No abstract

Parasexuality contributes to diversity and adaptive evolution of haploid (monokaryotic) fungi. However, non-sexual genetic exchange mechanisms are not defined in dikaryotic fungi (containing two distinct haploid nuclei). Newly emerged strains of the wheat stem rust pathogen, Puccinia graminis f. sp. tritici (Pgt), such as Ug99, are a major threat to global food security. Here, we provide genomics-based evidence supporting that Ug99 arose by somatic hybridisation and nuclear exchange between dikaryons. Fully haplotype-resolved genome assembly and DNA proximity analysis reveal that Ug99 shares one haploid nucleus genotype with a much older African lineage of Pgt, with no recombination or chromosome reassortment. These findings indicate that nuclear exchange between dikaryotes can generate genetic diversity and facilitate the emergence of new lineages in asexual fungal populations.

Breeding wheat with durable resistance to the fungal pathogen Puccinia graminis f. sp. tritici (Pgt), a major threat to cereal production, is challenging due to the rapid evolution of pathogen virulence. Increased durability and broad-spectrum resistance can be achieved by introducing more than one resistance gene, but combining numerous unlinked genes by breeding is laborious. Here we generate polygenic Pgt resistance by introducing a transgene cassette of five resistance genes into bread wheat as a single locus and show that at least four of the five genes are functional. These wheat lines are resistant to aggressive and highly virulent Pgt isolates from around the world and show very high levels of resistance in the field. The simple monogenic inheritance of this multigene locus greatly simplifies its use in breeding. However, a new Pgt isolate with virulence to several genes at this locus suggests gene stacks will need strategic deployment to maintain their effectiveness.Pgt continues to overcome resistant wheat cultivars, with three new, highly virulent isolates emerging in the last 20 years, and the disease reappearing in Europe and the UK 1 . Two classes of Pgt resistance genes have been cloned from wheat: all-stage resistance (ASR) genes and adult plant resistance (APR) genes 2 . ASR genes (for example, Sr22 (ref. 3 ), Sr35 (ref. 4 ), Sr45 (ref. 3 ) and Sr50 (ref. 5 )) generally encode nucleotide-binding, leucine-rich repeat (NLR) proteins that recognize a specific Pgt molecule (an effector) introduced into host plant cells by the fungus to promote parasitism, whereupon a plant defense response is activated 2 . The presence, absence or allelic variation of the fungal effector determines which Pgt isolates an ASR gene is effective against. ASR genes are extremely valuable for crop protection but, when deployed singly, often show transient resistance, as pathogen effectors rapidly evolve to avoid recognition. Combining ASR genes increases their durability, and theoretical estimates suggest that the chance of a single Pgt isolate gaining virulence for five or more ASR genes in wheat is infinitesimally small 6 .The second gene class, APR genes, can be remarkably durable and, in some cases, effective against multiple pathogen species. However, these genes generally provide partial resistance that is often insufficient for crop protection during severe pathogen epidemics.

The wild relatives and progenitors of wheat have been widely used as sources of disease resistance (R) genes. Molecular identification and characterization of these R genes facilitates their manipulation and tracking in breeding programmes. Here, we develop a reference-quality genome assembly of the wild diploid wheat relative Aegilops sharonensis and use positional mapping, mutagenesis, RNA-Seq and transgenesis to identify the stem rust resistance gene Sr62, which has also been transferred to common wheat. This gene encodes a tandem kinase, homologues of which exist across multiple taxa in the plant kingdom. Stable Sr62 transgenic wheat lines show high levels of resistance against diverse isolates of the stem rust pathogen, highlighting the utility of Sr62 for deployment as part of a polygenic stack to maximize the durability of stem rust resistance.

No abstract

Stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is a major biotic constraint to wheat production worldwide. Disease resistant cultivars are a sustainable means for the efficient control of this disease. To identify quantitative trait loci (QTLs) conferring resistance to stem rust at the seedling stage, an association mapping panel consisting of 230 tetraploid wheat accessions were evaluated for reaction to five Pgt races under greenhouse conditions. A high level of phenotypic variation was observed in the panel in response to all of the races, allowing for genome-wide association mapping of resistance QTLs in wild, landrace, and cultivated tetraploid wheats. Twenty-two resistance QTLs were identified, which were characterized by at least two marker-trait associations. Most of the identified resistance loci were coincident with previously identified rust resistance genes/QTLs; however, six regions detected on chromosomes 1B, 5A, 5B, 6B, and 7B may be novel. Availability of the reference genome sequence of wild emmer wheat accession Zavitan facilitated the search for candidate resistance genes in the regions where QTLs were identified, and many of them were annotated as NOD (nucleotide binding oligomerization domain)-like receptor (NLR) genes or genes related to broad spectrum resistance.

Fusarium crown rot (FCR) is an important disease of wheat and other grains that has had a signifi cant impact on cereal crop production worldwide. Fusarium species associated with FCR can also produce powerful trichothecenes mycotoxins that pose a considerable health risk to humans and animals that consume infected grains. In this study we examined Fusarium species of wheat from diff erent regions of Iraq that showed FCR symptoms. Twenty-nine isolates were collected overall, and the marker gene translation elongation factor 1 alpha (TEF-1α) was sequenced in order to determine their taxonomic identities. All isolates were determined to be F. culmorum, and primers targeting tri-cluster genes were used in order to further characterize isolates into specifi c trichothecene chemotype strains. Five of the 29 isolates were determined to be the nivalenol (NIV) chemotype, while the rest of the isolates recovered were the deoxynivalenol (DON) chemotype. All DON--type isolates produced 3Ac-DON, while the 15Ac-DON-type was not detected. Th e majority of the NIV-type isolates originated from wheat growing regions in the mid-latitudes of Iraq, while the DON-type isolates were recovered from areas distributed broadly across the country. To the best of our knowledge, this study is the fi rst to report on the distribution of specifi c F. culmorum chemotypes from FCR diseased wheat in Iraq.

A B S T R A C T This study was conducted to evaluate Iraqi propolis against gray mold on orange. Propolis Ethanolic Extract (PEE) in three concentrations 1, 2 and 3% were used to treated orange treatments separately and storage at 25±2°C for three weeks. Results showed a significant reduced in disease severity in the first, second and third week for all PEE concentrations compared with infected control. The best treatment was 3% in disease severity reduction. Disease incidence was significantly reduced with the treatment treated with PEE for all concentrations compared with the pathogen treatment, both concentration of PEE 2 and 3% was the most effective. Also, all PEE concentrations reduce depth of the mold area compared with pathogen treatment. There are no significant differences in patulin production between PEE and pathogen treatments was founded.