Al-resistance Genes Research Articles

SlSTOP1-regulated SlHAK5 expression confers Al tolerance in tomato by facilitating citrate secretion from roots

Abstract SENSITIVE TO PROTON RHIZOTOXICITY 1 (STOP1) is a core transcription factor that regulates the expression of aluminum (Al)-resistance genes to manage Al toxicity in plants. However, the genome-wide roles of SlSTOP1 in the Al stress response of tomato (Solanum lycopersicum) remain largely unknown. Here, we report that SlSTOP1 is crucial for Al tolerance in tomato, as loss-of-function mutants of SlSTOP1 displayed hypersensitivity to Al stress. Aluminum stress had no effect on SlSTOP1 mRNA expression, but promoted accumulation of SlSTOP1 protein in the nucleus. Through integrated DNA affinity purification sequencing and RNA sequencing analysis, we identified 39 SlSTOP1-targeted Al-responsive genes, some of which are homologous to known Al resistance genes in other plant species, suggesting that these SlSTOP1-targeted genes play essential roles in Al resistance in tomato. Furthermore, using peak enrichment analysis of SlSTOP1-targeted sequences, we identified a cis-acting element bound by SlSTOP1 and validated this finding via dual-luciferase reporter and electrophoretic mobility shift assay (EMSA). Additionally, we demonstrated SlHAK5 is one of direct targets of SlSTOP1 and functionally characterized it in terms of Al stress tolerance. Compared with wild-type plants, Slhak5 mutants developed by CRISPR/Cas9 technology presented increased sensitivity to Al stress, which was associated with reduced citrate secretion from the roots. Together, our findings demonstrate that SlSTOP1 directly interacts with cis-acting elements located in the promoters of target genes involved in diverse pathways contributing to Al resistance in tomato.

Genome-wide characterization, transcriptome profiling, and functional analysis of the ALMT gene family in Medicago for aluminum resistance

Aluminum (Al) is the major limiting factor affecting plant productivity in acidic soils. Al3+ ions exhibit increased solubility at a pH below 5, leading to plant root tip toxicity. Alternatively, plants can perceive very low concentrations of Al3+, and Al triggers downstream signaling even at pH 5.7 without causing Al toxicity. The ALUMINUM-ACTIVATED-MALATE-TRANSPORTER (ALMT) family members act as anion channels, with some regulating the secretion of malate from root apices to chelate Al, which is a crucial mechanism for plant Al resistance. To date, the role of the ALMT gene family within the legume Medicago species has not been fully characterized. In this study, we investigated the ALMT gene family in M. sativa and M. truncatula and identified 68 MsALMTs and 18 MtALMTs, respectively. Phylogenetic analysis classified these genes into five clades, and synteny analysis uncovered genuine paralogs and orthologs. The real-time quantitative reverse transcription PCR (qRT-PCR) analysis revealed that MtALMT8, MtALMT9, and MtALMT15 in clade 2-2b are expressed in both roots and root nodules, and MtALMT8 and MtALMT9 are significantly upregulated by Al in root tips. We also observed that MtALMT8 and MtALMT9 can partially restore the Al sensitivity of Atalmt1 in Arabidopsis. Moreover, transcriptome analysis examined the expression patterns of these genes in M. sativa in response to Al at both pH 5.7 and pH 4.6, as well as to protons, and found that Al and protons can independently induce some Al-resistance genes. Overall, our findings indicate that MtALMT8 and MtALMT9 may play a role in Al resistance, and highlight the resemblance between the ALMT genes in Medicago species and those in Arabidopsis.

High affinity promoter binding of STOP1 is essential for early expression of novel aluminum-induced resistance genes GDH1 and GDH2 in Arabidopsis.

Malate efflux from roots, which is regulated by the transcription factor STOP1 (SENSITIVE-TO-PROTON-RHIZOTOXICITY1) and mediates aluminum-induced expression of ALUMINUM-ACTIVATED-MALATE-TRANSPORTER1 (AtALMT1), is critical for aluminum resistance in Arabidopsis thaliana. Several studies showed that AtALMT1 expression in roots is rapidly observed in response to aluminum; this early induction is an important mechanism to immediately protect roots from aluminum toxicity. Identifying the molecular mechanisms that underlie rapid aluminum resistance responses should lead to a better understanding of plant aluminum sensing and signal transduction mechanisms. In this study, we observed that GFP-tagged STOP1 proteins accumulated in the nucleus soon after aluminum treatment. The rapid aluminum-induced STOP1-nuclear localization and AtALMT1 induction were detected in the presence of a protein synthesis inhibitor, suggesting that post-translational regulation is involved in these events. STOP1 also regulated rapid aluminum-induced expression for other genes that carry a functional/high-affinity STOP1-binding site in their promoter, including STOP2, GLUTAMATE-DEHYDROGENASE1 and 2 (GDH1 and 2). However STOP1 did not regulate Al resistance genes which have no functional STOP1-binding site such as ALUMINUM-SENSITIVE3, suggesting that the binding of STOP1 in the promoter is essential for early induction. Finally, we report that GDH1 and 2 which are targets of STOP1, are novel aluminum-resistance genes in Arabidopsis.

Regulation of Aluminum Resistance in Arabidopsis Involves the SUMOylation of the Zinc Finger Transcription Factor STOP1.

Aluminum (Al) is a primary constraint for crop production on acid soils, which make up more than 30% of the arable land in the world. Al resistance in Arabidopsis (Arabidopsis thaliana) is achieved by malate secretion mediated by the Al-ACTIVATED MALATE TRANSPORTER1 (AtALMT1) transporter. The C2H2-type transcription factor SENSITIVE TO PROTON RHIZOTOXICITY1 (STOP1) is essential and required for Al resistance, where it acts by inducing the expression of Al-resistance genes, including AtALMT1 In this study, we report that STOP1 protein function is modified by SUMOylation. The SMALL UBIQUITIN-LIKE MODIFIER (SUMO) protease ESD4, but not other SUMO proteases, specifically interacts with and deSUMOylates STOP1. Mutation of ESD4 increases the level of STOP1 SUMOylation and the expression of the STOP1-regulated gene AtALMT1, which contributes to the increased Al resistance in esd4 The esd4 mutation does not influence STOP1 protein abundance but increases the association of STOP1 with the AtALMT1 promoter, which might explain the elevated expression of AtALMT1 in esd4 We demonstrate that STOP1 is mono-SUMOylated at K40, K212, or K395 sites, and blocking STOP1 SUMOylation reduces STOP1 stability and the expression of STOP1-regulated genes, leading to the reduced Al resistance. Our results thus reveal the involvement of SUMOylation in the regulation of STOP1 and Al resistance in Arabidopsis.

Mutation of HPR1 encoding a component of the THO/TREX complex reduces STOP1 accumulation and aluminium resistance in Arabidopsis thaliana.

C2H2-type zinc finger transcription factor sensitive to proton rhizotoxicity1 (STOP1) plays an essential role in aluminium (Al) resistance in Arabidopsis thaliana by controlling the expression of a set of Al-resistance genes, including the malate transporter-encoding gene A.thaliana aluminium activated malate transporter1 (AtALMT1) that is critically required for Al resistance. STOP1 is suggested to be modulated by Al at post-transcriptional and/or post-translational levels. However, the underlying molecular mechanisms remain to be demonstrated. We carried out a forward genetic screen on an ethyl methanesulphonate mutagenized population, which contains the AtALMT1 promoter-driven luciferase reporter gene (pAtALMT1:LUC), and identified hyperrecombination protein1 (HPR1), which encodes a subunit of the THO/TREX complex. We investigate the effect of hpr1 mutations on the expression of Al-resistance genes and Al resistance, and we also examined the regulatory role of HPR1 in nuclear messenger RNA (mRNA) and protein accumulation of STOP1 gene. Mutation of HPR1 reduces the expression of STOP1-regulated genes and the associated Al resistance. The hpr1 mutations increase STOP1 mRNA retention in the nucleus and consequently decrease STOP1 protein abundance. Mutation of regulation of AtALMT1 expression1 (RAE1) that mediates STOP1 degradation in the hpr1 mutant background can partially rescue the deficient phenotypes of hpr1 mutants. Our results demonstrate that HPR1 modulates Al resistance partly through the regulation of nucleocytoplasmic STOP1 mRNA export.

A simple cultivar suitability index for low‐pH agricultural soils

AbstractPlanting wheat (Triticum aestivum L.) cultivars that carry the aluminum (Al) resistance gene TaAlmt1 is a potentially lower cost alternative to lime applications in acidic agricultural soils. However, the relative importance of TaAlmt1 expression and adaptedness (to regional environmental conditions) for preserving grain yield in acidic soils is not well understood. Adaptation trials were established in low‐pH soils to compare lime‐amended (YL) and unamended grain yield (YU) among regionally adapted spring wheat cultivars with and without TaAlmt1. Averaged across YL and YU (YAVG), yield of adapted TaAlmt1 carriers was similar to adapted noncarriers (p = .939) but greater than nonadapted noncarriers (p = .024). Soil pH‐driven spatial heterogeneity appears to inflate yield coefficients of variation and the probability of committing type II errors in cultivar yield comparisons. Our results support the use of YAVG as a suitability index and decision support tool for cultivar selection in acid‐affected soils.

Assessing How the Aluminum-Resistance Traits in Wheat and Rye Transfer to Hexaploid and Octoploid Triticale.

The mechanisms of aluminum (Al) resistance in wheat and rye involve the release of citrate and malate anions from the root apices. Many of the genes controlling these processes have been identified and their responses to Al treatment described in detail. This study investigated how the major Al resistance traits of wheat and rye are transferred to triticale (x Tritosecale Wittmack) which is a hybrid between wheat and rye. We generated octoploid and hexaploid triticale lines and compared them with the parental lines for their relative resistance to Al, organic anion efflux and expression of some of the genes encoding the transporters involved. We report that the strong Al resistance of rye was incompletely transferred to octoploid and hexaploid triticale. The wheat and rye parents contributed to the Al-resistance of octoploid triticale but the phenotypes were not additive. The Al resistance genes of hexaploid wheat, TaALMT1, and TaMATE1B, were more successfully expressed in octoploid triticale than the Al resistance genes in rye tested, ScALMT1 and ScFRDL2. This study demonstrates that an important stress-tolerance trait derived from hexaploid wheat was expressed in octoploid triticale. Since most commercial triticale lines are largely hexaploid types it would be beneficial to develop techniques to generate genetically-stable octoploid triticale material. This would enable other useful traits that are present in hexaploid but not tetraploid wheat, to be transferred to triticale.

Emerging Pleiotropic Mechanisms Underlying Aluminum Resistance and Phosphorus Acquisition on Acidic Soils.

Aluminum (Al) toxicity on acidic soils significantly damages plant roots and inhibits root growth. Hence, crops intoxicated by Al become more sensitive to drought stress and mineral nutrient deficiencies, particularly phosphorus (P) deficiency, which is highly unavailable on tropical soils. Advances in our understanding of the physiological and genetic mechanisms that govern plant Al resistance have led to the identification of Al resistance genes, both in model systems and in crop species. It has long been known that Al resistance has a beneficial effect on crop adaptation to acidic soils. This positive effect happens because the root systems of Al resistant plants show better development in the presence of soil ionic Al3+ and are, consequently, more efficient in absorbing sub-soil water and mineral nutrients. This effect of Al resistance on crop production, by itself, warrants intensified efforts to develop and implement, on a breeding scale, modern selection strategies to profit from the knowledge of the molecular determinants of plant Al resistance. Recent studies now suggest that Al resistance can exert pleiotropic effects on P acquisition, potentially expanding the role of Al resistance on crop adaptation to acidic soils. This appears to occur via both organic acid (OA)- and non-OA transporters governing a joint, iron-dependent interplay between Al resistance and enhanced P uptake, via changes in root system architecture. Current research suggests this interplay to be part of a P stress response, suggesting that this mechanism could have evolved in crop species to improve adaptation to acidic soils. Should this pleiotropism prove functional in crop species grown on acidic soils, molecular breeding based on Al resistance genes may have a much broader impact on crop performance than previously anticipated. To explore this possibility, here we review the components of this putative effect of Al resistance genes on P stress responses and P nutrition to provide the foundation necessary to discuss the recent evidence suggesting pleiotropy as a genetic linkage between Al resistance and P efficiency. We conclude by exploring what may be needed to enhance the utilization of Al resistance genes to improve crop production on acidic soils.

Transcriptome Analysis of Al-Induced Genes in Buckwheat (Fagopyrum esculentum Moench) Root Apex: New Insight into Al Toxicity and Resistance Mechanisms in an Al Accumulating Species.

Relying on Al-activated root oxalate secretion, and internal detoxification and accumulation of Al, buckwheat is highly Al resistant. However, the molecular mechanisms responsible for these processes are still poorly understood. It is well-known that root apex is the critical region of Al toxicity that rapidly impairs a series of events, thus, resulting in inhibition of root elongation. Here, we carried out transcriptome analysis of the buckwheat root apex (0–1 cm) with regards to early response (first 6 h) to Al stress (20 μM), which is crucial for identification of both genes and processes involved in Al toxicity and tolerance mechanisms. We obtained 34,469 unigenes with 26,664 unigenes annotated in the NCBI database, and identified 589 up-regulated and 255 down-regulated differentially expressed genes (DEGs) under Al stress. Functional category analysis revealed that biological processes differ between up- and down-regulated genes, although ‘metabolic processes’ were the most affected category in both up- and down-regulated DEGs. Based on the data, it is proposed that Al stress affects a variety of biological processes that collectively contributes to the inhibition of root elongation. We identified 30 transporter genes and 27 transcription factor (TF) genes induced by Al. Gene homology analysis highlighted candidate genes encoding transporters associated with Al uptake, transport, detoxification, and accumulation. We also found that TFs play critical role in transcriptional regulation of Al resistance genes in buckwheat. In addition, gene duplication events are very common in the buckwheat genome, suggesting a possible role for gene duplication in the species’ high Al resistance. Taken together, the transcriptomic analysis of buckwheat root apex shed light on the processes that contribute to the inhibition of root elongation. Furthermore, the comprehensive analysis of both transporter genes and TF genes not only deep our understanding on the responses of buckwheat roots to Al toxicity but provide a good start for functional characterization of genes critical for Al tolerance.

Isolation and Characterization of an Aluminum-resistant Mutant in Rice.

BackgroundAluminum (Al) toxicity represents a major constraint for crop production on acid soils. Rice is a high Al-resistant plant species among small-grain cereals, but its molecular mechanisms of Al resistance are not fully understood. We adopted a forward genetic screen strategy to uncover the Al-resistance mechanisms in rice. In this study, we screened an ethylmethylsulfone (EMS)-mutagenized library to isolate and characterize mutants with altered sensitivity to Al in rice.ResultsTreatment of an Al-intolerant indica variety Kasalath with 20 μM Al induced root swelling. This phenotype could be suppressed by the addition of aminoethoxyvinylglycine (AVG, an ethylene synthesis inhibitor), suggesting that increased production of ethylene is responsible for the root swelling under Al stress. By utilizing the root swelling as an indicator, we developed a highly effective method to screen Al-sensitive or -resistant mutants in rice. Through screening of ~5000 M2 lines, we identified 10 Al-sensitive mutants and one Al-resistant mutant ral1 (resistance to aluminum 1). ral1 mutant showed short root phenotype under normal growth condition, which was attributed to reduced cell elongation in the mutant. A dose-response experiment revealed that ral1 mutant was more resistant to Al than wild-type (WT) at all Al concentrations tested. The mutant was also more resistant to Al when grown in an acid soil. The mutant accumulated much lower Al in the root tips (0–1 cm) than WT. The mutant contained less Al in the cell wall of root tips than WT, whereas Al concentration in the cell sap was similar between WT and the mutant. In addition to Al, the mutant was also more resistant to Cd than WT. Quantitative RT-PCR analysis showed that the expression levels of known Al-resistance genes were not increased in the mutant compared to WT. Genetic analysis indicated that the Al-resistance phenotype in ral1 mutant was controlled by a single recessive gene mapped on the long arm of chromosome 6.ConclusionsWe have developed a highly efficient method for the screening of rice mutants with altered Al sensitivity. We identified a novel mutant ral1 resistant to Al by this screening. The increased resistance of ral1 to Al toxicity is caused by the reduced Al binding to the cell wall of root tips and the responsible gene is mapped on the long arm of chromosome 6.

Genome-Wide Association Mapping of Acid Soil Resistance in Barley (Hordeum vulgare L.).

Genome-wide association studies (GWAS) based on linkage disequilibrium (LD) have been used to detect QTLs underlying complex traits in major crops. In this study, we collected 218 barley (Hordeum vulgare L.) lines including wild barley and cultivated barley from China, Canada, Australia, and Europe. A total of 408 polymorphic markers were used for population structure and LD analysis. GWAS for acid soil resistance were performed on the population using a general linkage model (GLM) and a mixed linkage model (MLM), respectively. A total of 22 QTLs (quantitative trait loci) were detected with the GLM and MLM analyses. Two QTLs, close to markers bPb-1959 (133.1 cM) and bPb-8013 (86.7 cM), localized on chromosome 1H and 4H respectively, were consistently detected in two different trials with both the GLM and MLM analyses. Furthermore, bPb-8013, the closest marker to the major Al3+ resistance gene HvAACT1 in barley, was identified to be QTL5. The QTLs could be used in marker-assisted selection to identify and pyramid different loci for improved acid soil resistance in barley.

Plant Adaptation to Acid Soils: The Molecular Basis for Crop Aluminum Resistance.

Aluminum (Al) toxicity in acid soils is a significant limitation to crop production worldwide, as approximately 50% of the world's potentially arable soil is acidic. Because acid soils are such an important constraint to agriculture, understanding the mechanisms and genes conferring resistance to Al toxicity has been a focus of intense research interest in the decade since the last article on crop acid soil tolerance was published in this journal. An impressive amount of progress has been made during that time that has greatly increased our understanding of the diversity of Al resistance genes and mechanisms, how resistance gene expression is regulated and triggered by Al and Al-induced signals, and how the proteins encoded by these genes function and are regulated. This review examines the state of our understanding of the physiological, genetic, and molecular bases for crop Al tolerance, looking at the novel Al resistance genes and mechanisms that have been identified over the past ten years. Additionally, it examines how the integration of molecular and genetic analyses of crop Al resistance is starting to be exploited for the improvement of crop plants grown on acid soils via both molecular-assisted breeding and biotechnology approaches.

The role of aluminum sensing and signaling in plant aluminum resistance

As researchers have gained a better understanding in recent years into the physiological, molecular, and genetic basis of how plants deal with aluminum (Al) toxicity in acid soils prevalent in the tropics and sub-tropics, it has become clear that an important component of these responses is the triggering and regulation of cellular pathways and processes by Al. In this review of plant Al signaling, we begin by summarizing the understanding of physiological mechanisms of Al resistance, which first led researchers to realize that Al stress induces gene expression and modifies protein function during the activation of Al resistance responses. Subsequently, an overview of Al resistance genes and their function provides verification that Al induction of gene expression plays a major role in Al resistance in many plant species. More recent research into the mechanistic basis for Al-induced transcriptional activation of resistance genes has led to the identification of several transcription factors as well as cis-elements in the promoters of Al resistance genes that play a role in greater Al-induced gene expression as well as higher constitutive expression of resistance genes in some plant species. Finally, the post-transcriptional and translational regulation of Al resistance proteins is addressed, where recent research has shown that Al can both directly bind to and alter activity of certain organic acid transporters, and also influence Al resistance proteins indirectly, via protein phosphorylation.

Engineering greater aluminium resistance in wheat by over-expressing TaALMT1

Expected increases in world population will continue to make demands on agricultural productivity and food supply. These challenges will only be met by increasing the land under cultivation and by improving the yields obtained on existing farms. Genetic engineering can target key traits to improve crop yields and to increase production on marginal soils. Soil acidity is a major abiotic stress that limits plant production worldwide. The goal of this study was to enhance the acid soil tolerance of wheat by increasing its resistance to Al(3+) toxicity. Particle bombardment was used to transform wheat with TaALMT1, the Al(3+) resistance gene from wheat, using the maize ubiquitin promoter to drive expression. TaALMT1 expression, malate efflux and Al(3+) resistance were measured in the T(1) and T(2) lines and compared with the parental line and an Al(3+)-resistant reference genotype, ET8. Nine T(2) lines showed increased TaALMT1 expression, malate efflux and Al(3+) resistance when compared with untransformed controls and null segregant lines. Some T(2) lines displayed greater Al(3+) resistance than ET8 in both hydroponic and soil experiments. The Al(3+) resistance of wheat was increased by enhancing TaALMT1 expression with biotechnology. This is the first report of a major food crop being stably transformed for greater Al(3+) resistance. Transgenic strategies provide options for increasing food supply on acid soils.

Transgenic barley (Hordeum vulgare L.) expressing the wheat aluminium resistance gene (TaALMT1) shows enhanced phosphorus nutrition and grain production when grown on an acid soil

Barley (Hordeum vulgare L.), genetically modified with the Al(3+) resistance gene of wheat (TaALMT1), was compared with a non-transformed sibling line when grown on an acidic and highly phosphate-fixing ferrosol supplied with a range of phosphorus concentrations. In short-term pot trials (26 days), transgenic barley expressing TaALMT1 (GP-ALMT1) was more efficient than a non-transformed sibling line (GP) at taking up phosphorus on acid soil, but the genotypes did not differ when the soil was limed. Differences in phosphorus uptake efficiency on acid soil could be attributed not only to the differential effects of aluminium toxicity on root growth between the genotypes, but also to differences in phosphorus uptake per unit root length. Although GP-ALMT1 out-performed GP on acid soil, it was still not as efficient at taking up phosphorus as plants grown on limed soil. GP-ALMT1 plants grown in acid soil possessed substantially smaller rhizosheaths than those grown in limed soil, suggesting that root hairs were shorter. This is a probable reason for the lower phosphorus uptake efficiency. When grown to maturity in large pots, GP-ALMT1 plants produced more than twice the grain as GP plants grown on acid soil and 80% of the grain produced by limed controls. Expression of TaALMT1 in barley was not associated with a penalty in either total shoot or grain production in the absence of Al(3+), with both genotypes showing equivalent yields in limed soil. These findings demonstrate that an important crop species can be genetically engineered to successfully increase grain production on an acid soil.

Characterisation of genetic variation for aluminium resistance and polyphenol oxidase activity in genebank accessions of spelt wheat

Spelt wheat [Triticum aestivum ssp. spelta (L.) Thell.] is becoming a valuable crop due to its reputation as a healthy food. In Australia, this crop has not been targeted for systematic breeding. Identification of spelt genotypes having low PPO activity and resistance to acid soils (Al3+) are desirable attributes for future cultivar development. We evaluated 51 genebank accessions of spelts from the Australian Winter Cereals Collection for polyphenol oxidase (PPO) activity and for resistance to aluminium (Al). PPO activity was measured both visually and spectrophotometrically, using L-DOPA substrate. PPO activity for genotypes ranged from 0.15 to 1.3 and could be grouped into ‘low’, ‘medium’ and ‘high’ categories. At least eight accessions exhibited low PPO activity (not different to the durum check cultivar Arrivato). After measuring PPO activity, the same kernels were further evaluated for Al resistance using a nutrient solution culture method with haematoxylin staining test of root tips. Thirty-three accessions were resistant to Al. Functional gene markers associated with loci conditioning Al resistance gene (TaALMT1) and PPO activity (XPPO-2A) in common wheat confirmed their association with target phenotypes within spelt accessions. Genetic variation within spelt wheat for important agronomic and quality traits such as Al resistance and PPO suggested that progress in spelt improvement can be made by molecular plant breeding. Diversity Array Technology (DArT) based allelic data revealed that these spelt accessions are genetically diverse.

Analysis of TaALMT1 traces the transmission of aluminum resistance in cultivated common wheat (Triticum aestivum L.)

Allele diversities of four markers specific to intron three, exon four and promoter regions of the aluminum (Al) resistance gene of wheat (Triticum aestivum L.) TaALMT1 were compared in 179 common wheat cultivars used in international wheat breeding programs. In wheat cultivars released during the last 93 years, six different promoter types were identified on the basis of allele size. A previous study showed that Al resistance was not associated with a particular coding allele for TaALMT1 but was correlated with blocks of repeated sequence upstream of the coding sequence. We verified the linkage between these promoter alleles and Al resistance in three doubled haploid and one intercross populations segregating for Al resistance. Molecular and pedigree analysis suggest that Al resistance in modern wheat germplasm is derived from several independent sources. Analysis of a population of 278 landraces and subspecies of wheat showed that most of the promoter alleles associated with Al resistance pre-existed in Europe, the Middle East and Asia prior to dispersal of cultivated germplasm around the world. Furthermore, several new promoter alleles were identified among the landraces surveyed. The TaALMT1 promoter alleles found within the spelt wheats were consistent with the hypothesis that these spelts arose on several independent occasions from hybridisations between non-free-threshing tetraploid wheats and Al-resistant hexaploid bread wheats. The strong correlation between Al resistance and Al-stimulated malate efflux from the root apices of 49 diverse wheat genotypes examined was consistent with the previous finding that Al resistance in wheat is conditioned primarily by malate efflux. These results demonstrate that the markers based on intron, exon and promoter regions of TaALMT1 can trace the inheritance of the Al resistance locus within wheat pedigrees and track Al resistance in breeding programmes.

Identification of new sources of aluminum resistance in wheat

Aluminum (Al) toxicity is a major constraint for wheat production in acidic soils. An Al resistance gene on chromosome 4DL that traces to Brazilian wheat has been extensively studied, and can provide partial protection from Al damage. To identify potentially new sources of Al resistance, 590 wheat accessions, including elite wheat breeding lines from the United States and other American and European countries, landraces and commercial cultivars from East Asia, and synthetic wheat lines from CIMMYT, Mexico, were screened for Al resistance by measuring relative root elongation in culture with a nutrient solution containing Al, and by staining Al-stressed root tips with hematoxylin. Eighty-eight wheat accessions demonstrated at least moderate resistance to Al toxicity. Those selected lines were subjected to analysis of microsatellite markers linked to an Al resistance gene on 4DL and a gene marker for the Al-activated malate transporter (ALMT1) locus. Many of the selected Al-resistant accessions from East Asia did not have the Al-resistant marker alleles of ALMT1, although they showed Al resistance similar to the US Al-resistant cultivar, Atlas 66. Most of the cultivars derived from Jagger and Atlas 66 have the Al-resistant marker alleles of ALMT1. Cluster analysis separated the selected Al-resistant germplasm into two major clusters, labeled as Asian and American–European clusters. Potentially new germplasm of Al resistance different from those derived from Brazil were identified. Further investigation of Al resistance in those new germplasms may reveal alternative Al-resistance mechanisms in wheat.

QTL Analysis of Aluminum Resistance in Rice (Oryza sativa L.)

Aluminum (Al) toxicity is considered as one of the primary causes of low-rice productivity in acid soils. In the present study, quantitative trait loci (QTLs) controlling Al resistance based on relative root elongation (RRE) were dissected using a complete linkage map and a recombinant inbred lines (RILs) derived from a cross of Al-tolerant japonica cultivar Asominori (Oryza sativa L.) and Al-sensitive indica cultivar IR24 (O. sativa L.). A total of three QTLs (qRRE-1, qRRE-9, and qRRE-11) were detected on chromosomes 1, 9, and 11 with LOD score ranging from 2.64 to 3.60 and the phenotypic variance explained from 13.5 to 17.7%. The Asominori alleles were all associated with Al resistance at all the three QTLs. The existence of these QTLs was confirmed using Asominori chromosome segment substitution lines (CSSLs) in IR24 genetic background (IAS). By QTL comparative analysis, the two QTLs (qRRE-1and qRRE-9) on chromosomes 1 and 9 appeared to be consistent among different rice populations while qRRE-11 was newly detected and syntenic with a major Al resistance gene on chromosome 10 of maize. This region may provide an important case for isolating genes responsible for different mechanisms of Al resistance among different cereals. These results also provide the possibilities of enhancing Al resistance in rice breeding program by marker-assisted selection (MAS) and pyramiding QTLs.

Quantitative Trait Loci for Aluminum Resistance in Wheat Cultivar Chinese Spring

Aluminum (Al) toxicity is one of the major constrains for wheat production in many wheat growing areas worldwide. Further understanding of inheritance of Al resistance may facilitate improvement of Al resistance of wheat cultivars (Triticum aestivum L.). A set of ditelosomic lines derived from the moderately Al-resistant wheat cultivar Chinese Spring was assessed for Al resistance. The root growth of ditelosomic lines DT5AL, DT7AL, DT2DS and DT4DS was significantly lower than that of euploid Chinese Spring under Al stress, suggesting that Al-resistance genes might exist on the missing chromosome arms of 5AS, 7AS, 2DL and 4DL of Chinese Spring. A population of recombinant inbred lines (RILs) from the cross Annong 8455 × Chinese Spring-Sumai 3 7A substitution line was used to determine the effects of these chromosome arms on Al resistance. A genetic linkage map consisting of 381 amplified fragment length polymorphism (AFLP) markers and 168 simple sequence repeat (SSR) markers was constructed to determine the genetic effect of the quantitative trait loci (QTLs) for Al resistance in Chinese Spring. Three QTLs, Qalt.pser-4D, Qalt.pser-5A and Qalt.pser-2D, were identified that enhanced root growth under Al stress, suggesting that inheritance of Al resistance in Chinese Spring is polygenic. The QTL with the largest effect was flanked by the markers of Xcfd23 and Xwmc331 on chromosome 4DL and most probably is multi-allelic to the major QTL identified in Atlas 66. Two additional QTLs, Qalt.pser-5A and Qalt.pser-2D on chromosome 5AS and 2DL, respectively, were also detected with marginal significance in the population. Some SSR markers identified in this study would be useful for marker-assisted pyramiding of different QTLs for Al resistance in wheat cultivars.