Journal of Experimental Botany, Vol. 51, No. 342, pp. 19-27,
January 2000
© 2000 Oxford University Press
The development and application of molecular markers for abiotic stress tolerance in barley
1 Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, Scotland, UK
2 Dipartimento di Agronomia, Università delgi Studi di Bologna, Via Filippo Re 6, 40126Bologna, Italy
3 INRA-Montpellier, Génétique et Amélioration des Plantes, 2 place Pierre Viala, 34060 Montpellier Cedex, France
4 Barley Research Department, Field Crops Research Institute, 8 El-Gamaa Street, Giza 12619, Egypt
5 Départment d'Agronomie et d'Amélioration des Plantes, Ecole Nationale d'Agriculture de Meknès, PBS40 Meknès, Morocco
6 Laboratoire de Physiologie Végétale, INRAT 2049, rue Hedi Karray, Ariana, Tunis, Tunisia
Received 17 February 1999; Accepted 17 August 1999
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Abstract |
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 Top Abstract IntroductionEffects of developmental genesWild species/landrace variationQTL mappingOngoing work on marker...References |
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This article represents some current thinking and objectives in the use of molecular markers to abiotic stress tolerance. Barley has been chosen for study as it is an important crop species, as well as a model for genetic and physiological studies. It is an important crop and, because of its well-studied genetics and physiology, is an excellent candidate in which to devise more efficient breeding methods. Abiotic stress work on cultivated gene pools of small grain cereals frequently shows that adaptive and developmental genes are strongly associated with responses. Developmental genes have strong pleiotropic effects on a number of performance traits, not just abiotic stresses. One concern is that much of the genetic variation for improving abiotic stress tolerance has been lost during domestication, selection and modern breeding, leaving pleiotropic effects of the selected genes for development and adaptation. Such genes are critical in matching cultivars to their target agronomic environment, and since there is little leverage in changing these, other sources of variation may be required. In barley, and many other crops, greater variation to abiotic stresses exists in primitive landraces and related wild species gene pools. Wild barley, Hordeum spontaneum C. Koch is the progenitor of cultivated barley, Hordeum vulgare L. and is easily hybridized to H. vulgare. Genetic fingerprinting of H. spontaneum has revealed genetic m arker associations with site-of-origin ecogeographic factors and also experimentally imposed stresses. Genotypes and collection sites have been identified which show the desired variation for particular stresses. Doubled haploid and other segregating populations, including landrace derivatives have been used to map genetically the loci involved. These data can be used in molecular breeding approaches to improve the drought tolerance of barley. One strategy involves screening for genetic markers and physiological traits for drought tolerance, and the associated problem of drought relief-induced mildew susceptibility in naturally droughted fields of North Africa.
Key words: Molecular markers, abiotic stress, molecular breeding, cultivated barley, Hordeum vulgare, wild barley, Hordeum spontaneum.
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Introduction |
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TopAbstractIntroductionEffects of developmental genesWild species/landrace variationQTL mappingOngoing work on marker...References |
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Barley (Hordeum vulgare L.) is an established model species for genetic and physiological studies (Koornneef et al., 1997
). It is a convenient experimental organism because: (1) it is an annual with a short life cycle; (2) it is diploid with only seven pairs of chromosomes; (3) it is true breeding allowing multiple testing; (4) it exhibits wide diversity in physiology, morphology and genetics; (5) a wide range of genetic stocks is available; and (6) it has well-defined genetic maps. Barley is also an important cereal crop species ranking fourth in the world after rice, the wheats, and maize (Bengtsson, 1992). Major production areas include Europe, the Mediterranean rim of North Africa, Ethiopia, the Near East, the former USSR, China, India, Canada, the United States, South America, and Australia (Nevo, 1992; Harlan, 1995). Barley is an ancient crop species, it was domesticated about 10 000 years ago from the wild progenitor, H. spontaneum C. Koch somewhere in the Fertile Crescent (Zohary and Hopf, 1988; Harlan and Zohary, 1966) and is used for human consumption, fodder, brewing beer, and whisky production. The cultivated and wild species share a common genome (HH, n=7), are inter-fertile and can be recombined, they are considered by some taxonomists to be the same species.
The processes of domestication and selection have resulted in a drastic narrowing of the genetic variation of crop species (Tanksley and McCouch, 1997), including barley (Powell, 1997). In recent years, breeding for uniformity has accelerated this process and has led to greater susceptibility of many crops to diseases, pests and abiotic stresses (Plucknett et al., 1983). The genetic bottlenecks arising from the transitions between wild genotypes to early domesticated germplasm, and from early domesticated germplasm to modern cultivars has left behind many potentially useful genes. Until the late nineteenth century, all cultivated barleys existed as landraces (mixtures of inbred lines and hybrid segregants). Some landraces persist to the present day, especially in developing countries, though selection and breeding has largely displaced landraces with pure line cultivars. Barley is an inbreeding species and single plant selection, which promotes uniformity, has been common since the 1800s. The wild progenitor species and the primitive landraces of barley offer rich sources of genetic variation for crop improvement (Nevo, 1992; Ceccarelli et al., 1995). These gene pools can be exploited using conventional crossing procedures, but with the aid of genetic maps, markers and quantitative trait locations (QTL analysis) greater precision can be obtained in selecting desired genotypes.
Studies on the quantitative genetic control of abiotic stress have necessitated collaboration between geneticists and physiologists. Since quantitative geneticists typically score a few traits on many genotypes and physiologists do the opposite, collaborative experiments can be very large, assessing several traits in many genotypes. Nevertheless, the combination is powerful, productive and popular, evidenced by the many recent publications, whole journal issues and symposia (Thomas and Farrar, 1997). The interdependence of genetics and physiology is important in assessing phenotype, and determining how much of the phenotypic variation is controlled by genotype (G), how much is due to environmental effects (E ) and how much is a result of an interaction between the two (GxE ). If genotype explains large portions of the variation then genetic markers can be used to select for desired phenotypes: greater efficiency requires a shift from phenotypic selection to genotypic selection. If, however, phenotype is determined largely by environment then changes in agronomy may be more effective in producing the desired type. Agronomy cannot, however, change geographic location nor climate and the adaptation of barley to varied growing conditions around the world is largely a result of selection of specific genotypes. GxE interactions require attention to both factors. Many traits are, of course, controlled by genetic factors and improvement is still possible through genetic modification, and these include tolerance to abiotic stresses. In recent years breeders have become increasingly interested in exploiting genetic markers: ‘molecular breeding’, ‘accelerated breeding’, ‘marker assisted selection’ are terms used to describe new breeding methodologies based on genotypic selection. Barley is a prime example in which genetic markers are being used to devise new breeding methods (Powell et al., 1996a; Toojinda et al., 1998). Once genetic markers have been developed for a particular trait the markers can be used to evaluate the variation available to breeders. From these data more informed decisions can be made on parental combinations.
The genetic/physiology interaction also provides a potential means of identifying candidate genes/mechanisms. Genetic analyses, particularly of quantitative traits such as abiotic stress tolerance have associated errors. The detection of the loci involved is based on probabilities and can be crude (Hay and Ellis, 1998), with different results in different environments. Likewise physiological mechanisms can be difficult to dissect. More rigour is provided when genetic and physiological analyses point to the same gene function, allowing greater confidence in identifying a candidate gene.
This article describes some of the major aspects of abiotic stress tolerance research in barley including, developmental genes, exotic variation, genetic mapping, and breeding. Genetic markers have played a major role throughout and the various marker systems used, their pros and cons, are briefly described.
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Effects of developmental genes |
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TopAbstractIntroductionEffects of developmental genesWild species/landrace variationQTL mappingOngoing work on marker...References |
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Developmental genes are important in adapting crops to their agricultural environments and there has been strong selection for these genes during domestication and breeding. As stated earlier, there is a concern that genes for abiotic stress tolerance have been lost during the processes of domestication and breeding and, of the genes carried forward, the developmental genes often have pleiotropic effects on a number of traits including stress tolerance. Developmental genes in wheat, notably those involved in flowering time, are known to have pleiotropic effects on abiotic stress tolerance. When single chromosome recombinant lines carrying contrasting alleles of vernalization requirement (Vrn/vrn) and photoperiod requirement (Ppd/ppd ) genes were tested for salt-tolerance, lines carrying the dominant alleles (for early flowering) were found to be the more tolerant (Taeb et al., 1992
). Quantitative trait loci for salinity tolerance in wheat have been mapped (Semikhodskii et al., 1997) by testing a doubled haploid population, and it was found that the vernalization gene, Vrn1 had significant pleiotropic effects on a number of traits including those involved in salt tolerance. The ari-e.GP and sdw1 (denso) mutations are commercially important sources of semi-dwarfism in cultivated barley. In addition to plant stature they have large pleiotropic effects on salt tolerance and yield. As in the case of Vrn and Ppd in wheat, the positive effects of the ari-e.GP gene of barley appear to be constitutive, i.e. responses are not switched on by the stress.
Salt-tolerance tests on spring barley have showed that lines carrying the ari-e.GP mutation are more salt tolerant than non-ari-e.GP genotypes (Pakniyat et al., 1997a, c). The ari-e.GP mutation (syn. GPert) arose in the cultivar Golden Promise, which is a direct mutant of the cultivar Maythorpe (Sigurbjornsson and Micke, 1969). The mutation in Golden Promise (ari-e.GP) produces a semi-dwarf, erect phenotype with short awns, and has been of significant commercial importance. The mutation has been mapped to chromosome 5H (Thomas et al., 1984) and forms part of the ari-e (breviaristatum, short awned) mutation series (Franckowiak, 1991). Maythorpe is a relatively salt-sensitive spring cultivar (Forster et al., 1994) therefore the tolerance of Golden Promise must have arisen from the 
-ray mutagenic treatment. The isogenic relationship between Maythorpe and Golden Promise was confirmed using randomly amplified polymorphic DNA (RAPD) fingerprinting. RAPDs (Williams et al., 1990; Welsh and McClelland, 1990) are produced by PCR with arbitrary primers; polymorphism is a result of changes in the primer binding site in the DNA sequence.
The genetic make-up of crop plants, including barley has been finely tuned by decades of breeding. It is unlikely that large improvements for abiotic stress tolerance can be made by altering the composition of developmental genes. Stress tolerance genes, which are unlinked to essential agronomic traits need to be identified if realistic improvements of crops for abiotic stress tolerance are to be made. This has fuelled the search for sources of such variation among the wild relatives of crop plants.
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Wild species/landrace variation |
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TopAbstractIntroductionEffects of developmental genesWild species/landrace variationQTL mappingOngoing work on marker...References |
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Wild barley, Hordeum spontaneum C. Koch is the immediate progenitor of cultivated barley H. vulgare L. (Bothmer et al., 1995
; Harlan and Zohary, 1966; Zohary, 1969). The centre of diversity for H. spontaneum is the Fertile Crescent of the Middle East where wild barley is found in a wide range of environments varying in water supply, temperature, photoperiod, soil type, plant community, etc (Forster et al., 1997). Many types of genetic markers have been applied to diversity studies in wild barley. Interesting results came from some of the early work using biochemical markers (summarized in Nevo, 1992) which showed that wild barley possessed considerably more variation than the cultivated species, and that many alleles are associated with adaptation to specific environments. It has been shown that wild barleys from Israel possess seven isoforms of the enzyme ß-amylase, of which only two have been found in European cultivars (Chalmers et al., 1992). One isoform was restricted to desert locations, and 78% of the variation in the frequency of this isoform could be explained by site-of-origin rainfall and temperature. The ß-amylase gene, Bmy1 has been mapped to the long arm of chromosome 4H and found to be tightly linked to a QTL for height (Hackett et al., 1992) and a major spring habit gene, sh (syn. Vrn1, Chojecki et al., 1989; Hackett et al., 1992; Laurie et al., 1995). The tight linkage (2–4 cM) can be exploited in using Bmy1 as a genetic marker for spring/winter habit (Chojecki et al., 1989; Powell et al., 1991). The association of ß-amylase with adaptation to dry environments in wild barley may be due to linkage with variation at the sh locus. It may be that, as in cultivars, developmental genes in wild barley have major effects on adaptation and tolerance to abiotic stresses and control of development may provide an avoidance mechanism.
Genetic diversity in wild barley has also been studied using RFLPs (Saghai-Maroof et al., 1984), RAPDs (Dawson et al., 1993), simple sequence repeats (SSRs, also known as microsatellites; Saghai-Maroof et al., 1994), and AFLPs (Pakniyat et al., 1997b). A principal coordinate (PCO) plot of AFLP fingerprinting data of wild barleys from the Fertile Crescent showed that genotypes from Israel cluster separately from those of Turkey and Iran. Furthermore, the genotypic separations mirror those produced by a principal component analysis (PCA) plot of site-of-origin ecogeographic data (Forster et al., 1997) suggesting that there is specific genetic variation associated with specific environments. The wild barleys used in the AFLP study have been tested for responses to a number of abiotic stresses including, salinity, drought, N-starvation, cold, ozone, and daylength. The most complete data are those for salinity.
The strategy developed to identify genetic markers, genotypes and collection sites for the trait salt tolerance was to screen 39 wild barley lines from around the Fertile Crescent. Lines from Israel, Turkey and Iran were grown in hydroponics containing 100 mol m-3 NaCl. After 4 weeks of stress shoot Na+ and 
13C were measured. Shoot Na+ has been a standard indicator of salt tolerance in the Poaceae (Greenway and Munns, 1980), shoot 13C has been developed as an indicator of potential water-use efficiency (Farquhar and Richards, 1984), which has been correlated with salt tolerance in barley (Pakniyat et al., 1997a). From the genotypic data, 12 out of 204 polymorphic AFLPs were significantly associated with shoot Na+ content and shoot 13C. The 12 AFLPs were also associated with site-of-origin ecogeography. The most salt-tolerant line, from the Ilam site came from the south-eastern area of the Fertile Crescent whereas the most salt-susceptible line, from the Tabigha site came form the south-western portion (Pakniyat et al., 1997b). The strategy allows candidate genetic markers, genotypes and collection sites to be identified for a trait, in this case salt tolerance.
The AFLP technique generates a large number of polymorphic bands and, therefore, has a high multiplex ratio (Powell et al., 1996a) and homologous products map to the same location in different mapping populations (Waugh et al., 1996). Whilst this provides a means to link AFLP bands from the H. spontaneum survey to genetic maps, it proved difficult to do so in practice. Firstly, it is difficult to size bands to the exactitude required for cross-referencing, secondly, many of the bands were not represented in existing mapping populations as different base cutting enzymes had been used to prepare the template DNAs. AFLPs also have a big disadvantage in being dominant markers. For the vast majority of AFLPs only two phenotypes can be recognized; those in which the band is present and those in which the band is absent. Of the 12 AFLPs associated with salt tolerance only three were mappable. Interest has now centred on SSRs as a genetic marker system (Tautz and Renz, 1984). SSRs are PCR-based markers, which have the advantages of being single locus markers, co-dominant, multi-allelic, and widely dispersed over the genome. They have the disadvantages of having a time-consuming and costly developmental phase (Powell et al., 1996b). Fortunately, barley is one species in which SSRs have been developed and there are now over 500 mapped (Waugh et al., 1997; Ramsay et al., unpublished results). Initial work using SSRs in wild barley diversity studies involved just 11 mapped SSRs (Forster et al., 1997), one of which, in the Rubisco activase gene (Rca), was found to be correlated with site-of-origin water availability (Forster, 1999). Rubisco activase is physiologically important because primary CO2 assimilation in C3 species is mediated by Rubisco. It is the balance between Rubisco-mediated carboxylation and stomatal conductance (which regulates CO2
diffusion into and H2O diffusion out of the leaf) which determines 13C/12C discrimination and hence 13C (Farquhar et al., 1982). Rubisco activity is, in turn, regulated by Rubisco activase (Streusand and Portis, 1987). SSRs may not be necessarily biologically neutral and selection for SSR alleles may occur in relation to adaptation in wild barley.
The variation of SSRs in cultivars, landraces and wild barley shows that landrace and wild barley have unique alleles not found in the cultivated gene pool (Powell, 1997). The results show that wild barley offers a rich source of genes of enormous potential for crop improvement. Genetic loci known to be involved in the control of specific traits in cultivated barley can now be targeted and investigated in the wild gene pool to seek out novel and rare alleles.
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QTL mapping |
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TopAbstractIntroductionEffects of developmental genesWild species/landrace variationQTL mappingOngoing work on marker...References |
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Work described above on genetic marker associations with plant traits and site-of-origin ecogeography in wild barley is based on marker/trait regression analysis. The analysis provides an estimate of how well two parameters are associated, but association analysis can be imprecise and even misleading. Genetic mapping (Prioul et al., 1997
) provides a more rigorous assessment. Abiotic stress tolerances are quantitatively inherited traits, controlled by several genetic loci (QTLs). QTL analyses can be donein relation to mapped genetic markers and provide data on genome location and the relative effects, both positive and negative, of loci and alleles. Genetic maps in barley have been based on various markers, the most useful being those that are transferable from one mapping population to another, notably RFLPs and SSRs with the latter now being in the ascendancy. RLFPs, SSRs and other reference markers allow QTLs to be located and compared among mapping populations. For example, major barley QTLs for malting quality can be found in similar positions on chromosome 5H in Harrington/TR306 and Blenheim/E224 mapping populations, but distinct QTLs are found on chromosome 1H in Puffin/NSL92, 3H in Blenheim/E224, 5H in Harington/TR306, and 7H in Septoe/Morex mapping populations. These differences may, however, also reflect different environments and/or different QTL detection methods as well as germplasm differences.
Two doubled haploid populations have been tested for salinity tolerance at SCRI, Lina/HS92 DHs and Derkado/B83–12/21//5 DHs. The first of these is derived from a cross between a Swedish barley cultivar, Lina and a wild barley from Canada Park in Israel, HS92. The Lina/HS92 DHs segregate for many traits and more than 1000 genetic markers, AFLPs, RAPDs, and SSRs have been mapped in this population (Ellis et al., 1996; Waugh et al., 1996). Physiological traits associated with salt tolerance were mapped to chromosomes 1 (7H), 4 (4H), 5 (1H), and 6 (6H) (Ellis et al., 1997; the 4H map is shown in Fig. 1), and some QTLs were detected in both control and salt treatments. Others could be detected only in one or other of the two treatments indicating that gene action was induced by salt stress in some cases, or that normal activity was sufficiently reduced to reveal the action of other loci. The second test was set up to study the effects of the ari-e.GP and sdw1 dwarfing genes, both of which segregate in the Derkado/B83–12/21/5 DHs and for which there is a well-developed genetic map (Thomas et al., 1998). Initial results show that physiological traits associated with salt tolerance are found on all seven barley chromosomes, but with the greatest effects being clustered around the dwarfing genes. Yield QTLs have also been located in the Derkado/B83–12/21/5 DHs and many of the QTLs associated with physiological responses to salinity are coincident with those associated with yield, notably the dwarfing genes.
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Once QTL data from mapping populations are obtained, a comparison can be made between the QTL markers and those markers identified by regression analysis in populations of wild barley genotypes. The following questions can then be addressed:
- (1) Does regression analysis provide reliable marker information and to what extent can regression analysis be used as a predictor of abiotic stress response?
(2) How many loci are involved and how great are their individual and combined effects?
(3) Are loci associated with abiotic stress responses in wild barley the same as those identified in cultivated barley?
(4) How genetically variable are QTLs for stress response in cultivated compared to wild gene pools?
(5) Do QTLs with major effects on stress tolerance coincide with QTLs of adaptive or commercial importance, or can crop improvement for abiotic stress proceed using QTLs unlinked to these traits?
An aid to answering these questions would be the deployment of a common set of markers for both the wild barley association studies and the QTL mapping studies. RFLPs and SSRs evenly spaced across the barley genome offer ideal tools for such purposes.
QTL mapping has become a standard procedure in dissecting the genetic controls of a variety of traits. The next step is the identification of the genes, alleles and physiological processes that are biologically important: for abiotic stress tolerance in agriculture this translates into those which contribute to yield. The genomic regions identified by QTL analysis can be surveyed for coincident candidate genes of known function. Other, unlinked, functional genes can be eliminated at the same time. In abiotic stress work the long arm of chromosome 4H is often implicated; water use efficiency (Handley et al., 1994), adaptation to droughted environments (Chalmers et al., 1992), and salt tolerance (Ellis et al., 1997). Candidate genes include the vernalization gene (Vrn1, syn. sh) and Rubisco activase (Rca) which both map to the long arm of 4H (Fig. 1; Becker and Heun, 1995).
More detailed physiological studies can be performed by comparing genotypes which vary for critical genomic regions. In this way useful traits (morphological, developmental, physiological or biochemical) can be identified which contribute to yield under stressed conditions. The development of precise genetic stocks such as single chromosome recombinant lines or mutants, which contrast for critical regions of the genome, would be an advantage for such studies. The following questions could then be addressed:
- (1)How effective are the genetic changes? How important are background genetic effects?
(2) Which genes and physiological mechanisms are involved?
(3) Are these genes/physiological mechanisms specific? How many are common to a range of stresses?
(4) How may these genes/physiological traits be manipulated?
(5) Can markers be developed for application in molecular breeding?
(6) Where are the most potent sources of variation?
QTL maps for stress responses may be usefully compared with gene maps and QTL maps of important traits such yield, agronomy, quality and disease resistance. In Fig. 1 some genetic maps of one barley chromosome, 4 (4H) are compared. The first map in Fig. 1 is a classical consensus gene map, the rest are molecular marker maps which have been used to identify QTLs for flowering time, agronomic and commercially important traits, and traits for abiotic stress tolerance. The maps have been constructed using morphological and phenotypic data and a variety of molecular markers, RFLPs, SSRs, and AFLPs. By comparing the maps of 4H it is possible to identify a variety of genes and markers which may be associated with a mapped QTL for a trait of interest. By comparing the chromosome 4 (4H) maps it becomes apparent that genes associated with stress responses such as 13C may be coincident with or tightly linked with genes controlling development, yield and/or quality traits. If QTLs for stress responses are coincident with commercially important traits then there may be a penalty in changing these in favour of more potent stress response alleles. A more productive strategy may be to target stress QTLs unlinked to commercially important QTLs.
Information from other genomes may also be useful. Figure 1 shows the location of a gene controlling K+/Na+ discrimination (Kna1) in a 4B/4D wheat linkage map. This gene has been implicated in salt tolerance of wheat (Dvo
ák et al., 1994). Such information may be useful in finding homoeologues in barley, and may tie-in with pre-existing data on QTLs for stress tolerance in this genomic region. Comparative genome mapping has become a powerful technique in finding genes by using genetic information gained in another species (Ahn et al., 1993; Peng et al., 1999).
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Ongoing work on marker assisted breeding for abiotic stress tolerance |
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TopAbstractIntroductionEffects of developmental genesWild species/landrace variationQTL mappingOngoing work on marker...References |
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Genetic and physiological studies have been used to scan the components of the primary gene pool of barley (wild, landrace and modern cultivars) for tolerance of various abiotic stresses. Landrace and, particularly, wild barley genotypes exhibit huge variation of interest to breeders in improving the modern crop for specific traits. The introgression of this exotic variation into modern barley cultivars has been problematic in the past due to linkage drag (associated deleterious genes), difficulties in recognizing and testing for the presence of introgressed characters, and background genetic effects. Molecular marker procedures now provide a means of overcoming these problems. A programme of work has been initiated to apply molecular marker technologies to improve the drought tolerance and mildew resistance of the North African barley crop. The aim is to cross selected wild and landrace barleys to North African cultivars and to assess performance of subsequent backcross generations, doubled haploid lines and recombinant inbred lines (RILs) for drought tolerance in naturally droughted field conditions. Field performance (yield, morphology and physiology) from various sites in three countries, Morocco, Tunisia and Egypt, will then be compared with genotype (inherited genetic markers). The monitoring of genetic markers and physiological traits known to be associated with drought tolerance in natural habitats and in controlled environment experimentation will be of particular interest.
Four approaches are to be taken. In the first, wild barley is backcrossed to a barley cultivar with selection for agronomic characters and performance in the field. At the end of three years the best performing plants will be assessed for markers inherited from the wild donor species. In this approach (devised by Tanksley and Nelson, 1996), the discovery and transfer of desirable QTLs from unadapted to élite germplasm is simultaneous. The approach has already been tested in tomato (Tanksley et al., 1996), maize (Painter et al., 1997) and rice (Xiao et al., 1997). The second approach also uses wild barley as a donor for drought tolerance, but here genetic markers known to be associated with tolerance in the wild donor will be selected for (marker assisted selection). A selection pressure, to speed up the development of more agronomically acceptable types, will be applied to select for the genetic background of the recipient North African cultivars. A new method of fingerprinting, Copia-SSR (Provan et al., 1999) will be used to monitor genetic background. Copia-SSR is a simple PCR method based on primers directed towards two classes of multi-copy sequences, which are widely distributed, namely Ty1-copia retrotransposons and SSRs. As part of the second approach a doubled haploid population derived from an F1 from a cultivarxwild species cross will be developed and exploited in mapping QTLs for performance in naturally droughted fields. The third programme concentrates on physiological traits associated with drought tolerance. Here a selected line from an adapted Syrian landrace (Tadmor) is used as a source of genes for drought tolerance; the line is characterized by having high yield stability and is well adapted to dry conditions (Ceccarelli et al., 1987). Recombinant inbred lines (RILs) have been produced from an initial cross between Tadmor and the contrasting drought-susceptible line AR/Apm. Traitssuch as osmotic adjustment, plant architecture, growth habit, 13C, and chlorophyll content will be measured in the field and mapped as QTLs in the RILs. The exchange of information, expertise in genotyping and phenotyping will allow a high degree of accuracy in assessing QTL effects and may allow candidate genes (genes of known function) to be identified. A fourth component of work concerns drought relief-induced mildew susceptibility. This occurs following relief of water stress and includes cultivars that carry mlo or other resistance genes. The problem has been documented in Europe (Newton and Young, 1996), but has also been reported in North Africa (Yahyaoui et al., 1997). This trait will be treated as a QTL, genetically mapped and the presence of QTLs controlling the trait monitored in the other programmes of work with the aim of breaking any undesirable linkage.
Key questions posed by this research are:
- (1) Which genetic markers and physiological traits from the wild and landrace barleys are retained and which are lost?
(2) Do these markers and physiological traits correspond to those inherited from cultivars, landraces or wild germplasm?
(3) What are the similarities and contrasts of lines selected in the various North African sites?
(4) Are genes/physiological traits associated with drought tolerance also associated with yield penalties? Can undesirable linkages be broken? How important are background effects, e.g. epistasis?
(5) What is the potential for widening the gene pool to include landrace and wild species germplasm?
(6) How successful are the map-based approaches compared to conventional breeding? Can more efficient molecular breeding strategies be developed for barley breeding?
Key to all efforts in introgression is the choice of donor parent. Agriculture requires fast growing, high-yielding barley genotypes with good disease resistance. The introduction of additional traits such as drought tolerance should not alter established traits. There are many methods by which wild plants cope with drought. Avoidance is one mechanism and can be expressed as dormancy or retarded growth during periods of low water supply; such characters are normally deleterious in agriculture. The ideal donor would therefore be a genotype, which like the recipient cultivar is fast growing, high yielding, but is more tolerant of drought.
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Acknowledgments |
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SCRI receives grant-aided support from the Scottish Office Agriculture, Environment and Fisheries Department. The authors are recipients of a grant from the EC INCO programme to carry out work on ‘Stable yields in Mediterranean barley: application of molecular technologies in improving drought tolerance and mildew resistance’. Seed of the Lina/HS92 doubled haploids were originally supplied by Dr Stine Tuvesson of Svalöf Weibull AB, Svalöv, Sweden.
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Notes |
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7 To whom correspondence should be addressed. Fax: +44 1382 562426.
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