Al Tolerance In Maize Research Articles

Combining ability, gene action and heterosis analyses of maize lines that were developed for maize streak virus resistance and acidic soil tolerance

Aluminium toxicity in acidic soils (Al) and the maize streak virus disease (MSV) cause significant losses in maize productivity. These constraints can occur simultaneously in sub-Saharan Africa but there are no maize hybrids that combine resistance for both stresses. Therefore, the objectives of the study were to determine combining ability, gene action and heterosis for MSV and Al tolerance in maize. Ninety-nine experimental hybrids were generated from a 10 × 10 factorial mating design. The experimental maize hybrids, and 21 commercial hybrids were evaluated for MSV resistance under artificial infestation in Zimbabwe, and for grain yield and agronomic performance across eight environments in South Africa and Zimbabwe. Ninety-seven experimental hybrids and five standard control hybrids were evaluated for Al tolerance in a laboratory in South Africa. There were significant (p < 0.05) differences among hybrids for agronomic traits, grain yield, and MSV and Al toxicity tolerance. Only general combining ability effects were significant (p < 0.05) for grain yield and grain moisture content, while specific combining ability effects were significant (p < 0.05) for number of ears and ear prolificacy. Both GCA and SCA effects were significant for the Al toxicity tolerance related traits, indicating that Al tolerance was governed by both additive and non-additive gene action. Although hybrids displayed highly significant variation for MSV resistance, the GCA and SCA mean squares were not significant and only accounted for 59% of the hybrid sum of squares, suggesting that MSV resistance was partly under qualitative inheritance. The F1 hybrids displayed MSV severity scores similar to those of their MSV resistant parents, suggesting that probably simple dominance gene effects were involved. The hybrids also displayed significantly high levels of heterosis for both Al and MSV resistance, indicating that hybridisation would be effective in generating improved varieties. At least 60% of the hybrids were resistant to Al toxicity and more than 87% were MSV resistant, indicating that there is potential to develop varieties with resistance to both stresses.

Metabolic and physiological adjustments of maize leaves in response to aluminum stress

Acidic soils with elevated aluminum (Al) saturations are worldwide distributed and harm the crop production in most of the tropical and subtropical regions. Under these conditions, root elongation may be impaired and thus disturbs water and nutrient uptake. Consequently, physiological responses of plants challenged with excess Al may resemble those of drought stresses. Here, we hypothesized that drought tolerant plants are also Al tolerant due to changes in growth, metabolic and physiological adjustments in leaves. Two maize genotypes, BRS1010 and BRS1055, sensitive and tolerant to drought, respectively, were hydroponically grown under controlled conditions and challenged with two Al concentrations (0 and 100 µM AlCl3) for 5 days. After treatment with Al, BRS1055 plants displayed increased leaf and stem elongation whereas the relative root growth rate remained unchanged. This was accompanied by unaltered root structure, photosynthetic efficiency and leaf primary metabolism. In sharp contrast, the BRS1010 plants were sensitive to Al, exhibiting a reduction in leaf and stem elongation and biomass accumulation in shoot and root, as well as greater structural damages in root tips. Additionally, in response to Al, lipid peroxidation increased in BRS1010 leaves in parallel to inhibition of photosynthetic performance and dark respiration. Moreover, compared to control treatment, the genotype BRS1010 displayed a large accumulation of sugars, amino acid, proteins and organic acids in leaves under Al stress. Therefore, the leaf physiology and metabolism are pivotal players in modulating Al tolerance in maize.

Aluminum tolerance mechanisms in Kenyan maize germplasm are independent from the citrate transporter ZmMATE1

Aluminum (Al) toxicity on acid soils adversely affects maize yields, which can be overcome by combining soil amendments with genetic tolerance. In maize, ZmMATE1 confers Al tolerance via Al-activated citrate release, whereby citrate forms non-toxic complexes with Al3+ in the rhizosphere. Here, we investigated Al tolerance mechanisms in maize germplasm originated from Kenya based on quantitative trait loci (QTL) mapping. Five QTLs and four epistatic interactions explained ~51% of the phenotypic variation for Al tolerance. The lack of Al tolerance QTL on chromosome 6 and the much lower expression of ZmMATE1 in both Kenyan lines than in Cateto Al237, which donates the superior allele of ZmMATE1, strongly indicate that this gene does not play a significant role in Al tolerance in neither parent. In turn, maize homologs to genes previously implicated in Al tolerance in other species, ZmNrat1, ZmMATE3, ZmWRKY and ZmART1, co-localized with Al tolerance QTL and were more highly expressed in the parent that donate favorable QTL alleles. However, these candidate genes will require further studies for functional validation on maize Al tolerance. The existence of Al tolerance mechanisms independent from ZmMATE1 suggests it is possible to develop highly Al tolerant cultivars by pyramiding complementary Al tolerance genes in maize.

Genetic control of aluminum tolerance in tropical maize germplasm

ABSTRACT Aluminum (Al) toxicity is the most limiting factor to maize crop productivity in acid soils. Therefore, the understanding of inheritance of Al tolerance in maize is important for the developmentof more adequate procedures for Al tolerant genotypes selection. In this sense, the objectives of this study were to determine the inheritance, and the general (GCA) and specific combining ability (SCA) for Al tolerance in tropical maize. First, we evaluated diallel crosses of maize from landrace and hybrid germplasms for Al tolerance through the minimal solution methodology. The DIF data (root growth difference) were analyzed by Griffing diallel model. Later the additive-dominant genetic model proposed by Mather and Jinks (1971) was used to estimate the genetic effects. The results of the diallel analysis showed greater variability associated with the estimates of the SCA for both germplasm. The diallel crosses involving the V 06 (Dente de Ouro 2) landrace stood out by high SCA and GCA for Al tolerance. The generation mean analysis indicated quantitative inheritance of Al tolerance in this germplasm, with most of the variance explained by the additive effects. The heritability in the narrow sense varied from 47% to 71%, indicating the possibility of genetic gain with the selection of tolerant genotypes in F2 generation. Additive gene action associated with intermediate heritability and quantitative inheritance demonstrates the possibility of genetic gains with artificial selection for Al tolerance in this maize germplasm.

Aluminium Tolerance Assessment of 141 Maize Germplasms in a Solution Culture

Aluminum (Al) toxicity usually occurs in acidic soils with a pH of 5.5 or lower. The selection and breeding of Al-tolerant cultivars is a useful approach for protecting maize from Al toxicity. Rapid and reliable screening methods must be developed to discriminate Al-tolerant and Al-sensitive maize genotypes. The relative root growth (RRG) of the longest root in a toxic Al solution was used to classify 141 maize germplasm lines into three groups with varied Al sensitivity: Al-sensitive, moderately Al-tolerant, and Al-tolerant. Among these lines, the cultivars HZ85 and 178 had the highest RRG values and therefore the highest Al tolerance. Further root assessment of six representative lines using other methods, such as digital imaging analysis of total root length and superficial area or volume, biomass measurement, and hematoxylin staining, was roughly consistent with the classification based on RRG. These results indicated that the RRG of the longest root could be used as a reliable and reproducible phenotypic index for the evaluation of Al tolerance in maize genotypes. Cultivars with different Al tolerances can be used to improve breeding and explore the mechanism of Al tolerance in maize.

MECHANISM OF ALUMINUM TOXICITY AVOIDANCE IN TROPICAL RICE (Oryza sativa), MAIZE (Zea mays), AND SOYBEAN (Glycine max)

Planting Al tolerant crops is an economically justifiable approach in crop production on acid soils. Experiments were conducted to study the mechanisms of Al tolerance among species and varieties of tropical rice, maize, and soybean with previously known levels of Al tolerance. These varieties were hydroponically cultured in 0, 5, 10, and 30 mg l-1 Al with complete nutrient solution at pH 4. The results show that root/shoot ratio of dry weight at 10 mg l-1 Al treatment was an important parameter to indicate differential Al tolerance in maize. Oxalic acid exudation from roots cannot always explain the Al tolerance. Total organic acid concentration in roots at 10 mg l-1 Al treatment indicated a difference of Al tolerance in soybean and lowland rice. Aluminum translocation from roots to shoots was lower in tolerant varieties than in sensitive varieties of soybean. Increased Al concentration in shoots with increased Al level in the solution was larger in soybean and maize than in lowland or upland rice. Among varieties of soybean, the Al concentration in shoots increased drastically in Wilis (Al-sensitive variety) with increase Al level, while in Kitamusume (Al-tolerant variety) it did not.

MECHANISM OF ALUMINUM TOXICITY AVOIDANCE IN TROPICAL RICE (Oryza sativa), MAIZE (Zea mays), AND SOYBEAN (Glycine max)

Planting Al tolerant crops is an economically justifiable approach in crop production on acid soils. Experiments were conducted to study the mechanisms of Al tolerance among species and varieties of tropical rice, maize, and soybean with previously known levels of Al tolerance. These varieties were hydroponically cultured in 0, 5, 10, and 30 mg l-1 Al with complete nutrient solution at pH 4. The results show that root/shoot ratio of dry weight at 10 mg l-1 Al treatment was an important parameter to indicate differential Al tolerance in maize. Oxalic acid exudation from roots cannot always explain the Al tolerance. Total organic acid concentration in roots at 10 mg l-1 Al treatment indicated a difference of Al tolerance in soybean and lowland rice. Aluminum translocation from roots to shoots was lower in tolerant varieties than in sensitive varieties of soybean. Increased Al concentration in shoots with increased Al level in the solution was larger in soybean and maize than in lowland or upland rice. Among varieties of soybean, the Al concentration in shoots increased drastically in Wilis (Al-sensitive variety) with increase Al level, while in Kitamusume (Al-tolerant variety) it did not.

Physiological and molecular analysis of aluminum tolerance in selected Kenyan maize lines

Aluminum (Al) toxicity is an important limitation to maize production in many tropical and sub-tropical acid soil areas. The aim of this study was to survey the variation in Al tolerance in a panel of maize lines adapted for Kenya and look for novel sources of Al tolerance. 112 Kenyan maize accessions were phenotyped for Al tolerance in solution culture. Several Al tolerance-related parameters including relative net root growth (RNRG), root apex Al accumulation, Al-activated root organic acid exudation, and expression of the maize Al tolerance gene, ZmMATE1, were used to classify Kenyan maize accessions. Based on RNRG, 42 %, 28 %, and 30 % of the lines were classified as highly tolerant, moderately tolerant and sensitive, respectively. Tolerant accessions accumulated less Al in their root apices compared to sensitive lines. The Kenyan maize line, CON 5, and the Brazilian standard for tolerance, Cateto, exhibited the greatest Al tolerance based on RNRG, but CON 5 had only about 50 % of ZmMATE1 gene expression relative to Cateto. CON 5 also had low root apex Al content and high citrate exudation, suggesting that it may employ a citrate transporter other than ZmMATE1. We identified a very Al tolerant Kenyan maize line whose Al tolerance may be based in part on a novel tolerance gene. The maize lines identified in this study are useful germplasm for the development of varieties suitable for agriculture on acid soils in Kenya.

Genetic dissection of Al tolerance QTLs in the maize genome by high density SNP scan.

BackgroundAluminum (Al) toxicity is an important limitation to food security in tropical and subtropical regions. High Al saturation on acid soils limits root development, reducing water and nutrient uptake. In addition to naturally occurring acid soils, agricultural practices may decrease soil pH, leading to yield losses due to Al toxicity. Elucidating the genetic and molecular mechanisms underlying maize Al tolerance is expected to accelerate the development of Al-tolerant cultivars.ResultsFive genomic regions were significantly associated with Al tolerance, using 54,455 SNP markers in a recombinant inbred line population derived from Cateto Al237. Candidate genes co-localized with Al tolerance QTLs were further investigated. Near-isogenic lines (NILs) developed for ZmMATE2 were as Al-sensitive as the recurrent line, indicating that this candidate gene was not responsible for the Al tolerance QTL on chromosome 5, qALT5. However, ZmNrat1, a maize homolog to OsNrat1, which encodes an Al3+ specific transporter previously implicated in rice Al tolerance, was mapped at ~40 Mbp from qALT5. We demonstrate for the first time that ZmNrat1 is preferentially expressed in maize root tips and is up-regulated by Al, similarly to OsNrat1 in rice, suggesting a role of this gene in maize Al tolerance. The strongest-effect QTL was mapped on chromosome 6 (qALT6), within a 0.5 Mbp region where three copies of the Al tolerance gene, ZmMATE1, were found in tandem configuration. qALT6 was shown to increase Al tolerance in maize; the qALT6-NILs carrying three copies of ZmMATE1 exhibited a two-fold increase in Al tolerance, and higher expression of ZmMATE1 compared to the Al sensitive recurrent parent. Interestingly, a new source of Al tolerance via ZmMATE1 was identified in a Brazilian elite line that showed high expression of ZmMATE1 but carries a single copy of ZmMATE1.ConclusionsHigh ZmMATE1 expression, controlled either by three copies of the target gene or by an unknown molecular mechanism, is responsible for Al tolerance mediated by qALT6. As Al tolerant alleles at qALT6 are rare in maize, marker-assisted introgression of this QTL is an important strategy to improve maize adaptation to acid soils worldwide.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-153) contains supplementary material, which is available to authorized users.

Maize ZmALMT2 is a root anion transporter that mediates constitutive root malate efflux

Root efflux of organic acid anions underlies a major mechanism of plant aluminium (Al) tolerance on acid soils. This efflux is mediated by transporters of the Al-activated malate transporter (ALMT) or the multi-drug and toxin extrusion (MATE) families. ZmALMT2 was previously suggested to be involved in Al tolerance based on joint association-linkage mapping for maize Al tolerance. In the current study, we functionally characterized ZmALMT2 by heterologously expressing it in Xenopus laevis oocytes and transgenic Arabidopsis. In oocytes, ZmALMT2 mediated an Al-independent electrogenic transport product of organic and inorganic anion efflux. Ectopic overexpression of ZmALMT2 in an Al-hypersensitive Arabidopsis KO/KD line lacking the Al tolerance genes, AtALMT1 and AtMATE, resulted in Al-independent constitutive root malate efflux which partially restored the Al tolerance phenotype. The lack of correlation between ZmALMT2 expression and Al tolerance (e.g., expression not localized to the root tip, not up-regulated by Al, and higher in sensitive versus tolerance maize lines) also led us to question ZmALMT2's role in Al tolerance. The functional properties of the ZmALMT2 transporter presented here, along with the gene expression data, suggest that ZmALMT2 is not involved in maize Al tolerance but, rather, may play a role in mineral nutrient acquisition and transport.

Transcriptional profile of maize roots under acid soil growth.

BackgroundAluminum (Al) toxicity is one of the most important yield-limiting factors of many crops worldwide. The primary symptom of Al toxicity syndrome is the inhibition of root growth leading to poor water and nutrient absorption. Al tolerance has been extensively studied using hydroponic experiments. However, unlike soil conditions, this method does not address all of the components that are necessary for proper root growth and development. In the present study, we grew two maize genotypes with contrasting tolerance to Al in soil containing toxic levels of Al and then compared their transcriptomic responses.ResultsWhen grown in acid soil containing toxic levels of Al, the Al-sensitive genotype (S1587-17) showed greater root growth inhibition, more Al accumulation and more callose deposition in root tips than did the tolerant genotype (Cat100-6). Transcriptome profiling showed a higher number of genes differentially expressed in S1587-17 grown in acid soil, probably due to secondary effects of Al toxicity. Genes involved in the biosynthesis of organic acids, which are frequently associated with an Al tolerance response, were not differentially regulated in both genotypes after acid soil exposure. However, genes related to the biosynthesis of auxin, ethylene and lignin were up-regulated in the Al-sensitive genotype, indicating that these pathways might be associated with root growth inhibition. By comparing the two maize lines, we were able to discover genes up-regulated only in the Al-tolerant line that also presented higher absolute levels than those observed in the Al-sensitive line. These genes encoded a lipase hydrolase, a retinol dehydrogenase, a glycine-rich protein, a member of the WRKY transcriptional family and two unknown proteins.ConclusionsThis work provides the first characterization of the physiological and transcriptional responses of maize roots when grown in acid soil containing toxic levels of Al. The transcriptome profiles highlighted several pathways that are related to Al toxicity and tolerance during growth in acid soil. We found several genes that were not found in previous studies using hydroponic experiments, increasing our understanding of plant responses to acid soil. The use of two germplasms with markedly different Al tolerances allowed the identification of genes that are a valuable tool for assessing the mechanisms of Al tolerance in maize in acid soil.

Association and Linkage Analysis of Aluminum Tolerance Genes in Maize

BackgroundAluminum (Al) toxicity is a major worldwide constraint to crop productivity on acidic soils. Al becomes soluble at low pH, inhibiting root growth and severely reducing yields. Maize is an important staple food and commodity crop in acidic soil regions, especially in South America and Africa where these soils are very common. Al exclusion and intracellular tolerance have been suggested as two important mechanisms for Al tolerance in maize, but little is known about the underlying genetics.MethodologyAn association panel of 282 diverse maize inbred lines and three F2 linkage populations with approximately 200 individuals each were used to study genetic variation in this complex trait. Al tolerance was measured as net root growth in nutrient solution under Al stress, which exhibited a wide range of variation between lines. Comparative and physiological genomics-based approaches were used to select 21 candidate genes for evaluation by association analysis.ConclusionsSix candidate genes had significant results from association analysis, but only four were confirmed by linkage analysis as putatively contributing to Al tolerance: Zea mays AltSB like (ZmASL), Zea mays aluminum-activated malate transporter2 (ALMT2), S-adenosyl-L-homocysteinase (SAHH), and Malic Enzyme (ME). These four candidate genes are high priority subjects for follow-up biochemical and physiological studies on the mechanisms of Al tolerance in maize. Immediately, elite haplotype-specific molecular markers can be developed for these four genes and used for efficient marker-assisted selection of superior alleles in Al tolerance maize breeding programs.

Two functionally distinct members of the MATE (multi-drug and toxic compound extrusion) family of transporters potentially underlie two major aluminum tolerance QTLs in maize

Crop yields are significantly reduced by aluminum (Al) toxicity on acidic soils, which comprise up to 50% of the world's arable land. Al-activated release of ligands (such as organic acids) from the roots is a major Al tolerance mechanism in plants. In maize, Al-activated root citrate exudation plays an important role in tolerance. However, maize Al tolerance is a complex trait involving multiple genes and physiological mechanisms. Recently, transporters from the MATE family have been shown to mediate Al-activated citrate exudation in a number of plant species. Here we describe the cloning and characterization of two MATE family members in maize, ZmMATE1 and ZmMATE2, which co-localize to major Al tolerance QTL. Both genes encode plasma membrane proteins that mediate significant anion efflux when expressed in Xenopus oocytes. ZmMATE1 expression is mostly concentrated in root tissues, is up-regulated by Al and is significantly higher in Al-tolerant maize genotypes. In contrast, ZmMATE2 expression is not specifically localized to any particular tissue and does not respond to Al. [(14)C]-citrate efflux experiments in oocytes demonstrate that ZmMATE1 is a citrate transporter. In addition, ZmMATE1 expression confers a significant increase in Al tolerance in transgenic Arabidopsis. Our data suggests that ZmMATE1 is a functional homolog of the Al tolerance genes recently characterized in sorghum, barley and Arabidopsis, and is likely to underlie the largest maize Al tolerance QTL found on chromosome 6. However, ZmMATE2 most likely does not encode a citrate transporter, and could be involved in a novel Al tolerance mechanism.

The role of organic acids in the short- and long-term aluminum tolerance in maize seedlings (Zea mays 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.

Boron-induced amelioration of aluminium toxicity in a monocot and a dicot species

Previous research has reported inconsistent results from experiments on the influence of boron (B) on plant sensitivity to potentially toxic aluminium (Al) concentrations. Differences in B requirement and cell wall properties among species, especially between Poaceae and dicots, may account for this. This investigation reports amelioration by B of Al-induced inhibition of root elongation in Al-sensitive cucumber (Cucumis sativus), but not in Al-sensitive maize (Zea mays). Vital staining, however, also revealed a positive influence of B supply on Al tolerance in maize. In both species, adequate B supply decreased Al-induced damage of cell integrity. In cucumber, increasing B supply enhanced Al concentrations and haematoxylin staining in root tips. In maize, no differences for root Al among B treatments were observed. These results indicate that the positive effect of B on Al resistance was not due to less Al accumulation in root tips. Enhanced concentrations of reduced glutathione were found in roots of Al-stressed maize plants growing with adequate B. It is concluded that adequate B supply is essential for prevention of Al toxicity in both the dicot and the monocot species. In dicot cucumber, the B-induced amelioration of root elongation, despite higher Al accumulation in root tips, indicates B-induced change in either or both Al speciation and compartmentation in the tips. The protection by an adequate B supply of roots against Al-induced cell death suggests a role for B in the defence against oxidative stress. This is supported by the observation that Al induced enhanced levels of GSH in roots of maize plants growing with adequate B supply but not in those growing with either deficient or excess B concentrations.

Mapping QTLs for aluminum tolerance in maize

Aluminum toxicity is one of the major constraints for plant development in acid soils, limiting food production in many countries. Cultivars genetically adapted to acid soils may offer an environmental compatible solution, providing a sustainable agriculture system. The aim of this work was to identify genomic regions associated with Al tolerance in maize, and to quantify the genetic effects on the phenotypic variation. A population of 168F3:4 families derived from a cross between two contrasting maize inbred lines for Al tolerance was evaluated using the NSRL and RSRL parameters in nutrient solution containing toxic level of aluminum. Variance analyses indicated that the NSRL was the most reliable phenotypicindex to measure Al tolerance in the population, being used for further QTL mapping analysis. RFLP and SSR markers were selected for bulked segregant analysis, and additional SSR markers, flanking the polymorphisms of interest, were chosen in order to saturate the putative target regions. Seven linkage groups were constructed using 17 RFLP and 34 SSR markers. Five QTLs were mapped on chromosomes 2, 6 and 8, explaining 60% of the phenotypic variation. QTL4 and marker umc043 were located on chromosomes 8and 5, close to genes encoding for enzymes involved in the organic acids synthesis pathways, a widely proposed mechanism for Al tolerance in plants. QTL2 was mapped in the same region as Alm2,also associated with Al tolerance in maize. In addition, dominant and additive effects were important in the control of this trait in maize.

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.

Probing the role of calmodulin in Al toxicity in maize

The role of calmodulin on Al toxicity was studied in two maize ( Zea mays L.) inbred lines, Cat 100-6 (Al-tolerant) and S 1587-17 (Al-sensitive). Increasing levels of Al induced the release of malate at similar rate by roots of both genotypes, while the exudation of citrate, a stronger Al-binding compound, was 3.5 times higher in Cat 100-6 seedlings exposed to 16.2×10 −6 Al 3+ activity. The calmodulin inhibitor trifluoperazine significantly reduced the root growth in both genotypes, mimicking the main effect of Al. However, when Cat 100-6 and S 1587-17 seedlings were challenged with Al in conjunction with trifluoperazine, no further reduction in root growth or any other effect of Al toxicity was observed. The rate of Al-induced citrate exudation by both genotypes was not affected by treatment with trifluoperazine or calmidazolium, another calmodulin inhibitor. The Al 3+ interaction with cytoplasmic CaM was estimated using models for the binding of Al 3+ and Mg 2+ with CaM and physiological concentrations of citrate, CaM, InsP 3, ATP, ADP, Al 3+ and Mg 2+. In this simulation, Al 3+ associated with citrate and InsP 3, but not with CaM. We conclude that calmodulin is not relevant to the physiological processes leading to the Al tolerance in maize, nor is it a primary target for Al toxicity.

Eficiência de índices fenotípicos de comprimento de raiz seminal na avaliação de plantas individuais de milho quanto à tolerância ao alumínio

O objetivo deste trabalho foi testar a eficiência do comprimento relativo de raiz seminal (CRRS) e do comprimento líquido de raiz seminal (CLRS) como indicadores fenotípicos quanto à tolerância ao alumínio na avaliação de plantas individuais de milho. Plântulas de genótipos tolerantes e suscetíveis ao Al foram submetidas a solução nutritiva contendo nível tóxico deste elemento, por um período de sete dias, após o qual, determinaram-se os valores de CRRS e CLRS. Os resultados obtidos quando se utilizaram valores médios para CRRS e CLRS mostraram que ambos os índices foram capazes de discriminar com eficiência os materiais tolerantes dos suscetíveis. Entretanto, quando foram utilizados os valores de CRRS e CLRS obtidos a partir de plantas individuais, observou-se a existência de plantas tolerantes com valores típicos de plantas suscetíveis, o que indica que a avaliação fenotípica de plantas individuais pelos dois índices está sujeita a erros significativos, principalmente na caracterização de plantas suscetíveis. Portanto, em estudos para mapeamento de "quantitative trait loci" (QTLs) ligados à tolerância ao Al, nos quais utilizam-se estes índices fenotípicos, o mais apropriado é avaliar famílias F3, onde é possível obter valores médios para CRRS e CLRS e utilizar estas médias para representar os valores fenotípicos das respectivas plantas F2.

Aluminum-induced organic acids exudation by roots of an aluminum-tolerant tropical maize

Aluminum (Al) tolerant and sensitive plants selected from the tropical maize variety Taiúba were grown in complete nutrient and simple salt solutions in the presence and absence of phytotoxic concentrations of Al. During the first 20 hr of Al exposure, the root growth rate of both tolerant and sensitive plants was severely inhibited as a consequence of Al infiltration into the root tip cells. After this period, however, roots of Al-treated tolerant plants recovered to a growth rate similar to that of control plants, while the root growth rate of sensitive plants remained severely inhibited. The recovery of the root growth rate of tolerant plants coincided with the extrusion of the Al that had been absorbed in the first 20 hr of Al exposure. When the roots of tolerant and sensitive plants were grown in simple salt solutions containing a series of Al concentrations, a dose-dependent citrate and malate exudation was observed from tolerant but not from sensitive roots. The level of citrate exudation was two- to four-fold that observed for malate. The organic acid exudation was not influenced by the level of phosphate in the growth solution, suggesting a specific Al-inducing process involved in the Al tolerance in maize. We concluded from these results that the Al infiltration in the roots at the beginning of Al exposure induces the exudation of organic acids which may exclude the toxic ion from the root tip cells of tolerant plants.