Ptr ToxA Research Articles

Identification of quantitative trait loci conferring resistance to tan spot in a biparental population derived from two Nebraska hard red winter wheat cultivars

Tan spot, caused by Pyrenophora tritici-repentis (Ptr), is a destructive foliar disease in all types of cultivated wheat worldwide. Genetics of tan spot resistance in wheat is complex, involving insensitivity to fungal-produced necrotrophic effectors (NEs), major resistance genes, and quantitative trait loci (QTL) conferring race-nonspecific and race-specific resistance. The Nebraska hard red winter wheat (HRWW) cultivar ‘Wesley’ is insensitive to Ptr ToxA and highly resistant to multiple Ptr races, but the genetics of resistance in this cultivar is unknown. In this study, we used a recombinant inbred line (RIL) population derived from a cross between Wesley and another Nebraska cultivar ‘Harry’ (Ptr ToxA sensitive and highly susceptible) to identify QTL associated with reaction to tan spot caused by multiple races/isolates. Sensitivity to Ptr ToxA conferred by the Tsn1 gene was mapped to chromosome 5B as expected. The Tsn1 locus was a major susceptibility QTL for the race 1 and race 2 isolates, but not for the race 2 isolate with the ToxA gene deleted. A second major susceptibility QTL was identified for all the Ptr ToxC-producing isolates and located to the distal end of the chromosome 1A, which likely corresponds to the Tsc1 locus. Three additional QTL with minor effects were identified on chromosomes 7A, 7B, and 7D. This work indicates that both Ptr ToxA-Tsn1 and Ptr ToxC-Tsc1 interactions are important for tan spot development in winter wheat, and Wesley is highly resistant largely due to the absence of the two tan spot sensitivity genes.

Characterizing Virulence of the Pyrenophora tritici-repentis Isolates Lacking Both ToxA and ToxB Genes.

The fungus Pyrenophora tritici-repentis (Ptr) causes tan spot of wheat crops, which is an important disease worldwide. Based on the production of the three known necrotrophic effectors (NEs), the fungal isolates are classified into eight races with race 4 producing no known NEs. From a laboratory cross between 86–124 (race 2 carrying the ToxA gene for the production of Ptr ToxA) and DW5 (race 5 carrying the ToxB gene for the production of Ptr ToxB), we have obtained some Ptr isolates lacking both the ToxA and ToxB genes, which, by definition, should be classified as race 4. In this work, we characterized virulence of two of these isolates called B16 and B17 by inoculating them onto various common wheat (Triticum aestivum L.) and durum (T. turgidum L.) genotypes. It was found that the two isolates still caused disease on some genotypes of both common and durum wheat. Disease evaluations were also conducted in recombinant inbred line populations derived from two hard red winter wheat cultivars: Harry and Wesley. QTL mapping in this population revealed that three genomic regions were significantly associated with disease, which are different from the three known NE sensitivity loci. This result further indicates the existence of other NE-host sensitivity gene interactions in the wheat tan spot disease system.

Peculiarities of interaction of Tsn1 and ToxA genes in Triticum aestivum – Pyrenophora tritici-repentis pathosystem

The tan spot caused by the Pyrenophora tritici-repentis is a harmful and widespread disease in the Russian Federation. Last years, it has been proposed to carry out marker assisted selection (MAS) against the dominant allele of the gene Tsn1 determining the susceptibility to tan spot to speed up the breeding process. However, in these studies, the factor of the population diversity of the pathogen on the effector genes is underestimated, especially on the ToxA gene controlling the synthesis of necrosis inducing toxin Ptr ToxA in susceptible cultivars with the complementary gene Tsn1. In this paper, the results are analysed of inoculation of 24 soft winter wheat cultivars with dominant and recessive Tsn1 alleles by P. tritici-repentis isolates, characterized by the presence or absence of the ToxA (ToxA+ and ToxA–) effector gene. Using the example of the interaction between Tsn1 alleles and P. tritici-repentis ToxA effector gene in specific pairs of genetically characterized wheat samples and pathogen isolates, it was shown that the same sign of “formation of necrosis on leaves” in combinations of different genotypes of variety–isolate (Tsn1Tsn1– ToxA+/ToxA– and tsn1tsn1 – ToxA+/ToxA–) had a different genetic nature. 41.7 % of the cases of gene interactions did not correspond to the gene-to-gene scheme. This result is explained by either the presence of necrosis inducing toxins in isolates other than Ptr ToxA, or decrease in the level of ToxA gene expression in different wheat genotypes.

Reaction of Global Collection of Rye (Secale cereale L.) to Tan Spot and Pyrenophora tritici-repentis Races in South Dakota.

Rye (Secale cereale L.) serves as an alternative host of Pyrenophora tritici-repentis (PTR) the cause of tan spot on wheat. Rye is cultivated as a forage or cover crop and overlaps with a significant portion of wheat acreage in the U.S. northern Great Plains; however, it is not known whether the rye crop influences the evolution of PTR races. We evaluated a global collection of 211 rye accessions against tan spot and assessed the diversity in PTR population on rye in South Dakota. All the rye genotypes were inoculated with PTR races 1 and 5, and infiltrated with Ptr ToxA and Ptr ToxB, at seedling stage. We observed 21% of the genotypes exhibited susceptibility to race 1, whereas, 39% were susceptible to race 5. All 211 accessions were insensitive to both the Ptr toxins. It indicates that though rye exhibits diversity in reaction to tan spot, it lacks Ptr ToxA and ToxB sensitivity genes. This suggests that unknown toxins or other factors can lead to PTR establishment in rye. We characterized the race structure of 103 PTR isolates recovered from rye in South Dakota. Only 22% of the isolates amplified Ptr ToxA gene and were identified as race 1 based on their phenotypic reaction on the differential set. The remaining 80 isolates were noted to be race 4. Our results show that races 1 and 4 are prevalent on rye in South Dakota with a higher frequency of race 4, suggesting a minimal role of rye in the disease epidemiology.

Population Structure and Genomewide Association Analysis of Resistance to Disease and Insensitivity to Ptr Toxins in Canadian Spring Wheat Using 90K SNP Array

Leaf rust, yellow (stripe) rust, common bunt, and tan spot are economically significant diseases affecting wheat (Triticum aestivum L.) production in Canada. In this study, we investigated the genetic relationship and population structure of 81 Canadian western spring wheat cultivars released primarily between 1963 and 2011 and identified genomic regions associated with resistance to the above four diseases and insensitivity to three Pyrenophora tritici‐repentis (Ptr) toxins (Ptr ToxA, Ptr ToxB and Ptr ToxC). The cultivars were evaluated for field reaction to the four diseases and for reaction to the three Ptr toxins in a greenhouse and were genotyped with a subset of 19,919 of the wheat 90K single‐nucleotide polymorphic array and 11 gene‐specific markers. There were large genetic differences among pairwise comparisons of cultivars, except six pairs that showed <0.05 genetic distance. The cultivars exhibited clear population structure, generally in agreement with the major western Canada spring wheat classes. Using a threshold of p ≤ 5 × 10−5 and a weighted mixed linear model, we identified 94 markers from seven chromosomes associated with all traits except Ptr ToxC. Two major‐effect genomic regions on chromosomes 5B (71–74 cM) and 1A (52–53 cM) were associated with Ptr ToxA, of which the former coincided with the Tsn1 gene. For Ptr ToxB, we identified two other major‐effect regions on chromosomes 2B and 5B. The genomic regions associated with common bunt mapped on 2B, 4B, and 7A, whereas those associated with leaf rust mapped at two positions on 2B. We were only able to uncover a single marker‐trait association for tan spot on 7B and for yellow rust on 2A.

The sensitivity of Canadian wheat genotypes to the necrotrophic effectors produced by Pyrenophora tritici-repentis

Pyrenophora tritici-repentis (Ptr), the cause of tan spot of wheat, is a foliar pathogen of economic importance worldwide. Tan spot development is associated with the production of necrotrophic effectors (Ptr ToxA, Ptr ToxB and Ptr ToxC) that serve as fungal pathogenicity and/or virulence factors. The objective of this study was to investigate the sensitivity to each of these effectors in a collection of 100 Canadian wheat cultivars representing all wheat classes and developed over a century by various Canadian breeding programmes. Heterologously expressed, His-tagged Ptr ToxA and Ptr ToxB, and spore germination fluids possessing putative Ptr ToxC activity, were produced and used to screen the cultivars for sensitivity. Of the 100 wheat cultivars tested, 92 were sensitive to at least one effector, 68 were sensitive to Ptr ToxA, 63 were sensitive to Ptr ToxC and 24 were sensitive to Ptr ToxB. Only eight cultivars were insensitive to all three effectors. These results reflect the widespread sensitivity in Canadian wheat cultivars to these necrotrophic effectors, with sensitivity to Ptr ToxA and the putative Ptr ToxC found consistently to be more common than Ptr ToxB sensitivity. The predominance of Ptr ToxA and Ptr ToxC sensitivity in Canadian wheat is consistent with the prevalence of races 1 (Ptr ToxA + Ptr ToxC producer) and 2 (Ptr ToxA producer) of Ptr in North America, and could reflect a competitive advantage for Ptr ToxA-producing isolates in the pathogen population.

Insights into Tan Spot and Stem Rust Resistance and Susceptibility by Studying the Pre-Green Revolution Global Collection of Wheat.

Tan spot (TS), caused by the fungus Pyrenophora tritici-repentis (Died) Drechs, is an important foliar disease of wheat and has become a threat to world wheat production since the 1970s. In this study a globally diverse pre-1940s collection of 247 wheat genotypes was evaluated against Ptr ToxA, P. tritici-repentis race 1, and stem rust to determine if; (i) acquisition of Ptr ToxA by the P. tritici-repentis from Stagonospora nodorum led to increased pathogen virulence or (ii) incorporation of TS susceptibility during development stem rust resistant cultivars led to an increase in TS epidemics globally. Most genotypes were susceptible to stem rust; however, a range of reactions to TS and Ptr ToxA were observed. Four combinations of disease-toxin reactions were observed among the genotypes; TS susceptible-Ptr ToxA sensitive, TS susceptible-Ptr ToxA insensitive, TS resistant-Ptr ToxA insensitive, and TS resistant-Ptr ToxA toxin sensitive. A weak correlation (r = 0.14 for bread wheat and −0.082 for durum) was observed between stem rust susceptibility and TS resistance. Even though there were no reported epidemics in the pre-1940s, TS sensitive genotypes were widely grown in that period, suggesting that Ptr ToxA may not be an important factor responsible for enhanced prevalence of TS.

Characterization of Pyrenophora tritici-repentis (Tan Spot of Wheat) Races in Baltic States and Romania.

Tan spot, caused by the fungus Pyrenophora triticirepentis, is economically important foliar disease in Latvia, Lithuania, and Romania; however, race structure from Baltic States and Romania is not known. In this study, we performed genotypic and phenotypic race characterization of a large collection of P. triticirepentis isolates from these countries to determine race structure and utilize this information for better disease management and breeding wheat for tan spot resistance. We characterized 231 single spore isolates from Latvia (n = 15), Lithuania (n = 107), and Romania (n = 109) for Ptr ToxA and Ptr ToxB genes using two genes specific primers. A subset (139) of 231 isolates were further characterized for their race structure by inoculating them individually on tan spot wheat differentials set. Majority (83%) of the 231 isolates amplified Ptr ToxA gene suggesting prevalence of race 1 and 2. Further, phenotypic characterization of 139 isolates also showed wide prevalence of races 1 (68%), 2 (8%), 3 (11%), and 4 (5%) were also identified from Baltic States as well as Romania. Eighteen of the isolates (13%) did not seem to be of any of the eight known races as they lacked Ptr ToxA gene but they behaved like either race 1 or race 2, suggesting possibility of novel toxins in these isolates as their virulence tools.

Inverse gene-for-gene interactions contribute additively to tan spot susceptibility in wheat.

Tan spot susceptibility is conferred by multiple interactions of necrotrophic effector and host sensitivity genes. Tan spot of wheat, caused by Pyrenophora tritici-repentis, is an important disease in almost all wheat-growing areas of the world. The disease system is known to involve at least three fungal-produced necrotrophic effectors (NEs) that interact with the corresponding host sensitivity (S) genes in an inverse gene-for-gene manner to induce disease. However, it is unknown if the effects of these NE-S gene interactions contribute additively to the development of tan spot. In this work, we conducted disease evaluations using different races and quantitative trait loci (QTL) analysis in a wheat recombinant inbred line (RIL) population derived from a cross between two susceptible genotypes, LMPG-6 and PI 626573. The two parental lines each harbored a single known NE sensitivity gene with LMPG-6 having the Ptr ToxC sensitivity gene Tsc1 and PI 626573 having the Ptr ToxA sensitivity gene Tsn1. Transgressive segregation was observed in the population for all races. QTL mapping revealed that both loci (Tsn1 and Tsc1) were significantly associated with susceptibility to race 1 isolates, which produce both Ptr ToxA and Ptr ToxC, and the two genes contributed additively to tan spot susceptibility. For isolates of races 2 and 3, which produce only Ptr ToxA and Ptr ToxC, only Tsn1 and Tsc1 were associated with tan spot susceptibility, respectively. This work clearly demonstrates that tan spot susceptibility in this population is due primarily to two NE-S interactions. Breeders should remove both sensitivity genes from wheat lines to obtain high levels of tan spot resistance.

Allelic variation and effects of 16 candidate genes on disease resistance in western Canadian spring wheat cultivars

Recently, we mapped genomic regions associated with resistance to wheat diseases and insensitivity to Pyrenophora tritici-repentis (Ptr) toxins using 81 historical and modern Canadian western spring wheat cultivars genotyped with genome-wide single nucleotide polymorphic (SNP) markers. Here, we investigate the frequency and effects of allelic variants of 50 markers associated with 16 candidate genes that regulate resistance to leaf rust (Puccinia triticina), yellow or stripe rust (P. striiformis f. sp. tritici), tan spot (P. tritici-repentis), and Ptr ToxA reaction in a subset of 70 of the 81 spring wheat cultivars. We evaluated the 70 cultivars in the field for all diseases except Ptr ToxA, which was evaluated in a greenhouse. Using Spearman rank correlation, stepwise discriminant analysis, and partial least squares regression, we identified between 4 and 11 markers as best predictors of each phenotypic trait. Overall, 23 of the 50 markers were associated with one or more of the phenotypic traits of which analysis of variance showed significant differences between allelic variants of 19 markers. In most analyses, markers for Lr34/Yr18 and Tsn1 loci were identified consistently as the best predictor of disease resistance and Ptr ToxA sensitivity, respectively. The same alleles from two Lr34/Yr18 diagnostic SNP markers (wMAS000003 and wMAS000004) not only decreased stripe rust scores up to 1.6 (on a 1 to 9 scale), but also increased grain yield up to 196 kg ha−1 without affecting maturity. Results from this study could aid spring wheat breeders in selecting the best parental combinations and/or marker-assisted selection to integrate disease resistance with early maturity and short stature.

EVALUATION OF WHEAT CULTIVARS GROWING IN KAZAKHSTAN AND RUSSIA FOR RESISTANCE TO TAN SPOT

Tan spot, caused by Pyrenophora tritici-repentis, is a serious foliar disease of wheat in Kazakhstan and Russia. This study was conducted to screen elite wheat germplasms to the major virulence present in P. tritici-repentis in Kazakhstan and Russia; to evaluate sensitivity of the wheat germplasm to Ptr ToxA using molecular markers; and to identify resistance sources effective against tan spot and provide wheat breeders with sources of resistance. The study of wheat germplasm reaction to Ptr isolates revealed that 64.5% of the wheat cultivars were resistant when tested to isolate 11К-14 (race 1). Over 88% of cultivars tested to 18К-14 (race 2) of P. tritici-repentis were characterized as resistant. The largest number of Russian and Kazakh cultivars, 61.9% and 65.4% respectively, was susceptible to isolate 5K-14 (race 8). In 56 wheat cultivars, containing the recessive tsn1 allele, insensitive reaction to the Ptr ToxA toxin infiltration was predicted. When wheat material was evaluated for reaction to isolate 5K-14 (race 8) and genotyped with marker Xfcp394 the insensitive reaction to the Ptr ToxA was only predicted in 20 entries. The results of molecular screening made it possible to remove the toxin sensitivity alleles from studied material. The findings suggest that most of tested cultivars growing in Kazakhstan and Russia possess racespecific resistance. This fact should be taken into consideration during development of new wheat cultivars. Due to the threat of disease epiphytotic, it is necessary to create new donors of tan spot resistance and wheat breeding material based on them.

Race Diversity of Pyrenophora tritici-repentis in South Dakota and Response of Predominant Wheat Cultivars to Tan Spot

The fungus Pyrenophora tritici-repentis (Ptr) causing tan spot (TS) is an important pathogen of wheat in the US Northern-Great-Plains. Knowledge of physiological variation in the pathogen population is essential in the development of durable TS resistant cultivars. Eight Ptr races have been identified based on three host selective toxins (Ptr ToxA/Ptr ToxB/Ptr ToxC), which are associated with necrosis and chlorosis symptoms. The information about Ptr race structure and reaction of wheat cultivars grown in SD to tan spot is scarcely available. In this study, 569 isolates of Ptr collected from wheat were genotyped for Ptr ToxA and Ptr ToxB genes and a subset of 134 isolates were evaluated for their race identity on a wheat differential set. Ptr ToxA and Ptr ToxB genes were amplified in 89.6% and 0.4% isolates, respectively. The remaining 57 (10%) isolates lacked both toxins genes. The characterization of 134 isolates exhibited diverse race structure with 74.6%, 18.7%, 1.49% , and <1% isolates categorized as race 1, 4, 5, and 2, respectively. Another six (4.5%) isolates behaved like race 2 but lacked Ptr ToxA gene, hence could not fit under the currently known eight races. Our results determine the diversity of Ptr population that exists in SD and establish the presence of race 5 in SD for the first time. Since races 1 and 5 are most prevalent in the region, we screened 45 most predominant wheat cultivars against these races and Ptr ToxA. We observed eleven cultivars resistant or moderately resistant to both races, however, seven spring wheat cultivars showed susceptibility to both races 1 and 5. Continued cultivation of wheat cultivars susceptible to both races could play a role in the establishment and development of new races. Continuous germplasm enhancement and periodically monitoring Ptr population can help in better TS management.

Genetic relationships between race-nonspecific and race-specific interactions in the wheat-Pyrenophora tritici-repentis pathosystem.

We identified a major QTL conferring race-nonspecific resistance and revealed its relationships with race-specific interactions in the wheat- Pyrenophora tritici-repentis pathosystem. Tan spot, caused by the fungus Pyrenophora tritici-repentis (Ptr), is a destructive disease of wheat worldwide. The disease system is known to include inverse gene-for-gene, race-specific interactions involving the recognition of fungal-produced necrotrophic effectors (NEs) by corresponding host sensitivity genes. However, quantitative trait loci (QTLs) conferring race-nonspecific resistance have also been identified. In this work, we identified a major race-nonspecific resistance QTL and characterized its genetic relationships with the NE-host gene interactions Ptr ToxA-Tsn1 and Ptr ToxC-Tsc1 in a recombinant inbred wheat population derived from the cross between 'Louise' and 'Penawawa.' Both parental lines were sensitive to Ptr ToxA, but Penawawa and Louise were highly resistant and susceptible, respectively, to conidial inoculations of all races. Resistance was predominantly governed by a major race-nonspecific QTL on chromosome arm 3BL for resistance to all races. Another significant QTL was detected at the distal end of chromosome arm 1AS for resistance to the Ptr ToxC-producing isolates, which corresponded to the known location of the Tsc1 locus. The effects of the 3B and 1A QTLs were largely additive, and the 3B resistance QTL was epistatic to the Ptr ToxA-Tsn1 interaction. Resistance to race 2 in F1 plants was completely dominant; however, race 3-inoculated F1 plants were only moderately resistant because they developed chlorosis presumably due to the Ptr ToxC-Tsc1 interaction. This work provides further understanding of genetic resistance in the wheat-tan spot system as well as important guidance for tan spot resistance breeding.

Reaction of Five Non-cereal Grasses to Five Races and Two Host Selective Toxins of Pyrenophora tritici-repentis.

Alternative hosts increase the difficulty of disease management in crops because these alternate hosts provide additional sources of primary inoculum or refuges for diversity in the pathogen gene pool. Agropyron cristatum (crested wheatgrass), Bromus inermis (smooth bromegrass), Pascopyrum smithii (western wheatgrass), Stipa viridula (green needlegrass), and Thinopyrum intermedium (intermediate wheatgrass), commonly identified in range, prairie, verge, and soil reclamation habitats, serve as additional hosts for Pyrenophora tritici-repentis, the cause of tan spot in wheat (Triticum aestivum L.). A. cristatum (five lines), B. inermis (seven lines), P. smithii (four lines), S. viridula (two lines), and T. intermedium (six lines) were tested for their reactions to 30 representative P. tritici-repentis isolates from races 1–5. Plants were grown until the two-three-leaf stage in a greenhouse, inoculated individually with the 30 isolates, held at high humidity for 24 h, and rated after 7 days. All lines developed lesion types 1–2 (resistant) based on a 1–5 rating scale. Also, leaves from an additional plant set were infiltrated with two host selective toxins, Ptr ToxA as a pure preparation and Ptr ToxB as a dilute crude culture filtrate. All lines were insensitive to the toxins. Results indicate that these grass hosts have a limited or nonsignificant role in tan spot epidemiology on wheat in the northern Great Plains. Additionally, the resistant reactions demonstrated by the grass species in this research indicate the presence of resistance genes that can be valuable to wheat breeding programs for improving wheat resistance to P. tritici-repentis.

Proteome changes induced by Pyrenophora tritici-repentis ToxA in both insensitive and sensitive wheat indicate senescence-like signaling.

BackgroundPyrenophora tritici-repentis is a phytopathogenic fungus which causes tan spot on wheat. Some races of P. tritici-repentis produce host-specific toxins which present symptoms of chlorosis or necrosis on susceptible wheat cultivars. One such toxin is Ptr ToxA, which enters mesophyll cells through a putative toxin-receptor and localizes with chloroplasts, ultimately causing damage and necrosis on leaves. These symptoms can occur even in the absence of the pathogen. Insensitive cultivars lack the receptor and Ptr ToxA cannot enter cells. The molecular mechanisms surrounding this plant-pathogen interaction are still largely unknown, although some details have begun to emerge.ResultsUsing 2-D electrophoresis, fifteen protein changes were identified reproducibly in the leaf proteomes of a sensitive and an insensitive cultivar over three days after inoculation of purified Ptr ToxA. Functional analysis of the proteins indicated that senescence signals may be induced in the sensitive cultivar. In the insensitive cultivar proteins involved in some features of senescence inhibition were seen. Complementary responses at the biochemical level may be actively promoting a localized senescence-like response in sensitive wheat cultivars whilst actively inhibiting this response in insensitive cultivars.ConclusionThis is the first report of a biochemical response in an insensitive cultivar in this plant-pathogen interaction. Findings support the involvement of ethylene, and the activation of complementary pathways in sensitive versus insensitive wheat cultivars responding to Ptr ToxA. The nature of the system permits using purified toxin to mimic disease, which eliminates the pathogen proteome and ensures a synchronous response in inoculated leaves.Electronic supplementary materialThe online version of this article (doi:10.1186/s12953-014-0060-3) contains supplementary material, which is available to authorized users.

Genetics of tan spot resistance in wheat

Tan spot is a devastating foliar disease of wheat caused by the necrotrophic fungal pathogen Pyrenophora tritici-repentis. Much has been learned during the past two decades about the genetics of wheat-P. tritici-repentis interactions. Research has shown that the fungus produces at least three host-selective toxins (HSTs), known as Ptr ToxA, Ptr ToxB, and Ptr ToxC, that interact directly or indirectly with the products of the dominant host genes Tsn1, Tsc2, and Tsc1, respectively. The recent cloning and characterization of Tsn1 provided strong evidence that the pathogen utilizes HSTs to subvert host resistance mechanisms to cause disease. However, in addition to host-HST interactions, broad-spectrum, race non-specific resistance QTLs and recessively inherited qualitative 'resistance' genes have been identified. Molecular markers suitable for marker-assisted selection against HST sensitivity genes and for race non-specific resistance QTLs have been developed and used to generate adapted germplasm with good levels of tan spot resistance. Future research is needed to identify novel HSTs and corresponding host sensitivity genes, determine if the recessively inherited resistance genes are HST insensitivities, extend the current race classification system to account for new HSTs, and determine the molecular basis of race non-specific resistance QTLs and their relationships with host-HST interactions at the molecular level. Necrotrophic pathogens such as P. tritici-repentis are likely to become increasingly significant under a changing global climate making it imperative to further characterize the wheat-P. tritici-repentis pathosystem and develop tan spot resistant wheat varieties.

Host-Selective Toxins of Pyrenophora tritici-repentis Induce Common Responses Associated with Host Susceptibility

Pyrenophora tritici-repentis (Ptr), a necrotrophic fungus and the causal agent of tan spot of wheat, produces one or a combination of host-selective toxins (HSTs) necessary for disease development. The two most studied toxins produced by Ptr, Ptr ToxA (ToxA) and Ptr ToxB (ToxB), are proteins that cause necrotic or chlorotic symptoms respectively. Investigation of host responses induced by HSTs provides better insight into the nature of the host susceptibility. Microarray analysis of ToxA has provided evidence that it can elicit responses similar to those associated with defense. In order to evaluate whether there are consistent host responses associated with susceptibility, a similar analysis of ToxB-induced changes in the same sensitive cultivar was conducted. Comparative analysis of ToxA- and ToxB-induced transcriptional changes showed that similar groups of genes encoding WRKY transcription factors, RLKs, PRs, components of the phenylpropanoid and jasmonic acid pathways are activated. ROS accumulation and photosystem dysfunction proved to be common mechanism-of-action for these toxins. Despite similarities in defense responses, transcriptional and biochemical responses as well as symptom development occur more rapidly for ToxA compared to ToxB, which could be explained by differences in perception as well as by differences in activation of a specific process, for example, ethylene biosynthesis in ToxA treatment. Results of this study suggest that perception of HSTs will result in activation of defense responses as part of a susceptible interaction and further supports the hypothesis that necrotrophic fungi exploit defense responses in order to induce cell death.

Tan spot susceptibility governed by the Tsn1 locus and race-nonspecific resistance quantitative trait loci in a population derived from the wheat lines Salamouni and Katepwa

Wheat–tan spot interactions are known to have an inverse gene-for-gene relationship where pathogen-produced necrotrophic effectors are recognized by host sensitivity genes to cause susceptibility. However, broad-spectrum race-nonspecific resistance quantitative trait loci (QTL) that do not conform to the inverse gene-for-gene model have also been identified in this system. Here, we evaluated a population of wheat recombinant inbred lines derived from Salamouni (resistant) and Katepwa (susceptible) for reaction to two isolates of race 1 (Pti2 and Asc1) and one isolate of race 2 (86–124), which all produce the necrotrophic effector Ptr ToxA, and the isolate AR LonB2, which does not produce Ptr ToxA and does not conform to the current race classification system. As expected, the Tsn1 locus was significantly associated with disease caused by all three ToxA-producing isolates and was not associated with tan spot caused by AR LonB2. However, the amount of variation explained by Tsn1 varied considerably, with values of 5, 22, and 30 % for Asc1, Pti2, and 86–124, respectively, suggesting possible variability in ToxA gene regulation among these isolates. A locus on chromosome arm 7DS was specifically associated with isolate AR LonB2 but explained only 8 % of the variation. Additional QTL on 5DL and 7BS were race-nonspecific and associated with tan spot caused by multiple isolates. These results provide further evidence that race-nonspecific resistance QTL play important roles in governing reaction to tan spot, and they suggest that the wheat–tan spot pathosystem is more complicated than previously thought. The elimination of necrotrophic effector sensitivity genes and the addition of race-nonspecific resistance loci are needed to develop wheat cultivars with high levels of tan spot resistance.

Evaluation of CIMMYT germplasm for resistance to leaf spotting diseases of wheat

The leaf spotting disease complex is a major biotic constrain in enhancing grain production in the major wheat growing regions. Two leaf spotting diseases, tan spot, caused by an ascomycete fungus Pyrenophora tritici-repentis, and Stagonospora nodorum blotch besides causing average yield losses of 5&amp;ndash;10%, cause significant losses in grain quality by red smudge, black point and grain shriveling. Conservation agriculture in combination with wheat monoculture involving cultivation of susceptible cultivars has resulted in frequent onset of leaf spots epidemics worldwide. Development of resistant wheat cultivars, in conjunction with crop rotation, will provide an effective, economical, and environmentally safe means of controlling leaf spot. International Maize and Wheat Improvement Center (CIMMYT), Mexico has initiated major efforts to mitigate the threat of tan spot. Efforts include screening of wheat germplasm, identification of new sources of resistance, characterization of new tan spot resistance genes through classical and molecular genetic analysis, incorporation of resistance into adapted cultivars, and assessing the variability in the tan spot fungus. Screening studies reveal that elite CIMMYT germplasm has high level of resistance to tan spot caused by P. tritici-repentis race 1. These germplasm have diverse genetic make-up and the resistance is likely broad based. Association mapping studies done with CIMMYT germplasm reconfirmed the presence of previously identified genomic regions for tan spot resistance; however, novel genomic regions on long arm of chromosomes 6A and 7B have also been identified. Studies done to date indicate that CIMMYT germplasm possess high level diverse genetic based resistance to tan spot of wheat. Efforts are in place to develop desired wheat cultivars with tan spot resistance. Virulence studies indicate presence of P. tritici-repentis race 1 only with some variability in level of toxin Ptr ToxA produced in each of the 76 isolates studied.

Simple sequence repeats and diversity of globally distributed populations of Pyrenophora tritici-repentis

Genetic diversity and relatedness in Pyrenophora tritici-repentis, the causal agent of tan spot disease of wheat, were examined in a geographically diverse collection of 80 isolates representing the eight races of the pathogen. Thirty-one simple sequence repeat markers were designed from the P. tritici-repentis genome which amplified 31 different loci in the isolates tested. Data were examined using: (i) Nei's analysis of genetic diversity and genetic distance, (ii) analysis of molecular variance (AMOVA), and (iii) clustering by the unweighted pair group method using arithmetic averages (UPGMA). A significant genetic differentiation (ΦPT = 0.153, P ≤ 0.001) was detected among populations, with isolates from different origins having similar levels of genetic diversity. Nei's analysis of genetic distance revealed that isolates of P. tritici-repentis can be grouped into four distinct populations based on their region of origin. The AMOVA showed that Canadian isolates have less than 9% genetic dissimilarity with isolates from the Caucasus region, while they have up to 32% genetic dissimilarity with isolates from the Fertile Crescent. Isolates were also compared by race, genetic distance and cluster analysis based on pooled allele frequency. Ptr ToxA non-producing isolates clustered together but were distantly related to Ptr ToxA-producing isolates, suggesting that the host-specificity imposed by the different Ptr toxins could lead to differentiation among isolates of P. tritici-repentis.