Isolation of spinach leaf peroxisomes in 0.25 molar sucrose solution by percoll density gradient centrifugation

Sphingomonas sp. Hbc-6 alters Arabidopsis metabolites to improve plant growth and drought resistance by manipulating the microbiome

Drought significantly affects crop productivity and poses a considerable threat to agricultural ecosystems. Plant growth-promoting bacteria (PGPB) and plant microbiome play important roles in improving drought resistance and plant performance. However, the response of the rhizosphere microbiota to PGPB during the development of plants and the interaction between inoculum, microbiota, and plants under drought stress remain to be explored. In the present study, we used culturomic, microbiomic, and metabonomic analyses to uncover the mechanisms by which Sphingomonas sp. Hbc-6, a PGPB, promotes Arabidopsis growth and enhances drought resistance. We found that the rhizosphere microbiome assembly was interactively influenced by developmental stage, Hbc-6, and drought; the bacterial composition exhibited three patterns of shifts with developmental stage: resilience, increase, and decrease. Drought diminished microbial diversity and richness, whereas Hbc-6 increased microbial diversity and helped plants recruit specific beneficial bacterial taxa at each developmental stage, particularly during the bolting stage. Some microorganisms enriched by Hbc-6 had the potential to promote carbon and nitrogen cycling processes, and 86.79 % of the isolated strains exhibited PGP characteristics (for example Pseudomonas sp. TA9). They jointly regulated plant physiological metabolism (i.e., upregulated drought resistant-facilitating substances and reduced harmful substances), thereby stimulating the growth of Arabidopsis and increasing plant biomass under drought stress conditions. Collectively, these results indicate that Hbc-6 mediates plant growth and drought resistance by affecting the microbiome. The study thus provides novel insights and strain resources for drought-resistant, high-yielding crop cultivation and breeding.

Sphingomonas sp. Hbc-6 alters physiological metabolism and recruits beneficial rhizosphere bacteria to improve plant growth and drought tolerance

Drought poses a serious threat to plant growth. Plant growth-promoting bacteria (PGPB) have great potential to improve plant nutrition, yield, and drought tolerance. Sphingomonas is an important microbiota genus that is extensively distributed in the plant or rhizosphere. However, the knowledge of its plant growth-promoting function in dry regions is extremely limited. In this study, we investigated the effects of PGPB Sphingomonas sp. Hbc-6 on maize under normal conditions and drought stress. We found that Hbc-6 increased the biomass of maize under normal conditions and drought stress. For instance, the root fresh weight and shoot dry weight of inoculated maize increased by 39.1% and 34.8% respectively compared with non-inoculated plant, while they increased by 61.3% and 96.3% respectively under drought conditions. Hbc-6 also promoted seed germination, maintained stomatal morphology and increased chlorophyll content so as to enhance photosynthesis of plants. Hbc-6 increased antioxidant enzyme (catalase, superoxide, peroxidase) activities and osmoregulation substances (proline, soluble sugar) and up-regulated the level of beneficial metabolites (resveratrol, etc.). Moreover, Hbc-6 reshaped the maize rhizosphere bacterial community, increased its richness and diversity, and made the rhizosphere bacterial community more complex to resist stress; Hbc-6 could also recruit more potentially rhizosphere beneficial bacteria which might promote plant growth together with Hbc-6 both under normal and drought stress. In short, Hbc-6 increased maize biomass and drought tolerance through the above ways. Our findings lay a foundation for exploring the complex mechanisms of interactions between Sphingomonas and plants, and it is important that Sphingomonas sp. Hbc-6 can be used as a potential biofertilizer in agricultural production, which will assist finding new solutions for improving the growth and yield of crops in arid areas.

Changes of resveratrol and antioxidant enzymes during UV-induced plant defense response in peanut seedlings

Plants have evolved mechanisms to avoid and repair UV radiation damage, and the free radicals caused by UV tend to be involved in the induction of antioxidant defense systems. In this study, changes in resveratrol and antioxidant enzymes were investigated in relation to UV damage in peanut seedlings. Accumulation of endogenous resveratrol and stilbene synthase mRNA occurred rapidly and significantly in response to UV-C irradiation. Applying resveratrol before UV-C irradiation mitigated rusty spots and wilting of peanut leaves, and inhibition of resveratrol by applying 3,4-methylenedioxycinnamic acid worsened UV-C damage, an effect that was found to be concentration dependent. Correspondingly, the effect of resveratrol on malondialdehyde was similar to changes in the apparent morphology of seedling leaves. Changes in H(2)O(2), O(2)(-), and antioxidant enzymes showed some similarities after either UV-C irradiation or resveratrol treatment. Activities of superoxide dismutases, glutathione reductase, and catalase were more than 2-fold higher during the first 1h after treatments. Ascorbate peroxidase activity increased to more than 3-fold higher 24h after irradiation, whereas it was more than 2-fold higher 8h after resveratrol treatment. Activities of dehydroascorbate reductase and monodehydroascorbate reductase increased by 40% during 8-24h after treatments. Consequently, we proposed that changes in endogenous resveratrol and in antioxidant enzymes may have been involved in oxidative stress induced by UV-C exposure in peanut seedlings.

Protein organization in the matrix of leaf peroxisomes. A multi-enzyme complex involved in photorespiratory metabolism

This report is an investigation on how the compartmentation of peroxisomal metabolism, involved in the photorespiratory cycle, is accomplished. With isolated peroxisomes from spinach leaves the conversion of serine to glycerate, as coupled to the conversion of glycolate to glycine, was measured. Not only with intact but also with osmotically shocked peroxisomes, which had retained the aggregated state of the peroxisomal matrix but lost the integrity of the boundary membrane, the rates of glycerate synthesis were as high as required for the photorespiratory cycle in vivo. With both intact and shocked peroxisomes the intermediates glyoxylate and hydroxypyruvate did not equilibrate with the medium. It appears from these results that the apparent compartmentation of peroxisomal metabolism is not due to the function of the boundary membrane but to the organization of peroxisomal enzymes in multi-enzyme complexes. When glycolate was added to peroxisomes without transamination partners, glyoxylate was released from the peroxisomes while the peroxisomal matrix partially disintegrated. With solubilized peroxisomes a partial reconstitution of functional enzyme complexes was achieved by the addition of poly(ethylene glycol). The function of the apparently very stable peroxisomal multi-enzyme complexes in protecting the cells from the toxic intermediates H2O2 and glyoxylate is discussed.

Inhibition of plant and animal cytochrome oxidases by nitrous oxide as a function of cytochrome c concentration

Cytochrome oxidase from both pea leaves and bovine heart shows lower activity under a mixture of 79% N2O/21% O2 than under ambient air. This inhibition is not detectable below 5 microM cytochrome c but appears with increasing concentrations of cytochrome c. These results suggest that the N2O-induced inhibition of cytochrome c oxidase is modulated by cytochrome c concentration. This seems to concern only the lowest affinity site of the oxidase. Apparently, N2O and cytochrome c do not share the same site of fixation on the oxidase.

Compartmentation studies on spinach leaf peroxisomes : evidence for channeling of photorespiratory metabolites in peroxisomes devoid of intact boundary membrane

In concurrence with earlier results, the following enzymes showed latency in intact spinach (Spinacia oleracea L.) leaf peroxisomes: malate dehydrogenase (89%), hydroxypyruvate reductase (85%), serine glyoxylate aminotransferase (75%), glutamate glyoxylate aminotransferase (41%), and catalase (70%). In contrast, glycolate oxidase was not latent. Aging of peroxisomes for several hours resulted in a reduction in latency accompanied by a partial solubilization of the above mentioned enzymes. The extent of enzyme solubilization was different, being highest with glutamate glyoxylate aminotransferase and lowest with malate dehydrogenase. Osmotic shock resulted in only a partial reduction of enzyme latency. Electron microscopy revealed that the osmotically shocked peroxisomes remained compact, with smaller particle size and pleomorphic morphology but without a continuous boundary membrane. Neither in intact nor in osmotically shocked peroxisomes was a lag phase observed in the formation of glycerate upon the addition of glycolate, serine, malate, and NAD. Apparently, the intermediates, glyoxylate, hydroxypyruvate, and NADH, were confined within the peroxisomal matrix in such a way that they did not readily leak out into the surrounding medium. We conclude that the observed compartmentation of peroxisomal metabolism is not due to the peroxisomal boundary membrane as a permeability barrier, but is a function of the structural arrangement of enzymes in the peroxisomal matrix allowing metabolite channeling.

Photoinactivation of catalase in vitro and in leaves

Purified catalase from bovine liver and catalase of isolated intact peroxisomes from rye leaves were inactivated in vitro by irradiation with visible light. During photoinactivation the protein moiety of pure catalase was not cleaved; however, the electrophoretic mobility of the native enzyme was decreased, and a major portion of enzyme-bound heme was dissociated. In a suspension of isolated chloroplasts photoinactivation of pure or peroxisomal catalase was mediated by light absorption in the chloroplasts. Both the direct and the chloroplast-mediated photoinactivation of catalase were affected little by the presence of D2O or superoxide dismutase but were greatly retarded by formate. In isolated peroxisomes substantial photoinactivation of catalase occurred only in the presence of nonphotosynthesizing but not in the presence of photosynthesizing isolated chloroplasts. Substantial and selective photoinactivation of catalase was also observed in vivo when leaf sections from various plant species (rye, pea, sunflower, cucumber, maize) were irradiated with light of high intensity in the presence of the translation inhibitors cycloheximide or 2-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide, while catalase activity was much less or not affected in 3-(3,4-dichlorophenyl)-1,1-dimethylurea-treated or untreated control sections. The extent of photoinactivation of catalase in leaves depended on light intensity and also occurred in red light. The results suggest that photoinactivation of catalase generally occurs in leaves under high light intensity, though it is not apparent under normal physiological conditions because it is compensated for by new synthesis. Apparent photoinactivation of catalase has to be regarded as an early indication of photodamage in leaves and conceivably enhances its progress.

Conversion of serine to glycerate in intact spinach leaf peroxisomes: role of malate dehydrogenase

In photorespiration, leaf peroxisomes convert serine to glycerate via serine-glyoxylate aminotransferase and NADH-hydroxypyruvate reductase. We isolated intact spinach leaf peroxisomes in 0.25 M sucrose, and characterized their enzymatic conversion of serine to glycerate using physiological concentrations of substrates and coenzymes. In the presence of glycolate (glyoxylate), and NADH and NAD alone or together in physiological proportions, the rate of serine-to-glycerate conversion was enhanced and sustained by the addition of malate. The rate was similar at 1 and 5 mM serine, but was two to three times higher in 50 mM than 5 mM malate. In the presence of NAD and malate, there was 1:1 stoichiometric formation of glycerate and oxaloacetate. Addition of 1 or 5 mM glutamate resulted in a negligible enhancement of the conversion of hydroxypyruvate to glycerate. Intact peroxisomes produced glycerate from either serine or hydroxypyruvate at a rate two times higher than osmotically lysed peroxisomes. These results suggest that under physiological conditions, the peroxisomal malate dehydrogenase operates independent of aspartate-alpha-ketoglutarate aminotransferase in supplying NADH for hydroxypyruvate reduction. This supply of NADH is the rate-limiting step in the conversion of serine to glycerate. The compartmentation of hydroxypyruvate reductase and malate dehydrogenase in the peroxisomes confers a higher efficiency in the supply of NADH for hydroxypyruvate reduction under a normal, high NAD/NADH ratio in the cytosol.

Effects of CO(2) Enrichment and Carbohydrate Content on the Dark Respiration of Soybeans

During the period of most active leaf expansion, the foliar dark respiration rate of soybeans (Glycine max cv Williams), grown for 2 weeks in 1000 microliters CO(2) per liter air, was 1.45 milligrams CO(2) evolved per hour leaf density thickness, and this was twice the rate displayed by leaves of control plants (350 microliters CO(2) per liter air). There was a higher foliar nonstructural carbohydrate level (e.g. sucrose and starch) in the CO(2) enriched compared with CO(2) normal plants. For example, leaves of enriched plants displayed levels of nonstructural carbohydrate equivalent to 174 milligrams glucose per gram dry weight compared to the 84 milligrams glucose per gram dry weight found in control plant leaves. As the leaves of CO(2) enriched plants approached full expansion, both the foliar respiration rate and carbohydrate content of the CO(2) enriched leaves decreased until they were equivalent with those same parameters in the leaves of control plants. A strong positive correlation between respiration rate and carbohydrate content was seen in high CO(2) adapted plants, but not in the control plants.Mitochondria, isolated simultaneously from the leaves of CO(2) enriched and control plants, showed no difference in NADH or malate-glutamate dependent O(2) uptake, and there were no observed differences in the specific activities of NAD(+) linked isocitrate dehydrogenase and cytochrome c oxidase. Since the mitochondrial O(2) uptake and total enzyme activities were not greater in young enriched leaves, the increase in leaf respiration rate was not caused by metabolic adaptations in the leaf mitochondria as a response to long term CO(2) enrichment. It was concluded, that the higher respiration rate in the enriched plant's foliage was attributable, in part, to a higher carbohydrate status.

Reactivity of glyoxylate with hydrogen perioxide and simulation of the glycolate pathway of C3 plants and Euglena

The nonenzymatic reaction of glyoxylate and H2O2 was measured under physiological conditions of the pH and concentrations of reactants. The reaction of glyoxylate and H2O2 was secondorder, with a rate constant of 2.27 l mol(-1) s(-1) at pH 8.0 and 25° C. The rate constant increased by 4.4 times in the presence of Zn(2+) and doubled at 35°C. We propose a mechanism for the reaction between glyoxylate and H2O2. From a comparison of the rates of H2O2 decomposition by catalase and the reaction with glyoxylate, we conclude that H2O2 produced during glycolate oxidation in peroxisomes is decomposed by catalase but not by the reaction with glyoxylate, and that photorespiratory CO2 originates from glycine, but not from glyoxylate, in C3 plants. Simulation using the above rate constant and reported kinetic parameters leads to the same conclusion, and also makes it clear that alanine is a satisfactory amino donor in the conversion of glyoxylate to glycine. Some serine might be decomposed to give glycine and methylene-tetrahydrofolate; the latter is ultimately oxidized to CO2. In the simulation of the glycolate pathway of Euglena, the rate constant was high enough to ensure the decarboxylation of glyoxylate by H2O2 to produce photorespiratory CO2 during the glycolate metabolism of this organism.

Isolation of mitochondria from soybean leaves on discontinuous percoll gradients

A technique to isolate mitochondria from chamber-grown soybeans (Glycine max cv Williams) was developed. The mitochondria were isolated by centrifugation on discontinuous Percoll gradients which yielded a sharp band of mitochondria contaminated by only 4% of the total chlorophyll in the gradient. Contamination by peroxisomes was also slight. The isolated mitochondria oxidized malate plus glutamate, NADH, and malate with respiratory control. They also showed cyanide-insensitive, alternative pathway activity which was inhibited by salicylhydroxamic acid.

Conversion of glycerate to serine in intact spinach leaf peroxisomes

Intact spinach (Spinacia oleracea L.) leaf peroxisomes converted glycerate to serine in the presence of NAD and alanine. The reaction proceeded optimally at pH9. Addition of oxaloacetate or alpha-ketoglutarate plus aspartate enhanced the conversion about three-fold. Alteration of the concentration of one of the reaction components, consisting of 2 mM glycerate, 0.2 mM NAD, 0.5 mM oxaloacetate, and 2 mM alanine, revealed half-saturation constants of 0.45 mM for glycerate, 0.06 mM for NAD, 0.02 mM for oxaloacetate, and 0.33 mM for alanine. The conversion proceeded with the formation of hydroxypyruvate followed by serine; hydroxypyruvate did not accumulate to a high amount in the presence or absence of alanine. The amino group donor could be alanine (half-saturation constant, 0.33 mM), glycine (0.45 mM), or asparagine (0.67 mM); the three amino acids produced roughly similar Vmax values. The results indicate that, in the conversion of glycerate to serine, the transamination is catalyzed by a hydroxypyruvate aminotransferase with characteristics unknown among all other studied leaf peroxisomal aminotransferases. The peroxisomal membrane is sparsely permeable to NAD/NADH, and the participation of the peroxisomal malate dehydrogenase in an electron shuttle system across the membrane in the regeneration of NAD/NADH is suggested.

Purification of peroxisomes and mitochondria from spinach leaf by percoll gradient centrifugation

A procedure was developed to purify simultaneously peroxisomes and mitochondria from spinach (Spinacia oleracea L.) leaf under isoosmotic and low viscosity conditions. This method involved differential centrifugation and density gradient centrifugation on four layers of Percoll. Chlorophyll-free preparations of highly intact and active organelles were obtained and cross-contamination was negligible. Both organelles were stable for several hours, even if they remained in Percoll. Purified mitochondria were able to carry out the oxidation of different substrates with excellent respiratory control and ADP:O ratios. The method described in the present work was also suitable to purify mitochondria and peroxisomes from potato (Solanum tuberosum L.) tubers.

Glyoxylate transamination in intact leaf peroxisomes

Intact spinach (Spinacia oleracea L.) leaf peroxisomes were supplied with glycolate and one to three of the amino acids serine, glutamate, and alanine, and the amount of the respective alpha-keto acids formed in glyoxylate transamination was assayed. At 1 millimolar glycolate and 1 millimolar each of the three amino acids in combination, the transamination reaction reached saturation; reduction of either glycolate or amino acid concentration decreased the activity. The relative serine, glutamate, and alanine transamination at equal amino acid concentrations was roughly 40, 30, and 30%, respectively. The three amino acids exhibited mutual inhibition to one another in transamination due to the competition for the supply of glyoxylate. In addition to this competition for glyoxylate, competitive inhibition at the active site of enzymes occurred between glutamate and alanine, but not between serine and glutamate or alanine. Alteration of the relative concentrations of the three amino acids changed their relative transamination. Similar work was performed with intact oat (Avena sativa L.) leaf peroxisomes. At 1 millimolar of each of the three amino acids in combination, the relative serine, glutamate, and alanine transamination was roughly 60, 23, and 17%, respectively. Similarly, alteration of the relative concentration of the three amino acids changed their relative transamination. The contents of the three amino acids in leaf extracts were analyzed, and the relative contribution of the three amino acids in glycine production in photorespiration was assessed and discussed.

Metabolism of glycolate and glyoxylate in intact spinach leaf peroxisomes

Intact and broken (osmotically disrupted) spinach (Spinacia oleracea) leaf peroxisomes were compared for their enzymic activities on various metabolites in 0.25 molar sucrose solution. Both intact and broken peroxisomes had similar glycolate-dependent o(2) uptake activity. In the conversion of glycolate to glycine in the presence of serine, intact peroxisomes had twice the activity of broken peroxisomes at low glycolate concentrations, and this difference was largely eliminated at saturating glycolate concentrations. However, when glutamate was used instead of serine as the amino group donor, broken peroxisomes had slightly higher activity than intact peroxisomes. In the conversion of glyoxylate to glycine in the presence of serine, intact peroxisomes had only about 50% of the activity of broken peroxisomes at low glyoxylate concentrations, and this difference was largely overcome at saturating glyoxylate concentrations. In the transamination between alanine and hydroxypyruvate, intact peroxisomes had an activity only slightly lower than that of broken peroxisomes. In the oxidation of NADH in the presence of hydroxypyruvate, intact peroxisomes were largely devoid of activity. These results suggest that the peroxisomal membrane does not impose an entry barrier to glycolate, serine, and O(2) for matrix enzyme activity; such a barrier does exist to glutamate, alanine, hydroxypyruvate, glyoxylate, and NADH. Furthermore, in intact peroxisomes, glyoxylate generated by glycolate oxidase is channeled directly to glyoxylate aminotransferase for a more efficient glycolate-glycine conversion. In related studies, application of in vitro osmotic stress to intact or broken peroxisomes had little effect on their ability to metabolize glycolate to glycine.