Stress responses in alfalfa (Medicago sativa L.) XIX. Transcriptional activation of oxidative pentose phosphate pathway genes at the onset of the isoflavonoid phytoalexin response

6Pgdh polymorphism in wild bulb mite populations: prevalence, environmental correlates and life history trade-offs

Genetic polymorphism in key metabolic genes plays a pivotal role in shaping phenotypes and adapting to varying environments. Polymorphism in the metabolic gene 6-phosphogluconate dehydrogenase (6Pgdh) in bulb mites, Rhizoglyphus robini is characterized by two alleles, S and F, that differ by a single amino acid substitution and correlate with male reproductive fitness. The S-bearing males demonstrate a reproductive advantage. Although the S allele rapidly fixes in laboratory settings, the persistence of polymorphic populations in the wild is noteworthy. This study examines the prevalence and stability of 6Pgdh polymorphism in natural populations across Poland, investigating potential environmental influences and seasonal variations. We found widespread 6Pgdh polymorphism in natural populations, with allele frequencies varying across locations and sampling dates but without clear geographical or seasonal clines. This widespread polymorphism and spatio-temporal variability may be attributed to population demography and gene flow between local populations. We found some correlation between soil properties, particularly cation content (Na, K, Ca, and Mg) and 6Pgdh allele frequencies, showcasing the connection between mite physiology and soil characteristics and highlighting the presence of environment-dependent balancing selection. We conducted experimental fitness assays to determine whether the allele providing the advantage in male-male competition has antagonistic effects on life-history traits and if these effects are temperature-dependent. We found that temperature does not differentially influence development time or juvenile survival in different 6Pgdh genotypes. This study reveals the relationship between genetic variation, environmental factors, and reproductive fitness in natural bulb mite populations, shedding light on the dynamic mechanisms governing 6Pgdh polymorphism.

Changes in the Histology of Walnut (Juglans regia L.) Infected with Phomopsis capsici and Transcriptome and Metabolome Analysis

Phomopsis capsici (P. capsici) causes branch blight of walnuts, which leads to significant economic loss. The molecular mechanism behind the response of walnuts remains unknown. Paraffin sectioning and transcriptome and metabolome analyses were performed to explore the changes in tissue structure, gene expression, and metabolic processes in walnut after infection with P. capsici. We found that P. capsici caused serious damage to xylem vessels during the infestation of walnut branches, destroying the structure and function of the vessels and creating obstacles to the transport of nutrients and water to the branches. The transcriptome results showed that differentially expressed genes (DEGs) were mainly annotated in carbon metabolism and ribosomes. Further metabolome analyses verified the specific induction of carbohydrate and amino acid biosynthesis by P. capsici. Finally, association analysis was performed for DEGs and differentially expressed metabolites (DEMs), which focused on the synthesis and metabolic pathways of amino acids, carbon metabolism, and secondary metabolites and cofactors. Three significant metabolites were identified: succinic semialdehyde acid, fumaric acid, and phosphoenolpyruvic acid. In conclusion, this study provides data reference on the pathogenesis of walnut branch blight and direction for breeding walnut to enhance its disease resistance.

Hypolignification: A Decisive Factor in the Development of Hyperhydricity

One of the characteristics of hyperhydric plants is the reduction of cell wall lignification (hypolignification), but how this is related to the observed abnormalities of hyperhydricity (HH), is still unclear. Lignin is hydrophobic, and we speculate that a reduction in lignin levels leads to more capillary action of the cell wall and consequently to more water in the apoplast. p-coumaric acid is the hydroxyl derivative of cinnamic acid and a precursor for lignin and flavonoids in higher plant. In the present study, we examined the role of lignin in the development of HH in Arabidopsis thaliana by checking the wild-types (Ler and Col-0) and mutants affected in phenylpropanoid biosynthesis, in the gene coding for cinnamate 4-hydroxylase, C4H (ref3-1 and ref3-3). Exogenously applied p-coumaric acid decreased the symptoms of HH in both wild-type and less-lignin mutants. Moreover, the results revealed that exogenously applied p-coumaric acid inhibited root growth and increased the total lignin content in both wild-type and less-lignin mutants. These effects appeared to diminish the symptoms of HH and suggest an important role for lignin in HH.

Impaired chloroplast positioning affects photosynthetic capacity and regulation of the central carbohydrate metabolism during cold acclimation

Photosynthesis and carbohydrate metabolism of higher plants need to be tightly regulated to prevent tissue damage during environmental changes. The intracellular position of chloroplasts changes due to a changing light regime. Chloroplast avoidance and accumulation response under high and low light, respectively, are well known phenomena, and deficiency of chloroplast movement has been shown to result in photodamage and reduced biomass accumulation. Yet, effects of chloroplast positioning on underlying metabolic regulation are less well understood. Here, we analysed photosynthesis together with metabolites and enzyme activities of the central carbohydrate metabolism during cold acclimation of the chloroplast unusual positioning 1 (chup1) mutant of Arabidopsis thaliana. We compared cold acclimation under ambient and low light and found that maximum quantum yield of PSII was significantly lower in chup1 than in Col-0 under both conditions. Our findings indicated that net CO₂ assimilation in chup1 is rather limited by biochemistry than by photochemistry. Further, cold-induced dynamics of sucrose phosphate synthase differed significantly between both genotypes. Together with a reduced rate of sucrose cycling derived from kinetic model simulations our study provides evidence for a central role of chloroplast positioning for photosynthetic and metabolic acclimation to low temperature.

Overexpression of a Cytosolic 6-Phosphogluconate Dehydrogenase Gene Enhances the Resistance of Rice to Nilaparvata lugens

The pentose phosphate pathway (PPP) plays an important role in plant growth and development, and plant responses to biotic and abiotic stresses. Yet, whether the PPP regulates plant defenses against herbivorous insects remains unclear. In this study, we cloned a rice cytosolic 6-phosphogluconate dehydrogenase gene, Os6PGDH1, which encodes the key enzyme catalyzing the third step in the reaction involving the oxidative phase of the PPP, and explored its role in rice defenses induced by brown planthopper (BPH) Nilaparvata lugens. Levels of Os6PGDH1 transcripts were detected in all five examined tissues, with the highest in outer leaf sheaths and lowest in inner leaf sheaths. Os6PGDH1 expression was strongly induced by mechanical wounding, infestation of gravid BPH females, and jasmonic acid (JA) treatment. Overexpressing Os6PGDH1 (oe6PGDH) decreased the height of rice plants and the mass of the aboveground part of plants, but slightly increased the length of plant roots. In addition, the overexpression of Os6PGDH1 enhanced levels of BPH-induced JA, jasmonoyl-isoleucine (JA-Ile), and H₂O₂, but decreased BPH-induced levels of ethylene. Bioassays revealed that gravid BPH females preferred to feed and lay eggs on wild-type (WT) plants over oe6PGDH plants; moreover, the hatching rate of BPH eggs raised on oe6PGDH plants and the fecundity of BPH females fed on these were significantly lower than the eggs and the females raised and fed on WT plants. Taken together, these results indicate that Os6PGDH1 plays a pivotal role not only in rice growth but also in the resistance of rice to BPH by modulating JA, ethylene, and H₂O₂ pathways.

Chlororespiration Serves as Photoprotection for the Photo-Inactivated Oxygen-Evolving Complex in Zostera marina, a Marine Angiosperm

As an alternative electron sink, chlororespiration, comprising the NAD(P)H dehydrogenase complex and plastid terminal plastoquinone oxidase, may play a significant role in sustaining the redox equilibrium between stroma and thylakoid membrane. This study identified a distinct role for chlororespiration in the marine angiosperm Zostera marina, whose oxygen-evolving complex (OEC) is prone to photo-inactivation as a result of its inherent susceptibility to excess irradiation. The strong connectivity between OEC peripheral proteins and key chlororespiratory enzymes, as demonstrated in the interaction network of differentially expressed genes, suggested that the recovery of photo-inactivated OEC was connected with chlororespiration. Chlorophyll fluorescence, transcriptome and Western blot data verified a new physiological role for chlororespiration to function as photoprotection and generate a proton gradient across the thylakoid membrane for the recovery of photo-inactivated OEC. Chlororespiration was only activated in darkness following excess irradiation exposure, which might be related to electron deficiency in the electron transport chain because of the continuous impairment of the OEC. The activation of chlororespiration in Z. marina was prone to proactivity, which was also supported by the further activation of the oxidative pentose-phosphate pathway synthesizing NADPH to meet the demand of chlororespiration during darkness. This phenomenon is distinct from the common assumption that chlororespiration is prone to consuming redundant reducing power during the short transition phase from light to dark.

Signal transduction in artichoke [Cynara cardunculus L. subsp. scolymus (L.) Hayek] callus and cell suspension cultures under nutritional stress

Stimulated production of secondary phenolic metabolites and proline was studied by using cell cultures of artichoke [Cynara cardunculus L. subsp. scolymus (L.) Hayek] submitted to nutritional stress. Artichoke cell cultures accumulated phenolic secondary metabolites in a pattern similar to that seen in artichoke leaves and heads (capitula). This paper shows that both callus and cell suspension cultures under nutritional stress accumulated phenolic compounds and proline, at the same time their biomass production was negatively affected by nutrient deficiency. The results obtained strongly suggest that plant tissues respond to nutrient deprivation by a defensive costly mechanism, which determines the establishment of a mechanism of trade-off between growth and adaptive response. Furthermore, the results of this research suggest that perception of abiotic stress and increased phenolic metabolites are linked by a sequence of biochemical processes that also involves the intracellular free proline and the oxidative pentose phosphate pathway. The main conclusion of this paper is that, once calli and cell suspension cultures respond to nutrient deficiency, in acclimated cells the establishment of a negative correlation between primary metabolism (growth) and secondary metabolism (defence compounds) is observed.

Water deficit mechanisms in perennial shrubs Cerasus humilis leaves revealed by physiological and proteomic analyses

BACKGROUND

Drought (Water deficit, WD) poses a serious threat to extensively economic losses of trees throughout the world. Chinese dwarf cherry (Cerasus humilis) is a good perennial plant for studying the physiological and sophisticated molecular network under WD. The aim of this study is to identify the effect of WD on C. humilis through physiological and global proteomics analysis and improve understanding of the WD resistance of plants.

METHODS

Currently, physiological parameters were applied to investigate C. humilis response to WD. Moreover, we used two-dimensional gel electrophoresis (2DE) to identify differentially expressed proteins in C. humilis leaves subjected to WD (24 d). Furthermore, we also examined the correlation between protein and transcript levels.

RESULTS

Several physiological parameters, including relative water content and Pn were reduced by WD. In addition, the malondialdehyde (MDA), relative electrolyte leakage (REL), total soluble sugar, and proline were increased in WD-treated C. humilis. Comparative proteomic analysis revealed 46 protein spots (representing 43 unique proteins) differentially expressed in C. humilis leaves under WD. These proteins were mainly involved in photosynthesis, ROS scavenging, carbohydrate metabolism, transcription, protein synthesis, protein processing, and nitrogen and amino acid metabolisms, respectively.

CONCLUSIONS

WD promoted the CO2 assimilation by increase light reaction and Calvin cycle, leading to the reprogramming of carbon metabolism. Moreover, the accumulation of osmolytes (i.e., proline and total soluble sugar) and enhancement of ascorbate-glutathione cycle and glutathione peroxidase/glutathione s-transferase pathway in leaves could minimize oxidative damage of membrane and other molecules under WD. Importantly, the regulation role of carbohydrate metabolisms (e. g. glycolysis, pentose phosphate pathways, and TCA) was enhanced. These findings provide key candidate proteins for genetic improvement of perennial plants metabolism under WD.

NADPH from the oxidative pentose phosphate pathway drives the operation of cyclic electron flow around photosystem I in high-intertidal macroalgae under severe salt stress

Pyropia yezoensis (Bangiales, Rhodophyta) is a representative species of high-intertidal macroalgae, whose blades can tolerate extreme stresses, such as salt stress and desiccation. In this study, the photosystem (PS) responses of P. yezoensis blades under salt stress were studied. Our results showed that when the effective photochemical quantum yield of PS (Y) II decreased to almost zero under high salt stress, YI still had a relatively high activity rate. PSII was therefore more sensitive to salt stress than PSI. Furthermore, in the presence of 3-(3', 4'-dichlorophenyl)-1,1-dimethylurea (DCMU), YI rose as salinity increased. The YI values for DCMU-treated thalli decreased in the presence of glucose-6-phosphate dehydrogenase (EC 1.1.1.49, G6PDH) inhibitor (glucosamine, Glucm). The YI values were ∼0.09 in the presence of methyl viologen (MV) and almost zero in the presence of dibromothymoquinone (DBMIB). These results demonstrated that under severe salt stress (120‰ salinity) PSI activity was driven from a source other than PSII, and that stromal reductants probably supported the operation of PSI. Under salt stress, the starch content decreased and soluble sugar levels increased. The G6PDH and 6-phosphogluconate dehydrogenase (EC 1.1.1.44) activities increased, but cytosolic glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.12) activity decreased. Furthermore, the NADPH content increased, but NADH decreased, which suggested that soluble sugar entered the oxidative pentose phosphate pathway (OPPP). All these results suggested that NADPH from OPPP increases the cyclic electron flow around PSI in high-intertidal macroalgae under severe salt stress.

Carbon Fluxes between Primary Metabolism and Phenolic Pathway in Plant Tissues under Stress

Higher plants synthesize an amazing diversity of phenolic secondary metabolites. Phenolics are defined secondary metabolites or natural products because, originally, they were considered not essential for plant growth and development. Plant phenolics, like other natural compounds, provide the plant with specific adaptations to changing environmental conditions and, therefore, they are essential for plant defense mechanisms. Plant defensive traits are costly for plants due to the energy drain from growth toward defensive metabolite production. Being limited with environmental resources, plants have to decide how allocate these resources to various competing functions. This decision brings about trade-offs, i.e., promoting some functions by neglecting others as an inverse relationship. Many studies have been carried out in order to link an evaluation of plant performance (in terms of growth rate) with levels of defense-related metabolites. Available results suggest that environmental stresses and stress-induced phenolics could be linked by a transduction pathway that involves: (i) the proline redox cycle; (ii) the stimulated oxidative pentose phosphate pathway; and, in turn, (iii) the reduced growth of plant tissues.

Enzyme activity highlights the importance of the oxidative pentose phosphate pathway in lipid accumulation and growth of Phaeodactylum tricornutum under CO2 concentration

BACKGROUND

Rising CO2 concentration was reported to increase phytoplankton growth rate as well as lipid productivity. This has raised questions regarding the NADPH supply for high lipid synthesis as well as rapid growth of algal cells.

RESULTS

In this study, growth, lipid content, photosynthetic performance, the activity, and expression of key enzymes in Calvin cycle and oxidative pentose phosphate pathway (OPPP) were analyzed in the marine diatom Phaeodactylum tricornutum under three different CO2 concentrations (low CO2 (0.015 %), mid CO2 (atmospheric, 0.035 %) and high CO2 (0.15 %)). Both the growth rate and lipid content of P. tricornutum increased significantly under the high CO2 concentration. Enzyme activity and mRNA expression of three Calvin cycle-related enzymes (Rubisco, 3-phosphoglyceric phosphokinase (PGK), phosphoribulokinase (PRK)) were also increased under high CO2 cultivation, which suggested the enhancement of Calvin cycle activity. This may account for the observed rapid growth rate. In addition, high activity and mRNA expression of G6PDH and 6PGDH, which produce NADPH through OPPP, were observed in high CO2 cultured cells. These results indicate OPPP was enhanced and might play an important role in lipid synthesis under high CO2 concentration.

CONCLUSIONS

The oxidative pentose phosphate pathway may participate in the lipid accumulation in rapid-growth P. tricornutum cells in high CO2 concentration.

Positive correlation between PSI response and oxidative pentose phosphate pathway activity during salt stress in an intertidal macroalga

Studies have demonstrated that photosynthetic limitations and starch degradation are responses to stress; however, the relationship between the two is seldom described in detail. In this article, the effects of salt stress on photosynthesis, the levels of NADPH and total RNA, the starch content and the activities of glucose-6-phosphate dehydrogenase (G6PDH) and ribulose-5-phosphate kinase (RPK) were evaluated. In thalli that underwent salt treatments, the cyclic electron flow through PSI showed greater stress tolerance than the flow through PSII. Even though the linear electron flow was suppressed by DCMU, the cyclic electron flow still operated. The electron transport rate I (ETRI) increased as the salinity increased when the thalli recovered in seawater containing DCMU. These results suggested that PSI receives electrons from a source other than PSII. Furthermore, the starch content and RPK activity decreased, while the content of NADPH and total RNA, and the activity of G6PDH increased under salt stress. Soluble sugar from starch degradation may enter the oxidative pentose phosphate pathway (OPPP) to produce NADPH and ribose 5-phosphate. Data analysis suggests that NADPH provides electrons for PSI in Ulva prolifera during salt stress, the OPPP participates in the stress response and total RNA is synthesized in excess to assist recovery.

Centelloside accumulation in leaves of Centella asiatica is determined by resource partitioning between primary and secondary metabolism while influenced by supply levels of either nitrogen, phosphorus or potassium

In the present study we aimed to investigate the relevance of either N, P or K supply for herb and leaf yield and for centelloside concentrations in Centella asiatica L. Urban leaves. In this regard, we elucidated the causal relationship between assimilation rate, leaf N, P and K concentrations, herb and leaf production, and centelloside accumulation. The experiments were conducted consecutively in a greenhouse where C. asiatica was grown in hydroponic culture and fertigated with nutrient solutions at either 0, 30, 60, 100 or 150% of the N, P or K amount in a standard Hoagland solution. In general, the increase in N, P or K supply enhanced assimilation rate and herb and leaf yield. However, exceeding specific thresholds, the high availability of one single nutrient caused lower leaf N concentrations and a decline in assimilation rate and plant growth. Irrespective of N, P and K supply, the leaf centelloside concentrations were negatively associated with herb and leaf yield, which is in accordance with the assumptions of the carbon/nutrient balance and the growth differentiation balance hypotheses. Moreover, we found strong negative correlations between saponins and leaf N concentrations, while the respective sapogenins were negatively correlated with K concentrations. Using C. asiatica as model system, our experiments reveal for the first time that the accumulation of saponins and sapogenins is affected by resource allocation between primary and secondary metabolism and that besides carbon, also nutrient availability is relevant for the regulation of the centelloside synthesis. Finally, our results highlight the huge potential of optimized and carefully controlled mineral nutrition of medicinal plants for steering the bio-production of high-quality natural products.

Physiological and proteomic analysis of salinity tolerance in Puccinellia tenuiflora

Soil salinity poses a serious threat to agriculture productivity throughout the world. Studying mechanisms of salinity tolerance in halophytic plants will provide valuable information for engineering plants for enhanced salt tolerance. Monocotyledonous Puccinellia tenuiflora is a halophytic species that widely distributed in the saline-alkali soil of the Songnen plain in northeastern China. Here we investigate the molecular mechanisms underlying moderate salt tolerance of P. tenuiflora using a combined physiological and proteomic approach. The changes in biomass, inorganic ion content, osmolytes, photosynthesis, defense-related enzyme activities, and metabolites in the course of salt treatment were analyzed in the leaves. Comparative proteomic analysis revealed 107 identities (representing 93 unique proteins) differentially expressed in P. tenuiflora leaves under saline conditions. These proteins were mainly involved in photosynthesis, stress and defense, carbohydrate and energy metabolism, protein metabolism, signaling, membrane, and transport. Our results showed that reduction of photosynthesis under salt treatment was attributed to the down-regulation of the light-harvesting complex (LHC) and Calvin cycle enzymes. Selective uptake of inorganic ions, high K(+)/Na(+) ratio, Ca(2+) concentration changes, and an accumulation of osmolytes contributed to ion balance and osmotic adjustment in leaf cells. Importantly, P. tenuiflora plants developed diverse reactive oxygen species (ROS) scavenging mechanisms in their leaves to cope with moderate salinity, including enhancement of the photorespiration pathway and thermal dissipation, synthesis of the low-molecular-weight antioxidant α-tocopherol, and an accumulation of compatible solutes. This study provides important information toward improving salt tolerance of cereals.

Analysis of expressed sequence tags derived from a compatible Mycosphaerella fijiensis-banana interaction

Mycosphaerella fijiensis, a hemibiotrophic fungus, is the causal agent of black leaf streak disease, the most serious foliar disease of bananas and plantains. To analyze the compatible interaction of M. fijiensis with Musa spp., a suppression subtractive hybridization (SSH) cDNA library was constructed to identify transcripts induced at late stages of infection in the host and the pathogen. In addition, a full-length cDNA library was created from the same mRNA starting material as the SSH library. The SSH procedure was effective in identifying specific genes predicted to be involved in plant-fungal interactions and new information was obtained mainly about genes and pathways activated in the plant. Several plant genes predicted to be involved in the synthesis of phenylpropanoids and detoxification compounds were identified, as well as pathogenesis-related proteins that could be involved in the plant response against M. fijiensis infection. At late stages of infection, jasmonic acid and ethylene signaling transduction pathways appear to be active, which corresponds with the necrotrophic life style of M. fijiensis. Quantitative PCR experiments revealed that antifungal genes encoding PR proteins and GDSL-like lipase are only transiently induced 30 days post inoculation (dpi), indicating that the fungus is probably actively repressing plant defense. The only fungal gene found was induced 37 dpi and encodes UDP-glucose pyrophosphorylase, an enzyme involved in the biosynthesis of trehalose. Trehalose biosynthesis was probably induced in response to prior activation of plant antifungal genes and may act as an osmoprotectant against membrane damage.

Patterns of gene expression induced by oligoguluronates reveal conserved and environment-specific molecular defense responses in the brown alga Laminaria digitata

* Until now, no information has been available on the transcriptional response to the transduction of defense signals in brown seaweeds that leads to active resistance against pathogens or grazers. * Using a polymerase chain reaction (PCR)-based, subtractive cDNA approach combined with filter arrays we demonstrated that Laminaria digitata exhibits a rapid response to oligoguluronate elicitors. The transcription levels of several genes were validated by quantitative real-time reverse-transcription PCR and further analysed using pharmacological approaches. * Fifty upregulated genes were identified by differential screening in elicited algae over a 24-h time-course. These genes were related to oxidative stress responses, production of antimicrobial secondary compounds or cell wall strengthening. Moreover, pharmacological tests showed that intracellular signal transduction is likely to involve reactive oxygen species. A new oligoguluronate-inducible vanadium-dependent haloperoxidase (vHPO), specific to iodide was also characterized. The transcription of several vHPO genes was shown to be tightly regulated. * Taken together, our data show that early transcriptional defense responses in L. digitata are similar to those in land plants but also include novel defense responses that involve tightly regulated iodine metabolism.

Glucose-6-phosphate dehydrogenase plays a central role in modulating reduced glutathione levels in reed callus under salt stress

In the present study, we investigated the role of glucose-6-phosphate dehydrogenase (G6PDH) in regulating the levels of reduced form of glutathione (GSH) to the tolerance of calli from two reed ecotypes, Phragmites communis Trin. dune reed (DR) and swamp reed (SR), in a long-term salt stress. G6PDH activity was higher in SR callus than that of DR callus under 50-150 mM NaCl treatments. In contrast, at higher NaCl concentrations (300-600 mM), G6PDH activity was lower in SR callus. A similar profile was observed in GSH contents, glutathione reductase (GR) and glutathione peroxidase (GPX) activities in both salt-stressed calli. After G6PDH activity and expression were reduced in glycerol treatments, GSH contents and GR and GPX activity decreased strongly in both calli. Simultaneously, NaCl-induced hydrogen peroxide (H2O2) accumulation was also abolished. Exogenous application of H2O2 increased G6PDH, GR, and GPX activities and GSH contents in the control conditions and glycerol treatment. Diphenylene iodonium (DPI), a plasma membrane (PM) NADPH oxidase inhibitor, which counteracted NaCl-induced H(2)O(2) accumulation, decreased these enzymes activities and GSH contents. Furthermore, exogenous application of H2O2 abolished the N-acetyl-L: -cysteine (NAC)-induced decrease in G6PDH activity, and DPI suppressed the effect of buthionine sulfoximine (BSO) on induction of G6PDH activity. Western-blot analyses showed that G6PDH expression was stimulated by NaCl and H2O2, and blocked by DPI in DR callus. Taken together, G6PDH activity involved in GSH maintenance and H2O2 accumulation under salt stress. And H2O2 regulated G6PDH, GR, and GPX activities to maintain GSH levels. In the process, G6PDH plays a central role.

Glucose-6-phosphate dehydrogenase plays a pivotal role in nitric oxide-involved defense against oxidative stress under salt stress in red kidney bean roots

The pivotal role of glucose-6-phosphate dehydrogenase (G-6-PDH)-mediated nitric oxide (NO) production in the tolerance to oxidative stress induced by 100 mM NaCl in red kidney bean (Phaseolus vulgaris) roots was investigated. The results show that the G-6-PDH activity was enhanced rapidly in the presence of NaCl and reached a maximum at 100 mM. Western blot analysis indicated that the increase of G-6-PDH activity in the red kidney bean roots under 100 mM NaCl was mainly due to the increased content of the G-6-PDH protein. NO production and nitrate reductase (NR) activity were also induced by 100 mM NaCl. The NO production was reduced by NaN(3) (an NR inhibitor), but not affected by N(omega)-nitro-L-arginine (L-NNA) (an NOS inhibitor). Application of 2.5 mM Na(3)PO(4), an inhibitor of G-6-PDH, blocked the increase of G-6-PDH and NR activity, as well as NO production in red kidney bean roots under 100 mM NaCl. The activities of antioxidant enzymes in red kidney bean roots increased in the presence of 100 mM NaCl or sodium nitroprusside (SNP), an NO donor. The increased activities of all antioxidant enzymes tested at 100 mM NaCl were completely inhibited by 2.5 mM Na(3)PO(4). Based on these results, we conclude that G-6-PDH plays a pivotal role in NR-dependent NO production, and in establishing tolerance of red kidney bean roots to salt stress.

NADPH recycling systems in oxidative stressed pea nodules: a key role for the NADP+ -dependent isocitrate dehydrogenase

The symbiosis between legumes and rhizobia is characterised by the formation of dinitrogen-fixing root nodules. In natural conditions, nitrogen fixation is strongly impaired by abiotic stresses which generate over-production of reactive oxygen species. Since one of the nodule main antioxidant systems is the ascorbate-glutathione cycle, NADPH recycling that is involved in glutathione reduction is of great relevance under stress conditions. NADPH is mainly produced by glucose 6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (6PGDH; EC 1.1.1.44) from the oxidative pentose phosphate pathway, and also by NADP(+)-dependent isocitrate dehydrogenase (ICDH; EC 1.1.1.42). In this work, 10 microM paraquat (PQ) was applied to pea roots in order to determine the in vivo relationship between oxidative stress and the activity of the NADPH-generating enzymes in nodules. Whereas G6PDH and 6PGDH activities remained unchanged, a remarkable induction of ICDH gene expression and a dramatic increase of the ICDH activity was observed during the PQ treatment. These results support that ICDH has a key role in NADPH recycling under oxidative stress conditions in pea root nodules.

Molecular cloning and characterization of rice 6-phosphogluconate dehydrogenase gene that is up-regulated by salt stress

The pentose phosphate pathway (PPP) is the important metabolism pathway in plant. In the present study, a cDNA encoding one of the key enzymes of PPP, 6-phosphogluconate dehydrogenase(6PGDH), was isolated from rice and designated as Os6PGDH. The Os6PGDH encoding protein is a cytosolic isoenzyme according to the absence of plastid transit peptide at the N-terminus. The full-length cDNA of 1751 bp encodes 480 amino acids and its putative protein sequence is 94%, 84% and 83% identical to maize, spinach and alfalfa 6PGDHs respectively. Comparison of the cloned mRNA sequence with that of the genomic sequence from the Rice Genome Project showed a simple genomic organization devoid of introns in the translated region of the gene. RT-PCR experiments revealed that Os6PGDH expression was high in inflorescence, low in root and embryos but almost absent in leaves. Furthermore, Os6PGDH was up-regulated in the shoots under salt stress. It is suggested that 6PGDH in plant may play an important role in cell division and salt response.

Phenylpropanoids, phenylalanine ammonia lyase and peroxidases in elicitor-challenged cassava (Manihot esculenta) suspension cells and leaves

BACKGROUND AND AIMS

Control of diseases in the key tropical staple, cassava, is dependent on resistant genotypes, but the innate mechanisms are unknown. The aim was to study phenylpropanoids and associated enzymes as possible defence components.

METHODS

Phenylalanine ammonia-lyase (PAL), phenylpropanoids and peroxidases (POD) were investigated in elicited cassava suspension cells and leaves. Yeast elicitor was the most effective of several microbial and endogenous elicitors. Fungitoxicity was determined against the cassava pathogens Fusarium solani, F. oxysporum and the saprotroph Trichoderma harzianum.

KEY RESULTS

A single and rapid (> or =2-3 min) oxidative burst, measured as hydrogen peroxide, occurred in elicited cells. PAL activity was induced maximally at 15 h and was preceded by PAL mRNA accumulation, which peaked at 9 h. Symplasmic POD activity increased four-fold in cells, 48 h post-elicitation. POD isoforms (2-7 isoforms, pI 3.1-8.8) were detected in elicited and unelicited cells, extracellular medium and leaves but two extracellular isoforms were enhanced post-elicitation. Also expression of a cassava peroxidase gene MecPOD1 increased in elicited cells. Only anionic forms oxidized scopoletin, with highest activity by isoform pI 3.6, present in all samples. Unidentified phenolics and possibly scopolin increased post-elicitation, but there was no enhancement of scopoletin, rutin or kaempferol-3-O-rutinoside concentration. Fungal germ tube elongation was inhibited more than germination by esculetin, ferulic acid, quercetin and scopoletin. T. harzianum was generally more sensitive than the pathogens and was inhibited by > or =50 microg mL(-1) of ferulic acid and quercetin and > or =10 microg mL(-1) of scopoletin.

CONCLUSIONS

Phenolic levels in cells were not enhanced and were, theoretically, too low to be inhibitory. However, in combination and when oxidized they may contribute to defence, because oxidation of esculetin and scopoletin by peroxidase and of esculetin by tyrosinase enhanced their fungitoxicity up to 20-fold.

Double antisense plants lacking ascorbate peroxidase and catalase are less sensitive to oxidative stress than single antisense plants lacking ascorbate peroxidase or catalase

The plant genome is a highly redundant and dynamic genome. Here, we show that double antisense plants lacking the two major hydrogen peroxide-detoxifying enzymes, ascorbate peroxidase (APX) and catalase (CAT), activate an alternative/redundant defense mechanism that compensates for the lack of APX and CAT. A similar mechanism was not activated in single antisense plants that lacked APX or CAT, paradoxically rendering these plants more sensitive to oxidative stress compared to double antisense plants. The reduced susceptibility of double antisense plants to oxidative stress correlated with suppressed photosynthetic activity, the induction of metabolic genes belonging to the pentose phosphate pathway, the induction of monodehydroascorbate reductase, and the induction of IMMUTANS, a chloroplastic homologue of mitochondrial alternative oxidase. Our results suggest that a co-ordinated induction of metabolic and defense genes, coupled with the suppression of photosynthetic activity, can compensate for the lack of APX and CAT. In addition, our findings demonstrate that the plant genome has a high degree of plasticity and will respond differently to different stressful conditions, namely, lack of APX, lack of CAT, or lack of both APX and CAT.

The phenylpropanoid pathway and plant defence-a genomics perspective

Summary The functions of phenylpropanoid compounds in plant defence range from preformed or inducible physical and chemical barriers against infection to signal molecules involved in local and systemic signalling for defence gene induction. Defensive functions are not restricted to a particular class of phenylpropanoid compound, but are found in the simple hydroxycinnamic acids and monolignols through to the more complex flavonoids, isoflavonoids, and stilbenes. The enzymatic steps involved in the biosynthesis of the major classes of phenylpropanoid compounds are now well established, and many of the corresponding genes have been cloned. Less is understood about the regulatory genes that orchestrate rapid, coordinated induction of phenylpropanoid defences in response to microbial attack. Many of the biosynthetic pathway enzymes are encoded by gene families, but the specific functions of individual family members remain to be determined. The availability of the complete genome sequence of Arabidopsis thaliana, and the extensive expressed sequence tag (EST) resources in other species, such as rice, soybean, barrel medic, and tomato, allow, for the first time, a full appreciation of the comparative genetic complexity of the phenylpropanoid pathway across species. In addition, gene expression array analysis and metabolic profiling approaches make possible comparative parallel analyses of global changes at the genome and metabolome levels, facilitating an understanding of the relationships between changes in specific transcripts and subsequent alterations in metabolism in response to infection.

Oxidative pentose phosphate pathway and pyridine nucleotides in relation to heartwood formation in Robinia pseudoacacia L

Most tree species show in the inner parts of their woody axes often a dark colored zone, the heartwood. Its formation is a genetically determined, programmed cell death which is characterized by the activation of metabolic pathways which lead to the formation of phenolic heartwood extractives. In the present paper we report on the key position of the oxidative pentose phosphate pathway (OPP) for this process. The OPP plays a crucial role in anabolic processes and is involved in the interconversion and rearrangements of sugar-phosphates with the net production of NADPH. In tissues of Robinia pseudoacacia L. which are transferred to heartwood, enhanced activities of glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH) are present. A consequence of these increased enzyme activities is a shift in the pyridine nucleotide pool towards NADP+NADPH at the expense of NAD+NADH. These alterations in the metabolism and the redox status probably provide precursors and reduction equivalents being required for the synthesis of heartwood phenolics. The non heartwood forming species Acer pseudoplatanus L. shows neither a radial gradient nor seasonal changes in the amounts of pyridine nucleotides across the trunkwood. The results are discussed in connection with programmed cell death, mitochondrial activity, and heartwood formation.

Purification and cloning of chloroplast 6-phosphogluconate dehydrogenase from spinach. Cyanobacterial genes for chloroplast and cytosolic isoenzymes encoded in eukaryotic chromosomes

Previous attempts to purify chloroplast 6-phosphogluconate dehydrogenase (cp6PGDH), a key enzyme of the oxidative pentose phosphate pathway, have been unsuccessful due to rapid activity loss. An efficient purification protocol was developed and the enzyme from spinach leaves was purified 1000-fold to apparent homogeneity with a specific activity of 60 U.mg-1. The enzyme is a homodimer with subunits of 50 kDa. Antibodies raised against the purified cp6PGDH detected a 53-kDa protein from a crude extract, indicating alterations during purification. Purified cp6PGDH was microsequenced and the corresponding spinach cDNA was cloned using PCR techniques and degenerate primers. The cDNA for cytosolic 6PGDH from spinach was cloned for comparison. Phylogenetic analysis in the context of available homologues from eukaryotes and eubacteria revealed that animal and fungal cytosolic 6PGDH sequences are more similar to their homologues from gamma-proteobacteria, whereas plant 6PGDH is more similar to its cyanobacterial homologues. The ancestral gene for higher plant 6PGDH was acquired from the antecedent of plastids through endosymbiosis and gene transfer to the nucleus. A subsequent gene duplication gave rise to higher plant cytosolic 6PGDH, which assumed the function of its pre-existing cytosolic homologue through endosymbiotic gene replacement. The protein phylogeny of both 6PGDH and of the first enzyme of the oxidative pentose phosphate pathway, glucose-6-phosphate dehydrogenase, indicate a surprisingly close relationship between the plant and Trypanosoma brucei lineages, suggesting that T. brucei (a relative of Euglena gracilis) may be secondarily nonphotosynthetic.

Isolation and characterization of novel defence-related genes induced by copper, salicylic acid, methyl jasmonate, abscisic acid and pathogen infection in Brassica carinata

Summary To examine the defence response in Brassica carinata we differentially screened a cDNA library made from CuCl(2)-treated (Cu) leaves. The sequence of 17 of the 27 cDNA clones examined that showed Cu-induction had a high similarity to defence genes from other plant species. Among other clones that showed higher expression in the Cu leaves were two cDNAs encoding polypeptides of 351 and 250 amino acids, designated BcCJS1 and BcCJAS1. BcCJS1 had similarity to S-adenosyl-l-methionine: salicylic acid carboxyl methyltransferase from Clarkia breweri. However, the enzyme activity was not found in extracts from E. coli expressing BcCJS1. BcCJAS1 did not show extensive similarity to any genes with known function in the databases but it did contain three regions of amino acid sequence that are frequently found in amidotransferases. A third Cu-induced mRNA, Bcp6PGL, showed very high (86%) similarity to a putative 6-phosphogluconolactonase (6PGL) from Arabidopsis thaliana. In addition to Cu induction, BcCJS1 expression was induced by methyl jasmonate (MeJA) and salicylic acid (SA), BcCJAS1 expression by MeJA, SA and abscisic acid and Bcp6PGL expression by MeJA. The expression of all three genes increased after Alternaria brassicae infection. BcCJS1 and BcCJAS1 were induced within 1 h after MeJA- but not until 3 h after SA-treatment. The expression of both genes was systemically induced after infection with a compatible or incompatible fungal pathogen. SA systemically induced only BcCJAS1. The effects of various inhibitors of signalling pathways on expression of the three genes were studied.

Specific expression of glucose-6-phosphate dehydrogenase (G6PDH) gene by salt stress in wheat (Triticum aestivum L.)

We isolated three kinds of full-length cDNA clones for glucose-6-phosphate dehydrogenase (G6PDH) from wheat. They showed over 80% sequence homology with potato and alfalfa at the amino acid level, suggesting that the genes are highly conserved in angiosperms. The lack of a plastidic signal sequence, as well as a higher transcript accumulation level in roots than leaves, suggested that the wheat cDNA sequences encode the cytosolic isoform of the enzyme. Genomic Southern analysis revealed that the isoform is encoded by a few copies of the gene in the wheat genome. Southern analysis of nullisomic-tetrasomic lines suggested that at least one gene is located on chromosome 2B. The G6PDH transcript started to accumulate in the roots after 2 h of 0.15 M NaCl treatment, reaching its maximal level after 12 h of exposure. The G6PDH gene did not respond to either mannitol or abscisic acid (ABA) treatment. The NaCl-specific early response of the G6PDH gene is discussed in relation to the putative role of the enzyme in the salt stress response in plants.

Iron-superoxide dismutase expression in transgenic alfalfa increases winter survival without a detectable increase in photosynthetic oxidative stress tolerance

To determine whether overexpression of Fe-superoxide (SOD) dismutase would increase superoxide-scavenging capacity and thereby improve the winter survival of transgenic alfalfa (Medicago sativa L.) plants, two genotypes were transformed with the vector pEXSOD10, which contains a cDNA for Arabidopsis Fe-SOD with a chloroplast transit peptide and cauliflower mosaic virus 35S promoter. A novel Fe-SOD was detected by native PAGE in both greenhouse- and field-grown transgenic plants, but activity varied among independent transgenic plants. The increased Fe-SOD activity was associated with increased winter survival over 2 years in field trials, but not with oxidative stress tolerance as measured by resistance of leaves to methyl viologen, a superoxide generator. Total shoot dry matter production over 2 harvest years was not associated with Fe-SOD activity. There was no detectable difference in the pattern of primary freezing injury, as shown by vital staining, nor was there additional accumulation of carbohydrates in field-acclimated roots of the transgenic alfalfa plants. We did not detect any difference in growth of one transgenic plant with high Fe-SOD activity compared with a non-transgenic control. Therefore, the improvement in winter survival did not appear to be a consequence of improved oxidative stress tolerance associated with photosynthesis, nor was it a consequence of a change in primary freezing injury. We suggest that Fe-SOD overexpression reduced secondary injury symptoms and thereby enhanced recovery from stresses experienced during winter.

Prospects for the metabolic engineering of bioactive flavonoids and related phenylpropanoid compounds

The successful engineering of complex metabolic pathways will require, in addition to availability of cloned genes and promoters, knowledge of the regulatory mechanisms that control metabolic flux into the pathway including post-translational phenomena such as metabolite channeling. We are interested in modifying pathways for the synthesis of isoflavonoids and other bioactive phenylpropanoid compounds in transgenic plants. We describe studies on flux control utilizing transgenic tobacco plants that under- and over-express key biosynthetic enzymes, and outline experimental approaches for the molecular dissection of potential metabolic channels in the synthesis of antimicrobial flavonoid derivatives in alfalfa and other species.

Extensive reprogramming of primary and secondary metabolism by fungal elicitor or infection in parsley cells

The transcription rates of numerous plant genes have previously been shown to be strongly affected by pathogen infection or elicitor treatment. Here we estimate the extent and complexity of this response by analyzing the patterns of mRNA induction in fungal elicitor-treated parsley cells (Petroselinum crispum) for several representatives from various primary and secondary metabolic pathways, cytosolic as well as plastidic. As a reference, we use the biphasic accumulation curve for the coordinately induced mRNAs encoding the three core enzymes of general phenylpropanoid metabolism, phenylalanine ammonia-lyase, cinnamate 4-hydroxylase and 4-coumarate:CoA ligase. Coincidence with this curve was observed for the mRNA induction kinetics of several, but not all, phenylpropanoid branch pathway-related reactions, whereas seven selected mRNAs from the pentose phosphate, glycolytic and shikimate pathways, including various cytosolic and plastidic isoforms, were induced with great differences in timing. Likewise unique and dissimilar from the reference curve were the induction patterns for various mRNAs encoding enzymes or proteins that are either more distantly or not at all related to phenylpropanoid metabolism. None of over 40 mRNAs tested so far remained unaffected. Using one strongly elicitor-responsive mRNA from carbohydrate metabolism, encoding a cytosolic glucose 6-phosphate dehydrogenase, for in situ RNA/RNA hybridization in fungus-infected parsley leaf tissue, we observed again the previously reported, close simulation of metabolic changes in true plant/fungus interactions by elicitor treatment of cultured cells. In addition to demonstrating extensive, highly complex functional, temporal and spatial patterns of changes in gene expression in infected plant cells, these results provide valuable information for the identification of pathogen-responsive promoters suitable for gene technology-assisted resistance breeding.

Methylation reactions and the phytoalexin response in alfalfa suspension cultures

In order to determine why the activated methyl cycle is up-regulated in plants undergoing defence responses to fungal pathogens we have monitored the utilisation of methyl groups derived from methionine in cell-suspension cultures of alfalfa (Medicago sativa L.) treated for various times with fungal elicitor, by carrying out a parallel labelling study with [(35)S]methionine and [methyl-(3)H]methionine. The distribution of the two radiolabels among the medium, soluble cellular components and cell wall was then determined. In the absence of elicitor the utilisation of the two radiolabels was similar. However, in the presence of the elicitor the total incorporation of radioactivity from [methyl-(3)H]methionine into metabolites was far greater than from [(35)S]methionine, indicating that the methyl label had been utilised in methylation reactions. Elicitor treatment resulted in up to a sixfold increase in the use of (3)H-methyl groups in the methylation of hydrophobic metabolites. In the period 0-24 h after elicitor treatment, increased methylation was directed largely into the synthesis of the isoflavonoid phytoalexin medicarpin and related metabolites. Newly synthesized phytoalexins were exported into the medium, while a significant proportion of the medicarpin accumulating in the cell in the early stages of elicitation was derived from the hydrolysis of its respective conjugate. Elicitor treatment also modified the incorporation of (3)H-methyl groups into the cell wall. Between 0 and 24 h after elicitor treatment the methylation of pectin in the cell wall declined. After 24 h, pectin methylation recovered and was associated with an increase in the methylation of other wall-bound polysaccharide components. Since no other major metabolic sink for the increased methylation was determined we conclude that the increased activity of the activated methyl cycle during defence interactions in alfalfa is required to support phytoalexin synthesis and cell wall modifications.

Glutathione metabolism and protection against oxidative stress caused by peroxides in Penicillium chrysogenum

The filamentous fungus Penicillium chrysogenum showed remarkable resistance to the oxidative stress caused by high concentrations of either hydrogen peroxide (0.35-0.70 M) or tert-butyl hydroperoxide (tert-BOOH, 0.5-2.0 mM), which could be explained well with high levels of glutathione (GSH) peroxidase and catalase activities. The majority of exogenous H2O2 was likely removed by catalase from the cells while tert-BOOH was likely eliminated mainly by the GSH-dependent pathways. The GSH pool decreased considerably at high tert-BOOH concentrations, the glutathione disulphide (GSSG) pool increased at high H2O2 and tert-BOOH concentrations, meanwhile all the peroxide concentrations tested increased markedly the intracellular peroxide concentration. All the enzyme activities taking part in the glutathione metabolism (glutathione peroxidase, glutathione reductase, gamma-glutamyltranspeptidase and glutathione producing activities) except glutathione S-transferase increased significantly after exposing mycelia to both peroxides while the specific glucose-6-phosphate dehydrogenase and catalase activities remained unchanged. In the presence of 0.5 mM diamide both GSSG and GSH concentrations as well as the glutathione reductase and glutathione producing activities were elevated but no significant changes were found in the intracellular peroxide concentration or in any of the other enzyme activities examined.

Stress responses in alfalfa (Medicago sativa L.). XX. Transcriptional activation of phenlpropanoid pathway genes in elicitor-induced cell suspension cultures

Nuclear transcript run-on analysis was used to investigate++ the relative transcription rates of genes encoding enzymes of isoflavonoid phytoalexin biosynthesis and related pathways in elicitor-treated alfalfa (Medicago sativa L.) cell suspension cultures. Genes encoding L-phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS) and chalcone reductase (CHR) were most rapidly activated, with increases in transcription measurable within 10-20 min after elicitation. Cinnamic acid 4-hydroxylase (C4H), chalcone isomerase (CHI), isoflavone reductase (IFR) and caffeic acid 3-0-methyltransferase (COMT) genes were also rapidly activated, but at a slower initial rate. Transcription of chalcone 2'-O-methyltransferase (CHOMT), and 1,3-beta-D-glucanase genes was less rapid, with lag periods of 60 and 30 min post-elicitation, respectively. Treatment of cells with a PAL inhibitor L-alpha-aminooxy-beta-phenylpropionic acid (AOPP) resulted in increased transcription of PAL, CHS and CHR, but reduced transcription of CHOMT, indicating a role for phenylpropanoid products as both negative and positive regulators of gene expression within the phenylpropanoid pathway.