Breeding for salt tolerance in crop plants — the role of molecular biology

Salinity in soil affects about 7 % of the land’s surface and about 5 % of cultivated land. Most importantly, about 20 % of irrigated land has suffered from secondary salinisation and 50 % of irrigation schemes are affected by salts. In many hotter, drier countries of the world salinity is a concern in their agriculture and could become a key issue. Consequently, the development of salt resistant crops is seen as an important area of research. Although there has been considerable research into the effects of salts on crop plants, there has not, unfortunately, been a commensurate release of salt tolerant cultivars of crop plants. The reason is likely to be the complex nature of the effect of salts on plants. Given the rapid increase in molecular biological techniques, a key question is whether such techniques can aid the development of salt resistance in plants. Physiological and biochemical research has shown that salt tolerance depends on a range of adaptations embracing many aspects of a plant’s physiology: one of these the compartmentation of ions. Introducing genes for compatible solutes, a key part of ion compartmentation, in salt-sensitive species is, conceptually, a simple way of enhancing tolerance. However, analysis of the few data available suggests the consequences of transformation are not straightforward. This is not unexpected for a multigenic trait where the hierarchy of various aspects of tolerance may differ between and within species. The experimental evaluation of the response of transgenic plants to stress does not always match, in quality, the molecular biology. We have advocated the use of physiological traits in breeding programmes as a process that can be undertaken at the present while more knowledge of the genetic basis of salt tolerance is obtained. The use of molecular biological techniques might aid plant breeders through the development of marker aided selection.     

NO says more than ‘YES’ to salt tolerance: Salt priming and systemic nitric oxide signaling in plants

Nitric oxide (NO) is now recognized as an important signaling molecule and there has been an increasing bulk of studies regarding the various functions of NO in plants exposed to environmental stimulus. There is also emerging evidence, although not extensive, that NO plays systemic signaling roles during the establishment of salt tolerance in many plant species. In this mini-review, we highlight several candidate mechanisms as being functional in this NO systemic signaling action. In addition, we outline data supporting that plants possess prime-like mechanisms that allow them to memorize previous NO exposure events and generate defense responses following salt stress.Key words: nitric oxide, nitrosative stress, priming, salinity, systemic signaling     

Ornithine δ-aminotransferase: An enzyme implicated in salt tolerance in higher plants

This review deals with biochemical and physiological aspects of plant ornithine d-aminotransferase (OAT, EC 2.6.1.13). OAT is a mitochondrial enzyme containing pyridoxal-5′-phosphate as a cofactor, which catalyzes the conversion of L-ornithine to L-glutamate γ-semialdehyde using 2-oxoglutarate as a terminal amino group acceptor. It has been described in humans, animals, insects, plants and microorganisms. Based on the crystal structure of human OAT, both substrate binding and reaction mechanism of the enzyme are well understood. OAT shows a large structural and mechanistic similarity to other enzymes from the subgroup III of aminotransferases, which transfer an amino group from a carbon atom that does not carry a carboxyl function. In plants, the enzyme has been implicated in proline biosynthesis and accumulation (via pyrroline-5-carboxylate), which represents a way to regulate cellular osmolarity in response to osmotic stress. However, the exact metabolic pathway involving OAT remains a subject of controversy.Key words: ornithine δ-aminotransferase, osmotic stress, proline, Δ¹-pyrroline-5-carboxylate, pyridoxal-5′-phosphate, semialdehyde, transamination     

Allocation,stress tolerance and carbon transport in plants: how does phloem physiology affect plant ecology?

Despite the crucial role of carbon transport in whole plant physiology and its impact on plant–environment interactions and ecosystem function, relatively little research has tried to examine how phloem physiology impacts plant ecology. In this review, we highlight several areas of active research where inquiry into phloem physiology has increased our understanding of whole plant function and ecological processes. We consider how xylem–phloem interactions impact plant drought tolerance and reproduction, how phloem transport influences carbon allocation in trees and carbon cycling in ecosystems and how phloem function mediates plant relations with insects, pests, microbes and symbiotes. We argue that in spite of challenges that exist in studying phloem physiology, it is critical that we consider the role of this dynamic vascular system when examining the relationship between plants and their biotic and abiotic environment.     

Untangling metabolic and spatial interactions of stress tolerance in plants. 2. Accelerated method for measuring and predicting stress tolerance. Can we unravel the mysteries of the interactions between photosynthesis and respiration?

A simple method using the O₂ electrode that allows examination of the response of respiration and photosynthesis in leaf slices or algae to anoxia and high light under different temperatures useful for the examination of the interactions among photosynthesis, photorespiration, and respiration is described. The method provides a quantifiable assessment of stress tolerance that also permits us to examine fundamental biochemically and genetically related responses involved in stress tolerance and the cooperation among organelles. Additionally, we demonstrated a role for compounds, such as $ {\text{NO}}^{{\text{ - }}}_{{\text{3}}} $ and oxaloacetate, as protective agents against photoinhibition, and we examined the role of dark adaptation in the activation of photosynthesis and $ {\text{NO}}^{{\text{ - }}}_{{\text{3}}} $ -dependent O₂ oxygen evolution. A physiological and ecological role of a dark period (night) in stress tolerance is presented. Utilizing the method to follow changes in such metabolic activities as protein synthesis, protein conformation states, enzymes activity, carbon metabolism, and gene expression at different points during the treatments will be educational.     

Dynamic purine signaling and metabolism during neutrophil–endothelial interactions

During episodes of hypoxia and inflammation, polymorphonuclear leukocytes (PMN) move into underlying tissues by initially passing between endothelial cells that line the inner surface of blood vessels (transendothelial migration, TEM). TEM creates the potential for disturbances in vascular barrier and concomitant loss of extravascular fluid and resultant edema. Recent studies have demonstrated a crucial role for nucleotide metabolism and nucleoside signaling during inflammation. These studies have implicated multiple adenine nucleotides as endogenous tissue protective mechanisms invivo. Here, we review the functional components of vascular barrier, identify strategies for increasing nucleotide generation and nucleoside signaling, and discuss potential therapeutic targets to regulate the vascular barrier during inflammation.     

Origin of sucrose metabolism in higher plants: when,how and why?

Since the discovery of sucrose biosynthesis, considerable advances have been made in understanding its regulation and crucial role in the functional biology of plants. However, important aspects of this metabolism are still an enigma. Studies in cyanobacteria and the publication of the sequences of several complete genomes have recently significantly increased our knowledge of the structures of proteins involved in sucrose metabolism and given us new insights into their origin and further evolution.     

ROS signaling in the hypersensitive response: When,where and what for?

Plants generally react to the attack of non-host and incompatible host microorganisms by inducing pathogenesis-related (PR) genes and localised cell death (LCD) at the site of infection, a process collectively known as the hypersensitive response (HR). Reactive oxygen species (ROS) are generated in various sub-cellular compartments shortly after pathogen recognition, and proposed to cue subsequent orchestration of the HR. Although apoplast-associated ROS production by plasma membrane NADPH oxidases have been most thoroughly studied, recent observations suggest that ROS are generated in chloroplasts earlier in the response and play a key role in execution of LCD. A model is presented in which the initial outcome of successful pathogen detection is ROS accumulation in plastids, likely mediated by mitogen-activated protein kinases and caused by dysfunction of the photosynthetic electron transport chain. ROS signaling is proposed to spread from plastids to the apoplast, through the activation of NADPH oxidases, and from there to adjacent cells, leading to suicidal death in the region of attempted infection.Key words: biotic stress, chloroplasts, flavodoxin, hypersensitive response (HR), reactive oxygen species (ROS), ROS signaling     

Oxidative and nitrosative signaling in plants: Two branches in the same tree?

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) constitute key features underpinning the dynamic nature of cell signaling systems in plants. Despite their importance in many aspects of cell biology, our understanding of oxidative and especially of nitrosative signaling and their regulation remains poorly understood. Early reports have established that ROS and RNS coordinately regulate plant defense responses to biotic stress. In addition, evidence has accumulated demonstrating that there is a strong cross-talk between oxidative and nitrosative signaling upon abiotic stress conditions. The goal of this mini-review is to provide latest findings showing how both ROS and RNS comprise a coordinated oxidative and nitrosative signaling network that modulates cellular responses in response to environmental stimuli.Key words: abiotic stress, nitrosative stress, oxidative stress, reactive nitrogen species, reactive oxygen species, signaling     

Gene regulation and signal transduction in the ICE–CBF–COR signaling pathway during cold stress in plants

Low temperature is an abiotic stress that adversely affects the growth and production of plants. Resistance and adaptation of plants to cold stress is dependent upon the activation of molecular networks and pathways involved in signal transduction and the regulation of cold-stress related genes. Because it has numerous and complex genes, regulation factors, and pathways, research on the ICE–CBF–COR signaling pathway is the most studied and detailed, which is thought to be rather important for cold resistance of plants. In this review, we focus on the function of each member, interrelation among members, and the influence of manipulators and repressors in the ICE–CBF–COR pathway. In addition, regulation and signal transduction concerning plant hormones, circadian clock, and light are discussed. The studies presented provide a detailed picture of the ICE–CBF–COR pathway.     

Linking oxidative and salinity stress tolerance in barley: can root antioxidant enzyme activity be used as a measure of stress tolerance?

Aims

A causal relationship between salinity and oxidative stress tolerance and a suitability of using root antioxidant activity as a biochemical marker for salinity tolerance in barley was investigated.

Methods

Net ion fluxes were measured from the mature zone of excised roots of two barley varieties contrasting in their salinity tolerance using non-invasive MIFE technique in response to acute and prolonged salinity treatment. These changes were correlated with activity of major antioxidant enzymes; ascorbate peroxidase, catalase, and superoxide dismutase.

Results

It was found that genotypic difference in salinity tolerance was largely independent of root integrity, and observed not only for short-term but also long-term NaCl exposures. Higher K⁺ retention ability (and, hence, salinity tolerance) positively correlated with oxidative stress tolerance. At the same time, antioxidant activities were constitutively higher in a sensitive but not tolerant variety, and no correlation was found between SOD activity and salinity tolerance index during large-scale screening.

Conclusion

Although salinity tolerance in barley correlates with its oxidative stress tolerance, higher antioxidant activity at one particular time does not correlate with salinity tolerance and, as such, cannot be used as a biochemical marker in barley screening programs.     

Are there connections between phenol metabolism,ascorbate metabolism and membrane integrity in leaves of boron-deficient sunflower plants?

The influence of soluble phenol concentration and polyphenoloxidase activity in leaves of both B‐deficient and B‐sufficient sunflower plants ( Helianthus annuus L. cv. Frankasol) on plasma membrane permeability was investigated, A study was also undertaken as to whether or not the incubation of B‐deficient leaves in ascorbate‐ and calcium‐containing solutions has a beneficial effect on plasma membrane integrity. Plants were cultivated under controlled environmental conditions with deficient and sufficient B supply and different light intensity to provoke changes in phenol metabolism. Analysis of membrane permeability (measured by potassium efflux), soluble phenol concentration and polyphenoloxidase (EC 1.10.3.1) activity of leaves showed that there was no correlation between these parameters. Furthermore, incubation in solutions containing ascorbate and calcium did not decrease the enhanced membrane permeability due to B deficiency, which could, however, be lowered by boric acid application. In summary, the results suggest that B does not maintain plasma membrane integrity by complexing phenols or inhibiting polyphenoloxidase activity, thereby preventing damage by oxygen free radicals. Ascorbate metabolism or calcium‐related disorders seem also not to be involved. It is therefore likely that B has a direct function at the membrane, possibly by stimulating membrane‐related enzymes, or in a structural role similar to that reported for the cell wall.     

Metabolome-ionome-biomass interactions: What can we learn about salt stress by multiparallel phenotyping?

Long-term exposure of plants to saline soil results in mineral ion imbalance, altered metabolism and reduced growth. Currently, the interaction between ion content and plant metabolism under salt-stress is poorly understood. Here we present a multivariate correlation study on the metabolome, ionome and biomass changes of Lotus japonicus challenged by salt stress. Using latent variable models, we show that increasing salinity leads to reproducible changes of metabolite, ion and nutrient pools. Strong correlations between the metabolome and the ionome or biomass may allow one to estimate the degree of salt stress experienced by a plant based on metabolite profiles. Despite the apparently high predictive power of the models, it remains to be investigated whether such metabolite profiles of non- or moderately-stressed plants can be used by breeding programs as ideal ideotypes for the selection of enhanced salt-tolerant genotypes.Key words: acclimation, ionomic, lotus, metabolic, metabolomic, nutrients, salinity, salt stressAcclimation of plants to saline soils involves changes in the uptake, transport and/or partitioning of mineral ions.^1–3 These responses not only alter ion concentrations but also impair metabolism and growth.⁴ Exactly how metabolism as a whole changes in response to salinity is still unknown because of the complexity of the processes involved. Nevertheless, one might expect plant metabolism to respond in a predictable way to salt stress. With this in mind we carried out a multivariate correlation analysis of 137 metabolome and ionome profiles, and the corresponding biomass measurements, of shoot samples from Lotus japonicus exposed to two different salinity regimes.⁵Metabolome data obtained using GC/EI-TOF-MS technology were analyzed using the TagFinder software,⁶ resulting in a series of discrete metabolic-features. A metabolic-feature may be defined to represent a quantitative signal, measured by any analytical means or technology, which is distinct from analytical signals that arise as artefacts from electronic or chemical noise. A total of 1019 metabolic-features were obtained after filtering for those represented by 3 or more inter-correlated mass fragments.⁶ Corresponding ionomic data were obtained using ICP-AES technology and included measurements of Na and 10 macro- and micro-nutrients (K, Ca, S, P, Mg, B, Mn, Fe, Zn and Mo).⁵To integrate metabolomic and ionomic data, we used the statistical multivariate regression technique called orthogonal projections to latent structures (OPLS^7,8), which performs a regression of two matrices or a matrix versus a single variable and simultaneously corrects the resulting model for systematic, irrelevant variance. The metabolite profile matrix was regressed in three different models against the concentrations of Na, K and a matrix of all nutrients excluding K and Na. These regressions were designated metabolome-[ Na], metabolome-[K] and metabolome [nutrients-K] models, respectively. With OPLS it is possible to estimate how well associated the different metabolites are with the modeled variance of the different ions. The used measure is called the correlation loading and can be interpreted as a multivariate version of the standard Pearson correlation. In order to compare how the metabolic profiles may predict the different matrices, we regressed the correlation loadings vectors of the three different models amongst each other (Fig. 1). Remarkably, the loadings were highly correlated. Despite the high magnitude of change in K content compared to the other nutrients under salinity, the metabolome-[K] and metabolome-[nutrients-K] loadings were nearly identical, highlighting that K levels correlate strongly with the main metabolome correlated variance in the rest of the nutrient matrix. These observations suggest that salinity leads to reproducible changes in metabolite pools which match both the concentration of salt accumulated in the shoot and induced changes in the content of other elements.⁵ Since metabolome profiles have been considered a predictor of plant biomass under non-stressed growth conditions,⁹ a metabolome-biomass model was evaluated for the stress cue of our experimental setup. The metabolome data appeared to be correlated to shoot biomass in a manner similar to the predictability of [Na], [K] and [nutrients-K] (Fig. 2). Presumably, this observation reflects the property of the plant system to integrate in a highly interdependent process the nutritive elements, metabolism and growth.Open in a separate windowFigure 1Regression of the correlation loadings obtained from the models metabolome [Na], metabolome [K] and metabolome [nutrients-K].Open in a separate windowFigure 2Regression of the correlation loadings of the metabolome [Na], metabolome [K] and metabolome [nutrients-K] models with the metabolome biomass model.The correlation loadings of the models allowed a ranking of metabolite-features according to their contribution to the modeled regressions. We used the magnitude of the weight of each metabolic-feature to assess which metabolites may be more characteristic or diagnostic of salt stress, as determined by Na levels (Fig. 3).Open in a separate windowFigure 3Predictive power of the analysis, as revealed by a linear regression between the measured [Na] or biomass and the predicted [Na] or biomass from the models. The predictions were performed based on 10-fold crossvalidation, where in each segment the true values of Na content and biomass were held out and predicted from the corresponding metabolome data using the OPLS model.

Table 1

The top-most positively and negatively correlated metabolites of the metabolite [Na] regression model

β-Alanine metabolism and high salinity stress in the sea anemone,Bunodosoma cavernata

Summary During high salinity stress, -alanine accumulates to high levels in the sea anemone,Bunodosoma cavernata. Following a salinity increase from 26 to 40 -alanine increased 28-fold from 1.5 to 41.9 moles/g dry weight. Both whole animal studies and experiments with cell free homogenates indicate that under high salinity conditions an increase in the rate of -alanine synthesis from aspartic acid as well as a decrease in the rate of -alanine oxidation are responsible for the observed accumulation of -alanine. The rate of aspartic acid decarboxylation to -alanine is about 3 times greater in anemones acclimated to 40 than for those in normal salinity water (26). -alanine oxidation to CO₂ and acetyl-CoA proceeds 2.5 to 3 times slower in high salinity adaptedB. cavernata than in those acclimated to normal salinity. There is always a rapid degradation of uracil to -alanine, but this does not change with salinity.Abbreviations CASF cold acid soluble fraction - FAA free amino acids - MES 2(N-morpholino) ethane sulfonic acid - NPS ninhydrin positive substances - PCA perchloric acid - TCA trichloroacetic acid     

MAPK signalling in cellular metabolism: stress or wellness?

Mitogen-activated protein kinase (MAPK) signalling occurs in response to almost any change in the extracellular or intracellular milieu that affects the metabolism of the cell, organ or the entire organism. MAPK-dependent signal transduction is required for physiological metabolic adaptation, but inappropriate MAPK signalling contributes to the development of several interdependent pathological traits, collectively known as metabolic syndrome. Metabolic syndrome leads to life-threatening clinical consequences, such as type 2 diabetes. This Review provides an overview of the MAPK-signalling mechanisms that underly basic cellular metabolism, discussing their link to disease.