Inhibition of cell-wall autolysis and pectin degradation by cations.

Modification of cell wall components such as cellulose, hemicellulose and pectin plays an important role in cell expansion. Cell expansion is known to be diminished by cations but it is unknown if this results from cations reacting with pectin or other cell wall components. Autolysis of cell wall material purified from bean root (Phaseolus vulgaris L.) occurred optimally at pH 5.0 and released mainly neutral sugars but very little uronic acid. Autolytic release of neutral sugars and uronic acid was decreased when cell wall material was loaded with Ca, Cu, Sr, Zn, Al or La cations. Results were also extended to a metal-pectate model system, which behaved similarly to cell walls and these cations also inhibited the enzymatic degradation by added polygalacturonase (EC 3.2.1.15). The extent of sugar release from cation-loaded cell wall material and pectate gels was related to the degree of cation saturation of the substrate, but not to the type of cation. The binding strength of the cations was assessed by their influence on the buffer capacity of the cell wall and pectate. The strongly bound cations (Cu, Al or La) resulted in higher cation saturation of the substrate and decreased enzymatic degradability than the weakly held cations (Ca, Sr and Zn). The results indicate that the junction zones between pectin molecules can peel open with weakly held cations, allowing polygalacturonase to cleave the hairy region of pectin, while strongly bound cations or high concentrations of cations force the junction zone closed, minimising enzymatic attack on the pectin backbone.     

Phosphorus deficiency enhances aluminum tolerance of rice (Oryza sativa) by changing the physicochemical characteristics of root plasma membranes and cell walls

The negative charge at the root surface is mainly derived from the phosphate group of phospholipids in plasma membranes (PMs) and the carboxyl group of pectins in cell walls, which are usually neutralized by calcium (Ca) ions contributing to maintain the root integrity. The major toxic effect of aluminum (Al) in plants is the inhibition of root elongation due to Al binding tightly to these negative sites in exchange for Ca. Because phospholipid and pectin concentrations decrease in roots of some plant species under phosphorus (P)-limiting conditions, we hypothesized that rice (Oryza sativa L.) seedlings grown under P-limiting conditions would demonstrate enhanced Al tolerance because of their fewer sites on their roots. For pretreatment, rice seedlings were grown in a culture solution with (+P) or without (−P) P. Thereafter, the seedlings were transferred to a solution with or without Al, and the lipid, pectin, hemicellulose, and mineral concentrations as well as Al tolerance were then determined. Furthermore, the low-Ca tolerance of P-pretreated seedlings was investigated under different pH conditions. The concentrations of phospholipids and pectins in the roots of rice receiving −P pretreatment were lower than those receiving +P pretreatment. As expected, seedlings receiving the −P pretreatment showed enhanced Al tolerance, accompanied by the decrease in Al accumulation in their roots and shoots. This low P-induced enhanced Al tolerance was not explained by enhanced antioxidant activities or organic acid secretion from roots but by the decrease in phospholipid and pectin concentrations in the roots. In addition, low-Ca tolerance of the roots was enhanced by the −P pretreatment under low pH conditions. This low P-induced enhancement of low-Ca tolerance may be related to the lower Ca requirement to maintain PM and cell wall structures in roots of rice with fewer phospholipids and pectins.     

Does the degree of pectin esterification influence aluminium sorption by the root apoplast?

This study investigates the influence of the degree of pectin esterification (DE) on the sorption of aluminium (Al) by plant roots. Ca-pectates, with varying degrees of esterification, are major constituents of the soil–root interface and of the root apoplast. Ca-pectate networks (Ca–PG and Ca–Al–PG) were formed at three DEs (0%, 26%, 65%) with custom-made cells and used as a model system for the root cell wall. Sorption of Al was conducted for 24 h at a range of oxalic acid concentrations (0–500 μM) at pH 4.50 to examine two different metal resistance mechanisms of plants. In fact, plants release organic acids either to desorb or to complex metals to prevent their sorption by plant roots.Thermal analysis showed that Al sorption did not seem to affect the stability of the pectate gels and the presence of hydrophobic groups (–CH₃) at DE?>?0% seemed to even increase the stability of the gels decreasing thermal decomposition. Results suggest two potential Al tolerance mechanisms: (a) high oxalic acid concentrations (500 μM) were able to desorb almost 100% and 72% at DE 65 and 0%, respectively; (b) high oxalic acid concentrations (500 μM) and thus molar ratios of 5:1 (oxalate/Al) reduced Al sorption by 98% and 86% at DE 65 and 0%, respectively. In conclusion, both mechanisms indicate that high degrees of esterification as 65% are much more efficient in excluding Al from the apoplast and might therefore contribute to Al resistance in plants.     

Effect of aluminium on the activity of apoplastic acid phosphatase and the exudation of macromolecules by roots and suspension-culture cells ofZea mays L.

Aluminium accumulates predominantly in the root apoplast where it binds to the pectin matrix of the cell wall with its negative charges. In this study, we investigated whether short-term Al treatment (2 h) affects the activity of apoplastic acid phosphatase and the exudation of macromolecules by roots and suspension-culture cells of Zea mays L. The pectin content of the cell cultures was modified by long-term adaptation to NaCl stress or long-term adaptation to the cellulose-synthesis inhibitor 2,6-dichlorbenzonitrile (DCB), and by short-term treatment for up to 15 min with pectolyase. At pH 4.5, neither acid phosphatase activity of commercial enzyme preparations nor of exudates from root-tips and suspension-cells of Zea mays L. were affected directly by Al. However, the exudation and the activity of apoplastic acid phosphatase was reduced to a greater extent by Al cells with high pectin content than in cells with normal pectin content. The strongest reduction of acid phosphatase exudation was observed in pectolyase-treated cells with the lowest pectin content. Al reduced not only the release of acid phosphatase from the suspension cells, but also the release of total proteins and pectins. However, no relationship existed between the magnitude of Al-induced reduction of protein and pectin release and the cell pectin contents. These results support the assumption that Al modifies cell-wall and plasma-membrane transport-properties for macromolecules and the activity of apoplastic enzymes thus modifying Al sensitivity.     

Effects of low pH and aluminum stresses on common beans (Phaseolus vulgaris) differing in low-phosphorus and photoperiod responses

Using common beans differing greatly in the response to photoperiod and low-phosphorus (P) stress, we investigated their responses to acidity and aluminum (Al)toxicity and the relationship between Al tolerance and organic acid exudation under Al or low P stress. A genotype Ginshi was found to be sensitive to low pH treatment. When exposed to pH 4.5, serious curvature in the root tips of cv. Ginshi was observed; however, it was completely corrected by the application of 5 or 10 μmol/L AlCl3; increasing calcium (Ca) could ameliorate Al toxicity, but it could not correct root curvature at pH 4.5. Common beans showed significant differences in both root growth and Al tolerance, and the varieties from the Andes were more tolerant to Al toxicity than those from the Mesoamerican origin. In the presence of 50 μmol/L AlCl3,all the common bean genotypes exuded citrate, and a significant difference in the amounts of citrate was observed among genotypes. The genotypes originated in the Mesoamerica tended to release more citrate than other origins in the presence of Al. The P-inefficient genotype DOR364 exuded more citrate than the P-efficient genotype G19833 in the presence of 50 μmol/L AlCl3, whereas no organic acids were detected in root exudates under low-P stress. A reduction of citrate exudation in the DOR364, but a slight increase of citrate exudation in the G19833, was observed under Al stress after they were exposed to 6-d P starvation. These results suggest that different low-P or Al tolerance in common beans might not be associated with organic acid exudation.     

Cell-wall pectin and its degree of methylation in the maize root-apex: significance for genotypic differences in aluminium resistance

Cell-wall (CW) pectin content and its degree of methylation in root apices of selected maize cultivars were studied in relation to genotypic Al resistance. Maize cultivars differing in Al resistance were grown in nutrient solution treated with or without Al, and pectin content of the root tips was determined. Control plants did not differ in pectin content in the 5 mm root apex. Al treatment increased the pectin content of the root apex in all cultivars but more prominently in the Al-sensitive cultivars. Pectin and Al contents in 1 mm root sections decreased from the apex to the 3–4 mm zone. Pectin contents of the apical root sections were consistently higher although significantly different only in the 1–2 mm zone in the Al-sensitive cv Lixis. Al contents in most root sections were significantly higher in cv Lixis than in Al-resistant cv ATP-Y. Localization of pectins by immunofluorescence revealed that Al-sensitive cv. Lixis has a higher proportion of low-methylated pectin and thus a higher negativity of the cell wall than Al-resistant cv ATP-Y. This is in agreement with the higher Al content and Al sensitivity of cv Lixis. It is concluded that differences in CW pectin and its degree of methylation contribute to genotypic differences in Al resistance in maize in addition to the release of organic acid anions previously reported.     

A promoter-swap strategy between the AtALMT and AtMATE genes increased Arabidopsis aluminum resistance and improved carbon-use efficiency for aluminum resistance

The primary mechanism of Arabidopsis aluminum (Al) resistance is based on root Al exclusion, resulting from Al-activated root exudation of the Al(3+) -chelating organic acids, malate and citrate. Root malate exudation is the major contributor to Arabidopsis Al resistance, and is conferred by expression of AtALMT1, which encodes the root malate transporter. Root citrate exudation plays a smaller but still significant role in Arabidopsis Al resistance, and is conferred by expression of AtMATE, which encodes the root citrate transporter. In this study, we demonstrate that levels of Al-activated root organic acid exudation are closely correlated with expression of the organic acid transporter genes AtALMT1 and AtMATE. We also found that the AtALMT1 promoter confers a significantly higher level of gene expression than the AtMATE promoter. Analysis of AtALMT1 and AtMATE tissue- and cell-specific expression based on stable expression of promoter-reporter gene constructs showed that the two genes are expressed in complementary root regions: AtALMT1 is expressed in the root apices, while AtMATE is expressed in the mature portions of the roots. As citrate is a much more effective chelator of Al(3+) than malate, we used a promoter-swap strategy to test whether root tip-localized expression of the AtMATE coding region driven by the stronger AtALMT1 promoter (AtALMT1(P)::AtMATE) resulted in increased Arabidopsis Al resistance. Our results indicate that expression of AtALMT1(P)::AtMATE not only significantly increased Al resistance of the transgenic plants, but also enhanced carbon-use efficiency for Al resistance.     

Effects of low pH and aluminum stresses on common beans (Phaseolus vulgaris) differing in low-phosphorus and photoperiod responses

Using common beans differing greatly in the response to photoperiod and low-phosphorus (P) stress, we investigated their responses to acidity and aluminum (Al) toxicity and the relationship between Al tolerance and organic acid exudation under Al or low P stress. A genotype Ginshi was found to be sensitive to low pH treatment. When exposed to pH 4.5, serious curvature in the root tips of cv. Ginshi was observed; however, it was completely corrected by the application of 5 or 10 μmol/L AlCl₃; increasing calcium (Ca) could ameliorate Al toxicity, but it could not correct root curvature at pH 4.5. Common beans showed significant differences in both root growth and Al tolerance, and the varieties from the Andes were more tolerant to Al toxicity than those from the Mesoamerican origin. In the presence of 50 μmol/L AlCl₃, all the common bean genotypes exuded citrate, and a significant difference in the amounts of citrate was observed among genotypes. The genotypes originated in the Mesoamerica tended to release more citrate than other origins in the presence of Al. The P-inefficient genotype DOR364 exuded more citrate than the P-efficient genotype G19833 in the presence of 50 μmol/L AlCl₃, whereas no organic acids were detected in root exudates under low-P stress. A reduction of citrate exudation in the DOR364, but a slight increase of citrate exudation in the G19833, was observed under Al stress after they were exposed to 6-d P starvation. These results suggest that different low-P or Al tolerance in common beans might not be associated with organic acid exudation.     

Phosphorus and aluminum interactions in soybean in relation to aluminum tolerance. Exudation of specific organic acids from different regions of the intact root system

Aluminum (Al) toxicity and phosphorus (P) deficiency often coexist in acid soils that severely limit crop growth and production, including soybean (Glycine max). Understanding the physiological mechanisms relating to plant Al and P interactions should help facilitate the development of more Al-tolerant and/or P-efficient crops. In this study, both homogeneous and heterogeneous nutrient solution experiments were conducted to study the effects of Al and P interactions on soybean root growth and root organic acid exudation. In the homogenous solution experiments with a uniform Al and P distribution in the bulk solution, P addition significantly increased Al tolerance in four soybean genotypes differing in P efficiency. The two P-efficient genotypes appeared to be more Al tolerant than the two P-inefficient genotypes under these high-P conditions. Analysis of root exudates indicated Al toxicity induced citrate exudation, P deficiency triggered oxalate exudation, and malate release was induced by both treatments. To more closely mimic low-P acid soils where P deficiency and Al toxicity are often much greater in the lower soil horizons, a divided root chamber/nutrient solution approach was employed to impose elevated P conditions in the simulated upper soil horizon, and Al toxicity/P deficiency in the lower horizon. Under these conditions, we found that the two P-efficient genotypes were more Al tolerant during the early stages of the experiment than the P-inefficient lines. Although the same three organic acids were exuded by roots in the divided chamber experiments, their exudation patterns were different from those in the homogeneous solution system. The two P-efficient genotypes secreted more malate from the taproot tip, suggesting that improved P nutrition may enhance exudation of organic acids in the root regions dealing with the greatest Al toxicity, thus enhancing Al tolerance. These findings demonstrate that P efficiency may play a role in Al tolerance in soybean. Phosphorus-efficient genotypes may be able to enhance Al tolerance not only through direct Al-P interactions but also through indirect interactions associated with stimulated exudation of different Al-chelating organic acids in specific roots and root regions.     

The physiology and biophysics of an aluminum tolerance mechanism based on root citrate exudation in maize

Al-induced release of Al-chelating ligands (primarily organic acids) into the rhizosphere from the root apex has been identified as a major Al tolerance mechanism in a number of plant species. In the present study, we conducted physiological investigations to study the spatial and temporal characteristics of Al-activated root organic acid exudation, as well as changes in root organic acid content and Al accumulation, in an Al-tolerant maize (Zea mays) single cross (SLP 181/71 x Cateto Colombia 96/71). These investigations were integrated with biophysical studies using the patch-clamp technique to examine Al-activated anion channel activity in protoplasts isolated from different regions of the maize root. Exposure to Al nearly instantaneously activated a concentration-dependent citrate release, which saturated at rates close to 0.5 nmol citrate h(-1) root(-1), with the half-maximal rates of citrate release occurring at about 20 microM Al(3+) activity. Comparison of citrate exudation rates between decapped and capped roots indicated the root cap does not play a major role in perceiving the Al signal or in the exudation process. Spatial analysis indicated that the predominant citrate exudation is not confined to the root apex, but could be found as far as 5 cm beyond the root cap, involving cortex and stelar cells. Patch clamp recordings obtained in whole-cell and outside-out patches confirmed the presence of an Al-inducible plasma membrane anion channel in protoplasts isolated from stelar or cortical tissues. The unitary conductance of this channel was 23 to 55 pS. Our results suggest that this transporter mediates the Al-induced citrate release observed in the intact tissue. In addition to the rapid Al activation of citrate release, a slower, Al-inducible increase in root citrate content was also observed. These findings led us to speculate that in addition to the Al exclusion mechanism based on root citrate exudation, a second internal Al tolerance mechanism may be operating based on Al-inducible changes in organic acid synthesis and compartmentation. We discuss our findings in terms of recent genetic studies of Al tolerance in maize, which suggest that Al tolerance in maize is a complex trait.     

Nitric oxide enhances aluminum tolerance by affecting cell wall polysaccharides in rice roots

Nitric oxide (NO) is a key signal molecule involved in many physiological processes in plants. To study the mechanisms of exogenous NO contribution to alleviate the aluminum (Al) toxicity, roots of rice (Oryza sativa) seedlings pre-treated with sodium nitroprusside (SNP, a NO donor) were used to investigate the effect of Al in this study. Results indicated that NO alleviated the lipid peroxidation induced by Al and promoted the root elongation, whereas butylated hydroxyanisole (BHA), an efficient lipophilic antioxidant, alleviated the lipid peroxidation only. Rice seedling roots pre-treated with SNP followed by Al treatment had lower contents of pectin and hemicellulose, lower Al accumulation in root tips and cell walls, higher degree of methylation of pectin and lower wall Al-binding capacity than the roots with Al treatment only. Therefore, the decreased Al accumulation in the cell walls of rice roots is likely to be the reason for the NO-induced increase of Al tolerance in rice, and it seems that exogenous NO enhanced Al tolerance in rice roots by decreasing the contents of pectin and hemicellulose, increasing the degree of methylation of pectin, and decreasing Al accumulation in root cell walls.     

Effect of boron on the expression of aluminium toxicity in Phaseolus vulgaris

The interaction of boron (B) and aluminium (Al) was investigated in 5-day-old seedlings of soybean cv. Maple Arrow. Al treatment inhibited root elongation and callose formation in root tips particularly after 4-h Al treatment. After 10 and 24 h, both parameters indicated increasing recovery from Al stress. B deficiency aggravated Al toxicity compared with B sufficiency. B deficiency did lead to an increase in unmethylated pectin in the first 3 mm of the root tip. This increase in potential binding sites is reflected in generally higher Al contents in root tips of B-deficient plants. A fractionated extraction of Al from the root tips showed that citrate-exchangeable and non-exchangeable Al steeply increased up to 4 h, but then decreased after 10- and 24-h Al treatment faster in B-sufficient than in B-deficient plants. This decrease of Al contents can be explained by an Al-enhanced release of citrate from the root tips after 10-h Al treatment. However, the citrate exudation rate was the same (after 10 h) or even lower (after 24 h) in B-sufficient plants and thus cannot explain the faster decrease in Al contents of the root tips compared with the B-deficient plants. We, therefore, propose that under B deficiency, Al is more strongly bound by the pectic network of the cell wall of the root tips, which delays or prevents the recovery from initial Al stress through exudation of citrate, and thus explains the greater Al sensitivity of B-deficient common bean 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.     

Not all ALMT1-type transporters mediate aluminum-activated organic acid responses: the case of ZmALMT1 - an anion-selective transporter.

The phytotoxic effects of aluminum (Al) on root systems of crop plants constitute a major agricultural problem in many areas of the world. Root exudation of Al-chelating molecules such as low-molecular-weight organic acids has been shown to be an important mechanism of plant Al tolerance/resistance. Differences observed in the physiology and electrophysiology of root function for two maize genotypes with contrasting Al tolerance revealed an association between rates of Al-activated root organic acid release and Al tolerance. Using these genotypes, we cloned ZmALMT1, a maize gene homologous to the wheat ALMT1 and Arabidopsis AtALMT1 genes that have recently been described as encoding functional, Al-activated transporters that play a role in tolerance by mediating Al-activated organic acid exudation in roots. The ZmALMT1 cDNA encodes a 451 amino acid protein containing six transmembrane helices. Transient expression of a ZmALMT1::GFP chimera confirmed that the protein is targeted to the plant cell plasma membrane. We addressed whether ZmALMT1 might underlie the Al-resistance response (i.e. Al-activated citrate exudation) observed in the roots of the Al-tolerant genotype. The physiological, gene expression and functional data from this study confirm that ZmALMT1 is a plasma membrane transporter that is capable of mediating elective anion efflux and influx. However, gene expression data as well as biophysical transport characteristics obtained from Xenopus oocytes expressing ZmALMT1 indicate that this transporter is implicated in the selective transport of anions involved in mineral nutrition and ion homeostasis processes, rather than mediating a specific Al-activated citrate exudation response at the rhizosphere of maize roots.     

Not all ALMT1-type transporters mediate aluminum-activated organic acid responses: the case of ZmALMT1 – an anion-selective transporter

The phytotoxic effects of aluminum (Al) on root systems of crop plants constitute a major agricultural problem in many areas of the world. Root exudation of Al-chelating molecules such as low-molecular-weight organic acids has been shown to be an important mechanism of plant Al tolerance/resistance. Differences observed in the physiology and electrophysiology of root function for two maize genotypes with contrasting Al tolerance revealed an association between rates of Al-activated root organic acid release and Al tolerance. Using these genotypes, we cloned ZmALMT1 , a maize gene homologous to the wheat ALMT1 and Arabidopsis AtALMT1 genes that have recently been described as encoding functional, Al-activated transporters that play a role in tolerance by mediating Al-activated organic acid exudation in roots. The ZmALMT1 cDNA encodes a 451 amino acid protein containing six transmembrane helices. Transient expression of a ZmALMT1 ::GFP chimera confirmed that the protein is targeted to the plant cell plasma membrane. We addressed whether ZmALMT1 might underlie the Al-resistance response (i.e. Al-activated citrate exudation) observed in the roots of the Al-tolerant genotype. The physiological, gene expression and functional data from this study confirm that ZmALMT1 is a plasma membrane transporter that is capable of mediating elective anion efflux and influx. However, gene expression data as well as biophysical transport characteristics obtained from Xenopus oocytes expressing ZmALMT1 indicate that this transporter is implicated in the selective transport of anions involved in mineral nutrition and ion homeostasis processes, rather than mediating a specific Al-activated citrate exudation response at the rhizosphere of maize roots.     

Cell wall polysaccharides are specifically involved in the exclusion of aluminum from the rice root apex

Rice (Oryza sativa) is the most aluminum (Al)-resistant crop species among the small-grain cereals, but the mechanisms responsible for this trait are still unclear. Using two rice cultivars differing in Al resistance, rice sp. japonica 'Nipponbare' (an Al-resistant cultivar) and rice sp. indica 'Zhefu802' (an Al-sensitive cultivar), it was found that Al content in the root apex (0-10 mm) was significantly lower in Al-resistant 'Nipponbare' than in sensitive 'Zhefu802', with more of the Al localized to cell walls in 'Zhefu802', indicating that an Al exclusion mechanism is operating in 'Nipponbare'. However, neither organic acid efflux nor changes in rhizosphere pH appear to be responsible for the Al exclusion. Interestingly, cell wall polysaccharides (pectin, hemicellulose 1, and hemicellulose 2) in the root apex were found to be significantly higher in 'Zhefu802' than in 'Nipponbare' in the absence of Al, and Al exposure increased root apex hemicellulose content more significantly in 'Zhefu802'. Root tip cell wall pectin methylesterase (PME) activity was constitutively higher in 'Zhefu802' than in 'Nipponbare', although Al treatment resulted in increased PME activity in both cultivars. Immunolocalization of pectins showed a higher proportion of demethylated pectins in 'Zhefu802', indicating a higher proportion of free pectic acid residues in the cell walls of 'Zhefu802' root tips. Al adsorption and desorption kinetics of root tip cell walls also indicated that more Al was adsorbed and bound Al was retained more tightly in 'Zhefu802', which was consistent with Al content, PME activity, and pectin demethylesterification results. These responses were specific to Al compared with other metals (CdCl(2), LaCl(3), and CuCl(2)), and the ability of the cell wall to adsorb these metals was also not related to levels of cell wall pectins. All of these results suggest that cell wall polysaccharides may play an important role in excluding Al specifically from the rice root apex.     

Spatial characteristics of aluminum uptake and translocation in roots of buckwheat (Fagopyrum esculentum)

The detoxification of aluminum (Al) in root tips of the Al accumulator buckwheat by exudation of oxalate leading to reduced Al uptake (Al resistance) is difficult to reconcile with the Al accumulation (Al tolerance). The objective of this study was to analyze resistance and tolerance mechanisms at the same time evaluating particularly possible stratification of Al uptake, Al transport and oxalate exudation along the root apex. The use of a minirhizotron made it possible to differentiate between spatial responses to Al along the root apex with regard to Al uptake and organic acid anion exudation, but also to measure at the same time Al and organic acid transport in the xylem. Al accumulates particularly in the 3‐mm root apex. The study showed that Al taken up by the 10‐mm root apex is rapidly transferred to the xylem which differentiates in the 10 to 15‐mm root zone as revealed by a microscopic study. Al induces the release of oxalate from the root apex but particularly from the subapical 6–20 mm root zone even when Al was applied only to the 5‐mm root apex suggesting a basipetal signal transduction. Citrate proved to be the most likely ligand for Al in the xylem because Al and citrate transport rates were positively correlated. In conclusion, the data presented show that the Al‐induced release of oxalate, and Al uptake as well as Al accumulation are spatially not separated in the root apex.     

Overexpression of Malate Dehydrogenase in Transgenic Alfalfa Enhances Organic Acid Synthesis and Confers Tolerance to Aluminum

Al toxicity is a severe impediment to production of many crops in acid soil. Toxicity can be reduced through lime application to raise soil pH, however this amendment does not remedy subsoil acidity, and liming may not always be practical or cost-effective. Addition of organic acids to plant nutrient solutions alleviates phytotoxic Al effects, presumably by chelating Al and rendering it less toxic. In an effort to increase organic acid secretion and thereby enhance Al tolerance in alfalfa (Medicago sativa), we produced transgenic plants using nodule-enhanced forms of malate dehydrogenase and phosphoenolpyruvate carboxylase cDNAs under the control of the constitutive cauliflower mosaic virus 35S promoter. We report that a 1.6-fold increase in malate dehydrogenase enzyme specific activity in root tips of selected transgenic alfalfa led to a 4.2-fold increase in root concentration as well as a 7.1-fold increase in root exudation of citrate, oxalate, malate, succinate, and acetate compared with untransformed control alfalfa plants. Overexpression of phosphoenolpyruvate carboxylase enzyme specific activity in transgenic alfalfa did not result in increased root exudation of organic acids. The degree of Al tolerance by transformed plants in hydroponic solutions and in naturally acid soil corresponded with their patterns of organic acid exudation and supports the concept that enhancing organic acid synthesis in plants may be an effective strategy to cope with soil acidity and Al toxicity.     

Aluminium effects on mechanical properties of cell wall analogues

Aluminium (Al) toxicity adversely impacts plant productivity in acid soils by restricting root growth and although several mechanisms are involved the physiological basis of decreased root elongation remains unclear. Understanding the primary mechanisms of Al rhizotoxicity is hindered due to the rapid effects of soluble Al on root growth and the close proximity of many cellular components within the cell wall, plasma membrane, cytosol and nucleus with which Al may react. To overcome some of these difficulties, we report on a novel method for investigating Al interactions with Komagataeibacter xylinus bacterial cellulose (BC)‐pectin composites as cell wall analogues. The growth of K. xylinus in the presence of various plant cell wall polysaccharides, such as pectin, has provided a unique in vitro model system with which to investigate the interactions of Al with plant cell wall polysaccharides. The BC‐pectin composites reacted in a similar way with Al as do plant cell walls, providing insights into the effects of Al on the mechanical properties of the BC‐pectin composites as cell wall analogues. Our findings indicated that there were no significant effects of Al (4–160 μM) on the tensile stress, tensile strain or Young's modulus of the composites. This finding was consistent with cellulose, not pectin, being the major load bearing component in BC‐pectin composites, as is also the case in plant cell walls.