Durable resistance to the wheat rusts: Integrating systems biology and traditional phenotype-based research methods to guide the deployment of resistance genes

Seedling and adult plant resistance to leaf rust in some Bulgarian common wheat lines

The response of 250 common winter wheat breeding lines was investigated for resistance to the causative agent of Puccinia triticina under conditions of an infected field on the territory of Dobrudzha Agricultural Institute - General Toshevo, Bulgaria, during three successive seasons. Twenty lines with different degrees of resistance under field conditions were selected. Multi-pathotype testing was used to study the response of these lines at seedling stage under greenhouse conditions to individual pathotypes of P. triticina. Based on the response of the lines at seedling and adult stages, we found out that 20 % of them carried race-specific resistance. One of the lines (99/08-52) reacted with full resistance to the pathotypes used under greenhouse conditions. The reaction demonstrated by this line coincided with the response of isogenic lines carrying the genes Lr9, Lr19, Lr22a, Lr22b and Lr25. The other three lines (19/06- 108, 82/08-43 and 82/08-35) showed a resistant reaction to 6 or 5 of the pathotypes used in the study. Their response partially coincided with the reaction of 5 isogenic lines, and the presence of some of these genes in the above lines is quite possible. Lines carrying this type of resistance are to be subjected to further genetic and breeding investigations to prove the presence of a race-specific gene. Twenty-five percent of the lines combined partial race-specific resistance at seedling stage with the resistance of race non-specific nature at adult stage. Forty percent of all studied lines carried race non-specific resistance, and 15 % of the lines possessed resistance of the "slow rusting" type. As a result of the study we carried out, the lines that demonstrated stable resistance to leaf rust can provide sufficient protection of the host and can be included in the breeding programs for developing varieties resistant to P. triticina.

Transfer of the high-temperature adult-plant stripe rust resistance gene Yr62 in four Chinese wheat cultivars

Wheat stripe rust is one of the diseases that seriously affect wheat production worldwide. Breeding resistant cultivars is an effective way to control this disease. The wheat stripe rust resistance gene Yr62 has high-temperature adult-plant resistance (HTAP). In this study, PI 660,060, a single Yr62 gene line, was crossed with four Chinese wheat cultivars, LunXuan987 (LX987), Bainongaikang58 (AK58), ZhengMai9023 (ZM9023), and HanMai6172 (H6172). F1 seeds of four cross combinations were planted and self-crossed to develop the advance generations in the field. The seeds of each cross were mixed harvested and about 2400 to 3000 seeds were sown in each generation for F1 to F4 to maintain the maximum possible genotypes. Forty-five lines were selected and evaluated for resistance to stripe rust and agronomic traits, including plant height, number of grains per spike, and tiller number, in F5 and F6. Then, 33 lines with good agronomic traits and high disease resistance were developed to F9 generation. SSR markers Xgwm251 and Xgwm192 flank linked with the Yr62 were used to detect the presence of Yr62 in these 33 F9 lines. Of these, 22 lines were confirmed with the resistance gene Yr62. Finally, nine lines with good agronomic traits and disease resistance were successfully selected. The selected wheat lines in this study provide material support for the future breeding of wheat for stripe rust resistance.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01393-1.

QTL Mapping of Adult Plant Resistance to Stripe Rust in a Doubled Haploid Wheat Population

Stripe rust caused by Puccinia striiformis Westend. f. sp. tritici. is a major bread wheat disease worldwide with yield losses of up to 100% under severe disease pressure. The deployment of resistant cultivars with adult plant resistance to the disease provides a long-term solution to stripe rust of wheat. An advanced line from the International Winter Wheat Improvement Program (IWWIP) 130675 (Avd/Vee#1//1-27-6275/Cf 1770/3/MV171-C-17466) showed a high level of adult plant resistance to stripe rust in the field. To identify the adult plant resistance genes in this elite line, a mapping population of 190 doubled haploid (DH) lines was developed from a cross between line 130675 and the universal stripe rust-susceptible variety Avocet S. The DH population was evaluated at precision wheat stripe rust phenotyping platform, in Izmir during 2019, 2020, and 2021 cropping seasons under artificial inoculations. Composite interval mapping (CIM) identified two stable QTLs QYr.rcrrc-3B.1, and QYr.rcrrc-3B.2, which were detected in multiple years. In addition to these two QTLs, five more QTLs, QYr.rcrrc-1B, QYr.rcrrc-2A, QYr.rcrrc-3A, QYr.rcrrc-5A, and QYr.rcrrc-7D, were identified, which were specific to the cropping year (environment). All QTLs were derived from the resistant parent, except QYr.rcrrc-3A. The significant QTLs explained 3.4-20.6% of the phenotypic variance. SNP markers flanking the QTL regions can be amenable to marker-assisted selection. The best DH lines with high yield, end-use quality, and stripe rust resistance can be used for further selection for improved germplasm. SNP markers flanking the QTL regions can aid in identifying such lines.

Characterization and Use in Wheat Breeding of Leaf Rust Resistance Genes from Durable Varieties

Simple Summary

Wheat leaf rust is one of the most significant diseases worldwide, incited by a parasitic fungus which infects leaves, affecting grain yield. This pathogen is spread by the wind over large areas through microscopic spores. This huge number of spores favors the selection of virulent forms; therefore, there is a continuous need for new resistance genes to control this disease without fungicides. These resistant genes are naturally found in resistant wheat varieties and can be introduced by standard crosses. In this work, seven resistant genes were introduced into several commercial susceptible varieties. The selection of resistance genes was assisted by DNA markers that are close to these genes on the chromosome. Additionally, the selection of desirable traits from the commercial variety was also assisted by DNA markers to accelerate the process. In field testing, the varieties developed here were resistant to leaf rust, and suitable for commercial use.

Abstract

Leaf rust is one of the most significant diseases of wheat worldwide. In Argentina, it is one of the main reasons for variety replacement that becomes susceptible after large-scale use. Some varieties showed durable resistance to this disease, including Buck Manantial and Sinvalocho MA. RILs (Recombinant Inbred Lines) were developed for each of these varieties and used in genetics studies to identify components of resistance, both in greenhouse inoculations using leaf rust races, and in field evaluations under natural population infections. In Buck Manantial, the APR gene LrBMP1 was associated with resistance in field tests. In crosses involving Sinvalocho MA, four genes were previously identified and associated with resistance in field testing: APR (Adult Plant Resistance) gene LrSV1, the APR genetic system LrSV2 + LrcSV2 and the ASR (All Stage Resistance) gene LrG6. Using backcrosses, LrBMP1 was introgressed in four commercial susceptible varieties and LrSV1, LrSV2 + LrcSV2 and LrG6 were simultaneously introgressed in three susceptible commercial varieties. The use of molecular markers for recurrent parent background selection allowed us to select resistant lines with more than 80% similarity to commercial varieties. Additionally, progress towards positional cloning of the genetic system LrSV2 + LrcSV2 for leaf rust APR is reported.

Molecular Mapping and Analysis of an Excellent Quantitative Trait Loci Conferring Adult-Plant Resistance to Stripe Rust in Chinese Wheat Landrace Gaoxianguangtoumai

The Chinese wheat landrace "Gaoxianguangtoumai" (GX) has exhibited a high level of adult-plant resistance (APR) to stripe rust in the field for more than a decade. To reveal the genetic background for APR to stripe rust in GX, a set of 249 F_6:8 (F₆, F₇, and F₈) recombinant inbred lines (RILs) was developed from a cross between GX and the susceptible cultivar "Taichung 29." The parents and RILs were evaluated for disease severity at the adult-plant stage in the field by artificial inoculation with the currently predominant Chinese Puccinia striiformis f. sp. tritici races during three cropping seasons and genotyped using the Wheat 55K single-nucleotide polymorphism (SNP) array to construct a genetic map with 1,871 SNP markers finally. Two stable APR quantitative trait loci (QTL), QYr.GX-2AS and QYr.GX-7DS in GX, were detected on chromosomes 2AS and 7DS, which explained 15.5-27.0% and 11.5-13.5% of the total phenotypic variation, respectively. Compared with published Yr genes and QTL, QYr.GX-7DS and Yr18 may be the same, whereas QYr.GX-2AS is likely to be novel. Haplotype analysis revealed that QYr.GX-2AS is likely to be rare which presents in 5.3% of the 325 surveyed Chinese wheat landraces. By analyzing a heterogeneous inbred family (HIF) population from a residual heterozygous plant in an F₈ generation of RIL, QYr.GX-2AS was further flanked by KP2A_36.85 and KP2A_38.22 with a physical distance of about 1.37Mb and co-segregated with the KP2A_37.09. Furthermore, three tightly linked Kompetitive allele-specific PCR (KASP) markers were highly polymorphic among 109 Chinese wheat cultivars. The results of this study can be used in wheat breeding for improving resistance to stripe rust.

Wheat Pm4 resistance to powdery mildew is controlled by alternative splice variants encoding chimeric proteins

Crop breeding for resistance to pathogens largely relies on genes encoding receptors that confer race-specific immunity. Here, we report the identification of the wheat Pm4 race-specific resistance gene to powdery mildew. Pm4 encodes a putative chimeric protein of a serine/threonine kinase and multiple C2 domains and transmembrane regions, a unique domain architecture among known resistance proteins. Pm4 undergoes constitutive alternative splicing, generating two isoforms with different protein domain topologies that are both essential for resistance function. Both isoforms interact and localize to the endoplasmatic reticulum when co-expressed. Pm4 reveals additional diversity of immune receptor architecture to be explored for breeding and suggests an endoplasmatic reticulum-based molecular mechanism of Pm4-mediated race-specific resistance.

Genome-Wide Association Mapping for Stripe Rust Resistance in Pakistani Spring Wheat Genotypes

Stripe rust caused by the pathogen Puccinia striiformis f. sp. tritici (Pst) is a major threat for wheat, resulting in low yield and grain quality loss in many countries. Genetic resistance is a prevalent method to combat the disease. Mapping the resistant loci and their association with traits is highly exploited in this era. A panel of 465 Pakistani spring wheat genotypes were evaluated for their phenotypic response to stripe rust at the seedling and adult plant stages. A total of 765 single nucleotide polymorphism (SNP) markers were applied on 465 wheat genotypes to evaluate their stripe rust response against nine races during the seedling test and in three locations for the field test. Currently, twenty SNPs dispersed on twelve chromosomal regions (1A, 1B, 1D, 2A, 2B, 4A, 4B, 5B, 6A, 6B, 6D and 7B) have been identified that were associated with rust race-specific resistance at the seedling stage. Thirty SNPs dispersed on eighteen chromosomal regions (1A, 1B, 1D, 2A, 2B, 2D, 3A, 3B, 3D, 4B, 5A, 5B, 6A, 6B, 6D, 7A, 7B and 7D) are associated with adult plant resistance. SNP loci IWB3662 was linked with all three Pakistani races, and likewise IWA2344 and IWA4096 were found to be linked with three different USA races. The present research findings can be applied by wheat breeders to increase their resistant capability and yield potential of their cultivars, through marker-assisted selection.

Study of Androgenic Plant Families of Alloplasmic Introgression Lines (H. vulgare) -T. aestivum and the Use of Sister DH Lines in Breeding

One of the limitations in obtaining the genetic diversity of doubled haploid (DH) lines via anther culture is the development of families of regenerants, and each family represents a clone. This work examines the results of studying this phenomenon in anther culture of alloplasmic (H. vulgare)–T. aestivum and euplasmic lines with 1RS.1BL and 7DL-7Ai translocations and hybrids between them. Parameters of androgenesis such as the number of embryo-like structures, the total number of regenerants, and the number of green regenerants per 100 anthers varied depending on the genotype. In all genotypes from embryo-like structures, predominant development of families of plantlets rather than single plantlets was found. The source of family plantlets was polyembryos. About 75% of families consisted of regenerants at the same fertility level. On average, 37.74%4% of the R0 plants were fertile. The sister DH lines of three hybrid combinations were formed from seeds of R1 plants (2n = 42) with high fertility and in the presence of wheat–alien translocations. After four years of breeding trials, the sister DH lines of three families with fungal disease resistance increased yield, and some parameters of grain quality exceeding the controls were identified as promising for breeding.

Longin CF, Ganal M, Himmelbach A, Reif JC, Schulthess AW. Exome association analysis sheds light onto leaf rust (Puccinia triticina) resistance genes currently used in wheat breeding (Triticum aestivum L.)

Resistance breeding is crucial for a sustainable control of leaf rust (Puccinia triticina) in wheat (Triticum aestivum L.) while directly targeting functional variants is the Holy Grail for efficient marker-assisted selection and map-based cloning. We assessed the limits and prospects of exome association analysis for severity of leaf rust in a large hybrid wheat population of 1574 single-crosses plus their 133 parents. After imputation and quality control, exome sequencing revealed 202 875 single-nucleotide polymorphisms (SNPs) covering 19.7% of the high-confidence annotated gene space. We performed intensive data mining and found significant associations for 2171 SNPs corresponding to 50 different loci. Some of these associations mapped in the proximity of the already known resistance genes Lr21, Lr34-B, Lr1 and Lr10, while other associated genomic regions, such as those on chromosomes 1A and 3D, harboured several annotated genes putatively involved in resistance. Validation with an independent population helped to narrow down the list of putative resistance genes that should be targeted by fine-mapping. We expect that the proposed strategy of intensive data mining coupled with validation will significantly influence research in plant genetics and breeding.

YR36/WKS1-Mediated Phosphorylation of PsbO, an Extrinsic Member of Photosystem II, Inhibits Photosynthesis and Confers Stripe Rust Resistance in Wheat

Wheat stripe rust, due to infection by Puccinia striiformis f. sp. tritici (Pst), is a devastating disease that causes significant global grain yield losses. Yr36, which encodes Wheat Kinase START1 (WKS1), is an effective high-temperature adult-plant resistance gene and confers resistance to a broad spectrum of Pst races. We previously showed that WKS1 phosphorylates the thylakoid ascorbate peroxidase protein and reduces its ability to detoxify peroxides, which may contribute to the accumulation of reactive oxygen species (ROS). WKS1-mediated Pst resistance is accompanied by leaf chlorosis in Pst-infected regions, but the underlying mechanisms remain elusive. Here, we show that WKS1 interacts with and phosphorylates PsbO, an extrinsic member of photosystem II (PSII), to reduce photosynthesis, regulate leaf chlorosis, and confer Pst resistance. A point mutation in PsbO-A1 or reduction in its transcript levels by RNA interference resulted in chlorosis and reduced Pst sporulation. Biochemical analyses revealed that WKS1 phosphorylates PsbO at two conserved amino acids involved in physical interactions with PSII and reduces the binding affinity of PsbO with PSII. Presumably, phosphorylated PsbO proteins dissociate from the PSII complex and then undergo rapid degradation by cysteine and aspartic proteases. Taken together, these results demonstrate that perturbations of wheat PsbO by point mutation or phosphorylation by WKS1 reduce the rate of photosynthesis and delay the growth of Pst pathogen before the induction of ROS.

Rust pathogen effectors: perspectives in resistance breeding

Identification and functional characterization of plant pathogen effectors promise to ameliorate future research and develop effective and sustainable strategies for controlling or containing crop diseases. Wheat is the second most important food crop of the world after rice. Rust pathogens, one of the major biotic stresses in wheat production, are capable of threatening the world food security. Understanding the molecular basis of plant-pathogen interactions is essential for devising novel strategies for resistance breeding and disease management. Now, it has been established that effectors, the proteins secreted by pathogens, play a key role in plant-pathogen interactions. Therefore, effector biology has emerged as one of the most important research fields in plant biology. Recent advances in genomics and bioinformatics have allowed identification of a large repertoire of candidate effectors, while the evolving high-throughput tools have continued to assist in their functional characterization. The repertoires of effectors have become an important resource for better understanding of effector biology of pathosystems and resistance breeding of crop plants. In recent years, a significant progress has been made in the field of rust effector biology. This review describes the recent advances in effector biology of obligate fungal pathogens, identification and functional analysis of wheat rust pathogens effectors and the potential applications of effectors in molecular plant biology and rust resistance breeding in wheat.

Identification of a major QTL on chromosome arm 2AL for reducing yellow rust severity from a Chinese wheat landrace with evidence for durable resistance

A QTL on 2AL for reducing yellow rust severity was identified from a Chinese wheat landrace, being more effective than Yr18, with evidence for durable resistance from field observations. Utilization of wheat resistance is an important strategy to control yellow rust. The Chinese wheat landrace Hong Qimai (HQM) and the advanced breeding line AQ24788-83 (AQ; a progeny of HQM) can significantly reduce disease severity at the adult-plant growth stage. HQM has maintained adult-plant resistance for a prolonged period of time. To study the inheritance of the resistance, 126 recombinant inbred lines (RILs) derived from the cross Thatcher (TC) × HQM and 138 RILs from Luke × AQ were assessed for disease severity in six field trials. A genetic map of TC × HQM was constructed by genotyping these RILs using the 90 K wheat single-nucleotide polymorphism chip. Luke × AQ map was previously constructed for another disease study and also utilized here. Based on these maps and disease data, a quantitative trait locus (QTL) was detected on the chromosome arm 2AL from both TC × HQM and Luke × AQ and designated as QYr.cau-2AL. The resistance allele at QYr.cau-2AL came from HQM and AQ. QYr.cau-2AL was significantly effective across all the test environments and different genetic backgrounds, with its effect magnitude being higher than that of Yr18. QYr.cau-2AL synergistically acted with Yr18 and a QTL for high-temperature adult-plant resistance on 2BS, resulting in an elevated resistance from the juvenile plant growth stage onward, although QYr.cau-2AL alone displayed no substantial resistance at juvenile stage. Evidence indicates that QYr.cau-2AL is novel and confers durable resistance, and thus, should have high potential value for practical breeding.

Mapping of Genetic Loci Conferring Resistance to Leaf Rust From Three Globally Resistant Durum Wheat Sources

Genetic resistance in the host plant is the most economical and environmentally friendly strategy for controlling wheat leaf rust, caused by Puccinia triticina Eriks. The durum wheat lines Gaza (Middle East), Arnacoris (France) and Saragolla (Italy) express high levels of resistance to the Mexican races of P. triticina. Three recombinant inbred line (RIL) populations, derived from crosses of each of these resistance sources to the susceptible line ATRED #2, were evaluated for leaf rust reactions at CIMMYT's leaf rust nurseries in Mexico. Genetic analyses of host reactions suggested oligogenic control of resistance in all populations. The F₈ RILs from each cross were genotyped using the Illumina iSelect 90K array, and high-density genetic maps were constructed for each population. Using composite interval mapping, a total of seven quantitative trait loci (QTL) that provide resistance to leaf rust were identified. Two QTL designated as QLr.usw-6BS and QLr.usw-6BL were identified on chromosome 6B in Gaza, which explained up to 78.5% and 21.3% of the observed leaf rust severity variance, respectively. A major QTL designated as QLr.usw-7BL was detected on the long arm of chromosome 7B in Arnacoris, which accounted for up to 65.9% of the disease severity variance. Arnacoris also carried a minor QTL on chromosome 1BL, designated as QLr.usw-1BL.1 that explained up to 17.7% of the phenotypic variance. Three QTL conferred leaf rust resistance in Saragolla, namely QLr.usw-2BS, QLr.usw-3B, and QLr.usw-1BL.2, which accounted for up to 42.3, 9.4, and 7.1% of the phenotypic variance, respectively. Markers flanking each QTL were physically mapped against the durum wheat reference sequence and candidate genes involved in disease resistance were identified within the QTL intervals. The QTL identified in this study and their closely linked markers are useful resources for gene pyramiding and breeding for durable leaf rust resistance in durum wheat.

Quantitative Trait Loci Conferring Leaf Rust Resistance in Hexaploid Wheat

Leaf rust, caused by the fungal pathogen Puccinia triticina, is a major threat to wheat production in many wheat-growing regions of the world. The introduction of leaf rust resistance genes into elite wheat germplasm is the preferred method of disease control, being environmentally friendly and crucial to sustained wheat production. Consequently, there is considerable value in identifying and characterizing new sources of leaf rust resistance. While many major, qualitative leaf rust resistance genes have been identified in wheat, a growing number of valuable sources of quantitative resistance have been reported. Here we review the progress made in the genetic identification of quantitative trait loci (QTL) for leaf rust resistance detected primarily in field analyses, i.e., adult plant resistance. Over the past 50 years, leaf rust resistance loci have been assigned to genomic locations through chromosome analyses and genetic mapping in biparental mapping populations, studies that represent 79 different wheat leaf rust resistance donor lines. In addition, seven association mapping studies have identified adult plant and seedling leaf rust resistance marker trait associations in over 4,000 wheat genotypes. Adult plant leaf rust resistance QTL have been found on all 21 chromosomes of hexaploid wheat, with the B genome carrying the greatest number of QTL. The group 2 chromosomes are also particularly rich in leaf rust resistance QTL. The A genome has the lowest number of QTL for leaf rust resistance. Copyright © 2018 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

High-Density Mapping of an Adult-Plant Stripe Rust Resistance Gene YrBai in Wheat Landrace Baidatou Using the Whole Genome DArTseq and SNP Analysis

Stripe rust, caused by the biotrophic fungus Puccinia striiformis f. sp. tritici (Pst), is one of the most widespread and destructive wheat diseases worldwide. Growing resistant cultivars is an effective approach for controlling this disease. However, because host resistance genes were easily overcome by new virulent Pst races, there is a continuous demand for identifying new effective wheat stripe rust resistance genes and develop closely linked markers for marker-assisted selection (MAS). Baidatou, an old Chinese wheat landrace, has been grown for several decades in Longnan region, Gansu Province, where stripe rust epidemics are frequent and severe. In our previous study, a single dominant gene YrBai in Baidatou was identified to control the adult-plant resistance (APR) to Chinese prevalent Pst race CYR33. And the gene was located on wheat chromosome 6DS by four polymorphic simple sequence repeat (SSR) and two sequence-related amplified polymorphism (SRAP) markers, with the genetic distances of two closely linked markers 3.6 and 5.4 cM, respectively. To further confirm the APR gene in Baidatou and construct the high-density map for the resistance gene, adult plants of F1, F2, F3, and F5:6 populations derived from the cross Mingxian169/Baidatou and two parents were inoculated with CYR33 at Yangling field, Shaanxi Province during 2014-2015, 2015-2016, and 2016-2017 crop seasons, respectively. The field evaluation results indicated that a single dominant gene confers the APR to Pst race CYR33 in Baidatou. 92 F3 lines and parents were sequenced using DArTseq technology based on wheat GBS1.0 platform, and 31 genetic maps consisted of 2,131 polymorphic SilicoDArT and 952 SNP markers spanning 4,293.94 cM were constructed. Using polymorphic SilicoDArT, SNP markers and infection types (ITs) data of F3 lines, the gene YrBai was further located in 0.8 cM region on wheat chromosome 6D. These closely linked markers developed in this study should be useful for MAS for Baidatou in crop improvement and map-based clone this gene.

Identification and characterization of wheat stem rust resistance gene Sr21 effective against the Ug99 race group at high temperature

Wheat stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is a devastating foliar disease. The Ug99 race group has combined virulence to most stem rust (Sr) resistance genes deployed in wheat and is a threat to global wheat production. Here we identified a coiled-coil, nucleotide-binding leucine-rich repeat protein (NLR) completely linked to the Ug99 resistance gene Sr21 from Triticum monococcum. Loss-of-function mutations and transgenic complementation confirmed that this gene is Sr21. Sr21 transcripts were significantly higher at high temperatures, and this was associated with significant upregulation of pathogenesis related (PR) genes and increased levels of resistance at those temperatures. Introgression of Sr21 into hexaploid wheat resulted in lower levels of resistance than in diploid wheat, but transgenic hexaploid wheat lines with high levels of Sr21 expression showed high levels of resistance. Sr21 can be a valuable component of transgenic cassettes or gene pyramids combining multiple resistance genes against Ug99.

Wheat stem rust is a devastating disease that is threatening global wheat production. The emergence of new virulent races of this pathogen in Africa, including the Ug99 race group, has prompted global efforts to find effective resistance genes. We report here the identification of stem rust resistance gene Sr21 that is effective against the Ug99 race group. We developed a diagnostic marker to accelerate its deployment in wheat breeding programs and demonstrated that the introduction of two Sr21 copies in transgenic wheat results in high levels of resistance. An unusual characteristic of Sr21 is its increased resistance to stem rust at high temperatures. We show here that this is associated with the ability of Sr21 to coordinate the upregulation of multiple pathogenesis related genes at high temperatures. These genes slow down the growth of the pathogen and result in the characteristic Sr21 intermediate resistance reaction at high temperatures. A better understanding of this temperature dependent resistance mechanism will be useful for controlling the rust pathogens in our changing environments.

Parallel evolution of the POQR prolyl oligo peptidase gene conferring plant quantitative disease resistance

Plant pathogens with a broad host range are able to infect plant lineages that diverged over 100 million years ago. They exert similar and recurring constraints on the evolution of unrelated plant populations. Plants generally respond with quantitative disease resistance (QDR), a form of immunity relying on complex genetic determinants. In most cases, the molecular determinants of QDR and how they evolve is unknown. Here we identify in Arabidopsis thaliana a gene mediating QDR against Sclerotinia sclerotiorum, agent of the white mold disease, and provide evidence of its convergent evolution in multiple plant species. Using genome wide association mapping in A. thaliana, we associated the gene encoding the POQR prolyl-oligopeptidase with QDR against S. sclerotiorum. Loss of this gene compromised QDR against S. sclerotiorum but not against a bacterial pathogen. Natural diversity analysis associated POQR sequence with QDR. Remarkably, the same amino acid changes occurred after independent duplications of POQR in ancestors of multiple plant species, including A. thaliana and tomato. Genome-scale expression analyses revealed that parallel divergence in gene expression upon S. sclerotiorum infection is a frequent pattern in genes, such as POQR, that duplicated both in A. thaliana and tomato. Our study identifies a previously uncharacterized gene mediating QDR against S. sclerotiorum. It shows that some QDR determinants are conserved in distantly related plants and have emerged through the repeated use of similar genetic polymorphisms at different evolutionary time scales.

Validation and characterization of a QTL for adult plant resistance to stripe rust on wheat chromosome arm 6BS (Yr78)

KEY MESSAGE

This study validated one QTL for adult plant resistance to stripe rust, identified donor lines of the resistance allele, and demonstrated that it is different from previously named Yr genes. The spread of more virulent and aggressive races of Puccinia striiformis f. sp. tritici (Pst, causal pathogen of stripe rust) after the year 2000 has caused substantial yield losses worldwide. To find new sources of resistance, we previously performed a genome-wide association study and identified a strong QTL for adult plant resistance on the short arm of chromosome 6B (QYr.ucw-6B). In this study, we validated QYr.ucw-6B in ten biparental populations, and mapped it 0.6 cM proximal to IWA7257 and 3.9 cM distal to IWA4408. We showed that QYr.ucw-6B is located approximately 15 cM proximal to the all-stage resistance gene Yr35 and that none of the resistant lines carries the previously cloned Yr36 gene. Based on these results, QYr.ucw-6B was assigned the name Yr78. This gene was not effective against Pst at the seedling stage, suggesting that it is an adult plant resistance gene. Yr78 has been effective against Pst races present in field experiments performed in the Western USA between 2011 and 2016. Since this gene is predicted to be present at low frequency in wheat germplasm from this region, it can provide a useful tool to diversify the sources of resistance against this devastating pathogen.

Characterization and Mapping of Leaf Rust and Stripe Rust Resistance Loci in Hexaploid Wheat Lines UC1110 and PI610750 under Mexican Environments

Growing resistant wheat varieties is a key method of minimizing the extent of yield losses caused by the globally important wheat leaf rust (LR) and stripe rust (YR) diseases. In this study, a population of 186 F8 recombinant inbred lines (RILs) derived from a cross between a synthetic wheat derivative (PI610750) and an adapted common wheat line (cv. "UC1110") were phenotyped for LR and YR response at both seedling and adult plant stages over multiple seasons. Using a genetic linkage map consisting of single sequence repeats and diversity arrays technology markers, in combination with inclusive composite interval mapping analysis, we detected a new LR adult plant resistance (APR) locus, QLr.cim-2DS, contributed by UC1110. One co-located resistance locus to both rusts, QLr.cim-3DC/QYr.cim-3DC, and the known seedling resistance gene Lr26 were also mapped. QLr.cim-2DS and QLr.cim-3DC showed a marginally significant interaction for LR resistance in the adult plant stage. In addition, two previously reported YR APR loci, QYr.ucw-3BS and Yr48, were found to exhibit stable performances in rust environments in both Mexico and the United States and showed a highly significant interaction in the field. Yr48 was also observed to confer intermediate seedling resistance against Mexican YR races, thus suggesting it should be re-classified as an all-stage resistance gene. We also identified 5 and 2 RILs that possessed all detected YR and LR resistance loci, respectively. With the closely linked molecular markers reported here, these RILs could be used as donors for multiple resistance loci to both rusts in wheat breeding programs.

Characterization of molecular diversity and genome-wide mapping of loci associated with resistance to stripe rust and stem rust in Ethiopian bread wheat accessions

BACKGROUND

The narrow genetic basis of resistance in modern wheat cultivars and the strong selection response of pathogen populations have been responsible for periodic and devastating epidemics of the wheat rust diseases. Characterizing new sources of resistance and incorporating multiple genes into elite cultivars is the most widely accepted current mechanism to achieve durable varietal performance against changes in pathogen virulence. Here, we report a high-density molecular characterization and genome-wide association study (GWAS) of stripe rust and stem rust resistance in 190 Ethiopian bread wheat lines based on phenotypic data from multi-environment field trials and seedling resistance screening experiments. A total of 24,281 single nucleotide polymorphism (SNP) markers filtered from the wheat 90 K iSelect genotyping assay was used to survey Ethiopian germplasm for population structure, genetic diversity and marker-trait associations.

RESULTS

Upon screening for field resistance to stripe rust in the Pacific Northwest of the United States and Ethiopia over multiple growing seasons, and against multiple races of stripe rust and stem rust at seedling stage, eight accessions displayed resistance to all tested races of stem rust and field resistance to stripe rust in all environments. Our GWAS results show 15 loci were significantly associated with seedling and adult plant resistance to stripe rust at false discovery rate (FDR)-adjusted probability (P) <0.10. GWAS also detected 9 additional genomic regions significantly associated (FDR-adjusted P < 0.10) with seedling resistance to stem rust in the Ethiopian wheat accessions. Many of the identified resistance loci were mapped close to previously identified rust resistance genes; however, three loci on the short arms of chromosomes 5A and 7B for stripe rust resistance and two on chromosomes 3B and 7B for stem rust resistance may be novel.

CONCLUSION

Our results demonstrate that considerable genetic variation resides within the landrace accessions that can be utilized to broaden the genetic base of rust resistance in wheat breeding germplasm. The molecular markers identified in this study should be useful in efficiently targeting the associated resistance loci in marker-assisted breeding for rust resistance in Ethiopia and other countries.

Validation and characterization of a QTL for adult plant resistance to stripe rust on wheat chromosome arm 6BS (Yr78)

KEY MESSAGE

This study validated one QTL for adult plant resistance to stripe rust, identified donor lines of the resistance allele, and demonstrated that it is different from previously named Yr genes. The spread of more virulent and aggressive races of Puccinia striiformis f. sp. tritici (Pst, causal pathogen of stripe rust) after the year 2000 has caused substantial yield losses worldwide. To find new sources of resistance, we previously performed a genome-wide association study and identified a strong QTL for adult plant resistance on the short arm of chromosome 6B (QYr.ucw-6B). In this study, we validated QYr.ucw-6B in ten biparental populations, and mapped it 0.6 cM proximal to IWA7257 and 3.9 cM distal to IWA4408. We showed that QYr.ucw-6B is located approximately 15 cM proximal to the all-stage resistance gene Yr35 and that none of the resistant lines carries the previously cloned Yr36 gene. Based on these results, QYr.ucw-6B was assigned the name Yr78. This gene was not effective against Pst at the seedling stage, suggesting that it is an adult plant resistance gene. Yr78 has been effective against Pst races present in field experiments performed in the Western USA between 2011 and 2016. Since this gene is predicted to be present at low frequency in wheat germplasm from this region, it can provide a useful tool to diversify the sources of resistance against this devastating pathogen.

Loci associated with resistance to stripe rust (Puccinia striiformis f. sp. tritici) in a core collection of spring wheat (Triticum aestivum)

Stripe rust, caused by Puccinia striiformis Westend. f. sp. tritici Erikss. (Pst) remains one of the most significant diseases of wheat worldwide. We investigated stripe rust resistance by genome-wide association analysis (GWAS) in 959 spring wheat accessions from the United States Department of Agriculture-Agricultural Research Service National Small Grains Collection, representing major global production environments. The panel was characterized for field resistance in multi-environment field trials and seedling resistance under greenhouse conditions. A genome-wide set of 5,619 informative SNP markers were used to examine the population structure, linkage disequilibrium and marker-trait associations in the germplasm panel. Based on model-based analysis of population structure and hierarchical Ward clustering algorithm, the accessions were clustered into two major subgroups. These subgroups were largely separated according to geographic origin and improvement status of the accessions. A significant correlation was observed between the population sub-clusters and response to stripe rust infection. We identified 11 and 7 genomic regions with significant associations with stripe rust resistance at adult plant and seedling stages, respectively, based on a false discovery rate multiple correction method. The regions harboring all, except three, of the QTL identified from the field and greenhouse studies overlap with positions of previously reported QTL. Further work should aim at validating the identified QTL using proper germplasm and populations to enhance their utility in marker assisted breeding.

Novel Sources of Stripe Rust Resistance Identified by Genome-Wide Association Mapping in Ethiopian Durum Wheat (Triticum turgidum ssp. durum)

Stripe rust of wheat, caused by Puccinia striiformis f. sp. tritici (Pst), is a global concern for wheat production, and has been increasingly destructive in Ethiopia, as well as in the United States and in many other countries. As Ethiopia has a long history of stripe rust epidemics, its native wheat germplasm harbors potentially valuable resistance loci. Moreover, the Ethiopian germplasm has been historically underutilized in breeding of modern wheat worldwide and thus the resistance alleles from the Ethiopian germplasm represent potentially novel sources. The objective of this study was to identify loci conferring resistance to predominant Pst races in Ethiopia and the United States. Using a high-density 90 K wheat single nucleotide polymorphism array, a genome-wide association analysis (GWAS) was conducted on 182 durum wheat landrace accessions and contemporary varieties originating from Ethiopia. Landraces were detected to be more resistant at the seedling stage while cultivars were more resistant at the adult-plant stages. GWAS identified 68 loci associated with seedling resistance to one or more races. Six loci on chromosome arms 1AS, 1BS, 3AS, 4BL, and 5BL were associated with resistance against at least two races at the seedling stage, and five loci were previously undocumented. GWAS analysis of field resistance reactions identified 12 loci associated with resistance on chromosomes 1A, 1B, 2BS, 3BL, 4AL, 4B and 5AL, which were detected in at least two of six field screening nurseries at the adult-plant stage. Comparison with previously mapped resistance loci indicates that six of the 12 resistance loci are newly documented. This study reports effective sources of resistance to contemporary races in Ethiopia and the United States and reveals that Ethiopian durum wheat landraces are abundant in novel Pst resistance loci that may be transferred into adapted cultivars to provide resistance against Pst.

Expression of apoplast-targeted plant defensin MtDef4.2 confers resistance to leaf rust pathogen Puccinia triticina but does not affect mycorrhizal symbiosis in transgenic wheat

Rust fungi of the order Pucciniales are destructive pathogens of wheat worldwide. Leaf rust caused by the obligate, biotrophic basidiomycete fungus Puccinia triticina (Pt) is an economically important disease capable of causing up to 50 % yield losses. Historically, resistant wheat cultivars have been used to control leaf rust, but genetic resistance is ephemeral and breaks down with the emergence of new virulent Pt races. There is a need to develop alternative measures for control of leaf rust in wheat. Development of transgenic wheat expressing an antifungal defensin offers a promising approach to complement the endogenous resistance genes within the wheat germplasm for durable resistance to Pt. To that end, two different wheat genotypes, Bobwhite and Xin Chun 9 were transformed with a chimeric gene encoding an apoplast-targeted antifungal plant defensin MtDEF4.2 from Medicago truncatula. Transgenic lines from four independent events were further characterized. Homozygous transgenic wheat lines expressing MtDEF4.2 displayed resistance to Pt race MCPSS relative to the non-transgenic controls in growth chamber bioassays. Histopathological analysis suggested the presence of both pre- and posthaustorial resistance to leaf rust in these transgenic lines. MtDEF4.2 did not, however, affect the root colonization of a beneficial arbuscular mycorrhizal fungus Rhizophagus irregularis. This study demonstrates that the expression of apoplast-targeted plant defensin MtDEF4.2 can provide substantial resistance to an economically important leaf rust disease in transgenic wheat without negatively impacting its symbiotic relationship with the beneficial mycorrhizal fungus.

Development of a Greenhouse Screening Method for Adult Plant Response in Wheat to Stem Rust

Screening for adult plant resistance in wheat to stem rust, caused by Puccinia graminis f. sp. tritici, is generally conducted in field plots. Although such evaluations are successful if managed properly, field ratings are time consuming, expensive, weather dependent, and open to inoculum of unwanted races or other confounding diseases. The objective of this study was to develop a dependable system of screening the response of adult plants to stem rust under greenhouse conditions. A comparison of inoculation methods and incubation environments showed that plants inoculated with urediniospores suspended in water, followed by a 24 h dew period in a plastic chamber constructed in a greenhouse, gave the most consistent results. Measurements of response type, stem rust severity, and frequency in follow-up experiments indicated that the most reliable infection was obtained when plants sprayed with 1.25 mg urediniospores per ml water were incubated in the plastic chamber. Using the optimized protocol, a Kariega × Avocet S doubled haploid population was inoculated with two P. graminis f. sp. tritici races. Depending on the race, composite interval mapping showed flag leaf infection type to be significantly influenced by regions on chromosomes 6A, 6D, and 7D. Stem rust severity and reaction type mapped to chromosomes 6D and/or 6A. The Lr34/Yr18/Sr57 gene derived from Kariega on chromosome 7D affected the rust response on flag leaves but not on stems of greenhouse-grown plants. This study showed that phenotyping and genetic analysis of especially major effect stem rust resistance in adult wheat plants is possible and reproducible under controlled conditions in a greenhouse.

Wheat Stripe Rust Resistance Protein WKS1 Reduces the Ability of the Thylakoid-Associated Ascorbate Peroxidase to Detoxify Reactive Oxygen Species

Stripe rust is a devastating fungal disease of wheat caused by Puccinia striiformis f. sp tritici (Pst). The WHEAT KINASE START1 (WKS1) resistance gene has an unusual combination of serine/threonine kinase and START lipid binding domains and confers partial resistance to Pst. Here, we show that wheat (Triticum aestivum) plants transformed with the complete WKS1 (variant WKS1.1) are resistant to Pst, whereas those transformed with an alternative splice variant with a truncated START domain (WKS1.2) are susceptible. WKS1.1 and WKS1.2 preferentially bind to the same lipids (phosphatidic acid and phosphatidylinositol phosphates) but differ in their protein-protein interactions. WKS1.1 is targeted to the chloroplast where it phosphorylates the thylakoid-associated ascorbate peroxidase (tAPX) and reduces its ability to detoxify peroxides. Increased expression of WKS1.1 in transgenic wheat accelerates leaf senescence in the absence of Pst. Based on these results, we propose that the phosphorylation of tAPX by WKS1.1 reduces the ability of the cells to detoxify reactive oxygen species and contributes to cell death. This response takes several days longer than typical hypersensitive cell death responses, thus allowing the limited pathogen growth and restricted sporulation that is characteristic of the WKS1 partial resistance response to Pst.

A genome-wide association study of resistance to stripe rust (Puccinia striiformis f. sp. tritici) in a worldwide collection of hexaploid spring wheat (Triticum aestivum L.)

New races of Puccinia striiformis f. sp. tritici (Pst), the causal pathogen of wheat stripe rust, show high virulence to previously deployed resistance genes and are responsible for large yield losses worldwide. To identify new sources of resistance we performed a genome-wide association study (GWAS) using a worldwide collection of 1000 spring wheat accessions. Adult plants were evaluated under field conditions in six environments in the western United States, and seedlings were tested with four Pst races. A single-nucleotide polymorphism (SNP) Infinium 9K-assay provided 4585 SNPs suitable for GWAS. High correlations among environments and high heritabilities were observed for stripe rust infection type and severity. Greater levels of Pst resistance were observed in a subpopulation from Southern Asia than in other groups. GWAS identified 97 loci that were significant for at least three environments, including 10 with an experiment-wise adjusted Bonferroni probability < 0.10. These 10 quantitative trait loci (QTL) explained 15% of the phenotypic variation in infection type, a percentage that increased to 45% when all QTL were considered. Three of these 10 QTL were mapped far from previously identified Pst resistance genes and QTL, and likely represent new resistance loci. The other seven QTL mapped close to known resistance genes and allelism tests will be required to test their relationships. In summary, this study provides an integrated view of stripe rust resistance resources in spring wheat and identifies new resistance loci that will be useful to diversify the current set of resistance genes deployed to control this devastating disease.

Mapping of quantitative adult plant field resistance to leaf rust and stripe rust in two European winter wheat populations reveals co-location of three QTL conferring resistance to both rust pathogens

KEY MESSAGE

We detected several, most likely novel QTL for adult plant resistance to rusts. Notably three QTL improved resistance to leaf rust and stripe rust simultaneously indicating broad spectrum resistance QTL. The rusts of wheat (Puccinia spp.) are destructive fungal wheat diseases. The deployment of resistant cultivars plays a central role in integrated rust disease management. Durability of resistance would be preferred, but is difficult to analyse. The Austrian winter wheat cultivar Capo was released in the 1989 and grown on a large acreage during more than two decades and maintained a good level of quantitative leaf rust and stripe rust resistance. Two bi-parental mapping populations: Capo × Arina and Capo × Furore were tested in multiple environments for severity of leaf rust and stripe rust at the adult plant stage in replicated field experiments. Quantitative trait loci associated with leaf rust and stripe rust severity were mapped using DArT and SSR markers. Five QTL were detected in multiple environments associated with resistance to leaf rust designated as QLr.ifa-2AL, QLr.ifa-2BL, QLr.ifa-2BS, QLr.ifa-3BS, and QLr.ifa-5BL, and five for resistance to stripe rust QYr.ifa-2AL, QYr.ifa-2BL, QYr.ifa-3AS, QYr.ifa-3BS, and QYr.ifa-5A. For all QTL apart from two (QYr.ifa-3AS, QLr.ifa-5BL) Capo contributed the resistance improving allele. The leaf rust and stripe rust resistance QTL on 2AL, 2BL and 3BS mapped to the same chromosome positions, indicating either closely linked genes or pleiotropic gene action. These three multiple disease resistance QTL (QLr.ifa-2AL/QYr.ifa-2AL, QLr.ifa.2BL/QYr.ifa-2BL, QLr.ifa-3BS/QYr.ifa.3BS) potentially contribute novel resistance sources for stripe rust and leaf rust. The long-lasting resistance of Capo apparently rests upon a combination of several genes. The described germplasm, QTL and markers are applicable for simultaneous resistance improvement against leaf rust and stripe rust.

A time for more booms and fewer busts? Unraveling cereal-rust interactions

Recent advances in our understanding of the nature of resistance genes and rust fungus genomics are providing some insight into the basis of resistance and susceptibility to rust diseases in our cereal crops. Characterized rust resistance genes, for the most part, resemble other resistance genes that interact with effectors intracellularly, but some have unique features. Characterization of rust effectors is just beginning but genomic information and technical advances in rust functional genomics will accelerate their characterization. The ephemeral nature of resistance in past varieties has made the design of cultivars with durable resistance a major focus for geneticists and cereal breeders. This includes strategies for deploying race-specific resistance genes that prolong their effects and methods of predicting which will be difficult for the pathogen to defeat. Identification of resistance genes with race-nonspecific effects is another strategy where recent breakthroughs have been made. Routinely combining the numerous genes required for complex resistance, whether specific or nonspecific, in elite cultivars remains a primary constraint to realizing durable resistance in most programs.

Resistance to stem rust Ug99 in six bread wheat cultivars maps to chromosome 6DS

Identified SSR markers ( Xcfd49 and Xbarc183 ) linked with stem rust resistance for efficient use in marker-assisted selection and stacking of resistance genes in wheat breeding programs. More than 80 % of the worldwide wheat (Triticum aestivum L.) area is currently sown with varieties susceptible to the Ug99 race group of stem rust fungus. However, wheat lines Niini, Tinkio, Coni, Pfunye, Blouk, and Ripper have demonstrated Ug99 resistance at the seedling and adult plant stages. We mapped stem rust resistance in populations derived from crosses of a susceptible parent with each of the resistant lines. The segregation of resistance in each population indicated the presence of a single gene. The resistance gene in Niini mapped to short arm of chromosome 6D and was flanked by SSR markers Xcfd49 at distances of 3.9 cM proximal and Xbarc183 8.4 cM distal, respectively. The chromosome location of this resistance was validated in three other populations: PBW343/Coni, PBW343/Tinkio, and Cacuke/Pfunye. Resistance initially postulated to be conferred by the SrTmp gene in Blouk and Ripper was also linked to Xcfd49 and Xbarc183 on 6DS, but it was mapped proximal to Xbarc183 at a similar position to previously mapped genes Sr42 and SrCad. Based on the variation in diagnostic marker alleles, it is possible that Niini and Pfunye may carry different resistance genes/alleles. Further studies are needed to determine the allelic relationships between various genes located on chromosome arm 6DS. Our results provide valuable molecular marker and genetic information for developing Ug99 resistant wheat varieties in diverse germplasm and using these markers to tag the resistance genes in wheat breeding.

Comparative analysis of protein-protein interactions in the defense response of rice and wheat

Background

Despite the importance of wheat as a major staple crop and the negative impact of diseases on its production worldwide, the genetic mechanisms and gene interactions involved in the resistance response in wheat are still poorly understood. The complete sequence of the rice genome has provided an extremely useful parallel road map for genetic and genomics studies in wheat. The recent construction of a defense response interactome in rice has the potential to further enhance the translation of advances in rice to wheat and other grasses. The objective of this study was to determine the degree of conservation in the protein-protein interactions in the rice and wheat defense response interactomes. As entry points we selected proteins that serve as key regulators of the rice defense response: the RAR1/SGT1/HSP90 protein complex, NPR1, XA21, and XB12 (XA21 interacting protein 12).

Results

Using available wheat sequence databases and phylogenetic analyses we identified and cloned the wheat orthologs of these four rice proteins, including recently duplicated paralogs, and their known direct interactors and tested 86 binary protein interactions using yeast-two-hybrid (Y2H) assays. All interactions between wheat proteins were further tested using in planta bimolecular fluorescence complementation (BiFC). Eighty three percent of the known rice interactions were confirmed when wheat proteins were tested with rice interactors and 76% were confirmed using wheat protein pairs. All interactions in the RAR1/SGT1/ HSP90, NPR1 and XB12 nodes were confirmed for the identified orthologous wheat proteins, whereas only forty four percent of the interactions were confirmed in the interactome node centered on XA21. We hypothesize that this reduction may be associated with a different sub-functionalization history of the multiple duplications that occurred in this gene family after the divergence of the wheat and rice lineages.

Conclusions

The observed high conservation of interactions between proteins that serve as key regulators of the rice defense response suggests that the existing rice interactome can be used to predict interactions in wheat. Such predictions are less reliable for nodes that have undergone a different history of duplications and sub-functionalization in the two lineages.

Impacts of resistance gene genetics, function, and evolution on a durable future

Studies on resistance gene function and evolution lie at the confluence of structural and molecular biology, genetics, and plant breeding. However, knowledge from these disparate fields has yet to be extensively integrated. This review draws on ideas and information from these different fields to elucidate the influences driving the evolution of different types of resistance genes in plants and the concurrent evolution of virulence in pathogens. It provides an overview of the factors shaping the evolution of recognition, signaling, and response genes in the context of emerging functional information along with a consideration of the new opportunities for durable resistance enabled by high-throughput DNA sequencing technologies.

The Top 10 fungal pathogens in molecular plant pathology

The aim of this review was to survey all fungal pathologists with an association with the journal Molecular Plant Pathology and ask them to nominate which fungal pathogens they would place in a 'Top 10' based on scientific/economic importance. The survey generated 495 votes from the international community, and resulted in the generation of a Top 10 fungal plant pathogen list for Molecular Plant Pathology. The Top 10 list includes, in rank order, (1) Magnaporthe oryzae; (2) Botrytis cinerea; (3) Puccinia spp.; (4) Fusarium graminearum; (5) Fusarium oxysporum; (6) Blumeria graminis; (7) Mycosphaerella graminicola; (8) Colletotrichum spp.; (9) Ustilago maydis; (10) Melampsora lini, with honourable mentions for fungi just missing out on the Top 10, including Phakopsora pachyrhizi and Rhizoctonia solani. This article presents a short resumé of each fungus in the Top 10 list and its importance, with the intent of initiating discussion and debate amongst the plant mycology community, as well as laying down a bench-mark. It will be interesting to see in future years how perceptions change and what fungi will comprise any future Top 10.

The Top 10 fungal pathogens in molecular plant pathology

The aim of this review was to survey all fungal pathologists with an association with the journal Molecular Plant Pathology and ask them to nominate which fungal pathogens they would place in a 'Top 10' based on scientific/economic importance. The survey generated 495 votes from the international community, and resulted in the generation of a Top 10 fungal plant pathogen list for Molecular Plant Pathology. The Top 10 list includes, in rank order, (1) Magnaporthe oryzae; (2) Botrytis cinerea; (3) Puccinia spp.; (4) Fusarium graminearum; (5) Fusarium oxysporum; (6) Blumeria graminis; (7) Mycosphaerella graminicola; (8) Colletotrichum spp.; (9) Ustilago maydis; (10) Melampsora lini, with honourable mentions for fungi just missing out on the Top 10, including Phakopsora pachyrhizi and Rhizoctonia solani. This article presents a short resumé of each fungus in the Top 10 list and its importance, with the intent of initiating discussion and debate amongst the plant mycology community, as well as laying down a bench-mark. It will be interesting to see in future years how perceptions change and what fungi will comprise any future Top 10.

Mapping and validation of QTL which confer partial resistance to broadly virulent post-2000 North American races of stripe rust in hexaploid wheat

A mapping population of 186 recombinant inbred lines developed from a cross between UC1110, an adapted California spring wheat, and PI610750, a synthetic derivative from CIMMYT's Wide Cross Program, was evaluated for its response to current California races of stripe rust (Puccinia striiformis f. sp. tritici) in replicated field trials over four seasons (2007-2010) in the northern Sacramento Valley. A genetic map was constructed consisting of 1,494 polymorphic probes (SSRs, DArTs, and ESTs) mapped to 558 unique loci, and QTL analysis revealed the presence of four stripe rust resistance QTL segregating in this population, two from UC1110 (on chromosomes 3BS and 2BS) and two from PI610750 (5AL and 2AS). The two QTL of largest effects (on 3BS and 5AL) were validated in independent populations and their intervals narrowed to 2.5 and 5.3 cM, respectively. The 3BS QTL was shown, by allelism test and genotype, to carry a gene different from the Yr30/Sr2 complex. Mapped position also suggests that the 3BS QTL is associated with a gene different from either Yrns-B1 or YrRub, two stripe rust resistance genes mapped to this region in other studies. The 5AL QTL carries a previously unreported partial stripe rust resistance gene, designated here as Yr48. This paper discusses the individual contributions to resistance of these four QTL, their epistatic interactions, and their potential in durable resistance breeding strategies based on combinations of partial resistance genes.

Mapping and validation of QTL which confer partial resistance to broadly virulent post-2000 North American races of stripe rust in hexaploid wheat

A mapping population of 186 recombinant inbred lines developed from a cross between UC1110, an adapted California spring wheat, and PI610750, a synthetic derivative from CIMMYT's Wide Cross Program, was evaluated for its response to current California races of stripe rust (Puccinia striiformis f. sp. tritici) in replicated field trials over four seasons (2007-2010) in the northern Sacramento Valley. A genetic map was constructed consisting of 1,494 polymorphic probes (SSRs, DArTs, and ESTs) mapped to 558 unique loci, and QTL analysis revealed the presence of four stripe rust resistance QTL segregating in this population, two from UC1110 (on chromosomes 3BS and 2BS) and two from PI610750 (5AL and 2AS). The two QTL of largest effects (on 3BS and 5AL) were validated in independent populations and their intervals narrowed to 2.5 and 5.3 cM, respectively. The 3BS QTL was shown, by allelism test and genotype, to carry a gene different from the Yr30/Sr2 complex. Mapped position also suggests that the 3BS QTL is associated with a gene different from either Yrns-B1 or YrRub, two stripe rust resistance genes mapped to this region in other studies. The 5AL QTL carries a previously unreported partial stripe rust resistance gene, designated here as Yr48. This paper discusses the individual contributions to resistance of these four QTL, their epistatic interactions, and their potential in durable resistance breeding strategies based on combinations of partial resistance genes.