemin promoting adventitious root formation probably in a cGMP-dependent manner.

Mutation of Arabidopsis HY1 causes UV-C hypersensitivity by impairing carotenoid and flavonoid biosynthesis and the down-regulation of antioxidant defence

Previous pharmacological results confirmed that haem oxygenase-1 (HO-1) is involved in protection of cells against ultraviolet (UV)-induced oxidative damage in soybean [Glycine max (L.) Merr.] seedlings, but there remains a lack of genetic evidence. In this study, the link between Arabidopsis thaliana HO-1 (HY1) and UV-C tolerance was investigated at the genetic and molecular levels. The maximum inducible expression of HY1 in wild-type Arabidopsis was observed following UV-C irradiation. UV-C sensitivity was not observed in ho2, ho3, and ho4 single and double mutants. However, the HY1 mutant exhibited UV-C hypersensitivity, consistent with the observed decreases in chlorophyll content, and carotenoid and flavonoid metabolism, as well as the down-regulation of antioxidant defences, thereby resulting in severe oxidative damage. The addition of the carbon monoxide donor carbon monoxide-releasing molecule-2 (CORM-2), in particular, and bilirubin (BR), two catalytic by-products of HY1, partially rescued the UV-C hypersensitivity, and other responses appeared in the hy1 mutant. Transcription factors involved in the synthesis of flavonoid or UV responses were induced by UV-C, but reduced in the hy1 mutant. Overall, the findings showed that mutation of HY1 triggered UV-C hypersensitivity, by impairing carotenoid and flavonoid synthesis and antioxidant defences.

ZmHO-1, a maize haem oxygenase-1 gene, plays a role in determining lateral root development

Previous results revealed that haem oxygenase-1 (HO-1)/carbon monoxide (CO) system is involved in auxin-induced adventitious root formation. In this report, a cDNA for the gene ZmHO-1, encoding an HO-1 protein, was cloned from Zea mays seedlings. ZmHO-1 has a conserved HO signature sequence and shares highest homology with rice SE5 (OsHO-1) protein. We further discovered that N-1-naphthylacetic acid (NAA), haemin, and CO aqueous solution, led to the induction of ZmHO-1 expression as well as the thereafter promotion of lateral root development. These effects were specific for ZmHO-1 since the potent HO-1 inhibitor zinc protoporphyrin IX (ZnPPIX) differentially blocked the above actions. The addition of haemin and CO were able to reverse the auxin depletion-triggered inhibition of lateral root formation as well as the decreased ZmHO-1 transcripts. Molecular evidence showed that the haemin- or CO-mediated the modulation of target genes responsible for lateral root formation, including ZmCDK and ZmCKI2, could be blocked by ZnPPIX. Overexpression of ZmHO-1 in transgenic Arabidopsis plants resulted in promotion of lateral root development as well as the modulation of cell cycle regulatory gene expressions. Overall, our results suggested that a maize HO-1 gene is required for the lateral root formation.

Spatial-specific regulation of root development by phytochromes in Arabidopsis thaliana

Distinct tissues and organs of plants exhibit dissimilar responses to light exposure--cotyledon growth is promoted by light, whereas hypocotyl growth is inhibited by light. Light can have different impacts on root development, including impacting root elongation, morphology, lateral root proliferation and root tropisms. In many cases, light inhibits root elongation. There has been much attention given to whether roots themselves are the sites of photoperception for light that impacts light-dependent growth and development of roots. A number of approaches including photoreceptor localization in planta, localized irradiation and exposure of dissected roots to light have been used to explore the site(s) of light perception for the photoregulation of root development. Such approaches have led to the observation that photoreceptors are localized to roots in many plant species, and that roots are capable of light absorption that can alter morphology and/or gene expression. Our recent results show that localized depletion of phytochrome photoreceptors in Arabidopsis thaliana disrupts root development and root responsiveness to the plant hormone jasmonic acid. Thus, root-localized light perception appears central to organ-specific, photoregulation of growth and development in roots.