Effect of salinity on root-nodule conductance to the oxygen diffusion in the Medicago truncatula–Sinorhizobium meliloti symbiosis

In order to determine the effect of salinity on the nodule conductance, oxygen uptake by the nodulated roots was measured by registering the concentration of O₂ as a function of time in a tight incubator of known volume containing the nodulated roots of Medicago truncatula. Four lines, namely TN8.20 and TN6.18, originated from local populations, F83005.5 originated from Var (France) and Jemalong 6, a cultivar from Australia, were hydroponically grown in 250 ml glass bottles under semi-controlled conditions in a glasshouse, after germination and inoculation with the strain Sinorhizobium meliloti 2011. The saline treatment was applied gradually to reach 75 mM after 2 weeks. Results show that oxygen uptake increased significantly with salinity in TN6.18 and F83005.5, but not in Jemalong nor in TN8.20. Without salt, Jemalong showed a significantly higher O₂ uptake of 240 μmol O₂ per h per plant than the mean of 130 μmol O₂ per h per plant for other lines. Salinity increased significantly the nodule conductance in all genotypes. This salt effect was significantly higher for TN6.18 than for TN8.20, and for Jemalong than for F83005.5. Without salt there was less genotypic variation in nodule conductance in the range of 5–8 μm s^–1 for F83005.5 and TN8.20, respectively. Thus the sensitivity to salinity appears to be associated with an increase in nodule conductance that supports the increased respiration of N₂-fixing nodules under salinity.     

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 ¹⁸O 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 ¹⁸O in O₂ because of repeated measurements of the same incubation vial. COP and AOP respiration discriminated against ¹⁸O 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.     

The alternative respiratory pathway mediates carboxylate synthesis in white lupin cluster roots under phosphorus deprivation

Plant adaptations associated with a high efficiency of phosphorus (P) acquisition can be used to increase productivity and sustainability in a world with a growing population and decreasing rock phosphate reserves. White lupin (Lupinus albus) produces cluster roots that release carboxylates to efficiently mobilize P from P‐sorbing soils. It has been hypothesized that an increase in the activity of the alternative oxidase (AOX) would allow for the mitochondrial oxidation of NAD(P)H produced during citrate synthesis in cluster roots at a developmental stage when there is a low demand for ATP. We used the oxygen‐isotope fractionation technique to study the in vivo respiratory activities of the cytochrome oxidase pathway (COP) and the alternative oxidase pathway (AOP) in different root sections of white lupins grown hydroponically with and without P. In parallel, AOX protein levels and internal carboxylate concentrations were determined in cluster and non‐cluster roots. Higher in vivo AOP activity was measured in cluster roots when malate and citrate concentrations were also high, thus confirming our hypothesis. AOX protein levels were not always correlated with in vivo AOP activity, suggesting post‐translational regulation of AOX.     

Effect of salt stress on photosynthesis and chlorophyll fluorescence in Medicago truncatula

In the present study, photosynthetic parameters including gas exchanges, pigment contents, and chlorophyll fluorescence, were compared in two contrasting local Medicago truncatula lines TN6.18 and TN8.20, in response to salt added to the nutrient solution. Plants were cultivated under symbiotic nitrogen fixation (SNF) after inoculation with a reference strain Sinorhizobium meliloti 2011, a very tolerant strain to salinity (700 mM NaCl), and grown in a controlled glasshouse. On one month old plants (with active SNF), salt treatment (75 mM NaCl) was gradually applied. Photosynthesis, assimilating pigments and chlorophyll fluorescence were monitored throughout the experiment during both short and long terms, compared to control (non-saline) conditions. A genotypic variation in salt tolerance was found; TN6.18 was the more sensitive to salinity. The relative tolerance of TN8.20 was concomitant with the highest photochemical quenching coefficient (q_P) affecting the maximum quantum yield of PSII (Y); the real quantum yield (?_exc) was the most affected in the sensitive line. Moreover, stomatal and PSII reaction centers activities differed clearly between the studied lines. We found that the effect of salinity on photosynthesis of M. truncatula was related to PSII activity reduction rather than to stomatal conductance limitation. Photosynthesis was reduced by the inhibition of CO₂ assimilation caused by PSII damage. This was clearly estimated by the Y, ?_exc and especially by the quantum yield of electron transport of PSII (Φ_PSII). Thus, on the basis of our results on the two local M. truncatula lines, we recommend the use of chlorophyll fluorescence as non-destructive screening method to discriminate susceptible and resistant legumes to salt stress.     

The alternative oxidase pathway is involved in optimizing photosynthesis in Medicago truncatula infected by Fusarium oxysporum and Rhizoctonia solani

Phytopathogen infection alters primary metabolism status and plant development. The alternative oxidase (AOX) has been hypothesized to increase under pathogen attack preventing reductions, thus optimizing photosynthesis and growth. In this study, two genotypes of Medicago truncatula, one relatively resistant (Jemalong A17) and one susceptible (TN1.11), were infected with Fusarium oxysporum and Rhizoctonia solani. The in vivo foliar respiratory activities of the cytochrome oxidase pathway (COP) and the alternative oxidase pathway (AOP) were measured using the oxygen isotope fractionation. Gas exchange and photosynthesis-related parameters were measured and calculated together with antioxidant enzymes activities and organic acids contents. Our results show that the in vivo activity of AOX (v_alt) plays a role under fungal infection. When infected with R. solani, the increase of v_alt in A17 was concomitant to an increase in net assimilation, in mesophyll conductance, to an improvement in the maximum velocity of Rubisco carboxylation and to unchanged malate content. However, under F. oxysporum infection, the induced v_alt was accompanied by an enhancement in the antioxidant enzymes, superoxide dismutase (SOD; EC1.15.1.1), catalase (CAT; EC1.11.1.6) and guaiacol peroxidase (GPX; EC1.11.1.7), activities and to an unchanged tricarboxylic acid cycle intermediates. These results provide new insight into the role of the in vivo activity of AOX in coordinating primary metabolism interactions that, partly, modulate the relative resistance of M. truncatula to diseases caused by soil-borne pathogenic fungi.     

Growth, nitrogen fixation and ion distribution in Medicago truncatula subjected to salt stress

This study compared the growth, nodulation, N₂ fixation, and ion distribution in three Medicago truncatula lines, in response to salt in nutrient solution. Two local lines (TN8.20 and TN6.18) and a reference line (Jemalong 6) were inoculated with a reference strain Sinorhizobium meliloti 2011, a very tolerant strain to salinity (700 mM NaCl) and grown in a controlled glasshouse with or without 75 mM NaCl. A genotypic variation in tolerance to salt was found: TN6.18 was the most sensitive line whereas TN8.20 was the most tolerant. The relative tolerance of TN8.20 was concomitant with the lowest leaf Na⁺ concentration and the highest nodule biomass production. However, nodule efficiency (amount of nitrogen fixed per g dry weight nodule) decreased in all lines. Results suggest that the tolerance to salt seems to depend on the host plant ability to protect its leaves against an excessive Na⁺ (and Cl^?) accumulation, and its ability to maintain the development of an abundant nodular system, which in turn determines an important rate of nitrogen fixation and allows the plants to conserve their growth potentialities. The loss of the nodular efficiency under salt stress seems to be compensated by a large nodule biomass.     

The influence of alternative pathways of respiration that utilize branched‐chain amino acids following water shortage in Arabidopsis

During dark‐induced senescence isovaleryl‐CoA dehydrogenase (IVDH) and D‐2‐hydroxyglutarate dehydrogenase (D‐2HGDH) act as alternate electron donors to the ubiquinol pool via the electron‐transfer flavoprotein/electron‐transfer flavoprotein:ubiquinone oxidoreductase (ETF/ETFQO) pathway. However, the role of this pathway in response to other stresses still remains unclear. Here, we demonstrated that this alternative pathway is associated with tolerance to drought in Arabidopsis. In comparison with wild type (WT) and lines overexpressing D‐2GHDH, loss‐of‐function etfqo‐1, d2hgdh‐2 and ivdh‐1 mutants displayed compromised respiration rates and were more sensitive to drought. Our results demonstrated that an operational ETF/ETFQO pathway is associated with plants' ability to withstand drought and to recover growth once water becomes replete. Drought‐induced metabolic reprogramming resulted in an increase in tricarboxylic acid (TCA) cycle intermediates and total amino acid levels, as well as decreases in protein, starch and nitrate contents. The enhanced levels of the branched‐chain amino acids in loss‐of‐function mutants appear to be related to their increased utilization as substrates for the TCA cycle under water stress. Our results thus show that mitochondrial metabolism is highly active during drought stress responses and provide support for a role of alternative respiratory pathways within this response.     

工业真菌中的交替氧化酶

真菌是现代微生物发酵产业的主力军之一。交替呼吸途径(Alternative respiration pathway,ARP),以其非磷酸化电子传递途径,起到了能量溢流(Energy overflow)的作用,调节细胞能量代谢,平衡碳代谢和电子传递,有利于代谢产物的积累。此外,交替呼吸对真菌的抗逆反应和条件致病菌的生理作用也都具有非常重要的影响。交替氧化酶(Alternative oxidase,AOX)是线粒体中交替呼吸途径的末端氧化酶,广泛存在于高等植物及部分真菌和藻类中。由于交替氧化酶对水杨氧肟酸(Salicylhydroxamic acid,SHAM)敏感而对细胞色素呼吸抑制剂氰化物不敏感,交替氧化酶AOX介导的交替呼吸途径又被称为抗氰呼吸途径(Cyanide-resistant respiration,CRR)。近年来,研究交替呼吸途径和交替氧化酶已成为细胞呼吸代谢领域的热门课题。本文主要对真菌交替呼吸途径和交替氧化酶的结构与其在工业真菌体内功能的最新研究进展作一简要的综述。     

A ROP2‐RIC1 pathway fine‐tunes microtubule reorganization for salt tolerance in Arabidopsis

The reorganization of microtubules induced by salt stress is required for Arabidopsis survival under high salinity conditions. RIC1 is an effector of Rho‐related GTPase from plants (ROPs) and a known microtubule‐associated protein. In this study, we demonstrated that RIC1 expression decreased with long‐term NaCl treatment, and ric1‐1 seedlings exhibited a higher survival rate under salt stress. We found that RIC1 reduced the frequency of microtubule transition from shortening to growing status and knockout of RIC1 improved the reassembly of depolymerized microtubules caused by either oryzalin treatment or salt stress. Further investigation showed that constitutively active ROP2 promoted the reassembly of microtubules and the survival of seedlings under salt stress. A rop2‐1 ric1‐1 double mutant rescued the salt‐sensitive phenotype of rop2‐1, indicating that ROP2 functions in salt tolerance through RIC1. Although ROP2 did not regulate RIC1 expression upon salt stress, a quick but mild increase of ROP2 activity was induced, led to reduction of RIC1 on microtubules. Collectively, our study reveals an ROP2‐RIC1 pathway that fine‐tunes microtubule dynamics in response to salt stress in Arabidopsis. This finding not only reveals a new regulatory mechanism for microtubule reorganization under salt stress but also the importance of ROP signalling for salinity tolerance.     

Germination profiling of lentil genotypes subjected to salinity stress

• Salinity is one of the most severe environmental stresses, negatively affecting productivity of salt‐sensitive crop species. Given that germination is the most critical phase in the plant life cycle, the present study aimed to determine seed germination potential and associated traits under salt stress conditions as a simple approach to identify salt‐tolerant lentil genotypes.
• The genetic material consisted of six lentil genotypes whose adaptation to various agroclimatic conditions is not well elucidated. Salinity stress was applied by addition of NaCl at three different levels of stress, while non‐stressed plants were included as controls. Evaluation of tolerance was performed on the basis of germination percentage, seed water absorbance, root and shoot length, seedling water content, seedling vigour index and number of seedlings with an abnormal phenotype.
• Overall, our findings revealed that salinity stress substantially affects all traits associated with germination and early seedling growth, with the effect of salinity being dependent on the level of stress applied. It is noteworthy, however, that genotypes responded differently to the varying salinity levels. In this context, Samos proved the most salt‐tolerant genotype, indicating its possible use for cultivation under stress conditions.
• In conclusion, the determination of seed germination and early growth potential may be exploited as an efficient strategy to reveal genetic variation in lentil germplasm of unknown tolerance to salinity stress. This approach allows selection of desirable genotypes at early growth stages, thus enabling more efficient application of various breeding methods to achieve stress‐tolerant lentil genotypes.

     

The acclimation of photosynthesis and respiration to temperature in the C3–C4 intermediate Salsola divaricata: induction of high respiratory CO2 release under low temperature

Photosynthesis in C₃–C₄ intermediates reduces carbon loss by photorespiration through refixing photorespired CO₂ within bundle sheath cells. This is beneficial under warm temperatures where rates of photorespiration are high; however, it is unknown how photosynthesis in C₃–C₄ plants acclimates to growth under cold conditions. Therefore, the cold tolerance of the C₃–C₄ Salsola divaricata was tested to determine whether it reverts to C₃ photosynthesis when grown under low temperatures. Plants were grown under cold (15/10 °C), moderate (25/18 °C) or hot (35/25 °C) day/night temperatures and analysed to determine how photosynthesis, respiration and C₃–C₄ features acclimate to these growth conditions. The CO₂ compensation point and net rates of CO₂ assimilation in cold‐grown plants changed dramatically when measured in response to temperature. However, this was not due to the loss of C₃–C₄ intermediacy, but rather to a large increase in mitochondrial respiration supported primarily by the non‐phosphorylating alternative oxidative pathway (AOP) and, to a lesser degree, the cytochrome oxidative pathway (COP). The increase in respiration and AOP capacity in cold‐grown plants likely protects against reactive oxygen species (ROS) in mitochondria and photodamage in chloroplasts by consuming excess reductant via the alternative mitochondrial respiratory electron transport chain.     

The battle of two ions: Ca2+ signalling against Na+ stress

Soil salinity adversely affects plant growth, crop yield and the composition of ecosystems. Salinity stress impacts plants by combined effects of Na⁺ toxicity and osmotic perturbation. Plants have evolved elaborate mechanisms to counteract the detrimental consequences of salinity. Here we reflect on recent advances in our understanding of plant salt tolerance mechanisms. We discuss the embedding of the salt tolerance‐mediating SOS pathway in plant hormonal and developmental adaptation. Moreover, we review newly accumulating evidence indicating a crucial role of a transpiration‐dependent salinity tolerance pathway, that is centred around the function of the NADPH oxidase RBOHF and its role in endodermal and Casparian strip differentiation. Together, these data suggest a unifying and coordinating role for Ca²⁺ signalling in combating salinity stress at the cellular and organismal level.     

Effects of light on respiration and oxygen isotope fractionation in soybean cotyledons

Light effects on electron flow through the cyanide-resistant respiratory pathway, oxygen isotope fractionation and total respiration were studied in soybean (Glycine max L.) cotyledons. During the first 12 h of illumination there was an increase in both electron partitioning through the alternative pathway and oxygen isotope fractionation by the alternative oxidase. The latter probably indicates a change in the properties of the alternative oxidase. There was no engagement of the alternative oxidase in darkness and its fractionation was 27‰. In green cotyledons 60% of the respiration flux was through the alternative pathway and the alternative oxidase fractionation was 32‰. Exposing previously illuminated tissue to continuous darkness induced a decrease in the electron partitioning through the alternative pathway. However, this decrease was not directly linked with the low cellular sugar concentration resulting from the lack of light because 5 min of light every 12 h was sufficient to keep the alternative pathway engaged to the same extent as plants grown under control conditions.     

Whole-plant gas exchange responses of Spartina alterniflora (Poaceae) to a range of constant and transient salinities

A two-chamber-system was used to study whole-plant gas exchange responses of Spartina alterniflora to long-term and transient salinity treatments over the range of 5 to 40 ppt NaCl. Lower photosynthetic rates, leaf water vapor conductances, belowground respiration rates, and higher aboveground respiration rates in plants adapted to 40 ppt NaCl were observed. Area-specific leaf weight increased with salinity, although the salt content of leaf tissues did not. A reduced rate of gross photosynthesis and higher aboveground respiration rate in 40-ppt NaCl plants significantly lowered the net whole-plant CO₂ gain below that of 5-ppt NaCl plants, while the net CO₂ gain of 25-ppt NaCl plants was intermediate. Within 6 hr of increasing the salinity of 5- and 25-ppt NaCl plants by 20 and 15 ppt NaCl, S. alterniflora responded by reducing leaf water vapor conductance, which in turn reduced the photosynthetic rate. This response was reversed by returning the plants to their original salinity, which indicates that S. alterniflora adjusts water loss and gas exchange in response to transient salinity stress by regulating stomatal aperture. On the other hand, decreasing salinity of the growth media of plants cultured at 25 and 40 ppt NaCl had little or no effect on gas exchange characteristics. This suggests that S. alterniflora adapts to constant salinity through fixed, salinity-dependent structural modifications, such as stomatal density.     

Influence of ethanol on alternative oxidase in mitochondria from callus-forming potato tuber discs

Ethanol, when added to the incubation medium of callus-forming potato tuber discs, inhibits callus growth and causes an increase of the mitochondrial antimycin-A resistant respiration, expressed as a percentage of state III-respiration. This increase in resistance to antimycin-A is the result of a poor development of the cytochrome pathway in tissue discs treated with ethanol. The development of the antimycin-A resistant alternative oxidase sensitive to chelator is about the same for treated and untreated discs. The respiratory control (RC) ratio of the mitochondrial respiration increases after addition of a chelator, which inhibits the alternative pathway. The RC ratio of the uninhibited mitochondrial respiration appears to be inversely related to the capacity of the alternative pathway, when mitochondrial preparations with different capacities to transfer electrons via the alternative path are compared. From the experimentally observed relation between RC-ratio and alternative oxidase capacity, it was concluded that at least half of the capacity of the alternative path is used in uninhibited state IV respiration.