Effects of KCN and Salicylhydroxamic Acid on Respiration of Soybean Leaves at Different Ages

Effects of Superoxide Dismutase Inhibitors and Glucose on Cell Death and Generation of Reactive Oxygen Species in Pea Leaves

The effects of superoxide dismutase (SOD) inhibitors, diethyldithiocarbamate (DDC), triethylenetetramine (trien), and their combination with glucose on cells of the epidermis from pea leaves of different age (rapidly growing young leaves and slowly growing old leaves) was investigated. DDC and trien caused death of the guard cells as determined by destruction of their nuclei. Glucose did not affect destruction of the nuclei induced by SOD inhibitors in the cells from old leaves, but intensified it in the cells from young leaves. 2-Deoxyglucose, an inhibitor of glycolysis, and propyl gallate, SOD-mimic and antioxidant, suppressed destruction of the nuclei that was caused by SOD inhibitors and glucose in cells of the epidermis from the young, but not from the old leaves. Glucose and trien stimulated, and propyl gallate reduced generation of reactive oxygen species (ROS) in the pea epidermis as determined by the fluorescence of 2',7'-dichlorofluorescein (DCF). Carbonyl cyanide m-chlorophenylhydrazone (CCCP), a protonophoric uncoupler of oxidative and photosynthetic phosphorylation, suppressed the DCF fluorescence in the guard cells. Treatment of the cells with CCCP followed by its removal with washing increased destruction of the nuclei caused by SOD inhibitors and glucose. In young leaves, CCCP was less effective than in old ones. The findings demonstrate the effects of SOD inhibitors and glucose on the cell death and generation of ROS and could indicate glycolysis-dependent ROS production.

A Functional Approach towards Understanding the Role of the Mitochondrial Respiratory Chain in an Endomycorrhizal Symbiosis

Arbuscular mycorrhizal fungi (AMF) are crucial components of fertile soils, able to provide several ecosystem services for crop production. Current economic, social and legislative contexts should drive the so-called "second green revolution" by better exploiting these beneficial microorganisms. Many challenges still need to be overcome to better understand the mycorrhizal symbiosis, among which (i) the biotrophic nature of AMF, constraining their production, while (ii) phosphate acts as a limiting factor for the optimal mycorrhizal inoculum application and effectiveness. Organism fitness and adaptation to the changing environment can be driven by the modulation of mitochondrial respiratory chain, strongly connected to the phosphorus processing. Nevertheless, the role of the respiratory function in mycorrhiza remains largely unexplored. We hypothesized that the two mitochondrial respiratory chain components, alternative oxidase (AOX) and cytochrome oxidase (COX), are involved in specific mycorrhizal behavior. For this, a complex approach was developed. At the pre-symbiotic phase (axenic conditions), we studied phenotypic responses of Rhizoglomus irregulare spores with two AOX and COX inhibitors [respectively, salicylhydroxamic acid (SHAM) and potassium cyanide (KCN)] and two growth regulators (abscisic acid - ABA and gibberellic acid - Ga3). At the symbiotic phase, we analyzed phenotypic and transcriptomic (genes involved in respiration, transport, and fermentation) responses in Solanum tuberosum/Rhizoglomus irregulare biosystem (glasshouse conditions): we monitored the effects driven by ABA, and explored the modulations induced by SHAM and KCN under five phosphorus concentrations. KCN and SHAM inhibited in vitro spore germination while ABA and Ga3 induced differential spore germination and hyphal patterns. ABA promoted mycorrhizal colonization, strong arbuscule intensity and positive mycorrhizal growth dependency (MGD). In ABA treated plants, R. irregulare induced down-regulation of StAOX gene isoforms and up-regulation of genes involved in plant COX pathway. In all phosphorus (P) concentrations, blocking AOX or COX induced opposite mycorrhizal patterns in planta: KCN induced higher Arum-type arbuscule density, positive MGD but lower root colonization compared to SHAM, which favored Paris-type formation and negative MGD. Following our results and current state-of-the-art knowledge, we discuss metabolic functions linked to respiration that may occur within mycorrhizal behavior. We highlight potential connections between AOX pathways and fermentation, and we propose new research and mycorrhizal application perspectives.

Respiration and Alternative Oxidase in Corn Seedling Tissues during Germination at Different Temperatures

Respiration rates of Zea mays L. seedling tissues grown at 30 and 14 degrees C were measured at 25 degrees C at different stages of seedling growth. Accumulation of heat units was used to define the developmental stages to compare respiration between the two temperatures. At both temperatures, respiration rates of most tissues were highest at the youngest stages, then declined with age. Respiration rates of mesocotyl tissue were the most responsive to temperature, being nearly twofold higher when grown at 14 compared to 30 degrees C. Alternative pathway respiration increased concomitantly with respiration and was higher in mesocotyls grown in the cold. When seedlings were started at 30 then transferred to 14 degrees C, the increase in alternative pathway respiration due to cold was not observed unless the seedlings were transferred before 2 days of growth. Seedlings transferred to 14 degrees C after growth at 30 degrees C for 2 days had the same alternative oxidase capacity as seedlings grown at 30 degrees C. Seedlings grown at 14 degrees C for 10 to 12 days, then transferred to 30 degrees C, lost alternative pathway respiratory capacity over a period of 2 to 3 days. Western blots of mitochondrial proteins indicated that this loss of capacity was due to a loss of the alternative oxidase protein. Some in vitro characteristics of mitochondria were determined. The temperature optimum for measurement of alternative oxidase capacity was 15 to 20 degrees C. At 41 degrees C, very little alternative oxidase was measured, i.e., the mitochondrial oxygen uptake was almost completely sensitive to cyanide. This inactivation at 41 degrees C was reversible. After incubation at 41 degrees C, the alternative oxidase capacity measured at 25 degrees C was the similar to when it was measured at that temperature directly. Isolated mitochondria lost alternative oxidase capacity at the same rate when incubated at 41 degrees C as they did when incubated at 25 degrees C. Increasing the supply of electrons to isolated mitochondria increased the degree of engagement of the alternative pathway, whereas lower temperature decreased the degree of engagement. Lower temperatures did not increase the degree of engagement of the pathway in intact tissues. We interpret these observations to indicate that the greater capacity of alternative oxidase in cold-grown seedlings is a consequence of development at these low temperatures which results in elevated respiration rates. Low temperature itself does not cause greater capacity or engagement of the alternative oxidase in mitochondria that have developed under warm temperatures. Our hypothesis would be that the low growth temperatures require the seedlings to have a higher respiration rate for some reason, e.g., to prevent the accumulation of a toxic metabolite, and that the alternative pathway functions in that respiration.

Comparison of the Oxygen Exchange between Photosynthetic Cell Suspensions and Detached Leaves of Euphorbia characias L

Using a mass-spectrometric (16)O(2)/(18)O(2)-isotope technique, we compared the nature and the relative importance of oxygen exchange in photomixotrophic (PM) and photoautotrophic (PA) suspensions of Euphorbia characias L. with those in intact leaves of the same species. Young and mature leaves, dividing and nondividing cell suspensions were characterized in short-term experiments. On chlorophyll basis, the gross photosynthetic activities at CO(2) saturating concentration of PA and PM suspensions varied little from those of leaves. On dry weight basis, gross photosynthesis of PA suspensions was equal to that of leaves because of their similar chlorophyll content. This was not the case in PM suspensions where gross photosynthesis was lower and largely varied during the growth cycle. The CO(2) compensation point of PA cells (155-265 parts per million) was much higher than that of leaves (50-80 ppm). Oxygen uptakes were analyzed in terms of mitochondrial respiration, photorespiration and light stimulation of oxygen uptake (LSOU), often identified to Mehlertype reactions. In PA and PM suspensions, mitochondrial respiration rates were higher than in leaves by a factor of 1.5 to 4.5. In PM suspensions, photorespiration and LSOU were observed only in nondividing cells. Photorespiration and LSOU rates were comparable in PA suspensions and leaves. Our results demonstrate that photorespiration of PA suspensions has not been affected by the 2% CO(2) concentration imposed during 2 years of culture.

Whole Leaf Carbon Exchange Characteristics of Phosphate Deficient Soybeans (Glycine max L.)

Low phosphate nutrition results in increased chlorophyll fluorescence, reduced photosynthetic rate, accumulation of starch and sucrose in leaves, and low crop yields. This study investigated physiological responses of soybean (Glycine max [L.] Merr.) leaves to low inorganic phosphate (Pi) conditions. Responses of photosynthesis to light and CO(2) were examined for leaves of soybean grown at high (0.50 millimolar) or low (0.05 millimolar) Pi. Leaves of low Pi plants exhibited paraheliotropic orientation on bright sunny days rather than the normal diaheliotropic orientation exhibited by leaves of high Pi soybeans. Leaves of plants grown at high Pi had significantly higher light saturation points (1000 versus 630 micromole photons [400-700 nanometers] per square meter per second) and higher apparent quantum efficiency (0.062 versus 0.044 mole CO(2) per mole photons) at ambient (34 pascals) CO(2) than did low Pi leaves, yet stomatal conductances were similar. High Pi leaves also had significantly higher carboxylation efficiency (2.90 versus 0.49 micromole CO(2) per square meter per second per pascal), a lower CO(2) compensation point (6.9 versus 11.9 pascals), and a higher photosynthetic rate at 34 pascals CO(2) (19.5 versus 6.7 micromoles CO(2) per square meter per second) than did low Pi leaves. Soluble protein (0.94 versus 0.73 milligram per square centimeter), ribulose-1,5-bisphosphate carboxylase/oxygenase content (0.33 versus 0.25 milligram per square centimeter), and ribulose-1,5-bisphosphate carboxylase/oxygenase specific activity (25.0 versus 16.7 micromoles per square meter per second) were significantly greater in leaves of plants in the high Pi treatment. The data indicate that Pi stress alters the plant's CO(2) reduction characteristics, which may in turn affect the plant's capacity to accommodate normal radiation loads.

Variations in the Alternative Oxidase in Chlamydomonas Grown in Air or High CO(2)

Chlamydomonas in the resting phase of growth has an equal capacity of about 15 micromole O(2) uptake per hour per milligram of chlorophyll for both the cytochrome c, CN-sensitive respiration, and for the alternative, salicylhydroxamic acid-sensitive respiration. Alternative respiration capacity was measured as salicylhydroxamic acid inhibited O(2) uptake in the presence of CN, and cytochrome c respiration capacity as CN inhibition of O(2) uptake in the presence of salicylhydroxamic acid. Measured total respiration was considerably less than the combined capacities for respiration. During the log phase of growth on high (2-5%) CO(2), the alternative respiration capacity decreased about 90% but returned as the culture entered the lag phase. When the alternative oxidase capacity was low, addition of salicylic acid or cyanide induced its reappearance. When cells were grown on low (air-level) CO(2), which induced a CO(2) concentrating mechanism, the alternative oxidase capacity did not decrease during the growth phase. Attempts to measure in vivo distribution of respiration between the two pathways with either CN or salicylhydroxamic acid alone were inconclusive.

Cyanide-initiated oxygen consumption in autoclaved culture medium containing sugars

The consumption of oxygen initiated by KCN in an autoclaved sugar-containing rinse medium with protoplasts is described. The effect of autoclaving on several sugars was examined. Fructose solutions, followed in decreasing order by glucose, sucrose and sorbitol, were found to contain the largest amount of degraded products that could react with oxygen in the presence of KCN. Mannitol was found to be stable under the autoclaving conditions used in this investigation. KCN generally has an inhibitory effect on respiration, but in some plant tissues, respiration is stimulated by it. Under certain circumstances the degradation artefact described here may confuse interpretation of the results of respiration measurements. The use of autoclaved media containing sugars should be avoided in respiration studies that involve the application of KCN.

Changes in the Activity of the Chloroplastic and Cytosolic Forms of Dihydroxyacetone Phosphate Reductase during Maturation of Leaves

Young or mature rosette leaves from spinach (Spinacia oleracea L.) plants growing in the field, in the greenhouse, or in a growth chamber under a regimen of 8 hours light and 16 hours dark contained 15 to 50 nanomoles per minute per gram wet weight of NADH:dihydroxyacetone phosphate reductase activity. Of this activity, 75 to 87% was the chloroplastic isoform and 25 to 13% was the cytosolic form. When plants were induced to senesce, as measured by stem elongation and flowering, the percentage of the two reductase isoforms in rosette or stem leaves changed to about 12% as the chloroplastic and 88% as the cytosolic isoform. The change in enzyme activity of the rosette leaves occurred within 3 days, before phenotypic changes were observed. Likewise, when plants senesced in continuous darkness, the percentage of chloroplastic to cytosolic reductase changed from 80:20% to 25:75% after 62 hours before changes in total protein or chlorophyll occurred. The ratio of activities did not change in the first 16 hours of darkness or overnight. In each case the change in ratio resulted from about a 75% decrease in activity of the chloroplastic isoform and up to 14-fold increase in cytosolic isoform. In spinach leaves purchased at a local market primarily only the cytosolic isoform remained. When plants were returned to normal day-nights, after 62 hours in continuous darkness, the activity of the chloroplastic isoform increased, but not to control levels after 3 days, while the cytosolic enzyme decreased within 1 day to normal day-night values. Changes in activity were not due to changes during in vitro assays in activation by thioredoxin for the chloroplastic isoform or fructose 2,6-phosphate for the cytosolic isoform.

Occurrence of alternative respiratory capacity in soybean and pea

Capacity for the alternative respiratory pathway was assessed in leaf and root tissue of male-sterile and fertile soybean (Glycine max [L.] Merr.) plants and in leaf, embryonic axis, and epicotyl tissue as well as isolated mitochondria of pea (Pisum sativum L.) by measurement of oxygen uptake in the presence and absence of KCN and salicylhydroxamic acid. Male-sterile and fertile soybean tissues showed similar responses to the inhibitors, and both possessed a capacity for alternative respiration. We also found that tissue and isolated mitochondria from ;Progress No. 9' pea possessed alternative respiratory capacity similar to that of ;Alaska' pea.

Cyanide-resistant respiration in light- and dark-grown soybean cotyledons

Measurements of respiration were made on intact tissue and mitochondria isolated from soybean (Glycine max [L.] Merr. cv ;Corsoy') cotyledons from seedlings of different ages grown in light and darkness. Effects of cyanide (KCN) and salicylhydroxamic acid (SHAM) on O(2) uptake rates were determined. O(2) uptake was faster in light-grown tissue and was inhibited by both KCN and SHAM in all except light-grown tissue older than 9 days. Both inhibitors stimulated O(2) uptake in tissues more than 9 days old. Mitochondria in which O(2) uptake was coupled to ATP synthesis were isolated from all tissues. O(2) uptake by mitochondrial preparations from light- and dark-grown cotyledons was equally sensitive to KCN. Similarly, age did not affect KCN sensitivity, but sensitivity to SHAM declined with age both in the presence and absence of KCN. Estimated capacities of the cytochrome and alternative pathways of the mitochondrial preparations indicated considerably larger cytochrome than alternative pathway capacities. The cytochrome pathway capacities paralleled the state 3 mitochondrial respiration rates, which increased from day 5 to day 7 then declined thereafter. The alternative pathway capacities were not affected by light. The uncoupler, p-trifluoromethoxycarbonylcyanide phenylhydrazone (FCCP), increased the flow of electrons through the cytochrome pathway at the expense of flow through the alternative pathway in isolated mitochondria. However, the combined capacities did not exceed the rate in the presence of FCCP. The results are interpreted to indicate that the stimulation of respiration by KCN and SHAM observed in the 12-day-old green cotyledons and previously observed in older soybean leaves is not explained by characteristics of the mitochondria.

Redox activity and peroxidase activity associated with the plasma membrane of guard-cell protoplasts

Redox systems have been reported in the plasma membrane of numerous cell types and in cells from various species of higher plant. A search for a redox system in the plasma membrane of guard cells was therefore made in efforts to explain how blue light stimulates stomatal opening, a process which is coupled to guard cell H(+) efflux and K(+) uptake. The rates of O2 uptake by intact guard-cell protoplasts (GCP) of Commelina communis L., in the dark, were monitored in the presence of NAD(P)H since the stimulation of O2 consumption by reduced pyridine nucleotides is used as an indicator of the presence of a redox system in the plasma membrane. Oxygen consumption by intact GCP increased two- to threefold in the presence of NAD(P)H. The NAD(P)H-stimulation of O2 uptake was dependent on Mn(2+) and was stimulated 10- to 15-fold by salicylhydroxamic acid (SHAM). Catalase, cyanide and ascorbate, a superoxide scavenger, all individually inhibited the SHAM-stimulated O2 uptake. These are all characteristics of peroxidase activity although some of these features have been used to imply the presence of a redox system located in the plasma membrane. High levels of peroxidase activity (using guaiacol as a substrate) were also detected in the GCP and in the supernatant. The activity in the supernatant increased with time indicating that peroxidase was being excreted by the protoplasts. The properties of O2 uptake by the incubation medium after separation from the protoplasts were similar to those of the protoplast suspension. It is concluded that our observations can be more readily explained by peroxidase activity associated with the plasma membrane and secreted by the GCP than by the presence of a redox system in the plasma membrane of the protoplasts.