Evans S. Lagudah

A Putative ABC Transporter Confers Durable Resistance to Multiple Fungal Pathogens in Wheat

Agricultural crops benefit from resistance to pathogens that endures over years and generations of both pest and crop. Durable disease resistance, which may be partial or complete, can be controlled by several genes. Some of the most devastating fungal pathogens in wheat are leaf rust, stripe rust, and powdery mildew. The wheat gene Lr34 has supported resistance to these pathogens for more than 50 years. Lr34 is now shared by wheat cultivars around the world. Here, we show that the LR34 protein resembles adenosine triphosphate–binding cassette transporters of the pleiotropic drug resistance subfamily. Alleles of Lr34 conferring resistance or susceptibility differ by three genetic polymorphisms. The Lr34 gene, which functions in the adult plant, stimulates senescence-like processes in the flag leaf tips and edges.

Avenues for increasing salt tolerance of crops, and the role of physiologically based selection traits

Increased salt tolerance is needed for crops grown in areas at risk of salinisation. This requires new genetic sources of salt tolerance, and more efficient techniques for identifying salt-tolerant germplasm, so that new genes for tolerance can be introduced into crop cultivars. Screening a large number of genotypes for salt tolerance is not easy. Salt tolerance is achieved through the control of salt movement into and through the plant, and salt-specific effects on growth are seen only after long periods of time. Early effects on growth and metabolism are likely due to osmotic effects of the salt, that is to the salt in the soil solution. To avoid the necessity of growing plants for long periods of time to measure biomass or yield, practical selection techniques can be based on physiological traits. We illustrate this with current work on durum wheat, on selection for the trait of sodium exclusion. We have explored a wide range of genetic diversity, identified a new source of sodium exclusion, confirmed that the trait has a high heritability, checked for possible penalties associated with the trait, and are currently developing molecular markers. This illustrates the potential for marker-assisted selection based on sound physiological principles in producing salt-tolerant crop cultivars.

A Physical Map of the 1-Gigabase Bread Wheat Chromosome 3B

As the staple food for 35% of the worlds population, wheat is one of the most important crop species. To date, sequence-based tools to accelerate wheat improvement are lacking. As part of the international effort to sequence the 17–billion–base-pair hexaploid bread wheat genome (2n = 6x = 42 chromosomes), we constructed a bacterial artificial chromosome (BAC)–based integrated physical map of the largest chromosome, 3B, that alone is 995 megabases. A chromosome-specific BAC library was used to assemble 82% of the chromosome into 1036 contigs that were anchored with 1443 molecular markers, providing a major resource for genetic and genomic studies. This physical map establishes a template for the remaining wheat chromosomes and demonstrates the feasibility of constructing physical maps in large, complex, polyploid genomes with a chromosome-based approach.

A Sodium Transporter (HKT7) Is a Candidate for Nax1, a Gene for Salt Tolerance in Durum Wheat

Durum wheat (Triticum turgidum subsp. durum) is more salt sensitive than bread wheat (Triticum aestivum). A novel source of Na+ exclusion conferring salt tolerance to durum wheat is present in the durum wheat Line 149 derived from Triticum monococcum C68-101, and a quantitative trait locus contributing to low Na+ concentration in leaf blades, Nax1, mapped to chromosome 2AL. In this study, we used the rice (Oryza sativa) genome sequence and data from the wheat expressed sequence tag deletion bin mapping project to identify markers and construct a high-resolution map of the Nax1 region. Genes on wheat chromosome 2AL and rice chromosome 4L had good overall colinearity, but there was an inversion of a chromosomal segment that includes the Nax1 locus. Two putative sodium transporter genes (TmHKT7) related to OsHKT7 were mapped to chromosome 2AL. One TmHKT7 member (TmHKT7-A1) was polymorphic between the salt-tolerant and -sensitive lines, and cosegregated with Nax1 in the high-resolution mapping family. The other TmHKT7 member (TmHKT7-A2) was located within the same bacterial artificial chromosome contig of approximately 145 kb as TmHKT7-A1. TmHKT7-A1 and -A2 showed 83% amino acid identity. TmHKT7-A2, but not TmHKT7-A1, was expressed in roots and leaf sheaths of the salt-tolerant durum wheat Line 149. The expression pattern of TmHKT7-A2 was consistent with the physiological role of Nax1 in reducing Na+ concentration in leaf blades by retaining Na+ in the sheaths. TmHKT7-A2 could control Na+ unloading from xylem in roots and sheaths.

A locus for sodium exclusion (Nax1), a trait for salt tolerance, mapped in durum wheat

Salinity affects durum wheat [Triticum turgidum L. ssp. durum (Desf.)] more than it affects bread wheat (Triticum aestivum L.), and results in lower yield for durum wheat cultivars grown on salt-affected soils. A novel source of salt tolerance in the form of a sodium exclusion trait, identified previously in a screen of tetraploid wheat germplasm, was mapped using a QTL approach. The trait, measured as low Na+ concentration in the leaf blade, was mapped on a population derived from a cross between the low Na+ landrace and the cultivar Tamaroi. The use of AFLP, RFLP and microsatellite markers identified a locus, named Nax1 (Na exclusion), on chromosome 2AL, which accounted for approximately 38% of the phenotypic variation in the mapping population. Markers linked to the Nax1 locus also associated closely with low Na+ progeny in a genetically unrelated population. A microsatellite marker closely linked to the Nax1 locus was validated in genetically diverse backgrounds, and proven to be useful for marker-assisted selection in a durum wheat breeding program.

Cloning and characterisation of a family of disease resistance gene analogs from wheat and barley

Abstract The most common class of plant disease resistance (R) genes cloned so far belong to the NBS-LRR group which contain nucleotide-binding sites (NBS) and a leucine-rich repeat (LRR). Specific primer sequences derived from a previously isolated NBS-LRR sequence at the Cre3 locus, which confers resistance to cereal cyst nematode (CCN) in wheat (Triticum aestivum L.) were used in isolating a family of resistance gene analogs (RGA) through a polymerase chain reaction (PCR) cloning approach. The cloning, analysis and genetic mapping of a family of RGAs from wheat (cv ‘Chinese Spring’) and barley (Hordeum vulgare L. cvs ‘Chebec’ and ‘Harrington’) are presented. The wheat and barley RGAs contain other conserved motifs present in known R genes from other plants and share between 55–99% amino acid sequence identity to the NBS-LRR sequence at the Cre3 locus. Phylogenetic analysis of the RGAs with other cloned R genes and RGAs from various plant species indicate that they belong to a superfamily of NBS-containing genes. Two of the barley derived RGAs were mapped onto loci on chromosomes 2H (2), 5H (7) and 7H (1) using barley doubled haploid (DH) mapping populations. Some of these loci identified are associated with regions carrying resistance to CCN and corn leaf aphid.

Trends in genetic and genome analyses in wheat: a review

The size and structure of the wheat genome makes it one of the most complex crop species for genetic analysis. The development of molecular techniques for genetic analysis, in particular the use of molecular markers to monitor DNA sequence variation between varieties, landraces, and wild relatives of wheat and related grass species, has led to a dramatic expansion in our understanding of wheat genetics and the structure and behaviour of the wheat genome. This review provides an overview of these developments, examines some of the special issues that have arisen in applying molecular techniques to genetic studies in wheat, and looks at the applications of these technologies to wheat breeding and to improving our understanding of the genetic basis of traits such as disease resistance and processing quality. The review also attempts to foreshadow some of the key molecular issues and developments that may occur in wheat genetics and breeding over the next few years.

Leaf tip necrosis, molecular markers and β1-proteasome subunits associated with the slow rusting resistance genes Lr46/Yr29

Resistance based on slow-rusting genes has proven to be a useful strategy to develop wheat cultivars with durable resistance to rust diseases in wheat. However this type of resistance is often difficult to incorporate into a single genetic background due to the polygenic and additive nature of the genes involved. Therefore, markers, both molecular and phenotypic, are useful tools to facilitate the use of this type of resistance in wheat breeding programs. We have used field assays to score for both leaf and yellow rust in an Avocet-YrA × Attila population that segregates for several slow-rusting leaf and yellow rust resistance genes. This population was analyzed with the AFLP technique and the slow-rusting resistance locus Lr46/Yr29 was identified. A common set of AFLP and SSR markers linked to the Lr46/Yr29 locus was identified and validated in other recombinant inbred families developed from single chromosome recombinant populations that segregated for Lr46. These populations segregated for leaf tip necrosis (LTN) in the field, a trait that had previously been associated with Lr34/Yr18. We show that LTN is also pleiotropic or closely linked to the Lr46/Yr29 locus and suggest that a new Ltn gene designation should be given to this locus, in addition to the one that already exists for Lr34/Yr18. Coincidentally, members of a small gene family encoding β-1 proteasome subunits located on group 1L and 7S chromosomes implicated in plant defense were linked to the Lr34/Yr18 and Lr46/Yr29 loci.

Comparative mapping of HKT genes in wheat, barley, and rice, key determinants of Na+ transport, and salt tolerance

Salt tolerance of plants depends on HKT transporters (High-affinity K(+) Transporter), which mediate Na(+)-specific transport or Na(+)-K(+) co-transport. Gene sequences closely related to rice HKT genes were isolated from hexaploid bread wheat (Triticum aestivum) or barley (Hordeum vulgare) for genomic DNA southern hybridization analysis. HKT gene sequences were mapped on chromosomal arms of wheat and barley using wheat chromosome substitution lines and barley-wheat chromosome addition lines. In addition, HKT gene members in the wild diploid wheat ancestors, T. monococcum (A(m) genome), T. urartu (A(u) genome), and Ae. tauschii (D(t) genome) were investigated. Variation in copy number for individual HKT gene members was observed between the barley, wheat, and rice genomes, and between the different wheat genomes. HKT2;1/2-like, HKT2;3/4-like, HKT1;1/2-like, HKT1;3-like, HKT1;4-like, and HKT1;5-like genes were mapped to the wheat-barley chromosome groups 7, 7, 2, 6, 2, and 4, respectively. Chromosomal regions containing HKT genes were syntenic between wheat and rice except for the chromosome regions containing the HKT1;5-like gene. Potential roles of HKT genes in Na(+) transport in rice, wheat, and barley are discussed. Determination of the chromosome locations of HKT genes provides a framework for future physiological and genetic studies investigating the relationships between HKT genes and salt tolerance in wheat and barley.

Construction and characterisation of a large DNA insert library from the D genome of wheat

Abstract A large DNA fragment library consisting of 144 000 clones with an average insert size of 119 kb was constructed from nuclear DNA isolated from root and leaf tissue from Triticum tauschii (syn. Aegilops tauschii), the D-genome progenitor of wheat. The library was made in a binary vector that had previously been shown to stably maintain large inserts of foreign DNA in Escherichia coli. The use of root nuclei reduced considerably the proportion of the library containing clones derived from chloroplast DNA. Several experimental parameters were investigated and optimised, leading to a high cloning efficiency. Only three ligations were needed to construct the library which was estimated to be equivalent to 3.7 haploid genomes. The accuracy of this estimation was demonstrated by screening this library with three well-defined probes. One probe containing a glutenin gene sequence identified 5 clones covering at least 230 kb of the Glu-D1 locus and contained the two tightly linked high-molecular-weight glutenin genes Glu-D1x and -D1y. Each of the other two single-copy probes derived from the Cre3 cereal cyst nematode resistance gene locus hybridised with 4 clones containing gene sequences encoding nucleotide binding sites and a leucine-rich region. This is the first representative large-insert DNA library for wheat, and the results indicated that large molecules of wheat DNA can be efficiently cloned, stably maintained and manipulated in a bacterial system.