Barberry plays an active role as an alternate host of Puccinia graminis in Spain

Genomics Research on the Road of Studying Biology and Virulence of Cereal Rust Fungi

Rust fungi are highly destructive pathogens that pose a significant threat to crop production worldwide, especially cereals. Obligate biotrophy and, in many cases, complex life cycles make rust fungi particularly challenging to study. However, recent rapid advances in sequencing technologies and genomic analysis tools have revolutionised rust fungal research. It is anticipated that the increasing availability and ongoing substantial improvements in genome assemblies will propel the field of rust biology into the post-genomic era, instigating a cascade of research endeavours encompassing multi-omics and gene discoveries. This is especially the case for many cereal rust pathogens, for which continental-scale studies of virulence have been conducted over many years and historical collections of viable isolates have been sequenced and assembled. Genomic analysis plays a crucial role in uncovering the underlying causes of the high variability of virulence and the complexity of population dynamics in rust fungi. Here, we provide an overview of progress in rust genomics, discuss the strategies employed in genomic analysis, and elucidate the strides that will drive cereal rust biology into the post-genomic era.

Ornamental Barberry Twigs as an Underexploited Source of Berberine-Rich Extracts-Preliminary Research

Berberine is a natural substance obtained from the roots of common barberry which, due to its strong pharmacological activity, is a commonly tested ingredient of dietary supplements. However, ornamental barberries, which are widely available, have not been considered as a source of berberine so far. The research aimed to check whether the ornamental barberry leaves and twigs could be used as an easily accessible raw material for obtaining natural berberine-rich extract with biological activity. Twigs and leaves of seven cultivars of ornamental barberry extracts were assessed for their polyphenol content, antioxidant potential (FRAP and DPPH), and berberine content using high-performance thin layer chromatography (HPTLC). As a reference, commercially available roots of Berberis vulgaris were used. For the next step, selected extracts (two with high and two with low berberine content) were tested on three cell lines (HaCaT, A375, Caco-2) using neutral red assay, and pure berberine sulfate (1-100 μg mL-1) was used as a control. Although the antioxidant potential of aqueous-methanol extracts of tested barberry was higher for the leaves than for the twigs, the berberine content was determined only in the twig extracts (from 42 to 676 mg 100 g-1). Studies on cell lines have shown the general toxicity of barberry extracts, but the observed effect was not directly correlated with the content of the alkaloid. However, the extract showed greater activity compared to an analogous dose of pure berberine, suggesting a significant effect of the matrix composition. For the first time, it was shown that the twigs of selected cultivars of ornamental barberry can be considered as a promising berberine source for the pharmaceutical industry to develop new effective formulations. However, these findings require further studies.

Identification of Sr67, a new gene for stem rust resistance in KU168-2 located close to the Sr13 locus in wheat

KEY MESSAGE

Sr67 is a new stem rust resistance gene that represents a new resource for breeding stem rust resistant wheat cultivars Re-appearance of stem rust disease, caused by the fungal pathogen Puccinia graminis f. sp. tritici (Pgt), in different parts of Europe emphasized the need to develop wheat varieties with effective resistance to local Pgt populations and exotic threats. A Kyoto University wheat (Triticum aestivum L.) accession KU168-2 was reported to carry good resistance to leaf and stem rust. To identify the genomic region associated with the KU168-2 stem rust resistance, a genetic study was conducted using a doubled haploid (DH) population from the cross RL6071 × KU168-2. The DH population was phenotyped with three Pgt races (TTKSK, TPMKC, and QTHSF) and genotyped using the Illumina 90 K wheat SNP array. Linkage mapping showed the resistance to all three Pgt races was conferred by a single stem rust resistance (Sr) gene on chromosome arm 6AL, associated with Sr13. Presently, four Sr13 resistance alleles have been reported. Sr13 allele-specific KASP and STARP markers, and sequencing markers all showed null alleles in KU168-2. KU168-2 showed a unique combination of seedling infection types for five Pgt races (TTKSK, QTHSF, RCRSF, TMRTF, and TPMKC) compared to Sr13 alleles. The phenotypic uniqueness of the stem rust resistance gene in KU168-2 and null alleles for Sr13 allele-specific markers showed the resistance was conferred by a new gene, designated Sr67. Since Sr13 is less effective in hexaploid background, Sr67 will be a good source of stem rust resistance in bread wheat breeding programs.

Recombination in the wheat stem rust pathogen mediated by an indigenous barberry species in Spain

The comeback of wheat stem rust in Europe, caused by Puccinia graminis f. sp. tritici, and the prevalence of the alternate (sexual) host in local areas have recently regained attention as a potential threat to European wheat production. The aim of this study was to investigate a potential epidemiological link between the aecia found on an indigenous barberry species and stem rust infections on nearby cereals and grasses. Aecial infections collected from Berberis vulgaris subsp. seroi were inoculated on a panel of susceptible genotypes of major cereal crop species. In total, 67 stem rust progeny isolates were recovered from wheat (51), barley (7), and rye (9), but none from oat, indicating the potential of barberry derived isolates to infect multiple cereals. Molecular genotyping of the progeny isolates and 20 cereal and grass stem rust samples collected at the same locations and year, revealed a clear genetic relatedness between the progeny isolated from barberry and the stem rust infections found on nearby cereal and grass hosts. Analysis of Molecular Variance indicated that variation between the stem rust populations accounted for only 1%. A Principal Components Analysis using the 62 detected multilocus genotypes also demonstrated a low degree of genetic variation among isolates belonging to the two stem rust populations. Lastly, pairwise comparisons based on fixation index (Fst), Nei's genetic distances and number of effective migrants (Nm) revealed low genetic differentiation and high genetic exchange between the two populations. Our results demonstrated a direct epidemiological link and functionality of an indigenous barberry species as the sexual host of P. graminis in Spain, a factor that should be considered when designing future strategies to prevent stem rust in Europe and beyond.

Wheat Stem Rust Surveillance Reveals Two New Races of Puccinia graminis f. sp. tritici in South Africa During 2016 to 2020

Stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is an important disease of wheat in South Africa (SA) and is primarily controlled using resistant cultivars. Understanding virulence diversity of Pgt is essential for successful breeding of resistant cultivars. Samples of infected wheat stems were collected across the major wheat-growing regions of SA from 2016 to 2020 to determine the pathogenic variability of Pgt isolates. Seven races were identified from 517 isolates pathotyped. The most frequently found races were 2SA104 (BPGSC + Sr9h,27,Kw) (35% frequency) and 2SA88 (TTKSF + Sr8b) (33%). Race 2SA42 (PTKSK + Sr8b), which was found in 2017, and 2SA5 (BFGSF + Sr9h), identified in 2017, are new races. The Ug99 variant race 2SA42 is similar in its virulence to 2SA107 (PTKST + Sr8b) except for avirulence to Sr24 and virulence to Sr8155B1. Race 2SA5 is closely related in its virulence to existing races that commonly infect triticale. Certain races showed limited geographical distribution. Races 2SA5, 2SA105, and 2SA108 were found only in the Western Cape, whereas 2SA107 and 2SA42 were detected only in the Free State province. The new and existing races were compared using microsatellite (SSR) marker analysis and their virulence on commercial cultivars was also determined. Seedling response of 113 wheat entries against the new races, using 2SA88, 2SA88+9h, 2SA106, and 2SA107 as controls, revealed 2SA107 as the most virulent (67 entries susceptible), followed by 2SA42 (64), 2SA106 (60), 2SA88+9h (59), 2SA88 (25), and 2SA5 (17). Thus, 2SA5 may not pose a significant threat to local wheat production. SSR genotyping revealed that 2SA5 is genetically distinct from all other SA Pgt races.

Global dispersal and diversity of rust fungi in the context of plant health

Long-distance dispersal of plant pathogens at the continental scale may have strong implications on plant health, in particular when incursions result in spread of disease to new territories where the disease was previously absent or insignificant. These dispersions may be caused by airborne transmission of spores or accidental spread via human travel and trade. Recent surveillance efforts of cereal rust fungi have demonstrated that incursion of new strains with superior fitness into areas where the disease is already established may have similar implications on plant health. Since dispersal events are highly stochastic, irrespective of transmission mechanism, critical mitigation efforts include preparedness by coordinated pathogen surveillance activities, host crop diversification, and breeding for disease resistance with low vulnerability to sudden changes in the pathogen population.

Stem rust on barberry species in Europe: Host specificities and genetic diversity

The increased emergence of cereal stem rust in southern and western Europe, caused by the pathogen Puccinia graminis, and the prevalence of alternate (sexual) host, Berberis species, have regained attention as the sexual host may serve as source of novel pathogen variability that may pose a threat to cereal supply. The main objective of the present study was to investigate the functional role of Berberis species in the current epidemiological situation of cereal stem rust in Europe. Surveys in 11 European countries were carried out from 2018 to 2020, where aecial infections from five barberry species were collected. Phylogenetic analysis of 121 single aecial clusters of diverse origin using the elongation factor 1-α gene indicated the presence of different special forms (aka formae speciales) of P. graminis adapted to different cereal and grass species. Inoculation studies using aecial clusters from Spain, United Kingdom, and Switzerland resulted in 533 stem rust isolates sampled from wheat, barley, rye, and oat, which confirmed the presence of multiple special forms of P. graminis. Microsatellite marker analysis of a subset of 192 sexually-derived isolates recovered on wheat, barley and rye from the three populations confirmed the generation of novel genetic diversity revealed by the detection of 135 multilocus genotypes. Discriminant analysis of principal components resulted in four genetic clusters, which grouped at both local and country level. Here, we demonstrated that a variety of Berberis species may serve as functional alternate hosts for cereal stem rust fungi and highlights the increased risks that the sexual cycle may pose to cereal production in Europe, which calls for new initiatives within rust surveillance, epidemiological research and resistance breeding.