Magnesium deficiency-induced changes in organic acid metabolism of Citrus sinensis roots and leaves

Organic acid (OA) metabolisms are of fundamental importance but very limited data are available on the responses of plant OA metabolisms to Mg-deficiency. Seedlings of Citrus sinensis (L.) Osbeck cv. Xuegan were irrigated with Mg-deficient (0, 50, or 500 μM MgSO₄) or Mg-sufficient (2000 μM MgSO₄) nutrient solution every other day for 12 weeks. Thereafter, we investigated the content of Mg, malate, and citrate as well as the activities of acidmetabolizing enzymes in roots and leaves. Root malate content remained stable except for an increase in the highest Mg content and root citrate content increased with increasing root Mg content. As leaf Mg content increased, leaf malate and malate + citrate content decreased whereas leaf citrate content increased. Mg-deficiency decreased or did not affect activities of citrate synthase (CS), aconitase (ACO), phosphoenolpyruvate carboxylase (PEPC), NADP-isocitrate dehydrogenase (NADP-IDH), NAD-malate dehydrogenase (NAD-MDH), NADP-malic enzyme (NADP-ME), and pyruvate kinase (PK) in roots, whereas phosphoenolpyruvate phosphatase (PEPP) activity slightly increased. In contrast, Mg-deficient leaves had higher or similar activities of enzymes above mentioned except PEPP, NAD-MDH, and NADP-ME. In conclusion, both glycolysis and tricarboxylic acid (TCA) cycle may be up-regulated in Mg-deficient leaves but down-regulated in Mg-deficient roots.     

Immunogold localization of photosynthetic enzymes in leaves of various C4 plants, with particular reference to pyruvate orthophosphate dikinase

Cellular localization of photosynthetic enzymes was investigated by immunogold electron microscopy for leaves of nine C4 grasses (three NADP-malic enzyme (NADP-ME)subtype species, three NAD-malic enzyme (NAD-ME) subtype species, and three phosphoenolpyruvate carboxykinase (PCK) subtype species), two C4 sedges (NADP-ME subtype species) and two C4 dicots (an NADP-ME and an NADP/NAD-ME subtype species). In leaves of all species, immunogold labelling was present for phosphoenolpyruvate carboxylase in the cytosol of the mesophyll cells (MC) and for ribulose-1,5-bisphosphate carboxylase/oxygenase in the chloroplasts of the bundle sheath cells (BSC). However, considerable specific variation was found in the intercellular patterns of labelling for pyruvate orthophosphate dikinase (PPDK). In the NADP-ME grasses, two NAD-ME grasses, and the dicots, significant labelling for PPDK was present in the both the BSC and the MC chloroplasts. In the other NAD-ME grass, the PCK grasses, and the sedges, labelling for PPDK was present almost exclusively in the chloroplasts of the MC. These patterns were observed in the leaves of both young seedlings and mature plants. These results indicate that the accumulation of PPDK in leaves of C4 plants is not necessarily restricted to the MC, although the chloroplasts of the MC accumulate more than those of the BSC.Key words: C4 plants, immunolocalization, phosphoenolpyruvate carboxylase, pyruvate orthophosphate dikinase, ribulose-1,5-bisphosphate carboxylase/oxygenase.      

Organic acid exudation induced by phosphorus deficiency and/or aluminium toxicity in two contrasting soybean genotypes

Experiments in nutrient solution were conducted to investigate the exudation of organic acids (OAs) induced by phosphorus deficiency (–P) and/or aluminium toxicity (+Al) in two contrasting soybean genotypes as related to internal OA concentration and related enzyme activities. Baxi 10 (BX10), a known P‐efficient soybean (Glycine max[L] Merr.) genotype, was shown to be more resistant to +Al than a P‐inefficient genotype Bendi 2 (BD2), indicating the potential of selecting soybean cultivars with dual resistance to –P and +Al. The two contrasting genotypes were further characterized for root exudation and formation of oxalate, malate and citrate and their related enzyme activities in response to –P, +Al or both combined. –P significantly induced malate and oxalate exudation from both soybean genotypes, although the P‐efficient BX10 tended to excrete much more oxalate than the P‐inefficient BD2. The +Al treatment triggered citrate efflux from both genotypes, with BX10 having a much greater efflux rate than BD2. Interestingly, –P did not appear to induce citrate exudation, whereas +Al had no obvious effect on malate or oxalate exudation from the two genotypes. The exudation of OAs was generally diminished under the coupled stress of –P and +Al in comparison with either single stress, implying a possible antagonistic effect of the two stresses on OA exudation. Root malate content was negatively correlated with its exudation in BX10 but positively in BD2. A similar tendency was observed for oxalate content and exudation only with less magnitude. Determination of six related enzymes, phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvate phosphatase (PEPP), malate enzyme (ME), isocitrate dehydrogenase (ICDH), malate dehydrogenase (MDH), and pyruvate kinase (PK), in the root tips showed that their activities were not significantly altered during the early stage of treatments (2 and 4 days) whereas at 14 days after stress imposition, the activities of PEPC, PEPP, ME and ICDH were generally enhanced for both genotypes. However, the activity of these enzymes did not appear to be correlated with OA exudation or formation. This study clearly demonstrates that OA exudation is differentially induced by –P and +Al in soybean plants, with specific induction of oxalate and malate by –P and citrate by +Al. The lack of a close relationship between OA exudation and internal concentration or enzyme activities may suggest that the regulation of OA formation and exudation by –P and/or +Al could be imposed at different stages.     

Manganese requirement for optimum photosynthesis and growth in NAD-malic enzyme C-4 species

Despite the evidence for a critical role of Mn in malate decarboxylation and CO₂ release for carbon fixation reactions in C-4 plants, there is a lack of information on their Mn requirement. The objective of this study was to establish Mn levels needed for optimum growth and photosynthesis of four agriculturally important C-4 species, NAD-ME C-4 pearl millet and purple amaranth, and NADP-ME C-4 corn and sorghum, as compared to two C-3 species, wheat and squash. Plants were grown hydroponically in a complete nutrient solution with Mn concentrations ranging from 0 to 100 μM. We report that under these conditions, C-3 and NADP-ME C-4 plants reached their maximum biomass production with 2–5 μM Mn, the concentration commonly used in plant nutrient media. In contrast, Mn concentrations supporting maximum performance of NAD-ME C-4 plants were up to 20-fold higher and ranged between 50 and 100 μM. Although leaf tissue Mn concentrations increased in parallel with Mn nutrition in all plants, the higher leaf Mn had no effect on NADP-ME C-4 or C-3 plants, but it caused a large, up to 100%, increase in net photosynthetic rate in NAD-ME C-4 species. The highest photosynthetic rates across the spectrum of photon flux density were recorded for C-3 and NADP-ME C-4 plants receiving 2–5 μM Mn, and for NAD-ME C-4 species millet and amaranth supplied with 50 or 100 μM Mn, respectively. Squash (C-3) plants were the most sensitive to Mn and their photosynthetic rate was severely depressed with more than 10 μM Mn. The increase in photosynthetic rates of NAD-ME C-4 species occurred without an increase in stomatal conductance, eliminating CO₂ uptake as the main cause. We propose that the higher photosynthetic rates in NAD-ME C-4 species supplied with higher Mn were a result of increased activation of the Mn-dependent NAD-ME in bundle sheath cells, producing greater CO₂ supply for Calvin cycle reactions. This is, to our knowledge, the first report on a significantly higher Mn requirement for optimum photosynthesis and biomass production of NAD-ME C-4 species.     

Nickel-induced changes in carbon metabolism in wheat shoots

In this study, we analyzed the toxic effect of Ni during the development of wheat shoots. Typical developmental alterations in carbon metabolism-related parameters reflecting changes associated with the transition of the seedlings from heterotrophic to autotrophic metabolism were observed in the control shoots between the 1st and the 4th days. Adverse effects of 50 and 100 μM Ni became evident starting from the 4th day of growth on the metal-containing media. We found that Ni-induced stimulation of phosphoenolpyruvate carboxylase (PEPC) activity coincided with decrease in the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) level and with declines in net photosynthetic rate (P_N) and stomatal conductance (g_s). Application of Ni resulted in increased activities of several dehydrogenases: glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), isocitrate dehydrogenase (NADP-ICDH) and malate dehydrogenase (NADH-MDH). In contrast, the activities of malic enzymes (NADP-ME and NAD-ME) decreased due to Ni stress. Treatment with Ni led to accumulation of glucose and declined concentration of sucrose as well as considerable increases in concentrations of malic and citric acids. Our results indicate that Ni stress redirects the carbon metabolism of developing wheat shoots to provide carbon skeletons for synthesis of amino acids and organic acids as well as to supply reducing power to sustain normal metabolic processes and to support defense mechanisms against oxidative stress.     

Physiological adaptations to phosphorus deficiency during proteoid root development in white lupin

Release of large amounts of citric acid from specialized root clusters (proteoid roots) of phosphorus (P)-deficient white lupin (Lupinus albus L.) is an efficient strategy for chemical mobilization of sparingly available P sources in the rhizosphere. The present study demonstrates that increased accumulation and exudation of citric acid and a concomitant release of protons were predominantly restricted to mature root clusters in the later stages of P deficiency. Inhibition of citrate exudation by exogenous application of anion-channel blockers such as ethacrynic- and anthracene-9-carboxylic acids may indicate involvement of an anion channel. Phosphorus-deficiency-induced accumulation and subsequent exudation of citric acid seem to be a consequence of both increased biosynthesis and reduced metabolization of citric acid in the proteoid root tissue, indicated by increased in-vitro activity and enzyme protein levels of phosphoenolpyruvate carboxylase (EC 4.1.1.31), and reduced activity of aconitase (EC 4.2.1.3) and root respiration. Similar to citric acid, acid phosphatase, which is secreted by roots and involved in the mobilization of the organic soil P fraction, was released predominantly from proteoid roots of P-deficient plants. Also ³³Pi uptake per unit root fresh-weight was increased by approximately 50% in juvenile and mature proteoid root clusters compared to apical segments of non-proteoid roots. Kinetic studies revealed a K _m of 30.7 μM for Pi uptake of non-proteoid root apices in P-sufficient plants, versus K _m values of 8.5–8.6 μM for non-proteoid and juvenile proteoid roots under P-deficient conditions, suggesting the induction of a high-affinity Pi-uptake system. Obviously, P-deficiency-induced adaptations of white lupin, involved in P acquisition and mobilization of sparingly available P sources, are predominantly confined to proteoid roots, and moreover to distinct stages during proteoid root development. Received: 10 September 1998 / Accepted: 22 December 1998     

Use of inhibitors to distinguish between C4 acid decarboxylation mechanisms in bundle sheath cells of C4 plants

Both malate and aspartate were decarboxylated at the 4-carbonposition by isolated bundle sheath strands of C₄ plants butto different extents depending upon the species. In Digitariasanguinalis, an NADP-malic enzyme (NADP-ME) species, 100 µMoxalic acid blocked malate decarboxylation through NADP-ME withoutaffecting aspartate decarboxylation which apparently occursthrough NAD-ME. In several phosphoenolpyruvate carboxykinase(PEP-CK) type C₄ species, 200 µM 3-mercaptopicolinic acid(3-MPA), an inhibitor of PEP-CK, specifically inhibited themalate decarboxylation and partially inhibited aspartate decarboxylation.The aspartate decarboxylation insensitive to 3-MPA may occurthrough NAD-ME. Neither inhibitor prevented C₄ acid decarboxylationin bundle sheath cells of NAD-ME species. The inhibitors thusserved to differentiate between the decarboxylation of C₄ acidsin PEP-CK and NADP-ME type C₄ species through their major decarboxylasefrom that of their less active decarboxylation through NAD-ME. ¹ Present address: Department of Biochemistry and Microbiology,Rutgers University, New Brunswick, NJ 08903, U. S. A. (Received January 28, 1977; )     

Metabolic responses in iron deficient tomato plants

The effects of Fe deficiency on different metabolic processes were characterized in roots, xylem sap and leaves of tomato. The total organic acid pool increased significantly with Fe deficiency in xylem sap and leaves of tomato plants, whereas it did not change in roots. However, the composition of the pool changed with Fe deficiency, with major increases in citrate concentrations in roots (20-fold), leaves (2-fold) and xylem sap (17-fold). The activity of phosphoenolpyruvate carboxylase, an enzyme leading to anaplerotic C fixation, increased 10-fold in root tip extracts with Fe deficiency, whereas no change was observed in leaf extracts. The activities of the organic acid synthesis-related enzymes malate dehydrogenase, citrate synthase, isocitrate dehydrogenase, fumarase and aconitase, as well as those of the enzymes lactate dehydrogenase and pyruvate carboxylase, increased with Fe deficiency in root extracts, whereas only citrate synthase increased significantly with Fe deficiency in leaf extracts. These results suggest that the enhanced C fixation capacity in Fe-deficient tomato roots may result in producing citrate that could be used for Fe xylem transport. Total pyridine nucleotide pools did not change significantly with Fe deficiency in roots or leaves, although NAD(P)H/NAD(P) ratios were lower in Fe-deficient roots than in controls. Rates of O(2) consumption were similar in Fe-deficient and Fe-sufficient roots, but the capacity of the alternative oxidase pathway was decreased by Fe deficiency. Also, increases in Fe reductase activity with Fe deficiency were only 2-fold higher when measured in tomato root tips. These values are significantly lower than those found in other plant species, where Fe deficiency leads to larger increases in organic acid synthesis-related enzyme activities and flavin accumulation. These data support the hypothesis that the extent of activation of different metabolic pathways, including carbon fixation via PEPC, organic acid synthesis-related enzymes and oxygen consumption is different among species, and this could modulate the different levels of efficiency in Strategy I plants.     

Magnesium ameliorates aluminum rhizotoxicity in soybean by increasing citric acid production and exudation by roots

Superior effectiveness of Mg over Ca in alleviating Al rhizotoxicity cannot be accounted for by predicted changes in plasma membrane Al3+ activity. The influence of Ca and Mg on the production and secretion of citrate and malate, and on Al accumulation by roots was investigated with soybean genotypes Young and PI 416937 which differ in Al tolerance. In the presence of a solution Al3+ activity of 4.6 microM, citrate and malate concentrations of tap root tips of both genotypes increased with additions of either Ca up to 3 mM or Mg up to 50 microM. Citrate efflux rate from roots exposed to Al was only enhanced with Mg additions and exceeded malate efflux rates by as much as 50-fold. Maximum citrate release occurred within 12 h after adding Mg to solution treatments. Adding 50 microM Mg to 0.8 mM CaSO4 solutions containing Al3+ activities up to 4.6 microM increased citrate concentration of tap root tips by 3- to 5-fold and root exudation of citrate by 6- to 9-fold. Plants treated with either 50 microM Mg or 3 mM Ca had similar reductions in Al accumulation at tap root tips, which coincided with the respective ability of these ions to relieve Al rhizotoxicity. Amelioration of Al inhibition of soybean root elongation by low concentrations of Mg in solution involved Mg-stimulated production and efflux of citrate by roots.     

The role of phosphorus in aluminium-induced citrate and malate exudation from rape (Brassica napus)

Exudation of organic anions is believed to be a common tolerance mechanism for both aluminium toxicity and phosphorus deficiency. Nevertheless, which of these stresses that actually elicit the exudation of organic anions from rape ( Brassica napus L) remains unknown, and the combined effects of Al toxicity and P deficiency on rape have not been reported before. Therefore, in the current study, Brassica napus var. Natane nourin plants grown with or without 0.25 m M P were exposed to 0 or 50 µ M AlCl₃ and several parameters related to the exudation of organic anions from the roots were investigated. Eight days of P deficiency resulted in a significant growth reduction, but P deficiency alone did not induce exudation of organic anions. In contrast, Al strongly induced organic acid exudation, while simultaneously inhibiting root growth. Increased in-vitro activity of citrate synthase (CS, EC 4.1.3.7), malate dehydrogenase (MDH, EC 1.1.1.37) and phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), together with reduced root respiration, indicated that the Al-induced accumulation and subsequent exudation of citrate and malate were associated with both increased biosynthesis and reduced metabolism of citric and malic acid. Phosphorus-sufficient plants showed more pronounced aluminium-induced accumulation and exudation of organic anions than P-deficient plants. A divided root chamber experiment showed the necessity of direct contact between Al and roots to elicit exudation of organic anions. Prolonged exposure (10 days) to Al resulted in a decrease in the net exudation of citrate and malate, and the rate of decrease was much more rapid in P-deficient plants than in P-sufficient plants. It is concluded that P nutrition affects the level of Al-induced synthesis and exudation of organic anions. However, the mechanism needs further investigation.     

Malate decarboxylases: evolution and roles of NAD(P)-ME isoforms in species performing C(4) and C(3) photosynthesis

In the C(4) pathway of photosynthesis two types of malate decarboxylases release CO(2) in bundle sheath cells, NADP- and NAD-dependent malic enzyme (NADP-ME and NAD-ME), located in the chloroplasts and the mitochondria of these cells, respectively. The C(4) decarboxylases involved in C(4) photosynthesis did not evolve de novo; they were recruited from existing housekeeping isoforms. NADP-ME housekeeping isoforms would function in the control of malate levels during hypoxia, pathogen defence responses, and microspore separation, while NAD-ME participates in the respiration of malate in the tricarboxylic acid cycle. Recently, the existence of three enzymatic NAD-ME entities in Arabidopsis, occurring by alternative association of two subunits, was described as a novel mechanism to regulate NAD-ME activity under changing metabolic environments. The C(4) NADP-ME is thought to have evolved from a C(3) chloroplastic ancestor, which in turn would have evolved from an ancient cytosolic enzyme. In this way, the C(4) NADP-ME would have emerged through gene duplication, acquisition of a new promoter, and neo-functionalization. In contrast, there would exist a unique NAD-ME in C(4) plants, which would have been adapted to perform a dual function through changes in the kinetic and regulatory properties of the C(3) ancestors. In addition to this, for the evolution of C(4) NAD-ME, insertion of promoters or enhancers into the single-copy genes of the C(3) ancestors would have changed the expression without gene duplication.     

Immunocytochemical study of phosphoenolpyruvate carboxylase in nodulated Alnus glutinosa

The activities of phosphoenolpyruvate carboxylase (PEP carboxylase, EC 4.1.1.3.1) have been investigated in various organs of young nodulated Alnus glutinosa. The root nodules exhibited the highest specific enzyme activity when compared with the one in roots and leaves. Furthermore, in the root nodules the PEP carboxylase was predominantly localized in the cytosol of the large cortical cells containing the endophyte vesicles.Abbreviations PEP carboxylase phosphoenolpyruvate carboxylase - MDH malate dehydrogenase - PVP polyvinylpyrrolidone - PBS phosphate buffer saline     

Nitric oxide protects sour pummelo (Citrus grandis) seedlings against aluminum-induced inhibition of growth and photosynthesis

Limited data are available on the amelioration of nitric oxide (NO) on aluminum (Al)-toxicity. Sour pummelo (Citrus grandis) seedlings were irrigated for 18 weeks with nutrient solution containing 0 and 1.2 mM AlCl₃·6H₂O × 0 and 10 μM sodium nitroprusside (SNP, an NO donor). Under Al stress, SNP increased root phosphorus (P) and Al, but decreased shoot Al. Al decreased photosynthesis, maximum quantum yield of primary photochemistry (F_v/F_m) and total performance index (PI_tot,sbs), but increased inactivation of oxygen-evolving complex (OEC), K-band and relative variable fluorescence at I-steps (V_I). SNP alleviated Al-induced changes for all these parameters. SNP stimulated Al-induced secretion of malate and citrate by excised roots from Al-treated seedlings, while Al did not increase their contents in roots. Antioxidant system in leaves and roots was up- and down-regulated by Al, respectively. SNP prevented Al-induced accumulation of malondialdehyde (MDA) in roots and leaves. In conclusion, SNP alleviates Al-induced inhibition of growth and impairment of the whole photosynthetic electron transport chain. This occurs through increasing Al-immobilization and P level in roots and Al-induced secretion of malate and citrate from roots, and decreasing Al accumulation in shoots. Thus, the decrease of photosynthesis is prevented. Increased P level and Al-immobilization in roots through SNP may be effected through enhanced secretion of malate and citrate.     

Phosphorus Stress-Induced Proteoid Roots Show Altered Metabolism in Lupinus albus

Proteoid roots develop in Lupinus albus L. in response to nutrient stress, especially P. Proteoid roots excrete citrate and thus increase the availability of P, Fe, and Mn in the rhizosphere. In an effort to understand citrate synthesis and organic acid metabolism in proteoid roots of lupin, we have evaluated in vitro enzyme activities of citrate synthase (CS), malate dehydrogenase (MDH), and phosphoenolpyruvate carboxylase (PEPC) in proteoid and normal roots of plants grown with or without P. Organic acid concentrations, respiration rates, and dark 14CO2-labeling patterns were also determined. The in vitro specific activities of CS, MDH, and PEPC and in vivo dark 14CO2 fixation were higher in proteoid roots compared to normal roots, particularly under P stress. Western blot analysis showed that PEPC enzyme protein was more highly expressed in -P proteoid roots compared to other tissues. The majority of the fixed 14C was found in organic acids, predominantly malate and citrate. A larger fraction of citrate was labeled in P- stressed proteoid roots compared to other root tissue. Respiration rates of proteoid roots were 31% less than those of normal roots. The data provide evidence for increased synthesis of citrate in proteoid roots compared to normal roots, particularly under P stress. A portion of the carbon for citrate synthesis is derived from nonautotrophic CO2 fixation via PEPC in proteoid roots.     

Al-induced efflux of organic acid anions is poorly associated with internal organic acid metabolism in triticale roots

The secretion of organic acid anions from roots has been identified as a mechanism of resistance to Al. However, the process leading to the secretion of organic acid anions is poorly understood. The effect of Al on organic acid metabolism was investigated in two lines of triticale (xTriticosecale Wittmark) differing in Al-induced secretion of malate and citrate and in Al resistance. The site of Al-induced secretion of citrate and malate from a resistant line was localized to the root apices (terminal 5 mm). The levels of citrate (root apices and mature root segments) and malate (mature segments only) in roots increased during exposure to Al, but similar changes were observed in both triticale genotypes. The in vitro activities of four enzymes involved in malate and citrate metabolism (citrate synthase, phosphoenolpyruvate carboxylase, malate dehydrogenase, and NADP-isocitrate dehydrogenase) were similar for sensitive and resistant lines in both root apices and mature root segments. The response of these enzymes to pH did not differ between tolerant and sensitive lines or in the presence and absence of Al. Moreover, cytoplasmic and vacuolar pH were not affected by exposure to Al in either line. Together, these results indicate that the Al-dependent efflux of organic acid anions from the roots of triticale is not regulated by their internal levels in the roots or by the capacity of the root cells to synthesize malate and citrate.     

The role of organic acids in the short- and long-term aluminum tolerance in maize seedlings (<Emphasis Type="Italic">Zea mays</Emphasis> L.)

Aluminum (Al) affects numerous physiological processes in plants. However, Al tolerance mechanisms mediated by increased synthesis of organic acids (OAs) have been outlined recently. In this study, we examined the role of OAs in the short (1–8 h) and long-term (4 days) Al tolerance in maize seedlings. Exposure to Al stress for 4 days results in a rapid inhibition of root growth. Al induced morphological changes in the maize roots, especially at a higher solution of Al concentration (1,000 μM Al). The increase in Al accumulation in roots, including strongly elevated levels of Al accumulated in root cell walls suggests that Al tolerance in maize is mediated in part by higher accumulation of Al in the roots. The enhanced citrate exudation, which was only observed at 1,000 μM Al may lead to detoxification of Al by formation of OA–Al complexes in the root apoplast. This mechanism has been suggested to play a significant role in Al resistance response in maize. The short-term responses underlying internal detoxification via OA-chelators were also investigated. Succinate, malate, citrate and total root OA contents decreased markedly, 2 h after the Al exposure. At 4 and 8 h time points, OA contents increased or remained unchanged, except for that of malate which decreased. The level of OAs in shoots, on the other hand, showed alterations that were less pronounced in response to Al. Specifically, the citrate and total OA concentrations significantly increased at 4 h, but showed a pronounced decrease at the 8 h time point. Based on our findings, we propose that multiple responses, including Al exclusion by Al accumulation in root cells and citrate efflux, may contribute towards higher Al resistance in maize. The rapid OA changes in responses to short-term Al treatment may not be responsible for Al tolerance. However, increased OA synthesis observed in this study may be involved in diminishing the stress triggered by Al. The molecular aspects underlying Al resistance mechanism via Al-induced expression of the enzymes catalyzing OA synthesis and metabolism remain to be elucidated.     

Aluminium induces changes in organic acids metabolism in Coffea arabica suspension cells with differential Al-tolerance

The primary Al-tolerance mechanism in plants involves exudation and/or accumulation of specific organic acid species, which form non-phytotoxic complexes with Al³⁺ under physiological conditions. An evaluation was done of the role of organic acids in the tolerance mechanism of a cell suspension line of coffee Coffea arabica that exhibits Al-tolerance (LAMt) but for which the metabolic tolerance mechanism remains unknown. Significant differences existed in malate dehydrogenase and citrate synthase activities (key enzymes in organic acids metabolism) between protein extracts (day 7 of culture cycle) of the L2 (Al-sensitive) and LAMt (Al-tolerant) cells when cell suspensions were treated with 100 μM AlCl₃. HPLC analysis showed that the suspension cells of both lines exudate malate when incubated in a minimal solution but that exudation was not enhanced by treatment with AlCl₃ (100 μM). This is the first study demonstrating that plant Al-tolerance may be associated with down-regulation of malate dehydrogenase and citrate synthase activities.     

Differential aluminum tolerance in soybean: An evaluation of the role of organic acids

The role of organic acids in aluminum (Al) tolerance has been the object of intensive research. In the present work, we evaluated the roles of organic acid exudation and concentrations at the root tip on Al tolerance of soybean. Exposing soybean seedlings to Al³⁺ activities up to 4.7 μ M in solution led to different degrees of restriction of primary root elongation. Al tolerance among genotypes was associated with citrate accumulation and excretion into the external media. Citrate and malate efflux increased in all genotypes during the first 6 h of Al exposure, but only citrate efflux in Al-tolerant genotypes was sustained for an extended period. Tolerance to Al was correlated with the concentration of citrate in root tips of 8 genotypes with a range of Al sensitivities (r²=0.75). The fluorescent stain lumogallion indicated that more Al accumulated in root tips of the Al-sensitive genotype Young than the Al-tolerant genotype PI 416937, suggesting that the sustained release of citrate from roots of the tolerant genotype was involved in Al exclusion. The initial stimulation of citrate and malate excretion and accumulation in the tip of all genotypes suggested the involvement of additional tolerance mechanisms. The experiments included an examination of Al effects on lateral root elongation. Extension of lateral roots was more sensitive to Al than that of tap roots, and lateral root tips accumulated more Al and had lower levels of citrate.     

Boron-aluminum interactions affect organic acid metabolism more in leaves than in roots of <Emphasis Type="Italic">Citrus grandis</Emphasis> seedlings

Sour pummelo (Citrus grandis) seedlings were irrigated with nutrient solution containing four boron concentrations (i.e., 2.5, 10, 25 and 50 μM H₃BO₃) and two aluminum concentrations [i.e., 0 (-Al) and 1.2 mM AlCl₃ · 6 H₂O (+Al)]. It was found that B did not affect, but Al increased, the Al content in the roots. The Al and citrate contents in the -Al leaves either did not change or slightly increased with increasing B concentration. On the other hand, the Al and citrate contents in the +Al leaves rapidly decreased as B concentration increased from 2.5 to 50 μM, then decreased at the highest B concentration. The Al and citrate contents were higher in the +Al than in the -Al leaves, except for at 25 μM B when they were similar. The leaf malate content did not change in response to B or Al, except for an increase in the +Al leaves and a decrease in the -Al leaves at 2.5 μM B. Similarly, the root malate and citrate contents did not change in response to B with or without Al, except for a decrease in the malate and citrate contents in the +Al roots at 50 μM B and an increase in the citrate content in the -Al roots at 50 μM B. The activities of acid-metabolizing enzymes were less affected by B-Al interactions in the roots than in the leaves.