Porphyrin biosynthesis control under water stress: sustained porphyrin status correlates with drought tolerance in transgenic rice

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.

A quantitative shotgun proteomics analysis of germinated rice embryos and coleoptiles under low-temperature conditions

Background

At low temperatures, rice grains have a reduced germination rate and grow more slowly, which delays the emergence of rice seedlings from the paddy water surface and significantly increases seedling mortality. In this study, we conducted a shotgun proteomics analysis of geminated embryos and coleoptiles to compare the proteome expression pattern between the low-temperature resistant variety, Tong 88-7, and the low-temperature susceptible variety, Milyang 23.

Results

In a shotgun proteomics analysis of low-temperature resistant and susceptible embryos and coleoptiles in both cold and control temperatures, we discovered a total of 2626 non-redundant proteins, with a 0.01 false discovery rate. A comparison of protein expression patterns between resistant and susceptible embryos and coleoptiles under low-temperature and normal conditions revealed that 85 proteins and 196 proteins were expressed by the resistant and susceptible strains, respectively, in response to low temperature. Among them, 12 proteins overlapped. Proteins involved in stress responses, metabolism, and gene expression were expressed in both strains.

Conclusions

Similar molecular functions of the response were detected, suggesting that the resistant and susceptible strain have a similar proteome response to cold temperatures. The resistance of Tong 88-7 to cold-water germination may result from the efficiency of these proteins, rather than activation of additional or different molecular processes. A comparison of protein expression between the resistant and susceptible strains’ responses revealed that the more successful low-temperature germination of Tong 88-7 was associated with gibberellin signaling, protein trafficking, and the ABA-mediated stress response.

Electronic supplementary material

The online version of this article (doi:10.1186/s12953-015-0082-5) contains supplementary material, which is available to authorized users.

Resolving the Role of Plant NAD-Glutamate Dehydrogenase: III. Overexpressing Individually or Simultaneously the Two Enzyme Subunits Under Salt Stress Induces Changes in the Leaf Metabolic Profile and Increases Plant Biomass Production

NAD-dependent glutamate dehydrogenase (NAD-GDH) of higher plants has a central position at the interface between carbon and nitrogen metabolism due to its ability to carry out the deamination of glutamate. In order to obtain a better understanding of the physiological function of NAD-GDH under salt stress conditions, transgenic tobacco (Nicotiana tabacum L.) plants that overexpress two genes from Nicotiana plumbaginifolia individually (GDHA and GDHB) or simultaneously (GDHA/B) were grown in the presence of 50 mM NaCl. In the different GDH overexpressors, the NaCl treatment induced an additional increase in GDH enzyme activity, indicating that a post-transcriptional mechanism regulates the final enzyme activity under salt stress conditions. A greater shoot and root biomass production was observed in the three types of GDH overexpressors following growth in 50 mM NaCl, when compared with the untransformed plants subjected to the same salinity stress. Changes in metabolites representative of the plant carbon and nitrogen status were also observed. They were mainly characterized by an increased amount of starch present in the leaves of the GDH overexpressors as compared with the wild type when plants were grown in 50 mM NaCl. Metabolomic analysis revealed that overexpressing the two genes GDHA and GDHB, individually or simultaneously, induced a differential accumulation of several carbon- and nitrogen-containing molecules involved in a variety of metabolic, developmental and stress-responsive processes. An accumulation of digalactosylglycerol, erythronate and porphyrin was found in the GDHA, GDHB and GDHA/B overexpressors, suggesting that these molecules could contribute to the improved performance of the transgenic plants under salinity stress conditions.

Differential Antioxidant Responses and Perturbed Porphyrin Biosynthesis after Exposure to Oxyfluorfen and Methyl Viologen in Oryza sativa

We compared antioxidant responses and regulation of porphyrin metabolism in rice plants treated with oxyfluorfen (OF) or methyl viologen (MV). Plants treated with MV exhibited not only greater increases in conductivity and malondialdehyde but also a greater decline in Fv/Fm, compared to plants treated with OF. MV-treated plants had greater increases in activities of superoxide dismutase (SOD) and catalase (CAT) as well as transcript levels of SODA and CATA than OF-treated plants after 28 h of the treatments, whereas increases in ascorbate peroxidase (APX) activity and transcript levels of APXA and APXB were greater in OF-treated plants. Both OF- and MV-treated plants resulted in not only down-regulation of most genes involved in porphyrin biosynthesis but also disappearance of Mg-porphyrins during the late stage of photooxidative stress. By contrast, up-regulation of heme oxygenase 2 (HO2) is possibly part of an efficient antioxidant response to compensate photooxidative damage in both treatments. Our data show that down-regulated biosynthesis and degradation dynamics of porphyrin intermediates have important roles in photoprotection of plants from perturbed porphyrin biosynthesis and photosynthetic electron transport. This study suggests that porphyrin scavenging as well as strong antioxidative activities are required for mitigating reactive oxygen species (ROS) production under photooxidative stress caused by OF and MV.

Translocator proteins, porphyrins and abiotic stress: new light?

After four decades of extensive studies, the role of membrane-bound Translocator proteins (TSPOs) remains unclear and even controversial. In light of recent insights into the structure and activity of TSPOs, showing that they cannot only bind, but also enzymatically photodegrade protoporphyrin IX, we discuss their emerging physiological roles and regulation.

Tetrapyrrole-based drought stress signalling

Tetrapyrroles such as chlorophyll and heme play a vital role in primary plant metabolic processes such as photosynthesis and respiration. Over the past decades, extensive genetic and molecular analyses have provided valuable insights into the complex regulatory network of the tetrapyrrole biosynthesis. However, tetrapyrroles are also implicated in abiotic stress tolerance, although the mechanisms are largely unknown. With recent reports demonstrating that modified tetrapyrrole biosynthesis in plants confers wilting avoidance, a component physiological trait to drought tolerance, it is now timely that this pathway be reviewed in the context of drought stress signalling. In this review, the significance of tetrapyrrole biosynthesis under drought stress is addressed, with particular emphasis on the inter-relationships with major stress signalling cascades driven by reactive oxygen species (ROS) and organellar retrograde signalling. We propose that unlike the chlorophyll branch, the heme branch of the pathway plays a key role in mediating intracellular drought stress signalling and stimulating ROS detoxification under drought stress. Determining how the tetrapyrrole biosynthetic pathway is involved in stress signalling provides an opportunity to identify gene targets for engineering drought-tolerant crops.

Differential antioxidant defense and detoxification mechanisms in photodynamically stressed rice plants treated with the deregulators of porphyrin biosynthesis, 5-aminolevulinic acid and oxyfluorfen

This study focuses on differential molecular mechanisms of antioxidant and detoxification systems in rice plants under two different types of photodynamic stress imposed by porphyrin deregulators, 5-aminolevulinic acid (ALA) and oxyfluorfen (OF). The ALA-treated plants with white necrosis exhibited a greater decrease in photochemical quantum efficiency, Fv/Fm, as well as a greater increase in activity of superoxide dismutase, compared to the OF-treated plants. By contrast, the brown necrosis in OF-treated plants resulted in not only more widely dispersed H2O2 production and greater increases in H2O2-decomposing enzymes, catalase and peroxidase, but also lower ascorbate redox state. In addition, ALA- and OF-treated plants markedly up-regulated transcript levels of genes involved in detoxification processes including transport and movement, cellular homeostasis, and xenobiotic conjugation, with prominent up-regulation of serine/threonine kinase and chaperone only in ALA-treated plants. Our results demonstrate that different photodynamic stress imposed by ALA and OF developed differential actions of antioxidant enzymes and detoxification. Particularly, detoxification system may play potential roles in plant protection against photodynamic stress imposed by porphyrin deregulators, thereby contributing to alleviation of photodynamic damage.

Perturbed porphyrin biosynthesis contributes to differential herbicidal symptoms in photodynamically stressed rice (Oryza sativa) treated with 5-aminolevulinic acid and oxyfluorfen

This paper focuses on the molecular mechanism of deregulated porphyrin biosynthesis in rice plants under photodynamic stress imposed by an exogenous supply of 5-aminolevulinic acid (ALA) and oxyfluorfen (OF). Plants treated with 5 mM ALA or 50 µM OF exhibited differential herbicidal symptoms as characterized by white and brown necrosis, respectively, with substantial increases in cellular leakage and malondialdehyde production. Protoporphyrin IX accumulated to higher levels after 1 day of ALA and OF treatment, whereas it decreased to the control level after 2 days of ALA treatment. Plants responded to OF by greatly decreasing the levels of Mg-protoporphyrin IX (MgProto IX), MgProto IX methyl ester, and protochlorophyllide to levels lower than control, whereas their levels drastically increased 1 day after ALA treatment and then disappeared 2 days after the treatment. Enzyme activity and transcript levels of HEMA1, GSA and ALAD for ALA synthesis greatly decreased in ALA- and OF-treated plants. Transcript levels of PPO1, CHLH, CHLI, and PORB genes involving Mg-porphyrin synthesis continuously decreased in ALA- and OF-treated plants, with greater decreases in ALA-treated plants. By contrast, up-regulation of FC2 and HO2 genes in Fe-porphyrin branch was noticeable in ALA and OF-treated plants 1 day and 2 days after the treatments, respectively. Decreased transcript levels of nuclear-encoded genes Lhcb1, Lhcb6, and RbcS were accompanied by disappearance of MgProto IX in ALA- and OF-treated plants after 2 days of the treatments. Under photodynamic stress imposed by ALA and OF, tight control of porphyrin biosynthesis prevents accumulation of toxic metabolic intermediates not only by down-regulation of their biosynthesis but also by photodynamic degradation. The up-regulation of FC2 and HO2 also appears to compensate for the photodynamic stress-induced damage.

Increased expression of Fe-chelatase leads to increased metabolic flux into heme and confers protection against photodynamically induced oxidative stress

Fe-chelatase (FeCh, EC 4.99.1.1) inserts Fe(2+) into protoporphyrin IX (Proto IX) to form heme, which influences the flux through the tetrapyrrole biosynthetic pathway as well as fundamental cellular processes. In transgenic rice (Oryza sativa), the ectopic expression of Bradyrhizobium japonicum FeCh protein in cytosol results in a substantial increase of FeCh activity compared to wild-type (WT) rice and an increasing level of heme. Interestingly, the transgenic rice plants showed resistance to oxidative stress caused not only by the peroxidizing herbicide acifluorfen (AF) as indicated by a reduced formation of leaf necrosis, a lower conductivity, lower malondialdehyde and H2O2 contents as well as sustained Fv/Fm compared to WT plants, but also by norflurazon, paraquat, salt, and polyethylene glycol. Moreover, the transgenic plants responded to AF treatment with markedly increasing FeCh activity. The accompanying increases in heme content and heme oxygenase activity demonstrate that increased heme metabolism attenuates effects of oxidative stress caused by accumulating porphyrins. These findings suggest that increases in heme levels and porphyrin scavenging capacity support a detoxification mechanism serving against porphyrin-induced oxidative stress. This study also implicates heme as possibly being a positive signal in plant stress responses.

A pepper MSRB2 gene confers drought tolerance in rice through the protection of chloroplast-targeted genes

BACKGROUND

The perturbation of the steady state of reactive oxygen species (ROS) due to biotic and abiotic stresses in a plant could lead to protein denaturation through the modification of amino acid residues, including the oxidation of methionine residues. Methionine sulfoxide reductases (MSRs) catalyze the reduction of methionine sulfoxide back to the methionine residue. To assess the role of this enzyme, we generated transgenic rice using a pepper CaMSRB2 gene under the control of the rice Rab21 (responsive to ABA protein 21) promoter with/without a selection marker, the bar gene.

RESULTS

A drought resistance test on transgenic plants showed that CaMSRB2 confers drought tolerance to rice, as evidenced by less oxidative stress symptoms and a strengthened PSII quantum yield under stress conditions, and increased survival rate and chlorophyll index after the re-watering. The results from immunoblotting using a methionine sulfoxide antibody and nano-LC-MS/MS spectrometry suggest that porphobilinogen deaminase (PBGD), which is involved in chlorophyll synthesis, is a putative target of CaMSRB2. The oxidized methionine content of PBGD expressed in E. coli increased in the presence of H2O2, and the Met-95 and Met-227 residues of PBGD were reduced by CaMSRB2 in the presence of dithiothreitol (DTT). An expression profiling analysis of the overexpression lines also suggested that photosystems are less severely affected by drought stress.

CONCLUSIONS

Our results indicate that CaMSRB2 might play an important functional role in chloroplasts for conferring drought stress tolerance in rice.

Tetrapyrrole biosynthetic enzyme protoporphyrinogen IX oxidase 1 is required for plastid RNA editing

RNA editing is a posttranscriptional process that covalently alters the sequence of RNA molecules and plays important biological roles in both animals and land plants. In flowering plants, RNA editing converts specific cytidine residues to uridine in both plastid and mitochondrial transcripts. Previous studies identified pentatricopeptide repeat (PPR) motif-containing proteins as site-specific recognition factors for cytidine targets in RNA sequences. However, the regulatory mechanism underlying RNA editing was largely unknown. Here, we report that protoporphyrinogen IX oxidase 1 (PPO1), an enzyme that catalyzes protoporphyrinogen IX into protoporphyrin IX in the tetrapyrrole biosynthetic pathway, plays an unexpected role in editing multiple sites of plastid RNA transcripts, most of which encode subunits of the NADH dehydrogenase-like complex (NDH), in the reference plant Arabidopsis thaliana. We identified multiple organellar RNA editing factors (MORFs), including MORF2, MORF8, and MORF9, that interact with PPO1. We found that two conserved motifs within the 22-aa region at the N terminus of PPO1 are essential for its interaction with MORFs, its RNA editing function, and subsequently, its effect on NDH activity. However, transgenic plants lacking key domains for the tetrapyrrole biosynthetic activity of PPO1 exhibit normal RNA editing. Furthermore, MORF2 and MORF9 interact with three PPRs or related proteins required for editing of ndhB and ndhD sites. These results reveal that the tetrapyrrole biosynthetic enzyme PPO1 is required for plastid RNA editing, acting as a regulator that promotes the stability of MORF proteins through physical interaction.

The roles of two transcription factors, ABI4 and CBFA, in ABA and plastid signalling and stress responses

Genetic and physiological studies have revealed evidences for multiple signaling pathways by which the plastid exerts retrograde control over photosynthesis-associated-nuclear-genes. In this study we have examined the mechanisms of control of transcription by plastid signals, focusing on transcription factors. We have also further addressed the physical nature of plastid signals and the physiological role, in stress acclimation of this regulatory pathway. ABI4, a master Apetala 2 (AP2)-type transcription factor (TF), is targeted by multiple signalling pathways in plant cells, such as abscisic acid (ABA) signals, sugar signals and plastid signals derived from reactive oxygen species (ROS) and chlorophyll intermediates. ABI4 binds the promoter of target genes to prevent their transcription by competing with other competitive TFs. However, we found that once ABI4 bound the element (CCACGT), it may not be bound by other TFs, therefore making the signalling long-lasting. Downstream of ABI4, CBFA (CCAAT binding factor A) is a subunit of the HAP2/HAP3/HAP5 (Heme activator protein) trimeric transcription complex. CBFA however is a redundant HAP3 subunit. When emergency occurs (such as herbicide treatments or environmental stresses followed by ABA and ROS accumulation), the master transcription factor ABI4 down-regulates some TFs, like CBFA, and then some other TF subunits enter the transcription complex and transcriptional efficiency of stress-responsive genes (including the transcription co-factor CBP) is improved instantaneously. abi4, cbfA and cbp mutants showed weaker drought-tolerance after a herbicide norflurazon treatment, which indicated the physiological role of these key transcription factors.

Reactive oxygen species, ascorbate-glutathione pool, and enzymes of their metabolism in drought-sensitive and tolerant indica rice (Oryza sativa L.) seedlings subjected to progressing levels of water deficit

Water deficit for rice is a worldwide concern, and to produce drought-tolerant varieties, it is essential to elucidate molecular mechanisms associated with water deficit tolerance. In the present study, we investigated the differential responses of nonenzymatic antioxidants ascorbate (AsA), glutathione (GSH), and their redox pool as well as activity levels of enzymes of ascorbate-glutathione cycle in seedlings of drought-sensitive rice (Oryza sativa L.) cv. Malviya-36 and drought-tolerant cv. Brown Gora subjected to water deficit treatment of -1.0 and -2.1 MPa for 24-72 h using PEG-6000 in sand cultures. Water deficit caused increased production of reactive oxygen species such as O2[Symbol: see text](-), H2O2, and HO[Symbol: see text] in the tissues, and the level of production was higher in the sensitive than the tolerant cultivar. Water deficit caused reduction in AsA and GSH and decline in their redox ratios (AsA/DHA and GSH/GSSG) with lesser decline in tolerant than the sensitive seedlings. With progressive level of water deficit, the activities of monodehydroascorbate reductase, dehydroascorbate reductase, ascorbate peroxidase (APX), and glutathione transferase increased in the seedlings of both rice cultivars, but the increased activity levels were higher in the seedlings of drought-tolerant cv. Brown Gora compared to the sensitive cv. Malviya-36. Greater accumulation of proline was observed in stressed seedlings of tolerant than the sensitive cultivar. In-gel activity staining of APX revealed varying numbers of their isoforms and their differential expression in sensitive and tolerant seedlings under water deficit. Results suggest that an enhanced oxidative stress tolerance by a well-coordinated cellular redox state of ascorbate and glutathione in reduced forms and induction of antioxidant defense system by elevated activity levels of enzymes of ascorbate-glutathione cycle is associated with water deficit tolerance in rice.