Cytoplasmic malate levels in maize root tips during K+ ion uptake determined by 13C-NMR spectroscopy

Potassium Improves Drought Stress Tolerance in Plants by Affecting Root Morphology, Root Exudates and Microbial Diversity

Potassium (K) reduces the deleterious effects of drought stress on plants. However, this mitigation has been studied mainly in the aboveground plant pathways, while the effect of K on root-soil interactions in the underground part is still underexplored. Here, we conducted the experiments to investigate how K enhances plant resistance and tolerance to drought by controlling rhizosphere processes. Three culture methods (sand, water, and soil) evaluated two rapeseed cultivars’ root morphology, root exudates, soil nutrients, and microbial community structure under different K supply levels and water conditions to construct a defensive network of the underground part. We found that K supply increased the root length and density and the organic acids secretion. The organic acids were significantly associated with the available potassium decomposition, in order of formic acid > malonic acid > lactic acid > oxalic acid > citric acid. However, the mitigation had the hormesis effect, as the appropriate range of K facilitated the morphological characteristic and physiological function of the root system with increases of supply levels, while the excessive input of K could hinder the plant growth. The positive effect of K-fertilizer on soil pH, available phosphorus and available potassium content, and microbial diversity index was more significant under the water stress. The rhizosphere nutrients and pH further promoted the microbial community development by the structural equation modeling, while the non-rhizosphere nutrients had an indirect negative effect on microbes. In short, K application could alleviate drought stress on the growth and development of plants by regulating the morphology and secretion of roots and soil ecosystems.

Transcriptomic and metabolomic analyses reveal mechanisms of adaptation to salinity in which carbon and nitrogen metabolism is altered in sugar beet roots

BACKGROUND

Beta vulgaris L. is one of the main sugar-producing crop species and is highly adaptable to saline soil. This study explored the alterations to the carbon and nitrogen metabolism mechanisms enabling the roots of sugar beet seedlings to adapt to salinity.

RESULTS

The ionome, metabolome, and transcriptome of the roots of sugar beet seedlings were evaluated after 1 day (short term) and 7 days (long term) of 300 mM Na+ treatment. Salt stress caused reactive oxygen species (ROS) damage and ion toxicity in the roots. Interestingly, under salt stress, the increase in the Na+/K+ ratio compared to the control ratio on day 7 was lower than that on day 1 in the roots. The transcriptomic results showed that a large number of differentially expressed genes (DEGs) were enriched in various metabolic pathways. A total of 1279 and 903 DEGs were identified on days 1 and 7, respectively, and were mapped mainly to 10 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. Most of the genes were involved in carbon metabolism and amino acid (AA) biosynthesis. Furthermore, metabolomic analysis revealed that sucrose metabolism and the activity of the tricarboxylic acid (TCA) cycle increased in response to salt stress. After 1 day of stress, the content of sucrose decreased, whereas the content of organic acids (OAs) such as L-malic acid and 2-oxoglutaric acid increased. After 7 days of salt stress, nitrogen-containing metabolites such as AAs, betaine, melatonin, and (S)-2-aminobutyric acid increased significantly. In addition, multiomic analysis revealed that the expression of the gene encoding xanthine dehydrogenase (XDH) was upregulated and that the expression of the gene encoding allantoinase (ALN) was significantly downregulated, resulting in a large accumulation of allantoin. Correlation analysis revealed that most genes were significantly related to only allantoin and xanthosine.

CONCLUSIONS

Our study demonstrated that carbon and nitrogen metabolism was altered in the roots of sugar beet plants under salt stress. Nitrogen metabolism plays a major role in the late stages of salt stress. Allantoin, which is involved in the purine metabolic pathway, may be a key regulator of sugar beet salt tolerance.

Exogenous silicon alters organic acid production and enzymatic activity of TCA cycle in two NaCl stressed indica rice cultivars

The activities of TCA cycle enzymes viz., pyruvate dehydrogenase, citrate synthase, isocitrate dehydrogenase, succinate dehydrogenase and malate dehydrogenase as well as levels of different organic acids viz., pyruvic acid, citric acid, succinic acid and malic acid were studied in two rice cultivars viz. cv. Nonabokra and cv. MTU 1010 differing in salt tolerance grown under 25, 50 and 100 mM NaCl salinity levels. A contrasting response to salt stress on enzyme activities of TCA cycle and accumulation of organic acid was observed between two cultivars during twenty-one days period of study. Salinity caused enhanced organic acid production and increase in all five enzyme activities in cv. Nonabokra whereas in cv. MTU 1010 decrease in both organic acid production and enzymes activities were noted. Joint application of exogenous silicon along with NaCl, altered the organic acids levels and activities of enzymes in both cultivars of rice seedlings conferring tolerance against salt induced stress. Rice cv. MTU 1010 showed better response to exogenous silicon on parameters tested compared to cv. Nonabokra.

Purification and functional characterization of the vacuolar malate transporter tDT from Arabidopsis

The exact transport characteristics of the vacuolar dicarboxylate transporter tDT from Arabidopsis are elusive. To overcome this limitation, we combined a range of experimental approaches comprising generation/analysis of tDT overexpressors, ¹³CO₂ feeding and quantification of ¹³C enrichment, functional characterization of tDT in proteoliposomes, and electrophysiological studies on vacuoles. tdt knockout plants showed decreased malate and increased citrate concentrations in leaves during the diurnal light-dark rhythm and after onset of drought, when compared with wildtypes. Interestingly, under the latter two conditions, tDT overexpressors exhibited malate and citrate levels opposite to tdt knockout plants. Highly purified tDT protein transports malate and citrate in a 1:1 antiport mode. The apparent affinity for malate decreased with decreasing pH, whereas citrate affinity increased. This observation indicates that tDT exhibits a preference for dianion substrates, which is supported by electrophysiological analysis on intact vacuoles. tDT also accepts fumarate and succinate as substrates, but not α-ketoglutarate, gluconate, sulfate, or phosphate. Taking tDT as an example, we demonstrated that it is possible to reconstitute a vacuolar metabolite transporter functionally in proteoliposomes. The displayed, so far unknown counterexchange properties of tDT now explain the frequently observed reciprocal concentration changes of malate and citrate in leaves from various plant species. tDT from Arabidopsis is the first member of the well-known and widely present SLC13 group of carrier proteins, exhibiting an antiport mode of transport.

2-Hydroxy Acids in Plant Metabolism

Glycolate, malate, lactate, and 2-hydroxyglutarate are important 2-hydroxy acids (2HA) in plant metabolism. Most of them can be found as D- and L-stereoisomers. These 2HA play an integral role in plant primary metabolism, where they are involved in fundamental pathways such as photorespiration, tricarboxylic acid cycle, glyoxylate cycle, methylglyoxal pathway, and lysine catabolism. Recent molecular studies in Arabidopsis thaliana have helped elucidate the participation of these 2HA in in plant metabolism and physiology. In this chapter, we summarize the current knowledge about the metabolic pathways and cellular processes in which they are involved, focusing on the proteins that participate in their metabolism and cellular/intracellular transport in Arabidopsis.

Selective induction and subcellular distribution of ACONITASE 3 reveal the importance of cytosolic citrate metabolism during lipid mobilization in Arabidopsis

Arabidopsis thaliana has three genes that encode distinct aconitases (ACO), but little is known about the function of each isoenzyme during plant development. In newly emerged seedlings of Arabidopsis, transcript and protein levels for ACO3 were selectively induced to yield more than 80% of total aconitase activity. Characterization of knockout mutants for each of the three ACOs suggests a major role for only ACO3 in citrate metabolism. The aco3 mutant showed delayed early seedling growth, altered assimilation of [14C]acetate feeding and elevated citrate levels, which were nearly 4-fold greater than in wild-type, aco1 or aco2. However, both ACO1 and ACO2 are active in seedlings as shown by inhibition of aco3 growth by the toxin monofluoroacetate, and altered [14C]acetate assimilation and metabolite levels in aco1 and aco2. Relative levels of fumarate and malate differed between aco2 and aco3, indicating metabolically isolated pools of these metabolites in seedlings. Our inability to enrich ACO protein through mitochondria isolation, and the reduced cytosolic ACO activity of the iron-sulfur centre assembly mutant atm3-1, indicated a cytosolic localization of ACO3 in 3-day-old seedlings. Subsequently, we determined that more than 90% of ACO3 was cytosolic. We conclude that ACO3 is cytosolic in young seedlings and functions in citrate catabolism consistent with the operation of the classic glyoxylate and not direct catabolism of citrate within mitochondria.

Comparative metabolite profiling of two rice genotypes with contrasting salt stress tolerance at the seedling stage

BACKGROUND

Rice is sensitive to salt stress, especially at the seedling stage, with rice varieties differing remarkably in salt tolerance (ST). To understand the physiological mechanisms of ST, we investigated salt stress responses at the metabolite level.

METHODS

Gas chromatography-mass spectrometry was used to profile metabolite changes in the salt-tolerant line FL478 and the sensitive variety IR64 under a salt-stress time series. Additionally, several physiological traits related to ST were investigated.

RESULTS

We characterized 92 primary metabolites in the leaves and roots of the two genotypes under stress and control conditions. The metabolites were temporally, tissue-specifically and genotype-dependently regulated under salt stress. Sugars and amino acids (AAs) increased significantly in the leaves and roots of both genotypes, while organic acids (OAs) increased in roots and decreased in leaves. Compared with IR64, FL478 experienced greater increases in sugars and AAs and more pronounced decreases in OAs in both tissues; additionally, the maximum change in sugars and AAs occurred later, while OAs changed earlier. Moreover, less Na+ and higher relative water content were observed in FL478. Eleven metabolites, including AAs and sugars, were specifically increased in FL478 over the course of the treatment.

CONCLUSIONS

Metabolic responses of rice to salt stress are dynamic and involve many metabolites. The greater ST of FL478 is due to different adaptive reactions at different stress times. At early salt-stress stages, FL478 adapts to stress by decreasing OA levels or by quickly depressing growth; during later stages, more metabolites are accumulated, thereby serving as compatible solutes against osmotic challenge induced by salt stress.

Phosphorus and cadmium interactions in Kandelia obovata (S. L.) in relation to cadmium tolerance

This study focused on the cadmium (Cd) tolerance of mangroves with application of phosphate (P) in order to explore whether exogenous P can alleviate Cd stress on these intertidal species. Kandelia obovata (S. L.) seedlings were cultivated in rhizoboxes under different levels of Cd and P concentrations. The speciation distributions of Cd in the rhizosphere and non-rhizosphere sediments were examined by sequential extraction procedures; organic acid in plant tissues and soil solution was measured by high-performance liquid chromatography; Cd and P accumulation in the plants was also determined. Results showed that considerable differences existed in Cd speciation distributions between rhizosphere and non-rhizosphere sediments. Root activity influenced the dynamics of Cd, P application increased the organic acid content in root tissues, P also increased Cd accumulation in roots whilst lowering Cd translocation from root to the above-ground tissues, and a significant positive correlation was found between Cd and P in roots (r = 0.905). It is postulated that Cd detoxification of K. obovata (S. L.) is associated with higher Cd immobilization in the presence of higher P and organic acid contents in root tissue.

Maize cytosolic NADP-malic enzyme (ZmCytNADP-ME): a phylogenetically distant isoform specifically expressed in embryo and emerging roots

Two maize plastidic NADP-malic enzyme isoforms have been characterized: the bundle sheath-located photosynthetic isoform (ZmC(4)-NADP-ME) and a constitutively expressed one (Zm-nonC(4)-NADP-ME). In this work, the characterization of the first maize cytosolic NADP-ME (ZmCytNADP-ME) is presented, which transcript is exclusively found in embryo and emerging roots. ZmCytNADP-ME expression in roots decreases with development, while Zm-nonC ( 4 ) -NADP-ME increases concomitantly. On the other hand, ZmCytNADP-ME accumulation is differentially modulated by several stress conditions and shows coordination with that of Zm-nonC ( 4 ) -NADP-ME in maize young roots. Recombinant ZmCytNADP-ME displays clearly distinct kinetic parameters and metabolic regulation than the plastidic isoforms. The particular properties and the specific-expression pattern of this novel isoform suggest that it may be involved in the control of cytosolic malate levels in emerging roots, e.g. during hypoxia. ZmCytNADP-ME is phylogenetically related to other cytosolic mono and dicot NADP-MEs, and data indicate that it belongs to an ancestral unique group among plant NADP-MEs.

Four rice genes encoding NADP malic enzyme exhibit distinct expression profiles

In plants, the NADP malic enzymes (NADP-MEs) are encoded by small gene families. These NADP-ME gene families are relatively well described in C4 plants but not well studied in C3 plants. In this study, we investigated the NADP-ME gene family in a model C3 monocot plant (rice, Oryza sativa) based on its recently released genomic DNA sequence. We found that the rice NADP-ME family is composed of four members, one plastidic NADP-ME and three cytosolic versions. Although the rice NADP-ME genes identified share a high degree of similarity with one another, one cytosolic NADP-ME (OscytME3) contains several unique amino acid substitutions within highly conserved amino acid regions. Phylogenetic analysis showed that OscytME3 might be derived from a different evolutionary branch than the other three rice genes. Expression analysis of the four rice NADP-ME genes indicated that each had a different tissue-specific and developmental profile, although all four responded to stress stimuli.

Impaired pH homeostasis in Arabidopsis lacking the vacuolar dicarboxylate transporter and analysis of carboxylic acid transport across the tonoplast

Arabidopsis (Arabidopsis thaliana) mutants lacking the tonoplastic malate transporter AttDT (A. thaliana tonoplast dicarboxylate transporter) and wild-type plants showed no phenotypic differences when grown under standard conditions. To identify putative metabolic changes in AttDT knock-out plants, we provoked a metabolic scenario connected to an increased consumption of dicarboxylates. Acidification of leaf discs stimulated dicarboxylate consumption and led to extremely low levels of dicarboxylates in mutants. To investigate whether reduced dicarboxylate concentrations in mutant leaf cells and, hence, reduced capacity to produce OH(-) to overcome acidification might affect metabolism, we measured photosynthetic oxygen evolution under conditions where the cytosol is acidified. AttDT::tDNA protoplasts showed a much stronger inhibition of oxygen evolution at low pH values when compared to wild-type protoplasts. Apparently citrate, which is present in higher amounts in knock-out plants, is not able to replace dicarboxylates to overcome acidification. To raise more information on the cellular level, we performed localization studies of carboxylates. Although the total pool of carboxylates in mutant vacuoles was nearly unaltered, these organelles contained a lower proportion of malate and fumarate and a higher proportion of citrate when compared to wild-type vacuoles. These alterations concur with the observation that radioactively labeled malate and citrate are transported into Arabidopsis vacuoles by different carriers. In addition, wild-type vacuoles and corresponding organelles from AttDT::tDNA mutants exhibited similar malate channel activities. In conclusion, these results show that Arabidopsis vacuoles contain at least two transporters and a channel for dicarboxylates and citrate and that the activity of AttDT is critical for regulation of pH homeostasis.

Abiotic stresses affecting water balance induce phosphoenolpyruvate carboxylase expression in roots of wheat seedlings

Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) plays an important role in CO(2) fixation in C4 and CAM plants. In C3 plants, PEPC is widely expressed in most organs; however, its function is not yet clearly established. With the aim of providing clues on the function of PEPC in C3 plants, we have analyzed its pattern of expression in wheat ( Triticum aestivum L.) seedlings. Roots showed almost double the level of PEPC activity of shoots. Further analysis of PEPC expression in roots by in situ localization techniques showed a high accumulation of PEPC transcripts and polypeptides in meristematic cells, whereas in the rest of the root PEPC localized preferentially to the vascular tissue. Treatment with NaCl and LiCl induced PEPC expression in roots. Similarly, other abiotic stresses affecting water status, such as drought or cold, induced PEPC expression. Induction was root-specific except for the cold treatment, which also induced PEPC in shoots, although to a lesser extent. In contrast, hypoxia, which does not affect water balance, did not promote any induction of PEPC expression. These results suggest an important role for this enzyme in the adaptation of plants to environmental changes.

Contribution of malic enzyme, pyruvate kinase, phosphoenolpyruvate carboxylase, and the krebs cycle to respiration and biosynthesis and to intracellular pH regulation during hypoxia in maize root tips observed by nuclear magnetic resonance imaging and gas chromatography-mass spectrometry

In vivo pyruvate synthesis by malic enzyme (ME) and pyruvate kinase and in vivo malate synthesis by phosphoenolpyruvate carboxylase and the Krebs cycle were measured by 13C incorporation from [1-13C]glucose into glucose-6-phosphate, alanine, glutamate, aspartate, and malate. These metabolites were isolated from maize (Zea mays L.) root tips under aerobic and hypoxic conditions. 13C-Nuclear magnetic resonance spectroscopy and gas chromatography-mass spectrometry were used to discern the positional isotopic distribution within each metabolite. This information was applied to a simple precursor-product model that enabled calculation of specific metabolic fluxes. In respiring root tips, ME was found to contribute only approximately 3% of the pyruvate synthesized, whereas pyruvate kinase contributed the balance. The activity of ME increased greater than 6-fold early in hypoxia, and then declined coincident with depletion of cytosolic malate and aspartate. We found that in respiring root tips, anaplerotic phosphoenolpyruvate carboxylase activity was high relative to ME, and therefore did not limit synthesis of pyruvate by ME. The significance of in vivo pyruvate synthesis by ME is discussed with respect to malate and pyruvate utilization by isolated mitochondria and intracellular pH regulation under hypoxia.