Diferulic acids in the cell wall may contribute to the suppression of shoot growth in the first phase of salt stress in maize

Mechanisms Involved in Cell Wall Remodeling in Etiolated Rice Shoots Grown Under Osmotic Stress

Osmotic stress impacts the cell wall properties in plants. This study aimed to elucidate the mechanisms involved in cell wall remodeling in etiolated (dark-grown) rice (Oryza sativa L.) shoots grown under polyethylene glycol (PEG)-induced osmotic stress conditions. Shoot growth was inhibited by 70% by the treatment with 60 mM PEG for 2 days. However, when the stressed seedlings were transferred to a solution without PEG, their shoot growth rate increased significantly. A measurement of the cell wall mechanical properties revealed that the cell walls of the stressed shoots became looser and more extensible than those of unstressed shoots. Among the cell wall constituents, the amounts of cell wall-bound phenolic acids, such as ferulic acid (FA), p-coumaric acid (p-CA), and diferulic acid (DFA), per shoot and per unit of matrix polysaccharide content were significantly reduced in the stressed shoots compared to those in the unstressed shoots. Concerning the formation of cell wall-bound phenolic acids, the activity of cell wall-bound peroxidase (CW-PRX) per unit of cell wall content, which is responsible for the coupling reaction of FA to produce DFA, was 3.5 times higher in stressed shoots than in unstressed shoots, while the activity was reduced by 20% on a shoot basis in stressed shoots compared to that in unstressed shoots. The expression levels of the major class III peroxidase genes in stressed shoots were either comparable to or slightly lower than those in unstressed shoots. Conversely, the phenylalanine ammonia-lyase (PAL) activity, which contributes to the biosynthesis of FA and p-CA, was reduced by 55% and 30% on a shoot and unit-of-protein-content basis, respectively, in stressed shoots compared to that in unstressed shoots. The expression levels of abundantly expressed PAL genes decreased by 14-46% under osmotic stress. Moreover, the gene expression levels of specific BAHD acyltransferases, which are responsible for the addition of FA and p-CA to form ester-linked moieties on cell wall constituents, decreased by 15-33% under osmotic stress. These results suggest that the downregulation of the expression of specific PAL and BAHD acyltransferase genes in osmotically stressed rice shoots is responsible for a reduction in the formation of cell wall-bound phenolic acid monomers. This, in turn, may result in a decrease in the levels of DFAs. The reduction in the formation of DFA-mediated cross-linking structures within the cell wall may contribute to an increase in the mechanical extensibility of the cell wall. The remodeling of cell walls in an extensible and loosened state could assist in maintaining the growth capacity of etiolated rice shoots grown under osmotic stress and contribute to rapid growth recovery following the alleviation of osmotic stress.

Metabolite analysis of peach (Prunus persica L. Batsch) branches in response to freezing stress

Cold resistance in fruit trees has a direct impact on food production and scientific studies. 'Donghe No.1' is an excellent cold-tolerant peach variety. Metabolomic changes under freezing stress were examined to understand the mechanisms of cold adaptation. The UPLC-MS/MS system was used to identify differentially expressed metabolites (DEMs) in branches of 'Donghe No.1' under freezing stress for 12 h at -5°C, -20°C, -25°C, or -30°C. In total, 1096 metabolites and 196 DEMs were obtained at -5°C vs -20°C, -25°C, and - 30°C, while 179 DEMs and eight shared DEMs obtained at -5°C vs -20°C, -20°C vs -25°C, and -25°C vs -30°C. KEGG enrichment identified 196 DEMs associated with amino acid metabolism, linoleic acid metabolism, alpha-linolenic acid metabolism, phenylpropanoid biosynthesis, and flavonoid biosynthesis under freezing stress. A metabolic network in 1-year-old peach branches under freezing stress is proposed. Moreover, these results enhance understanding of metabolite responses and mechanisms to freezing stress in peach and will help in future breeding of freezing-tolerant varieties and investigating tolerance mechanisms.

Cell size and xylem differentiation regulating genes from Salicornia europaea contribute to plant salt tolerance

Cell wall is involved in plant growth and plays pivotal roles in plant adaptation to environmental stresses. Cell wall remodelling may be crucial to salt adaptation in the euhalophyte Salicornia europaea. However, the mechanism underlying this process is still unclear. Here, full-length transcriptome indicated cell wall-related genes were comprehensively regulated under salinity. The morphology and cell wall components in S. europaea shoot were largely modified under salinity. Through the weighted gene co-expression network analysis, SeXTH2 encoding xyloglucan endotransglucosylase/hydrolases, and two SeLACs encoding laccases were focused. Meanwhile, SeEXPB was focused according to expansin activity and the expression profiling. Function analysis in Arabidopsis validated the functions of these genes in enhancing salt tolerance. SeXTH2 and SeEXPB overexpression led to larger cells and leaves with hemicellulose and pectin content alteration. SeLAC1 and SeLAC2 overexpression led to more xylem vessels, increased secondary cell wall thickness and lignin content. Notably, SeXTH2 transgenic rice exhibited enhanced salt tolerance and higher grain yield. Altogether, these genes may function in the succulence and lignification process in S. europaea. This work throws light on the regulatory mechanism of cell wall remodelling in S. europaea under salinity and provides potential strategies for improving crop salt tolerance and yields.

Correlation between plant cell wall stiffening and root extension arrest phenotype in the combined abiotic stress of Fe and Al

The plasticity and growth of plant cell walls (CWs) remain poorly understood at the molecular level. In this work, we used atomic force microscopy (AFM) to observe elastic responses of the root transition zone of 4-day-old Arabidopsis thaliana wild-type and almt1-mutant seedlings grown under Fe or Al stresses. Elastic parameters were deduced from force-distance curve measurements using the trimechanic-3PCS framework. The presence of single metal species Fe2+ or Al3+ at 10 µM exerts no noticeable effect on the root growth compared with the control conditions. On the contrary, a mix of both the metal ions produced a strong root-extension arrest concomitant with significant increase of CW stiffness. Raising the concentration of either Fe2+ or Al3+ to 20 µM, no root-extension arrest was observed; nevertheless, an increase in root stiffness occurred. In the presence of both the metal ions at 10 µM, root-extension arrest was not observed in the almt1 mutant, which substantially abolishes the ability to exude malate. Our results indicate that the combination of Fe2+ and Al3+ with exuded malate is crucial for both CW stiffening and root-extension arrest. However, stiffness increase induced by single Fe2+ or Al3+ is not sufficient for arresting root growth in our experimental conditions.

Impact of Single and Combined Salinity and High-Temperature Stresses on Agro-Physiological, Biochemical, and Transcriptional Responses in Rice and Stress-Release

Here, for the first time, we aimed to identify in rice the key mechanisms and processes underlying tolerance to high-temperature (HT) or salt stress (SS) alone, the co-occurrence of both stresses, and recovery using physiological and biochemical measurements and gene expression analysis. We also investigated whether recovery from the two stressors depended on the relative intensities/relief of each stressor. Wild type ('Yukinkomai') rice plants were found to be more susceptible to salinity or heat applied individually. SS leads to a depletion of cellular water content, higher accumulation of Na⁺, and alterations in photosynthetic pigments. The stress-tolerant cultivar 'YNU31-2-4' (YNU) displayed a lower Na⁺/K⁺ ratio, higher water content in cells and improved photosynthetic traits, antioxidant system, and expression of defence genes. Strikingly, the SS + HT combination provided a significant level of protection to rice plants from the effects of SS alone. The expression pattern of a selected set of genes showed a specific response and dedicated pathways in plants subjected to each of the different stresses, while other genes were explicitly activated when the stresses were combined. Aquaporin genes were activated by SS, while stress-related (P5CS, MSD1, HSPs, and ions transporters) genes were shaped by HT. Hierarchical clustering and principal component analyses showed that several traits exhibited a gradually aggravating effect as plants were exposed to the combined stresses and identified heat as a mitigating factor, clearly separating heat + salt-stressed from salt-non-heat-stressed plants. Furthermore, seedling recovery was far more dependent on the relative intensities of stressors and cultivars, demonstrating the influence of one stressor over another upon stress-release. Taken together, our data show the uniqueness and complexity of the physiological and molecular network modules used by rice plants to respond to single and combined stresses and recovery.

Not Just a Simple Sugar: Arabinose Metabolism and Function in Plants

Growth, development, structure as well as dynamic adaptations and remodeling processes in plants are largely controlled by properties of their cell walls. These intricate wall structures are mostly made up of different sugars connected through specific glycosidic linkages but also contain many glycosylated proteins. A key plant sugar that is present throughout the plantae, even before the divergence of the land plant lineage, but is not found in animals, is l-arabinose (l-Ara). Here, we summarize and discuss the processes and proteins involved in l-Ara de novo synthesis, l-Ara interconversion, and the assembly and recycling of l-Ara-containing cell wall polymers and proteins. We also discuss the biological function of l-Ara in a context-focused manner, mainly addressing cell wall-related functions that are conferred by the basic physical properties of arabinose-containing polymers/compounds. In this article we explore these processes with the goal of directing future research efforts to the many exciting yet unanswered questions in this research area.

Dynamic changes in cell wall composition of mature sunflower leaves under distinct water regimes affect photosynthesis

In previous work, we identified that exposure to limited water availability induced changes in cell wall composition of mature Helianthus annuus L. leaves that affected mesophyll conductance to CO2 diffusion (gm). However, it is unclear on which timescale these changes in cell wall composition occurred. Here, we subjected H. annuus to control (i.e. water availability), different levels of short-term water deficit stress (ST), long-term water deficit stress (LT), and long-term water deficit stress followed by gradual recoveries addressed at different timescales (LT-Rec) to evaluate the dynamics of modifications in the main composition of cell wall (cellulose, hemicelluloses, pectins and lignins) affecting photosynthesis. During gradual ST treatments, pectins enhancement was associated with gm decline. However, during LT-Rec, pectins content decreased significantly after only 5 h, while hemicelluloses and lignins amounts changed after 24 h, all being uncoupled from gm. Surprisingly, lignins increased by around 200% compared with control and were related to stomatal conductance to gas diffusion (gs) during LT-Rec. Although we suspect that the accuracy of the protocols to determine cell wall composition should be re-evaluated, we demonstrate for the first time that a highly dynamic cell wall composition turnover differently affects photosynthesis in plants subjected to distinct water regimes.

Characterization of Maize Hybrids (Zea mays L.) for Detecting Salt Tolerance Based on Morpho-Physiological Characteristics, Ion Accumulation and Genetic Variability at Early Vegetative Stage

Increasing soil salinity due to global warming severely restricts crop growth and yield. To select and recommend salt-tolerant cultivars, extensive genotypic screening and examination of plants' morpho-physiological responses to salt stress are required. In this study, 18 prescreened maize hybrid cultivars were examined at the early growth stage under a hydroponic system using multivariate analysis to demonstrate the genotypic and phenotypic variations of the selected cultivars under salt stress. The seedlings of all maize cultivars were evaluated with two salt levels: control (without NaCl) and salt stress (12 dS m^-1 simulated with NaCl) for 28 d. A total of 18 morpho-physiological and ion accumulation traits were dissected using multivariate analysis, and salt tolerance index (STI) values of the examined traits were evaluated for grouping of cultivars into salt-tolerant and -sensitive groups. Salt stress significantly declined all measured traits except root-shoot ratio (RSR), while the cultivars responded differently. The cultivars were grouped into three clusters and the cultivars in Cluster-1 such as Prabhat, UniGreen NK41, Bisco 51, UniGreen UB100, Bharati 981 and Star Beej 7Star exhibited salt tolerance to a greater extent, accounting for higher STI in comparison to other cultivars grouped in Cluster-2 and Cluster-3. The high heritability (h²_bs, >60%) and genetic advance (GAM, >20%) were recorded in 13 measured traits, indicating considerable genetic variations present in these traits. Therefore, using multivariate analysis based on the measured traits, six hybrid maize cultivars were selected as salt-tolerant and some traits such as Total Fresh Weight (TFW), Total Dry Weight (TDW), Total Na⁺, Total K⁺ contents and K⁺-Na⁺ Ratio could be effectively used for the selection criteria evaluating salt-tolerant maize genotypes at the early seedling stage.

Silicon-mediated growth promotion in maize (Zea mays L.) occurs via a mechanism that does not involve activation of the plasma membrane H+-ATPase

Silicon (Si)-mediated growth promotion of various grasses is well documented. In the present study, Si-induced changes in maize shoot growth and its underlying mechanisms were studied. Maize plants were grown with various concentrations of Si (0-3 mM) in the nutrient solution. Silicon nutrition improved plant expansion growth. Silicon-supplied maize plants (0.8 and 1.2 mM) showed higher plant height and leaf area compared to no-Si amended plants. It was assumed that Si-induced expansion growth was due to positive Si effects on plasma membrane (PM) H⁺-ATPase. In this context, western blot analysis revealed an increase in PM H⁺-ATPase abundance by 77% under Si nutrition. However, in vitro measurements of enzyme activities showed no significant effect on apoplast pH, proton pumping, passive H⁺ efflux and enzyme kinetics such as K_m, V_max, and activation energy. Further, these results were confirmed by in vivo ratiometric analysis of apoplastic pH, which showed non-significant changes upon Si supply. In contrast, 1 mM Si altered the relative transcripts of specific PM H⁺-ATPase isoforms. Silicon application resulted in a significant decrease of MHA3, and this decrease in transcription seems to be compensated by an increased concentration of H⁺-ATPase protein. From these results, it can be concluded that changes in cell wall composition and PM H⁺-ATPase may be responsible for Si-mediated growth improvement in maize.

Coffee cell walls—composition, influence on cup quality and opportunities for coffee improvements

The coffee beverage is the second most consumed drink worldwide after water. In coffee beans, cell wall storage polysaccharides (CWSPs) represent around 50 per cent of the seed dry mass, mainly consisting of galactomannans and arabinogalactans. These highly abundant structural components largely influence the organoleptic properties of the coffee beverage, mainly due to the complex changes they undergo during the roasting process. From a nutritional point of view, coffee CWSPs are soluble dietary fibers shown to provide numerous health benefits in reducing the risk of human diseases. Due to their influence on coffee quality and their health-promoting benefits, CWSPs have been attracting significant research attention. The importance of cell walls to the coffee industry is not restricted to beans used for beverage production, as several coffee by-products also present high concentrations of cell wall components. These by-products include cherry husks, cherry pulps, parchment skin, silver skin, and spent coffee grounds, which are currently used or have the potential to be utilized either as food ingredients or additives, or for the generation of downstream products such as enzymes, pharmaceuticals, and bioethanol. In addition to their functions during plant development, cell walls also play a role in the plant’s resistance to stresses. Here, we review several aspects of coffee cell walls, including chemical composition, biosynthesis, their function in coffee’s responses to stresses, and their influence on coffee quality. We also propose some potential cell wall–related biotechnological strategies envisaged for coffee improvements.

Cell wall remodeling under salt stress: Insights into changes in polysaccharides, feruloylation, lignification, and phenolic metabolism in maize

Although cell wall polymers play important roles in the tolerance of plants to abiotic stress, the effects of salinity on cell wall composition and metabolism in grasses remain largely unexplored. Here, we conducted an in-depth study of changes in cell wall composition and phenolic metabolism induced upon salinity in maize seedlings and plants. Cell wall characterization revealed that salt stress modulated the deposition of cellulose, matrix polysaccharides and lignin in seedling roots, plant roots and stems. The extraction and analysis of arabinoxylans by size-exclusion chromatography, 2D-NMR spectroscopy and carbohydrate gel electrophoresis showed a reduction of arabinoxylan content in salt-stressed roots. Saponification and mild acid hydrolysis revealed that salinity also reduced the feruloylation of arabinoxylans in roots of seedlings and plants. Determination of lignin content and composition by nitrobenzene oxidation and 2D-NMR confirmed the increased incorporation of syringyl units in lignin of maize roots. Salt stress also induced the expression of genes and the activity of enzymes enrolled in phenylpropanoid biosynthesis. The UHPLC-MS-based metabolite profiling confirmed the modulation of phenolic profiling by salinity and the accumulation of ferulate and its derivatives 3- and 4-O-feruloyl quinate. In conclusion, we present a model for explaining cell wall remodeling in response to salinity.

Phenolic cross-links: building and de-constructing the plant cell wall

Covering: Up to 2019Phenolic cross-links and phenolic inter-unit linkages result from the oxidative coupling of two hydroxycinnamates or two molecules of tyrosine. Free dimers of hydroxycinnamates, lignans, play important roles in plant defence. Cross-linking of bound phenolics in the plant cell wall affects cell expansion, wall strength, digestibility, degradability, and pathogen resistance. Cross-links mediated by phenolic substituents are particularly important as they confer strength to the wall via the formation of new covalent bonds, and by excluding water from it. Four biopolymer classes are known to be involved in the formation of phenolic cross-links: lignins, extensins, glucuronoarabinoxylans, and side-chains of rhamnogalacturonan-I. Lignins and extensins are ubiquitous in streptophytes whereas aromatic substituents on xylan and pectic side-chains are commonly assumed to be particular features of Poales sensu lato and core Caryophyllales, respectively. Cross-linking of phenolic moieties proceeds via radical formation, is catalyzed by peroxidases and laccases, and involves monolignols, tyrosine in extensins, and ferulate esters on xylan and pectin. Ferulate substituents, on xylan in particular, are thought to be nucleation points for lignin polymerization and are, therefore, of paramount importance to wall architecture in grasses and for the development of technology for wall disassembly, e.g. for the use of grass biomass for production of 2^nd generation biofuels. This review summarizes current knowledge on the intra- and extracellular acylation of polysaccharides, and inter- and intra-molecular cross-linking of different constituents. Enzyme mediated lignan in vitro synthesis for pharmaceutical uses are covered as are industrial exploitation of mutant and transgenic approaches to control cell wall cross-linking.

(Poly)phenols, Carotenoids, and Tocochromanols in Corn (Zea mays L.) Kernels As Affected by Phosphate Fertilization and Sowing Time

Corn (Zea mays L.) growth and development is often limited by the availability of phosphate. We thus hypothesized that phosphate fertilization may increase the contents of (poly)phenols, carotenoids, and tocochromanols (vitamin E) in corn grains. Corn plants cultivated on a soil fertilized with 44 kg phosphorus/ha were compared to plants grown on soil with low plant-available phosphate (1.6 mg CAL-P/100 g of soil), each sown early (April) and late (May) in a randomized field experiment. HPLC-DAD-(HR)-ESI-MSⁿ revealed 19 soluble and 10 insoluble (poly)phenols, comprising phenolic acids, phenolic amines, diferulic, and triferulic acids in corn grains. Contents of individual (poly)phenols, carotenoids, and tocochromanols in whole grains were significantly (p < 0.05) increased by sowing time, but not by phosphate fertilization. In conclusion, low phosphate availability did not impair the biosynthesis of (poly)phenols, carotenoids, and tocochromanols in corn grains.

The different tolerance to magnesium deficiency of two grapevine rootstocks relies on the ability to cope with oxidative stress

BACKGROUND

Magnesium (Mg) deficiency causes physiological and molecular responses, already dissected in several plant species. The study of these responses among genotypes showing a different tolerance to the Mg shortage can allow identifying the mechanisms underlying the resistance to this nutritional disorder. To this aim, we compared the physiological and molecular responses (e.g. changes in root metabolome and transcriptome) of two grapevine rootstocks exhibiting, in field, different behaviors with respect to Mg shortage (1103P, tolerant and SO4 susceptible).

RESULTS

The two grapevine rootstocks confirmed, in a controlled growing system, their behavior in relation to the tolerance to Mg deficiency. Differences in metabolite and transcriptional profiles between the roots of the two genotypes were mainly linked to antioxidative compounds and the cell wall constituents. In addition, differences in secondary metabolism, in term of both metabolites (e.g. alkaloids, terpenoids and phenylpropanoids) and transcripts, assessed between 1103P and SO4 suggest a different behavior in relation to stress responses particularly at early stages of Mg deficiency.

CONCLUSIONS

Our results suggested that the higher ability of 1103P to tolerate Mg shortage is mainly linked to its capability of coping, faster and more efficiently, with the oxidative stress condition caused by the nutritional disorder.

Transcription profile analysis of Lycopersicum esculentum leaves, unravels volatile emissions and gene expression under salinity stress

Salinity stress can impede development and plant growth adversely. However, there is very little molecular information on NaCl resistance and volatile emissions in Lycopersicum esculentum. In order to investigate the effects of salt stress on the release of volatile compounds, we quantified and compared transcriptome changes by RNA-Seq analysis and volatile constituents with gas chromatography/mass spectrometry (GC/MS) coupled with solid-phase microextraction (SPME) after exposure to continuous salt stress. Chemical analysis by GC-MS analysis revealed that NaCl stress had changed species and quantity of volatile compounds released. In this research, 21,578 unigenes that represented 44,714 assembled unique transcripts were separated from tomato leaves exposed to NaCl stress based on de novo transcriptome assembly. The total number of differentially expressed genes was 7210 after exposure to NaCl, including 6200 down-regulated and 1208 up-regulated genes. Among these differentially expressed genes (DEGs), there were eighteen differentially expressed genes associated with volatile biosynthesis. Of the unigenes, 3454 were mapped to 131 KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways, mainly those are involved in RNA transport, plant-pathogen interactions, and plant hormone signal transduction. qRT-PCR analysis showed that NaCl exposure affected the expression profiles of the biosynthesis genes for eight volatile compounds (IPI, GPS, and TPS, etc.), which corresponded well with the RNA-Seq analysis and GC-MS results. Our results suggest that NaCl stress affects the emission of volatile substances from L. esculentum leaves by regulating the expression of genes that are involved in volatile organic compounds' biosynthesis.

Relevance, structure and analysis of ferulic acid in maize cell walls

Phenolic compounds in foods have been widely studied due to their health benefits. In cereals, phenolic compounds are extensively linked to cell wall polysaccharides, mainly arabinoxylans, which cross-link with each other and with other cell wall components. In maize, ferulic acid is the phenolic acid present in the highest concentration, forming ferulic acid dehydrodimers, trimers and tetramers. The cross-linking of polysaccharides is important for the cell wall structure and growth, and may protect against pathogen invasion. In addition to the importance for maize physiology, ferulic acid has been recognized as an important chemical structure with a wide range of health benefits when consumed in a diet rich in fibre. This review paper presents the different ways ferulic acid can be present in maize, the importance of ferulic acid derivatives and the methodologies that can be used for their analysis.

Relation between salt tolerance and biochemical changes in cumin ( Cuminum cyminum L.) seeds

In this study, the effects of salinity on growth, fatty acid, essential oil, and phenolic composition of cumin (Cuminum cyminum L.) seeds as well as the antioxidant activities of their extracts were investigated. Plants were treated with different concentrations of NaCl treatment: 0, 50, 75, and 125 mmoL. Plant growth was significantly reduced with the severity of saline treatment. This also caused important reductions in the seed yield and yield components. Besides, NaCl treatments affected fatty acid composition. Petroselinic and linoleic acids proportions diminished consistently with the increase in NaCl concentration, whereas palmitic acid proportion increased. Furthermore, NaCl enhanced essential oil production in C. cyminum seeds and induced marked changes on the essential oil quality. Essential oil chemotype was modified from γ-terpinene/1-phenyl-1,2 ethanediol in control to γ-terpinene/β-pinene in salt stressed plants. Total polyphenol content was higher in treated seeds, and salinity improved the amount of individual phenolic compounds. Moreover, antioxidant activities of the extracts were determined by four different test systems, namely 2,2-diphenyl-1-picrylhydrazyl, β-carotene/linoleic acid chelating, and reducing power assays. The highest antioxidant activities were reveled in severe stressed plants. In this case, cumin seeds produced under saline conditions may function as a potential source of essential oil and antioxidant compounds, which could support the utilization of this plant in a large field of applications such as food industry.

Different allocation of carbohydrates and phenolics in dehydrated leaves of triticale

Carbohydrates are used in plant growth processes, osmotic regulation and secondary metabolism. A study of the allocation of carbohydrates to a target set of metabolites during triticale acclimation to soil drought was performed. The study included a semi-dwarf cultivar 'Woltario' and a long-stemmed cultivar 'Moderato', differing in the activity of the photosynthetic apparatus under optimum growth conditions. Differences were found in the quantitative and qualitative composition of individual carbohydrates and phenolic compounds, depending on the developmental stage and water availability. Soluble carbohydrates in the semi-dwarf 'Woltario' cv. under soil drought were utilized for synthesis of starch, soluble phenolic compounds and an accumulation of cell wall carbohydrates. In the typical 'Moderato' cv., soluble carbohydrates were primarily used for the synthesis of phenolic compounds that were then incorporated into cell wall structures. Increased content of cell wall-bound phenolics in 'Moderato' cv. improved the cell wall tightness and reduced the rate of leaf water loss. In 'Woltario' cv., the increase in cell osmotic potential due to an enhanced concentration of carbohydrates and proline was insufficient to slow down the rate of leaf water loss. The mechanism of cell wall tightening in response to leaf desiccation may be the main key in the process of triticale acclimation to soil drought.

Mechanisms involved in the regulation of photosynthetic efficiency and carbohydrate partitioning in response to low- and high-temperature flooding triggered in winter rye (Secale cereale) lines with distinct pink snow mold resistances

In terms of climate changes and global warming, winter hardiness could be determined by unfavorable environmental conditions other than frost. These could include flooding from melting snow and/or rain, coincident with fungal diseases. Therefore, we designed an experiment to identify potential common mechanisms of flooding tolerance and snow mold resistance, involving the regulation of photosynthetic efficiency and carbohydrate metabolism at low temperatures. Snow mold-resistant and susceptible winter rye (Secale cereale) plants were characterized by considerably different patterns of response to flooding. These differences were clearer at low temperature, thus confirming a possible role of the observed changes in snow mold tolerance. The resistant plants were characterized by lower PSII quantum yields at low temperature, combined with much higher energy flux for energy dissipation from the PSII reaction center. During flooding, the level of soluble carbohydrates increased in the resistant plants and decreased in the susceptible ones. Thus increase in resistant line was connected with a decrease in the energy dissipation rate in PSII/increased photosynthetic activity (energy flux for electron transport), a lower rate of starch degradation and higher rates of sucrose metabolism in leaves. The resistant lines accumulated larger amounts of total soluble carbohydrates in the crowns than in the leaves. Irrespective of flooding treatment, the resistant lines allocated more sugars for cell wall composition, both in the leaves and crowns. Our results clearly indicated that studies on carbohydrate changes at low temperatures or during anoxia should investigate not only the alterations in water-soluble and storage carbohydrates, but also cell wall carbohydrates. The patterns of changes observed after low and high-temperature flooding were different, indicating separate control mechanisms of these responses. These included changes in the photosynthetic apparatus, starch accumulation and cell wall carbohydrate accumulation.

Cell Wall Metabolism in Response to Abiotic Stress

This review focuses on the responses of the plant cell wall to several abiotic stresses including drought, flooding, heat, cold, salt, heavy metals, light, and air pollutants. The effects of stress on cell wall metabolism are discussed at the physiological (morphogenic), transcriptomic, proteomic and biochemical levels. The analysis of a large set of data shows that the plant response is highly complex. The overall effects of most abiotic stress are often dependent on the plant species, the genotype, the age of the plant, the timing of the stress application, and the intensity of this stress. This shows the difficulty of identifying a common pattern of stress response in cell wall architecture that could enable adaptation and/or resistance to abiotic stress. However, in most cases, two main mechanisms can be highlighted: (i) an increased level in xyloglucan endotransglucosylase/hydrolase (XTH) and expansin proteins, associated with an increase in the degree of rhamnogalacturonan I branching that maintains cell wall plasticity and (ii) an increased cell wall thickening by reinforcement of the secondary wall with hemicellulose and lignin deposition. Taken together, these results show the need to undertake large-scale analyses, using multidisciplinary approaches, to unravel the consequences of stress on the cell wall. This will help identify the key components that could be targeted to improve biomass production under stress conditions.

Cell wall remodeling under abiotic stress

Plants exposed to abiotic stress respond to unfavorable conditions on multiple levels. One challenge under drought stress is to reduce shoot growth while maintaining root growth, a process requiring differential cell wall synthesis and remodeling. Key players in this process are the formation of reactive oxygen species (ROS) and peroxidases, which initially cross-link phenolic compounds and glycoproteins of the cell walls causing stiffening. The function of ROS shifts after having converted all the peroxidase substrates in the cell wall. If ROS-levels remain high during prolonged stress, OH°-radicals are formed which lead to polymer cleavage. In concert with xyloglucan modifying enzymes and expansins, the resulting cell wall loosening allows further growth of stressed organs.