Abstract

Yellow rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a devastating fungal disease threatening much of global wheat production. Yr15 is a broad-spectrum resistance gene, derived from wild emmer wheat (WEW) Triticum dicoccoides. Yr15 encodes a protein with kinase-pseudokinase domain architecture, designated as Wheat Tandem Kinase 1 (WTK1) that comprises an unusual R-gene structure in wheat (Klymiuk et al. Nature Communications 2018). WTK1 orthologs and paralogs are found in all group 1 and 6 wheat chromosomes. The exon-intron structure of WTK1 orthologues copies is similar to that of the WEW functional copy but differ in indels that cause changes in reading frames and in numerous SNPs. Proteins with similarity to WTK1’s domain architecture were also identified in 92 putative proteins across the plant kingdom, as well as in the wheat and barley stem rust resistance genes Sr60 (WTK2) and RPG1, and a candidate for the barley true smut resistance gene (Un8), suggesting that they are members of a distinct family of plant proteins, termed here tandem kinase-pseudokinases (TKPs). We found that 175 out of 184 kinase/pseudokinase domains of these TKPs are associated with receptor-like kinases (RLKs), suggesting that TKPs are involved in plant defense mechanisms. Further phylogenetic analysis indicated that TKP family members originated from either gene duplication or gene fusion events, implying a polyphyletic origin of the TKPs by convergent evolutionary processes. Here, we propose a decoy model to describe the potential function of the pseudokinase domain in the TKP family. According to this model, the pathogen secreted effector proteins to suppress the activity of host kinases involved in the resistance response, while pseudokinase domains serve as decoys for these effectors. After interacting with the pathogen effector, the pseudokinase protein may act as a “molecular switch” to activate the kinase domain, which then phosphorylates downstream substrates resulting in disease resistance. This model can explain the molecular function of the TKP protein family and provide support for their polyphyletic origin and evolution. This “molecular switch” has a crucial role in controlling the activation of local programmed cell death in the presence of pathogen, while preventing its activation in the absence of the pathogen. Further studies are underway in the wheat-Pst pathosystem in order to identify the WTK1 partner proteins and elucidate the mechanism of resistance conferred by this extraordinary protein family.

Abstract

Breeding for host resistance is a critical component in controlling yellow rust worldwide. However, in recent years there have been dramatic shifts in the patterns of varietal resistance, resulting from an incursion of exotic pathogen populations, and the subsequent evolution of these populations within Europe. Our ‘Yellowhammer’ project* aims to support breeding for durable yellow rust resistance by determining the most effective combinations of adult plant resistance (APR) loci, having complementary mechanisms of resistance, for stacking in breeding programmes. In previous and ongoing work, we have identified several promising APR loci in northern European germplasm, but have found that their effectiveness in combination with other loci is variable. In this project we are systematically investigating the most effective combinations of APR loci against a wide range of yellow rust pathotypes. We are using near-isogenic line (NIL) pairs differing only at APR loci, an association mapping panel of European lines, and bi-parental populations developed for the project to: (a) characterize field resistance across multiple sites over 5 years, including testing for resistance expression in heterozygotes to aid in hybrid breeding, and to identify whether resistance alleles have any pleiotropic effects on yield, (b) undertake micro-phenotyping and disease progression studies to characterize the timing, nature and location of the host resistance response for different APR loci (c) characterize differential gene expression profiles associated with different APR loci, to understand the genetic pathways underlying the resistance response, to both to aid in selection of complementary APR loci for stacking, as well as suggest novel target genes for resistance breeding. By the end of the project, we aim to know the best combinations of yellow rust resistance genes for northern European wheat breeders to use, but also why they are the best combinations, providing a more general strategy for rust resistance breeding in cereals. Some preliminary results from the first 18 months of the project are presented. *The project is a collaboration between NIAB, the UK Agriculture and Horticulture Development Board and the breeding companies: DSV UK Ltd, KWS UK Ltd., Lantmännen Lantbruk, Limagrain UK Ltd., RAGT Seeds Ltd., Sejet Plant Breeding, Syngenta UK Ltd.

Abstract

Spain could serve as a potential bridge for cereal rust movement between Africa and Europe. Two sub-species of Berberis vulgaris (seroi and australis) are indigenous to Spain and could serve as the alternate hosts for Puccinia graminis f. sp. tritici and P. striiformis f. sp. tritici, the wheat stem rust and stripe rust pathogens, respectively. Barberry populations, wild grasses from the Poaceae family and cereal crops were surveyed for rust infections in several provinces in Spain since 2015, but more intensively in 2018 and 2019. In northern Spain, B. vulgaris subsp. seroi grows at altitudes from 470 to 2,000 m, occasionally in close proximity to cereal crops by growing along fields. Aecial infections on barberry occur before stem rust uredinia are observed on wild grasses. Grasses infected with stem rust included Aegilops ventricosa, Avena sterilis, Dactylis glomerata, Elymus spp., Helictotrichon bromoides, Hordeum murinum, and Polypogon monspeliensis. Stem rust infections on crops occur occasionally, while yellow rust infects crops regularly, both under rainfed and irrigated conditions. Impact on yield is wide-ranging as observed by farmers. Puccinia graminis was identified to be the main component in the aecial samples collected from this region. In southeastern Spain, B. vulgaris subsp. australis grows at high altitudes (1,000-2,500 m) and it is rarely found close to cereal crops. Aecia on barberry occurs in late June-early July after cereal crops have been already harvested in the region. Wheat is usually affected by yellow rust during spring, but stem rust is rare. Summer wheat nurseries (under irrigation) often show considerable infections of stem rust by the end of August. However, the source of stem rust inoculum infecting summer wheat nurseries is still unknown. Additional information may be found in this workshop, about species identification of aecial samples (see abstract by Rodriguez-Algaba et al.), and about stem rust races sampled in this survey (see abstract by Olivera et al.). Further investigations are needed to ascertain the roles of alternate hosts (Berberis spp.) and accessory hosts (wild grasses) in the disease epidemiology of cereal rusts in different regions of Spain so that appropriate rust control strategies may be developed.

Abstract

Abstract

Barberry (Berberis spp.) is an alternate host for both the wheat stripe rust pathogen (Puccinia striiformis f. sp. tritici, Pst) and the stem rust pathogen (P. graminis f. sp. tritici, Pgt). To determine if barberry can be infected by either of the pathogens, the weather data of 606 locations in 37 countries in Asia and Southeastern Europe were analyzed to assess risks of barberry infection. Weather conditions related to barberry growth were also analyzed for these locations. Potential risks for Pst and Pgt infections on barberry for individual countries were assessed separately based on the rust favorable index and barberry growth favorable index. Individual countries were categorized into no (N), low (L), moderate (M), high (H), and very high (VH) risks based on the concurrence of category of barberry favorable growth and rust favorable indices. In East Asia, Bhutan, China, and Nepal were found to have low risk of barberry infection by Pst, but high risk by Pgt. In Central Asia, Azerbaijan, Iran, Kazakhstan, Russia, and Uzbekistan were found to have low to high risks of barberry infection for both Pst and Pgt, but not necessarily at the similar risk levels. In Northwest Asia, high risk was found for both pathogens in Turkey and very high risk for both pathogens in Georgia. In Southwest Asia, no risk or low risk was generally found, except few locations. In Southeastern Europe, similar high or very high risks for both pathogens were found for all studied countries except moderate risk for Pst but high risk for Pgt in Bulgaria and Romania. The potential risks of barberry infection by Pst and/or Pgt should provide guidelines for monitoring barberry infection and also should be valuable for developing rust management programs in these regions. The framework used in this study may be useful to predict rust infection risks in other regions.