The alternative oxidase in roots of poa annua after transfer from high-light to low-light conditions

ATP yield of plant respiration: potential, actual and unknown

BACKGROUND AND AIMS

The ATP yield of plant respiration (ATP/hexose unit respired) quantitatively links active heterotrophic processes with substrate consumption. Despite its importance, plant respiratory ATP yield is uncertain. The aim here was to integrate current knowledge of cellular mechanisms with inferences required to fill knowledge gaps to generate a contemporary estimate of respiratory ATP yield and identify important unknowns.

METHOD

A numerical balance sheet model combining respiratory carbon metabolism and electron transport pathways with uses of the resulting transmembrane electrochemical proton gradient was created and parameterized for healthy, non-photosynthesizing plant cells catabolizing sucrose or starch to produce cytosolic ATP.

KEY RESULTS

Mechanistically, the number of c subunits in the mitochondrial ATP synthase Fo sector c-ring, which is unquantified in plants, affects ATP yield. A value of 10 was (justifiably) used in the model, in which case respiration of sucrose potentially yields about 27.5 ATP/hexose (0.5 ATP/hexose more from starch). Actual ATP yield often will be smaller than its potential due to bypasses of energy-conserving reactions in the respiratory chain, even in unstressed plants. Notably, all else being optimal, if 25 % of respiratory O2 uptake is via the alternative oxidase - a typically observed fraction - ATP yield falls 15 % below its potential.

CONCLUSIONS

Plant respiratory ATP yield is smaller than often assumed (certainly less than older textbook values of 36-38 ATP/hexose) leading to underestimation of active-process substrate requirements. This hinders understanding of ecological/evolutionary trade-offs between competing active processes and assessments of crop growth gains possible through bioengineering of processes that consume ATP. Determining the plant mitochondrial ATP synthase c-ring size, the degree of any minimally required (useful) bypasses of energy-conserving reactions in the respiratory chain, and the magnitude of any 'leaks' in the inner mitochondrial membrane are key research needs.

Integrated analysis on biochemical profiling and transcriptome revealed nitrogen-driven difference in accumulation of saponins in a medicinal plant Panax notoginseng

The medicinal plant Panax notoginseng is considered a promising source of secondary metabolites due to its saponins. However, there are relatively few studies on the response of saponins to nitrogen (N) availability and the mechanisms underlying the N-driven regulation of saponins. Saponins content and saponins -related genes were analyzed in roots of P. notoginseng grown under low N (LN), moderate N (MN) and high N (HN). Saponins was obviously increased in LN individuals with a reduction in β-glucosidase activity. LN facilitated root architecture and N uptake rate. Compared with the LN individuals, 2872 and 1122 genes were incorporated into as differently expressed genes (DEGs) in the MN and HN individuals. Clustering and enrichment showed that DEGs related to "carbohydrate biosynthesis", "plant hormone signal transduction", "terpenoid backbone biosynthesis", "sesquiterpenoid and triterpenoid biosynthesis" were enriched. The up-regulation of some saponins-related genes and microelement transporters was found in LN plants. Whereas the expression of IPT3, AHK4 and GS2 in LN plants fell far short of that in HN ones. Anyways, LN-induced accumulation of C-based metabolites as saponins might derive from the interaction between N and phytohormones in processing of N acquisition, and HN-induced reduction of saponins might be result from an increase in the form of β-glucosidase activity and N-dependent cytokinins (CKs) biosynthesis.

A Comprehensive Biophysical Model of Ion and Water Transport in Plant Roots. III. Quantifying the Energy Costs of Ion Transport in Salt-Stressed Roots of Arabidopsis

Salt stress defense mechanisms in plant roots, such as active Na+ efflux and storage, require energy in the form of ATP. Understanding the energy required for these transport mechanisms is an important step toward achieving an understanding of salt tolerance. However, accurate measurements of the fluxes required to estimate these energy costs are difficult to achieve by experimental means. As a result, the magnitude of the energy costs of ion transport in salt-stressed roots relative to the available energy is unclear, as are the relative contributions of different defense mechanisms to the total cost. We used mathematical modeling to address three key questions about the energy costs of ion transport in salt-stressed Arabidopsis roots: are the energy requirements calculated on the basis of flux data feasible; which transport steps are the main contributors to the total energy costs; and which transport processes could be altered to minimize the total energy costs? Using our biophysical model of ion and water transport we calculated the energy expended in the trans-plasma membrane and trans-tonoplast transport of Na+, K+, Cl-, and H+ in different regions of a salt-stressed model Arabidopsis root. Our calculated energy costs exceeded experimental estimates of the energy supplied by root respiration for high external NaCl concentrations. We found that Na+ exclusion from, and Cl- uptake into, the outer root were the major contributors to the total energy expended. Reducing the leakage of Na+ and the active uptake of Cl- across outer root plasma membranes would lower energy costs while enhancing exclusion of these ions. The high energy cost of ion transport in roots demonstrates that the energetic consequences of altering ion transport processes should be considered when attempting to improve salt tolerance.

Improved in vivo measurement of alternative oxidase respiration in field-collected pine roots

Cellular respiration via the alternative oxidase pathway (AOP) leads to a considerable loss in efficiency. Compared to the cytochrome pathway (COP), AOP produces 0-50% as much ATP per carbon (C) respired. Relative partitioning between the pathways can be measured in vivo based on their differing isotopic discriminations against ¹⁸ O in O₂ . Starting from published methods, we have refined and tested a new protocol to improve measurement precision and efficiency. The refinements detect an effect of tissue water content (P < 0.0001), which we have removed, and yield precise discrimination endpoints in the presence of pathway-specific respiratory inhibitors [CN^- and salicylhydroxamic acid (SHAM)], which improves estimates of AOP/COP partitioning. Fresh roots of Pinus sylvestris were sealed in vials with a CO₂ trap. The air was replaced to ensure identical starting conditions. Headspace air was repeatedly sampled and isotopically analyzed using isotope-ratio mass spectrometry. The method allows high-precision measurement of the discrimination against ¹⁸ O in O₂ because of repeated measurements of the same incubation vial. COP and AOP respiration discriminated against ¹⁸ O by 15.1 ± 0.3‰ and 23.8 ± 0.4‰, respectively. AOP contributed to root respiration by 23 ± 0.2% of the total in an unfertilized stand. In a second, nitrogen-fertilized, stand AOP contribution was only 14 ± 0.2% of the total. These results suggest the improved method can be used to assess the relative importance of COP and AOP activities in ecosystems, potentially yielding information on the role of each pathway for the carbon use efficiency of organisms.

An In Vivo Perspective of the Role(s) of the Alternative Oxidase Pathway

Despite intense research on the in vitro characterization of regulatory factors modulating the alternative oxidase (AOX) pathway, the regulation of its activity in vivo is still not fully understood. Advances concerning in vivo regulation of AOX based on the oxygen-isotope fractionation technique are reviewed, and regulatory factors that merit future research are highlighted. In addition, we review and discuss the main biological functions assigned to the plant AOX, and suggest future experiments involving in vivo activity measurements to test different hypothesized physiological roles.

Arbuscular Mycorrhizal Symbiosis with Arundo donax Decreases Root Respiration and Increases Both Photosynthesis and Plant Biomass Accumulation

The effect of arbuscular mycorrhiza (AM) symbiosis on plant growth is associated with the balance between costs and benefits. A feedback regulation loop has been described in which the higher carbohydrate cost to plants for AM symbiosis is compensated by increases in their photosynthetic rates. Nevertheless, plant carbon balance depends both on photosynthetic carbon uptake and respiratory carbon consumption. The hypothesis behind this research was that the role of respiration in plant growth under AM symbiosis may be as important as that of photosynthesis. This hypothesis was tested in Arundo donax L. plantlets inoculated with Rhizophagus irregularis and Funneliformis mosseae. We tested the effects of AM inoculation on both photosynthetic capacity and in vivo leaf and root respiration. Additionally, analyses of the primary metabolism and ion content were performed in both leaves and roots. AM inoculation increased photosynthesis through increased CO₂ diffusion and electron transport in the chloroplast. Moreover, respiration decreased only in AM roots via the cytochrome oxidase pathway (COP) as measured by the oxygen isotope technique. This decline in the COP can be related to the reduced respiratory metabolism and substrates (sugars and tricarboxylic acid cycle intermediates) observed in roots.

Transcriptome Analysis Reveals the Response of Iron Homeostasis to Early Feeding by Small Brown Planthopper in Rice

It has been documented that planthopper attacks change iron (Fe) content of rice plants. To investigate whether planthopper attacks change rice Fe homeostasis at the molecular level, the response of rice Fe homeostasis to early feeding by small brown planthopper (SBPH) was examined by transcriptome profiling. Results showed that the concentration of Fe and nicotianamine decreased in resistant rice genotype and increased in susceptible rice genotype after attack by SBPH. Transcriptome profiling showed that 13 and 21 Fe homeostasis-related genes encoded enzymes that were involved in phytosiderophore biosynthesis and that Fe transporters and regulators displayed altered expression in resistant and susceptible rice genotypes, respectively, after attack by SBPH. This revealing response of Fe homeostasis to planthopper attack in rice at the molecular level provided new insight into rice plants' response to planthopper attack and uncovered a novel physiological response of rice to planthopper attack, which extended our knowledge of the early interaction between rice and SBPH.

Overexpression of alternative oxidase gene confers aluminum tolerance by altering the respiratory capacity and the response to oxidative stress in tobacco cells

Aluminum (Al) stress represses mitochondrial respiration and produces reactive oxygen species (ROS) in plants. Mitochondrial alternative oxidase (AOX) uncouples respiration from mitochondrial ATP production and may improve plant performance under Al stress by preventing excess accumulation of ROS. We tested respiratory changes and ROS production in isolated mitochondria and whole cell of tobacco (SL, ALT 301) under Al stress. Higher capacities of AOX pathways relative to cytochrome pathways were observed in both isolated mitochondria and whole cells of ALT301 under Al stress. AOX1 when studied showed higher AOX1 expression in ALT 301 than SL cells under stress. In order to study the function of tobacco AOX gene under Al stress, we produced transformed tobacco cell lines by introducing NtAOX1 expressed under the control of the cauliflower mosaic virus (CaMV) 35 S promoter in sensitive (SL) Nicotiana tabacum L. cell lines. The enhancement of endogenous AOX1 expression and AOX protein with or without Al stress was in the order of transformed tobacco cell lines > ALT301 > wild type (SL). A decreased respiratory inhibition and reduced ROS production with a better growth capability were the significant features that characterized AOX1 transformed cell lines under Al stress. These results demonstrated that AOX plays a critical role in Al stress tolerance with an enhanced respiratory capacity, reducing mitochondrial oxidative stress burden and improving the growth capability in tobacco cells.

Seasonal variation of leaf respiration and the alternative pathway in field-grown Populus × canadensis

The temperature response of plant respiration varies between species and can acclimate to changing temperatures. Mitochondrial respiration in plants has two terminal oxidases: the cytochrome c oxidase (COX) and the cyanide-resistant alternative oxidase (AOX). In Populus × canadensis var. italica, a deciduous tree species, we investigated the temperature response of leaf respiration via the alternative and cytochrome pathways, as well as seasonal changes in these pathways, using the oxygen isotope fractionation technique. The electron partitioning through the alternative pathway (τ(a) ) increased from 0 to 30-40% with measurement temperatures from 6 to 30°C at all times measured throughout the growing season. τ(a) at the growth temperature (the average temperature during 3 days prior to sampling) increased from 12 to 29% from spring until late summer and decreased thereafter. Total respiration declined throughout the growing season by 50%, concomitantly with decreases in both AOX (64%) and COX (32%) protein abundances. Our results provide new insight into the natural variability of AOX protein abundances and alternative respiration electron partitioning over immediate and seasonal timescales.

Distinct responses of the mitochondrial respiratory chain to long- and short-term high-light environments in Arabidopsis thaliana

In order to ensure the cooperative function with the photosynthetic system, the mitochondrial respiratory chain needs to flexibly acclimate to a fluctuating light environment. The non-phosphorylating alternative oxidase (AOX) is a notable respiratory component that may support a cellular redox homeostasis under high-light (HL) conditions. Here we report the distinct acclimatory manner of the respiratory chain to long- and short-term HL conditions and the crucial function of AOX in Arabidopsis thaliana leaves. Plants grown under HL conditions (HL plants) possessed a larger ubiquinone (UQ) pool and a higher amount of cytochrome c oxidase than plants grown under low light conditions (LL plants). These responses in HL plants may be functional for efficient ATP production and sustain the fast plant growth. When LL plants were exposed to short-term HL stress (sHL), the UQ reduction level was transiently elevated. In the wild-type plant, the UQ pool was re-oxidized concomitantly with an up-regulation of AOX. On the other hand, the UQ reduction level of the AOX-deficient aox1a mutant remained high. Furthermore, the plastoquinone pool was also more reduced in the aox1a mutant under such conditions. These results suggest that AOX plays an important role in rapid acclimation of the respiratory chain to sHL, which may support efficient photosynthetic performance.

Metabolic and transcriptional responses of glycerolipid pathways to a perturbation of glycerol 3-phosphate metabolism in Arabidopsis

Glycerolipid synthesis in plants involves two major metabolic pathways compartmentalized in the chloroplasts and cytosol, respectively. Although these two parallel pathways are regulated with considerable flexibility, the factors mediating this process remain unclear. To investigate the influence of glycerol 3-phosphate (Gly-3-P) on the interactions of the glycerolipid pathways, we generated transgenic Arabidopsis lines with a feedback-resistant Gly-3-P dehydrogenase gene (gpsA(FR)) from Escherichia coli. gpsA(FR) was detected in the cytosol, but augmented Gly-3-P levels were observed in the cytosol as well as in chloroplasts. Glycerolipid composition and fatty acid positional distribution analyses revealed an altered fatty acid flux that affected not only the molar ratios of glycerolipid species but also their fatty acid composition. To decipher this complex pathway, a transgenic line was subjected to lipidomic analysis and a global gene-expression survey. The results revealed that changes in Gly-3-P metabolism caused altered expression of a broad array of genes. When viewed from the perspective of glycerolipid metabolism, coherent networks emerged, revealing that many enzymatic components of the glycerolipid pathways operate in a modular manner under the influence of Gly-3-P. Transcript levels of the enzymes involved in the prokaryotic pathway were mostly induced, whereas genes of the eukaryotic pathway enzymes were largely suppressed. Hence, the gene-expression changes were consistent with the detected biochemical phenotype. Our results suggest that Gly-3-P modulates the balance of the two glycerolipid pathways in Arabidopsis by influencing both metabolic flux and gene transcription.

Changes of alternative oxidase activity, capacity and protein content in leaves of Cucumis sativus wild-type and MSC16 mutant grown under different light intensities

In vitro studies demonstrated that alternative oxidase (AOX) is biochemically regulated by a sulfhydryl-disulfide system, interaction with alpha-ketoacids, ubiquinone pool redox state and protein content among others. However, there is still scarce information about the in vivo regulation of the AOX. Cucumis sativus wild-type (WT) and MSC16 mutant plants were grown under two different light intensities and were used to analyze the relationship between the amount of leaf AOX protein and its in vivo capacity and activity at night and day periods. WT and MSC16 plants presented lower total respiration (V(t)), cytochrome oxidase pathway (COP) activity (v(cyt)) and alternative oxidase pathway (AOP) activity (v(alt)) when grown at low light (LL), although growth light intensity did not change the amount of cytochrome oxidase (COX) nor AOX protein. Changes of v(cyt) related to growing light conditions suggested a substrate availability and energy demand control. On the other hand, the total amount of AOX protein present in the tissue does not play a role in the regulation neither of the capacity nor of the activity of the AOP in vivo. Soluble carbohydrates were directly related to the activity of the AOP. However, although differences in soluble sugar contents mostly regulate the capacity of the AOP at different growth light intensities, additional regulatory mechanisms are necessary to fully explain the observed results.

Alternative oxidases in Arabidopsis: A comparative analysis of differential expression in the gene family provides new insights into function of non-phosphorylating bypasses

The emergence of Arabidopsis as a model plant provides an opportunity to gain insights into the role of the alternative oxidase that cannot be as readily achieved in other plant species. The analysis of extensive mRNA expression data indicates that all five Aox genes (Aox1a, 1b, 1c, 1d and 2) are expressed, but organ and developmental regulation are evident, suggesting regulatory specialisation of Aox gene members. The stress-induced nature of the alternative pathway in a variety of plants is further supported in Arabidopsis as Aox1a and Aox1d are amongst the most stress responsive genes amongst the hundreds of known genes encoding mitochondrial proteins. Analysis of genes co-expressed with Aoxs from studies of responses to various treatments altering mitochondrial functions and/or from plants with altered Aox levels reveals that: (i) this gene set encodes more functions outside the mitochondrion than functions in mitochondria, (ii) several pathways for induction exist and there is a difference in the magnitude of the induction in each pathway, (iii) the magnitude of induction may depend on the endogenous levels of Aox, and (iv) induction of Aox can be oxidative stress-dependent or -independent depending on the gene member and the tissue analysed. An overall role for Aox in re-programming cellular metabolism in response to the ever changing environment encountered by plants is proposed.

Involvement of a glycerol-3-phosphate dehydrogenase in modulating the NADH/NAD+ ratio provides evidence of a mitochondrial glycerol-3-phosphate shuttle in Arabidopsis

A mitochondrial glycerol-3-phosphate (G-3-P) shuttle that channels cytosolic reducing equivalent to mitochondria for respiration through oxidoreduction of G-3-P has been extensively studied in yeast and animal systems. Here, we report evidence for the operation of such a shuttle in Arabidopsis thaliana. We studied Arabidopsis mutants defective in a cytosolic G-3-P dehydrogenase, GPDHc1, which, based on models described for other systems, functions as the cytosolic component of a G-3-P shuttle. We found that the gpdhc1 T-DNA insertional mutants exhibited increased NADH/NAD+ ratios compared with wild-type plants under standard growth conditions, as well as impaired adjustment of NADH/NAD+ ratios under stress simulated by abscisic acid treatment. The altered redox state of the NAD(H) pool was correlated with shifts in the profiles of metabolites concerning intracellular redox exchange. The impairment in maintaining cellular redox homeostasis was manifest by a higher steady state level of reactive oxygen species under standard growth conditions and by a significantly augmented hydrogen peroxide production under stress. Loss of GPDHc1 affected mitochondrial respiration, particularly through a diminished capacity of the alternative oxidase respiration pathway. We propose a model that outlines potential involvements of a mitochondrial G-3-P shuttle in plant cells for redox homeostasis.

Contribution of current photosynthates to root respiration of non-nodulated Medicago sativa: effects of light and nitrogen supply

The effects of light (PFD) and nitrogen (N) supply on root respiration of new C (currently assimilated carbon, R(new)) and old C ( R(old)) were analysed in non-nodulated Medicago sativa. Plants were pre-treated with high/low PFD and high/low N supply with a regular 16/8 h light/dark cycle. Five to eight weeks after planting current photosynthates were labelled with (13)C and their contribution to root respiration was continuously measured during a 24 h day/night cycle. PFD conditions during labelling were either those of the pre-treatments (control, 25 or 6 mol m(-2) d(-1)) or, for high PFD plants, 6 mol m(-2) d(-1) by shortening the photoperiod or reducing irradiance. The fraction of new C in the respiratory CO2 increased during the light period, but remained constant in the dark period. In control plants, R(new) contributed 40 % to the daily root respiration in high PFD/high N conditions. Continuously low PFD increased (50 %) and low N decreased (26 %) the contribution of R(new). Exposing plants from high PFD pre-treatments to a short photoperiod or to low PFD stimulated R(old), indicating mobilisation of reserve C. This stimulation was more pronounced in plants with high N supply than in those with low N supply. Comparison with other legumes suggested that R(new) in root respiration was mainly defined by the ratio between the assimilatory capacity of the shoots and the maintenance costs of roots with a short-term capacity of buffering respiratory demand by mobilisation of reserves in situations of fluctuating PFD.

Role of carbohydrate supply in white and brown root respiration of ponderosa pine

•  Respiration of intact ponderosa pine (Pinus ponderosa) fine roots (< 2.5 mm) was measured to determine the role of recently fixed carbohydrate in maintaining root metabolism of growing white (WR) and recently suberized brown roots (BR). •  The CO₂ efflux and O₂ uptake of individual roots were followed continuously over 24 h after carbohydrate supply was altered by exposing shoots to light/dark treatments and by root excision. •  In situ respiration of individual WR and BR averaged 86.0 ± 2.6 and 21.1 ± 1.5 mol CO₂  g^-1  h^-1 , respectively. Growth respiration was estimated to be approximately two-thirds the rate of WR respiration. Attached WR and BR respiration did not decline significantly over 24 h under continuous light. The WR respiration significantly decreased during a dark period. All roots maintained relatively constant respiration rates for at least 6 h after excision. Respiratory quotient (RQ; CO₂  : O₂ ) was not different between attached (0.84 ± 0.014) and detached (0.85 ± 0.017) roots. CO₂ environment of the cuvette did not influence WR or BR respiration. •  The WR appear to be more sensitive to supply of current photosynthate than BR. Shoot light environment needs to be considered when measuring root and soil CO₂ efflux.

Thermal acclimation and the dynamic response of plant respiration to temperature

Temperature-mediated changes in plant respiration (R) are now accepted as an important component of the biosphere's response to global climate change. Here we discuss the underlying mechanisms responsible for the dynamic response of plant respiration to short and long-term temperature changes. The Q(10) is often assumed to be 2.0 (i.e. R doubles per 10 degrees C rise in temperature); however, the Q(10) is not constant (e.g. it declines near-linearly with increasing temperature). The temperature dependence of Q(10) is linked to shifts in the control exerted by maximum enzyme activity at low temperature and substrate limitations at high temperature. In the long term, acclimation of R to temperature is common, in effect reducing the temperature sensitivity of R to changes in thermal environment, with the temperature during plant development setting the maximal thermal acclimation of R.

Regulation of alternative oxidase activity during phosphate deficiency in bean roots (Phaseolus vulgaris)

Cyanide-resistant respiration was studied in mitochondria isolated from the roots of bean plants (Phaseolus vulgaris L. cv. Złota Saxa) grown hydroponically up to 16 days on a phosphate-sufficient (+P, control) or phosphate-deficient (-P) medium. Western blotting indicated that the alternative oxidase (AOX) was present only in its reduced (active) form, both in phosphate-sufficient and phosphate-deficient roots, but in the latter, the amount of AOX protein was greater. Addition of pyruvate to the isolation, washing and reaction media made mitochondria from +P roots cyanide-insensitive, similar to mitochondria from -P roots. The doubled activity of NAD-malic enzyme (NAD-ME) in -P compared with +P root mitochondria may suggest increased pyruvate production in -P mitochondria. Lower cytochrome c oxidase (COX) activity and no uncoupler effect on respiration indicated limited cytochrome chain activity in -P mitochondria. In -P mitochondria, the oxygen uptake decreased and the level of Q reduction increased from 60 to 80%. With no pyruvate present (AOX not fully activated), inhibition of the cytochrome pathway resulted in an increased level of the ratio of reduced ubiquinone (Qr) to total ubiquinone (Qt) (Qr/Qt) in +P mitochondria, but did not change Qr/Qt in -P mitochondria. When pyruvate was present, the kinetics for AOX were similar in mitochondria from -P and +P roots. It is suggested that AOX participation in -P respiration may provide an acclimation to phosphate deficiency. Stabilization of the ubiquinone reduction level by AOX might prevent the harmful effect of an increased formation of reactive oxygen species.

Regulation of alternative oxidase activity in six wild monocotyledonous species. An in vivo study at the whole root level

The activity of the alternative pathway is affected by a number of factors, including the level and reduction state of the alternative oxidase (AOX) protein, and the reduction state of the ubiquinone pool. To investigate the significance of these factors for the rate of alternative respiration in vivo, we studied root respiration of six wild monocotyledonous grass species that were grown under identical controlled conditions. The activity of the alternative pathway was determined using the oxygen isotope fractionation technique. In all species, the AOX protein was invariably in its reduced (high activity) state. There was no correlation between AOX activity and AOX protein concentration, ubiquinone (total, reduced, or oxidized) concentration, or the reduction state of the ubiquinone pool. However, when some of these factors are combined in a linear regression model, a good fit to AOX activity is obtained. The function of the AOX is still not fully understood. It is interesting that we found a positive correlation between the activity of the alternative pathway and relative growth rate; a possible explanation for this correlation is discussed. Inhibition of the AOX (with salicylhydroxamic acid) decreases respiration rates less than the activity present before inhibition (i.e. measured with the 18O-fractionation technique).