An aluminum-activated citrate transporter in barley

Soluble ionic aluminum (Al) inhibits root growth and reduces crop production on acid soils. Al-resistant cultivars of barley (Hordeum vulgare L.) detoxify Al by secreting citrate from the roots, but the responsible gene has not been identified yet. Here, we identified a gene (HvAACT1) responsible for the Al-activated citrate secretion by fine mapping combined with microarray analysis, using an Al-resistant cultivar, Murasakimochi, and an Al-sensitive cultivar, Morex. This gene belongs to the multidrug and toxic compound extrusion (MATE) family and was constitutively expressed mainly in the roots of the Al-resistant barley cultivar. Heterologous expression of HvAACT1 in Xenopus oocytes showed efflux activity for (14)C-labeled citrate, but not for malate. Two-electrode voltage clamp analysis also showed transport activity of citrate in the HvAACT1-expressing oocytes in the presence of Al. Overexpression of this gene in tobacco enhanced citrate secretion and Al resistance compared with the wild-type plants. Transiently expressed green fluorescent protein-tagged HvAACT1 was localized at the plasma membrane of the onion epidermal cells, and immunostaining showed that HvAACT1 was localized in the epidermal cells of the barley root tips. A good correlation was found between the expression of HvAACT1 and citrate secretion in 10 barley cultivars differing in Al resistance. Taken together, our results demonstrate that HvAACT1 is an Al-activated citrate transporter responsible for Al resistance in barley.     

Identification of AFLP and microsatellite markers linked with an aluminium tolerance gene in barley ( Hordeum vulgare L.)

Barley is the most sensitive among the cereals to aluminium (Al) stress and breeding for more tolerant cultivars is a priority. To enhance selection efficiency for Al tolerance in barley, PCR-based AFLP and microsatellite markers linked to a locus conferring tolerance to aluminium were identified. The study used F(2) progeny derived from a single cross between Yambla (moderately tolerant of Al) and WB229 (tolerant of Al) and developed hydroponic pulse-recovery screening methods to assess tolerance of phenotypes based on root growth. The segregation ratios of tolerant and sensitive genotypes and F(3) progeny testing suggest that a single major gene controlled Al tolerance ( Alt). In order to determine the chromosomal location of the Alt gene, we used the AFLP technique coupled with bulk segregant analysis. We evaluated tolerant and sensitive bulks using 30 combinations of EcoRI/ MseI primers, and 12 of these permitted differentiation of the sensitive and tolerant bulks. More than 1,000 amplified fragments were obtained, and 98 polymorphic bands were scored. AFLP analysis of wheat-barley chromosome addition lines indicated that the Alt gene was located on barley chromosome 4H. Four chromosome 4H-specific microsatellite markers (Bmac310, Bmag353, HVM68 and HVMCABG) were tightly linked to Alt. The large allelic variation detected with microsatellite marker Bmag353 allowed us to implement this marker for routine marker-assisted selection for Al tolerance, and 396 plants could be screened on a single gel.     

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. Electronic supplementary material The online version of this article (doi:contains supplementary material, which is available to authorized users. Responsible Editor: Thomas B. Kinraide.     

High-resolution mapping of the <Emphasis Type="Italic">Alp</Emphasis> locus and identification of a candidate gene <Emphasis Type="Italic">HvMATE</Emphasis> controlling aluminium tolerance in barley (<Emphasis Type="Italic">Hordeum vulgare</Emphasis> L.)

Aluminium (Al) tolerance in barley is conditioned by the Alp locus on the long arm of chromosome 4H, which is associated with Al-activated release of citrate from roots. We developed a high-resolution map of the Alp locus using 132 doubled haploid (DH) lines from a cross between Dayton (Al-tolerant) and Zhepi 2 (Al-sensitive) and 2,070 F₂ individuals from a cross between Dayton and Gairdner (Al-sensitive). The Al-activated efflux of citrate from the root apices of Al-tolerant Dayton was 10-fold greater than from the Al-sensitive parents Zhepi 2 and Gairdner. A suite of markers (ABG715, Bmag353, GBM1071, GWM165, HvMATE and HvGABP) exhibited complete linkage with the Alp locus in the DH population accounting 72% of the variation for Al tolerance evaluated as relative root elongation. These markers were used to map this genomic region in the Dayton/Gairdner population in more detail. Flanking markers HvGABP and ABG715 delineated the Alp locus to a 0.2 cM interval. Since the HvMATE marker was not polymorphic in the Dayton/Gairdner population we instead investigated the expression of the HvMATE gene. Relative expression of the HvMATE gene was 30-fold greater in Dayton than Gardiner. Furthermore, HvMATE expression in the F_2:3 families tested, including all the informative recombinant lines identified between HvGABP and ABG715 was significantly correlated with Al tolerance and Al-activated citrate efflux. These results identify HvMATE, a gene encoding a multidrug and toxic compound extrusion protein, as a candidate controlling Al tolerance in barley.     

Quantitative trait loci for aluminum resistance in wheat

Quantitative trait loci (QTL) for wheat resistance to aluminum (Al) toxicity were analyzed using simple sequence repeats (SSRs) in a population of 192 F₆ recombinant inbred lines (RILs) derived from a cross between an Al-resistant cultivar, Atlas 66 and an Al-sensitive cultivar, Chisholm. Wheat reaction to Al was measured by relative root growth and root response to hematoxylin stain in nutrient-solution culture. After screening 1,028 SSR markers for polymorphisms between the parents and bulks, we identified two QTLs for Al resistance in Atlas 66. One major QTL was mapped on chromosome 4D that co-segregated with the Al-activated malate transporter gene (ALMT1). Another minor QTL was located on chromosome 3BL. Together, these two QTLs accounted for about 57% of the phenotypic variation in hematoxylin staining score and 50% of the variation in net root growth (NRG). Expression of the minor QTL on 3BL was suppressed by the major QTL on 4DL. The two QTLs for Al resistance in Atlas 66 were also verified in an additional RIL population derived from Atlas 66/Century. Several SSR markers closely linked to the QTLs were identified and have potential to be used for marker-assisted selection (MAS) to improve Al-resistance of wheat cultivars in breeding programs.     

A Second Mechanism for Aluminum Resistance in Wheat Relies on the Constitutive Efflux of Citrate from Roots

The first confirmed mechanism for aluminum (Al) resistance in plants is encoded by the wheat (Triticum aestivum) gene, TaALMT1, on chromosome 4DL. TaALMT1 controls the Al-activated efflux of malate from roots, and this mechanism is widespread among Al-resistant genotypes of diverse genetic origins. This study describes a second mechanism for Al resistance in wheat that relies on citrate efflux. Citrate efflux occurred constitutively from the roots of Brazilian cultivars Carazinho, Maringa, Toropi, and Trintecinco. Examination of two populations segregating for this trait showed that citrate efflux was controlled by a single locus. Whole-genome linkage mapping using an F₂ population derived from a cross between Carazinho (citrate efflux) and the cultivar EGA-Burke (no citrate efflux) identified a major locus on chromosome 4BL, Xce_c, which accounts for more than 50% of the phenotypic variation in citrate efflux. Mendelizing the quantitative variation in citrate efflux into qualitative data, the Xce_c locus was mapped within 6.3 cM of the microsatellite marker Xgwm495 locus. This linkage was validated in a second population of F_2:3 families derived from a cross between Carazinho and the cultivar Egret (no citrate efflux). We show that expression of an expressed sequence tag, belonging to the multidrug and toxin efflux (MATE) gene family, correlates with the citrate efflux phenotype. This study provides genetic and physiological evidence that citrate efflux is a second mechanism for Al resistance in wheat.     

Differential Al resistance and citrate secretion in barley (<Emphasis Type="Italic">Hordeum vulgare</Emphasis> L.)

While barley ( Hordeum vulgare L.) is the most sensitive species to Al toxicity among small-grain crops, variation in Al resistance between cultivars does exist. We examined the mechanism responsible for differential Al resistance in 21 barley varieties. Citrate was secreted from the roots in response to Al stress. A positive correlation between citrate secretion and Al resistance [(root elongation with Al)/(root elongation without Al)] and a negative correlation between citrate secretion and Al content of root apices, were obtained, suggesting that citrate secretion from the root apices plays an important role in excluding Al and thereby detoxifying Al. The Al-induced secretion of citrate was characterized using an Al-resistant variety (Sigurdkorn) and an Al-sensitive variety (Kearney). In Sigurdkorn, Al-induced secretion of citrate occurred within 20 min, and the secretion did not increase with increasing external Al concentration. The Al-induced citrate secretion ceased at low temperature (6 degrees C) and was inhibited by anion-channel inhibitors. Internal citrate content of root apices was increased by Al exposure in Sigurdkorn, but was not affected in Kearney. The activity of citrate synthase was unaffected by Al in both Al-resistant and Al-sensitive varieties. The secretion rate of organic acid anions from barley was the lowest among wheat, rye and triticale.     

Isolation and characterization of a rice mutant hypersensitive to Al

Rice (Oryza sativa L.) is a highly Al-resistant species among small grain crops, but the mechanism responsible for the high Al resistance has not been elucidated. In this study, rice mutants sensitive to Al were isolated from M(3) lines derived from an Al-resistant cultivar, Koshihikari, irradiated with gamma-rays. Relative root elongation was used as a parameter for evaluating Al resistance. After initial screening plus two rounds of confirmatory testing, a mutant (als1) was isolated from a total of 560 lines. This mutant showed a phenotype similar to the wild-type plant in the absence of Al. However, in the presence of 10 microM Al, root elongation was inhibited 70% in the mutant, but only 8% in the wild-type plant. The mutant also showed poorer root growth in acid soil. The Al content of root apices (0-1 cm) was much lower in the wild-type plant. The sensitivity to other metals including Cd and La did not differ between the mutant and the wild-type plants. A small amount of citrate was secreted from the roots of the mutant in response to Al stress, but there was no difference from that secreted by the wild-type plant. Genetic analysis of F(2) populations between als1 and wild-type plants showed that the Al-resistant seedlings and Al-sensitive seedlings segregated at a 3 : 1 ratio, indicating that the high sensitivity to Al in als1 is controlled by a single recessive gene. The gene was mapped to the long arm of chromosome 6, flanked by InDel markers MaOs0619 and MaOs0615.     

Fine-mapping of the BaMMV, BaYMV-1 and BaYMV-2 resistance of barley (Hordeum vulgare) accession PI1963

Barley yellow mosaic disease caused by the bymoviruses barley mild mosaic virus (BaMMV) and barley yellow mosaic virus (BaYMV) is one of the economically most important diseases of winter barley in Europe. In European barley breeding programmes, resistance is currently due to only two genes—rym4, which is effective against viruses BaMMV and BaYMV-1, and rym5, which is effective against BaYMV-2. Diversification of resistance is therefore an important task. Because the accession PI1963 confers immunity against all European strains of barley yellow mosaic disease and is not allelic to rym5, we have attempted to develop closely linked markers in order to facilitate the efficient introgression of this resistance into adapted germplasm. By means of restriction fragment length polymorphism analysis, we located a gene locus for resistance to BaMMV, BaYMV-1 and BaYMV-2 of PI1963 on chromosome 4HL using a mapping population (W757) comprising 57 doubled haploid (DH) lines. Subsequent tests for allelism indicated that the BaMMV resistance gene in PI1963 is allelic to rym11. Two DH populations, IPK1 and IPK2, comprising 191 and 161 DH lines, respectively, were derived from the initial mapping population W757 and used for further analysis. As random amplified polymorphic DNA development did not facilitate the identification of more closely linked markers, simple sequence repeat (SSR) analyses were conducted. For population IPK1, the closest SSRs detected were Bmac181 and Bmag353, which flank the gene at 2.1 cM and 2.7 cM, respectively. For the IPK2 population, the SSR markers HVM3 and Bmag353 are located proximally at 2.5 cM and distally at 8.2 cM, respectively. In order to develop markers more tightly linked to rym11, a targeted amplified fragment length polymorphism (AFLP) marker identification approach was adopted using bulks comprising lines carrying recombination events proximal and distal to the target interval. Using this approach we identified six AFLP markers closely linked to rym11, with the two markers, E56M32 and E49M33, co-segregating with rym11 in both populations. The SSRs and AFLPs identified in this study represent useful tools for marker-assisted selection.     

Molecular mapping of <Emphasis Type="Italic">Rym17</Emphasis>, a dominant and <Emphasis Type="Italic">rym18</Emphasis> a recessive barley yellow mosaic virus (BaYMV) resistance genes derived from <Emphasis Type="Italic">Hordeum vulgare</Emphasis> L.

PK23-2, a line of six-rowed barley (Hordeum vulgare L.) originating from Pakistan, has resistance to Japanese strains I and III of the barley yellow mosaic virus (BaYMV). To identify the source of resistance in this line, reciprocal crosses were made between the susceptible cultivar Daisen-gold and PK23-2. Genetic analyses in the F₁ generation, F₂ generation, and a doubled haploid population (DH45) derived from the F₁ revealed that PK23-2 harbors one dominant and one recessive resistance genes. A linkage map was constructed using 61 lines of DH45 and 127 DNA markers; this map covered 1268.8 cM in 10 linkage groups. One QTL having a LOD score of 4.07 and explaining 26.8% of the phenotypic variance explained (PVE) for resistance to BaYMV was detected at DNA marker ABG070 on chromosome 3H. Another QTL having a LOD score of 3.53 and PVE of 27.2% was located at marker Bmag0490 on chromosome 4H. The resistance gene on chromosome 3H, here named Rym17, showed dominant inheritance, whereas the gene on chromosome 4H, here named rym18, showed recessive inheritance in F₁ populations derived from crosses between several resistant lines of DH45 and Daisen-gold. The BaYMV recessive resistance genes rym1, rym3, and rym5, found in Japanese barley germplasm, were not allelic to rym18. These results revealed that PK23-2 harbors two previously unidentified resistance genes, Rym17 on 3H and rym18 on 4H; Rym17 is the first dominant BaYMV resistance gene to be identified in primary gene pool. These new genes, particularly dominant Rym17, represent a potentially valuable genetic resource against BaYMV disease.     

Aluminum resistance in maize cannot be solely explained by root organic acid exudation. A comparative physiological study

Root apical aluminum (Al) exclusion via Al-activated root citrate exudation is widely accepted as the main Al-resistance mechanism operating in maize (Zea mays) roots. Nonetheless, the correlation between Al resistance and this Al-exclusion mechanism has not been tested beyond a very small number of Al-resistant and Al-sensitive maize lines. In this study, we conducted a comparative study of the physiology of Al resistance using six different maize genotypes that capture the range of maize Al resistance and differ significantly in their genetic background (three Brazilian and three North American genotypes). In these maize lines, we were able to establish a clear correlation between root tip Al exclusion (based on root Al content) and Al resistance. Both Al-resistant genotypes and three of the four Al-sensitive lines exhibited a significant Al-activated citrate exudation, with no evidence for Al activation of root malate or phosphate release. There was a lack of correlation between differential Al resistance and root citrate exudation for the six maize genotypes; in fact, one of the Al-sensitive lines, Mo17, had the largest Al-activated citrate exudation of all of the maize lines. Our results indicate that although root organic acid release may play a role in maize Al resistance, it is clearly not the only or the main resistance mechanism operating in these maize roots. A number of other potential Al-resistance mechanisms were investigated, including release of other Al-chelating ligands, Al-induced alkalinization of rhizosphere pH, changes in internal levels of Al-chelating compounds in the root, and Al translocation to the shoot. However, we were unsuccessful in identifying additional Al-resistance mechanisms in maize. It is likely that a purely physiological approach may not be sufficient to identify these novel Al-resistance mechanisms in maize and this will require an interdisciplinary approach integrating genetic, molecular, and physiological investigations.     

Mapping quantitative trait loci associated with barley net blotch resistance

Net blotch of barley, caused by Pyrenophora teres Drechs., is an important foliar disease worldwide. Deployment of resistant cultivars is the most economic and eco-friendly control method. This report describes mapping of quantitative trait loci (QTL) associated with net blotch resistance in a doubled-haploid (DH) barley population using diversity arrays technology (DArT) markers. One hundred and fifty DH lines from the cross CDC Dolly (susceptible)/TR251 (resistant) were screened as seedlings in controlled environments with net-form net blotch (NFNB) isolates WRS858 and WRS1607 and spot-form net blotch (SFNB) isolate WRS857. The population was also screened at the adult-plant stage for NFNB resistance in the field in 2005 and 2006. A high-density genetic linkage map of 90 DH lines was constructed using 457 DArT and 11 SSR markers. A major NFNB seedling resistance QTL, designated QRpt6, was mapped to chromosome 6H for isolates WRS858 and WRS1607. QRpt6 was associated with adult-plant resistance in the 2005 and 2006 field trials. Additional QTL for NFNB seedling resistance to the more virulent isolate WRS858 were identified on chromosomes 2H, 4H, and 5H. A seedling resistance QTL (QRpts4) for the SFNB isolate WRS857 was detected on chromosome 4H as was a significant QTL (QRpt7) on chromosome 7H. Three QTL (QRpt6, QRpts4, QRpt7) were associated with resistance to both net blotch forms and lines with one or more of these demonstrated improved resistance. Simple sequence repeat (SSR) markers tightly linked to QRpt6 and QRpts4 were identified and validated in an unrelated barley population. The major 6H QTL, QRpt6, may provide adequate NFNB field resistance in western Canada and could be routinely selected for using molecular markers in a practical breeding program.     

Diverse origins of aluminum-resistance sources in wheat

Aluminum (Al) toxicity is a major constraint for wheat production in acidic soils. Wheat producers now routinely use Al-resistant cultivars as one cost-effective means to reduce risks associated with acidic soils. To date, diverse Al-resistant materials have been identified, but their genetic relationship has not been well characterized. A total of 57 wheat accessions, including the majority of the parents of Al-resistant accessions we identified in a previous study, were evaluated for Al resistance and analyzed with 49 simple sequence repeat (SSR) markers and 4 markers for Al-activated malate transporter (ALMT1). Pedigree and principle coordinate analysis (PCA) both separated Al-resistant accessions into four groups labeled according to common ancestry or geographical origin: US-Fultz, Polyssu, Mexican and Chinese. Al resistance in the four groups may have three independent origins given their distinct geographic origins and gene pools. Fultz originated in the USA as a major ancestor to soft red winter wheat, Polyssu originated in Brazil as a major source of Al resistance used in most genetic studies worldwide, and the Chinese group originated in China. Based on ALMT1 marker haplotypes, the Al resistance in the Polyssu and Mexico groups was likely derived from Polyssu, while most Al-resistant cultivars developed in the USA most likely inherited most of Al resistance from Fultz. Fultz was released about 50 years earlier than Polyssu. Norin 10 likely played a pivotal role in passing Al-resistant gene(s) from Fultz to better adapted, semi-dwarf wheat cultivars developed in the USA. Further characterization of Al resistance in the three different sources could reveal multiple Al-resistant mechanisms in wheat. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Identification of quantitative trait loci for resistance to powdery mildew in a Spanish barley landrace

The Spanish landrace-derived inbred line SBCC97, together with other lines from the Spanish Barley Core Collection, displays high resistance to powdery mildew, caused by the fungus Blumeria graminis f. sp. hordei. The objective of this study was to map quantitative trait loci (QTLs) for resistance to powdery mildew in a recombinant inbred line population derived from a cross between SBCC97 and the susceptible cultivar ‘Plaisant’. Phenotypic analysis was performed using four B. graminis isolates, and genetic maps were constructed with mainly simple sequence repeat (SSR) markers, following a sequential genotyping strategy. Two major QTLs with large effects were identified on chromosome 7H, and they accounted for up to 45% of the total phenotypic variance. The alleles for resistance at each QTL were contributed by the Spanish parent SBCC97. One locus was mapped to the short arm of chromosome 7HS, and was flanked by the resistance gene analogue (RGA) marker S9202 and the SSR GBM1060. This corresponded to the same chromosomal region in which a major race-specific resistance gene from Hordeum vulgare ssp. spontaneum, designated as mlt, had been identified previously. The second QTL was linked tightly to marker EBmac0755, and it shared its chromosomal location with the qualitative resistance gene Mlf, which has only been described previously in the wild ancestor H. spontaneum. This is the first report of these two QTLs occurring together in cultivated barley, and it paves the way for their use in barley breeding programs that are designed to transfer resistance alleles into elite cultivars.     

Genetic analysis of scab resistance QTL in wheat with microsatellite and AFLP markers.

Three chromosomal regions associated with scab resistance were detected in a common cultivar, Ning7840, by microsatellite and AFLP analysis. Six microsatellites on chromosome 3BS, Xgwm389, Xgwm533, Xbarc147, Xgwm493, Xbarc102, and Xbarc131, were integrated into an amplified fragment length polymorphism (AFLP) linkage group containing a major quantitative trait locus (QTL) for scab resistance in a mapping population of 133 recombinant inbred lines (RILs) derived from 'Ning7840' x 'Clark'. Based on single-factor analysis of variance of scab infection data from four experiments, Xgwm533 and Xbarc147 were the two microsatellite markers most tightly associated with the major scab resistance QTL. Interval analysis based on the integrated map of AFLP and microsatellite markers showed that the major QTL was located in a chromosome region about 8 cM in length around Xgwm533 and Xbarc147. Based on mapping of six microsatellite markers on eight 3BS deletion lines, the major QTL was located distal to breakage point 3BS-8. In total, 18 microsatellites were physically located on different subarm regions on 3BS. Two microsatellites, Xgwm120 and Xgwm614, were significantly associated with QTL for scab resistance on chromosome 2BL and 2AS, respectively. The resistance alleles on 3BS, 2BL, and 2AS were all derived from 'Ning7840'. Significant interaction between the major QTL on 3BS and the QTL on 2BL was detected based on microsatellite markers linked to them. Using these microsatellite markers would facilitate marker-assisted selection to improve scab resistance in wheat.     

Inheritance of the number and thickness of cell layers in barley aleurone tissue (<Emphasis Type="Italic">Hordeum vulgare </Emphasis>L.): an approach using F2–F3 progeny

The aleurone tissue of cereal grains, nutritionally rich in minerals and vitamins, is an important target for the improvement of cereals. Inheritance of the thickness and the number of cell layers in barley aleurone was studied on the F2–F3 progeny of an Erhard Frederichen × Criolla Negra cross in which the parental lines have three or two aleurone layers, respectively. F3 grain was sampled from each F2 plant and 96.8% of the entire F3 grain population was classified as being either the 2- or 3-layer type. Using microsatellite, single nucleotide polymorphism (SNP) and morphological markers on 190 F2 plants, a linkage map was built. Three quantitative trait loci (QTLs) affecting aleurone traits were revealed on chromosome 5H (max. LOD = 5.83) and chromosome 7H (max. LOD = 4.45) by interval mapping, and on chromosome 2H by marker analysis with an unmapped marker. These QTLs were consistent with genetic sub-models involving either 2-cell type dominance for 7H and 2H, or putative partial dominance for 5H where 2-cell-layer dominance and additivity gave similar LODs. The number of aleurone cell layers and aleurone thickness were strongly correlated and QTL results for these traits were alike. An SNP marker of sal1, an orthologue of the maize multilayer aleurone gene was mapped to the 7HL chromosome arm. However, the 7H QTL did not co-locate with the barley sal1 SNP, suggesting that an additional gene is involved in determining aleurone traits. These new mapping data allow comparisons to be made with related studies.     

Isolate-specific QTLs of resistance to leaf stripe (<Emphasis Type="Italic">Pyrenophora graminea</Emphasis>) in the 'Steptoe' × 'Morex' spring barley cross

Leaf stripe caused by the fungus Pyrenophora graminea represents a serious threat to grain yield in organically grown barley and in conventional Nordic and Mediterranean districts, for which resistant cultivars are necessary. A medium-density, molecular marker map derived from a 'Steptoe' (partially resistant) x 'Morex' (susceptible) spring barley cross and its derived doubled-haploid mapping population inoculated with the fungus made it possible to identify QTLs of resistance to leaf stripe. In order to investigate isolate-specificity of partial resistance, the 'Steptoe' x 'Morex' segregating population was inoculated with two highly virulent P. graminea isolates, Dg2 and Dg5. The present study demonstrates that partial resistance to leaf stripe of cv 'Steptoe' is governed in part by shared loci and in part by isolate-specific ones. One QTL is common to the resistance for the two isolates, on the long arm of chromosome 2 (2H), two QTLs are linked on chromosome 3 (3H), and the remaining two are isolate-specific, respectively for isolate Dg2 on chromosome 2 (2H) and for isolate Dg5 on chromosome 7 (5H). The QTL in common is that with the major effect on the resistance for each isolate, explaining 18.3% and 30.9% R(2) respectively for Dg2 and Dg5. The isolate-specific QTLs mapped in the 'Steptoe' x 'Morex' barley reference map support the assumption of Parlevliet and Zadoks (1977) that partial resistance may be due to minor gene-for-minor-gene interactions. Map comparisons of the QTLs with the known qualitative resistance genes to leaf stripe, Rdg1 (2H) and Rdg2 (7H), as well as with other QTLs of partial resistance in barley, show that the QTL for resistance to both isolates mapped on the long arm of chromosome 2 (2H) does not coincide with the qualitative Rdg1 gene but is linked to it at about 30 cM. One isolate-specific QTL of resistance to P. graminea, mapped on the short arm of chromosome 2 (2H), is coincident with a QTL for resistance to Pyrenophora teres previously mapped in the 'Steptoe' x 'Morex' cross.     

Genetic and physiological analysis of doubled-haploid,aluminium-resistant lines of wheat provide evidence for the involvement of a 23 kD,root exudate polypeptide in mediating resistance

We have made use of a genetic approach to develop homozygous, near-isogenic germplasm for investigating aluminium (Al) resistance in Triticum aestivum L. A conventional backcross program was used to transfer Al resistance from the Al-resistant cultivar, Maringa, to a locally-adapted, Al-sensitive cultivar, Katepwa. At the third backcross stage, a single, resistant isoline (Alikat = Katepwa*3/Maringa) was chosen on the basis of superior root growth after 14 days of exposure to a broad range of Al concentrations (0 to 600 µM). Genetic analysis of doubled-haploid lines (DH) developed from this isoline suggested that resistance is controlled by a single dominant gene. Crosses between DH Alikat and DH Katepwa yielded an Al-resistant F1 population. Backcrossing this F1 population to DH Katepwa produced a population which segregated 1:1 for Al resistance, while selfing produced a population segregating 3 : 1 for Al resistance. Under conditions of Al stress, Al-resistant F2 plants released a suite of novel low molecular weight polypeptides into the rhizosphere. One of these polypeptides (23 kD) shows substantive Al-binding capacity and segregates with the resistant phenotype. While the precise mechanisms that mediate Al resistance are still unknown, this research has provided support for a possible role of the 23 kD exudate polypeptide in mediating resistance to Al. To more fully understand the role that this polypeptide plays in Al-resistance, we are attempting to clone this gene from microsequence data obtained from purified protein.     

Quantitative trait loci for aluminum resistance in Chinese wheat landrace FSW

Aluminum (Al) toxicity is a major constraint for wheat production in acid soils worldwide. Chinese landrace FSW demonstrates a high level of Al resistance. A population of recombinant inbred lines (RILs) was developed from a cross between FSW and an Al-sensitive Chinese line, ND35, using single seed descent, to map quantitative trait loci (QTLs) for Al resistance. Wheat reaction to Al stress was measured by net root growth (NRG) in a nutrient solution culture containing Al(3+) and hematoxylin staining score (HSS) of root after Al stress. After 1,437 simple sequence repeats (SSRs) were screened using bulk segregant analysis, three QTLs were identified to control Al resistance in FSW. One major QTL (Qalt.pser-4DL) was mapped on chromosome 4DL that co-segregated with Xups4, a marker for the promoter of the Al-activated malate transporter (ALMT1) gene. The other two QTLs (Qalt.pser-3BL, Qalt.pser-2A) were located on chromosomes 3BL and 2A, respectively. Together, the three QTLs accounted for up to 81.9% of the phenotypic variation for HSS and 78.3% of the variation for NRG. The physical positions of flanking markers for Qalt.pser-4DL and Qalt.pser-3BL were determined by analyzing these markers in corresponding nulli-tetrasomic, ditelosomic, and 3BL deletion lines of Chinese Spring. Qalt.pser-3BL is a novel QTL with a major effect on Al resistance discovered in this study. The two major QTLs on 4DL and 3BL demonstrated an additive effect. The SSR markers closely linked to the QTLs have potential to be used for marker-assisted selection (MAS) to improve Al resistance of wheat cultivars in breeding programs.