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Wheat is the largest produced grain crop world-wide and has been extensively studied for a wide range of agronomic traits located across the genome. Its large chromosomes and the capacity of the polyploid genome to tolerate the addition or loss of chromosomes facilitated a fast progress in early wheat genetics using cytogenetic techniques. However, these same characteristics have limited the progress in genomic studies that have been focused on diploid model species with smaller genomes such as rice and Arabidopsis. These model systems are seen as a launching pad for comparative genomic strategies to tackle the challenges associated with the larger genomes of other plants such as wheat. The strengths and limitations of comparative genomic approaches to study the wheat genome are presented in this review. Structural genomic studies in wheat started by the development of large DNA insert libraries (BAC libraries) for the progenitor A and D genomes using diploid species and for the B genome using tetraploid wheat. Sequencing of the first wheat BAC clones has confirmed cytogenetic results indicating the presence of portions of the wheat genome that contain high density gene regions approximating that of the model species. These high-density gene regions also exhibited higher-than-average rates of genetic to physical distances. Functional genomic approaches, focused on the expressed portion of the wheat genome, have recently led to an exponential growth of expressed sequence tagged (EST) databases from both the public and private sector. Assigning gene function to these ESTs is now one of the major challenges in wheat genomics. Hypothesis testing strategies based on high-resolution EST mapping, candidate gene analysis, gene expression profiling, and proteomics are being used as entry points towards assigning gene function. Improving transformation efficiency as well as selection strategies for high throughput mutagenesis experiments will be two critical areas of research for the ultimate assignment of function to the numerous wheat genes that are being discovered using the new genomic tools. © 2001 Éditions scientifiques et médicales Elsevier SAS.

Segregating [F.sub.3] families, derived from a cross between oat cultivar Swan and the putative single gene line PC68, were used to determine the association of seed storage protein loci and resistance gene analogues (RGAs) with the crown rust resistance gene Pc68. SDS-PAGE analysis detected three avenin loci, AveX, AveY, and AveZ, closely linked to Pc68. Their diagnostic alleles are linked in coupling to Pc68 and were also detected in three additional lines carrying Pc68. Another protein locus was linked in repulsion to Pc68. In complementary studies, three wheat RGA clones (W2, W4, and W10) detected restriction fragment length polymorphisms (RFLPs) between homozygous resistant and homozygous susceptible [F.sub.3] DNA bulks. Four oat homologues of W2 were cloned and sequenced. RFLPs detected with two of them were mapped using [F.sub.3] and [F.sub.4] populations. Clone 18 detected a locus, Orga2, linked in repulsion to Pc68. Clone 22 detected several RFLPs including Orgal (the closest locus to Pc68) and three RGA loci (Orga22-2, Orga22-3, and Orga22-4) loosely linked to Pc68. The diagnostic RFLPs linked in coupling to Pc68 were detected by clone 22 in three additional oat lines carrying Pc68 and have potential utility in investigating and improving crown rust resistance of oat. Key words: Avena sativa L., storage proteins, avenin, resistance gene analogues, Puccinia coronata avenae, SDS-PAGE. Des familles [F.sub.3] en segregation, derivees d'un croisement entre le cultivar d'avoine Swan et une lignee a simple introgression du gene PC68, ont ete employees pour examiner l'association entre les locus codant pour les proteines de reserve ainsi que des analogues de genes de resistance (RGA) et le gene de resistance a la rouille couronnee Pc68. Une analyse SDS-PAGE a revele une etroite association entre trois locus d'avenines, AveX, AveY et AveZ, et le gene Pc68. Leurs alleles diagnostiques etaient lies en couplage avec Pc68 et ils etaient egalement detectes chez trois autres lignees portant Pc68. Un autre locus pour une proteine de reserve etait lie en repulsion avec Pc68. Dans des travaux complementaires, trois clones RGA du ble (W2, W4 et W10) ont permis de detecter des polymorphismes de taille des fragments de restriction (RFLP) entre des composites d'ADN provenant d'individus homozygotes resistants ou sensibles. Quatre homologues de W2 chez l'avoine ont ete clones et sequences. Les RFLP detectes avec deux de ceux-ci ont ete cartographies au sein de populations [F.sub.3] et [F.sub.4]. Le clone 18 a permis de detecter un locus, Orga2, qui etait lie en repulsion avec Pc68. Le clone 22 a permis de detecter plusieurs RFLP incluant Orga1, le locus le plus proche de Pc68, et trois locus RGA (Orga22-2, Orga22-3 et Orga22-4) faiblement lies a Pc68. Les RFLP diagnostiques lies en couplage a Pc68 ont ete detectes par le clone 22 chez trois autres lignees d'addition portant le gene Pc68 et seront potentiellement utiles pour etudier et augmenter la resistance a la rouille couronnee chez l'avoine. Mots-cles: Avena sativa, proteines de reserve, avenine, analogues de genes de resistance, Puccinia coronata avenae, SDS PAGE. [Traduit par la Redaction], Introduction Oat (primarily Avena sativa L.) is an important cereal crop for human and animal nutrition. In Australia, oat has the third largest area sown of any cereal. Crown (leaf) [...]

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.

The Lr34/Yr18 adult plant resistance gene contributes significantly to durable leaf rust (caused by Puccinia triticina Eriks.) resistance. Simple and robust molecular markers that enable early detection of Lr34/Yr18 are a major advancement in wheat (Triticum aestivum L.) breeding. An insertion/deletion size variant located at the csLV34 locus on chromosome 7D within an intron sequence of a sulfate transporter-like gene tightly linked to the Lr34/Yr18 dual rust resistance gene was used to examine a global collection of wheat cultivars, landraces, and D genome-containing diploid and polyploid species of wheat relatives. Two predominant allelic size variants, csLV34a and b, found among the wheat cultivars showed disparate variation in different wheat growing zones. A strong association was observed between the presence of Lr34/Yr18 and the csLV34b allele and wheat lines known to have Lr34/Yr18 that had the csLV34a allele were rare. All landraces with the exception of those from China were predominantly of the csLV34a type. Only one size variant, csLV34a, was detected among the diploid and polyploid D genome-containing species, indicating that csLV34b arose subsequent to hexaploid bread wheat synthesis. The lineage of the csLV34b allele associated with Lr34/Yr18 in modern wheat cultivars from North and South America, CIMMYT, Australia, and Russia was tracked back to the cultivars Mentana and Ardito developed in Italy by Nazareno Strampelli in the early 1900s. The robustness of the csLV34 marker in postulating the likely occurrence of Lr34/Yr18 across a wide range of wheat germplasm and its utility in wheat breeding was confirmed.

Sequencing was used to investigate the origin of the D genome of the allopolyploid species Tritiicum aestivum and Aegilops cylindrica. A 247-bp region of the wheat D-genome Xwrye838 locus, encoding ADP-glucopyrophosphorylase, and a 326-bp region of the wheat D-genome Gss locus, encoding granule-bound starch synthase, were sequenced in a total 564 lines of hexaploid wheat (T. aestivum, genome AABBDD) involving all its subspecies and 203 lines of Aecilops tauschii, the diploid source of the wheat D genome. In Ae. tauschii, two SNP variants were detected at the Xwye838 locus and 11 haplotypes at the Gss locus. Two haplotypes with contrasting frequencies were found at each locus in wheat. Both wheat X~e838 variants, but only one of the Gss haplotypes seen in wheat, were found among the Ae. tauschii lines. The other wheat Gss haplotype was not found in either Ae. tauschii or 70 lines of tetraploid Ae. cylindrica (genomes CCDD), which is known to hybridize with wheat. It is concluded that both T. aestivum and Ae. cylindrica originated recurrently, with at least two genetically distinct progenitors contributing to the formation of the D genome in both species.

In the last 20 years, stem rust caused by the fungus Puccinia graminis f. sp. tritici (Pgt), has re‐emerged as a major threat to wheat and barley production in Africa and Europe. In contrast to wheat with 60 designated stem rust (Sr) resistance genes, barley's genetic variation for stem rust resistance is very narrow with only ten resistance genes genetically identified. Of these, only one complex locus consisting of three genes is effective against TTKSK, a widely virulent Pgt race of the Ug99 tribe which emerged in Uganda in 1999 and has since spread to much of East Africa and parts of the Middle East. The objective of this study was to assess the functionality, in barley, of cloned wheat Sr genes effective against race TTKSK. Sr22, Sr33, Sr35 and Sr45 were transformed into barley cv. Golden Promise using Agrobacterium‐mediated transformation. All four genes were found to confer effective stem rust resistance. The barley transgenics remained susceptible to the barley leaf rust pathogen Puccinia hordei, indicating that the resistance conferred by these wheat Sr genes was specific for Pgt. Furthermore, these transgenic plants did not display significant adverse agronomic effects in the absence of disease. Cloned Sr genes from wheat are therefore a potential source of resistance against wheat stem rust in barley. [ABSTRACT FROM AUTHOR]