Inoculum Sources of Puccinia striiformis f. sp. tritici for Stripe Rust Epidemics on the Eastern Coast of China.

Stripe rust, a fungal disease caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most destructive diseases affecting wheat production areas worldwide. In recent years in China, wheat stripe rust has caused huge yield losses throughout the vast Huang-Huai-Hai region, including the eastern coast regions, especially Shandong province. The aim of the present study was to explore the population structure and potential inoculum sources of the pathogen in this region. A total of 234 Pst isolates in 2021 were collected and isolated from seven provinces and identified for virulence phenotypes using 19 Chinese differentials and for genotypes using 17 single-nucleotide polymorphism-based Kompetitive allele-specific PCR markers. The virulence phenotype tests identified predominant races CYR34 (18.0%) and CYR32 (16.0%) in Shandong, which were similar to the results in Henan province, also with the predominant races CYR34 (21.9%) and CYR32 (18.8%). Based on the virulence data of phenotyping, the Pst populations in Shandong, Hubei, and Henan were similar. The genotypic analysis revealed remarkable gene flows among the Shandong, Hubei, Henan, Yunnan, and Guizhou populations, showing a migration of Pst from the southwestern oversummering regions to Shandong through the winter spore production regions. Genetic structure analysis also indicated an additional migration route from the northwestern oversummering regions through winter spore production regions to Shandong. The results are useful for understanding stripe rust epidemiology in the eastern coast region and improving control of the disease throughout the country.

Virulence Characterization of Puccinia striiformis f. sp. tritici in China Using the Chinese and Yr Single-Gene Differentials.

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most important diseases of wheat. Identifying Pst races is essential for developing resistant cultivars and managing the disease. In this study, 608 isolates collected from China in 2021 were tested with the Chinese set of 19 wheat variety differentials and the set of 18 Yr single-gene differentials. Of the 119 races detected with the Chinese set of differentials, 94 were new. A higher number (149) of races were identified using the Yr single-gene differentials. The frequencies of virulence factors to 17 of the 19 Chinese differential varieties and to 10 of the 18 Yr single-gene differentials were high (>60%). None of the isolates were virulent to the differentials Zhong 4 (Yr genes unknown) and Triticum spelta Album (Yr5) in the Chinese set and the Yr5 and Yr15 lines in the single-gene set of differentials, indicating that these genes or varieties are effective against the Pst population detected in 2021. Using Nei's genetic distance, the 16 provincial Pst populations were clustered into six groups based on the Chinese set and eight groups based on the Yr single-gene set of differentials. In addition, we found that the same races identified using the Chinese differentials could be further differentiated into different races using the Yr single-gene differentials, suggesting a higher differential capability than the Chinese set of differentials. The results provide a scientific basis for monitoring Pst populations and guiding resistance breeding in China.

Evaluation of Stripe Rust Resistance and Chip Detection Resistance Genes in 286 Xinjiang Wheat Cultivars and Breeding Lines.

Wheat stripe rust is a destructive disease worldwide, caused by Puccinia striiformis f. sp. tritici (Pst). Resistance breeding is the most effective method of controlling stripe rust. Xinjiang is a relatively independent epidemic region of wheat stripe rust in China. In recent years, wheat stripe rust in this area has shown an upward trend. Therefore, the purpose of this study was to evaluate the resistance level of wheat cultivars (lines) to the prevalent Pst races and determine the genetic background of stripe rust resistance genes in Xinjiang. Six predominant Pst races in China were used to study resistance of 286 wheat cultivars (lines) at both seedling under controlled conditions and adult-plant stages under field conditions. In the seedling tests, 175 (61.19%) entries were resistant to races CYR23, 125 (43.71%) to CYR29, 153 (53.50%) to CYR31, 88 (30.77%) to CYR32, 174 (60.84%) to CYR33, and 98 (34.27%) to CYR34. Among the resistant entries, 23 (8.04%) were resistant to all six races. In the field test, 135 (47.20%) entries were resistant to the tested mixed races. Through comparing the responses in the seedling and adult-plant stages, 109 (38.11%) entries were found to have adult-plant resistance (APR), and 14 (4.90%) entries have all-stage resistance (ASR). The 286 wheat entries were also tested using a wheat breeder chip containing 12 Yr resistance loci. Among these entries, 44 (15.38%) were found to have single gene, 221 (77.27%) have two or more genes, and 21 (7.34%) have none of the 12 genes, including 144 (50.35%) with Yr30 and 5 (1.75%) with YrSP. Entries with two or more genes have stronger resistance to Pst. Overall, the majority of entries have all-stage and/or adult-plant resistance, but their genes for resistance in addition to the 12 tested Yr genes need to be determined. It is also necessary to introduce more effective resistance genes in the breeding programs to improve stripe rust resistance in wheat cultivars in Xinjiang.

Simultaneous editing of three homoeologues of TaCIPK14 confers broad-spectrum resistance to stripe rust in wheat.

Wheat stripe rust caused by the fungus Puccinia striiformis f. sp. tritici (Pst) is one of the most destructive wheat diseases resulting in significant losses to wheat production worldwide. The development of disease-resistant varieties is the most economical and effective measure to control diseases. Altering the susceptibility genes that promote pathogen compatibility via CRISPR/Cas9-mediated gene editing technology has become a new strategy for developing disease-resistant wheat varieties. Calcineurin B-like protein (CBL)-interacting protein kinases (CIPKs) has been demonstrated to be involved in defence responses during plant-pathogen interactions. However, whether wheat CIPK functions as susceptibility factor is still unclear. Here, we isolated a CIPK homoeologue gene TaCIPK14 from wheat. Knockdown of TaCIPK14 significantly increased wheat resistance to Pst, whereas overexpression of TaCIPK14 resulted in enhanced wheat susceptibility to Pst by decreasing different aspects of the defence response, including accumulation of ROS and expression of pathogenesis-relative genes. We generated wheat Tacipk14 mutant plants by simultaneous modification of the three homoeologues of wheat TaCIPK14 via CRISPR/Cas9 technology. The Tacipk14 mutant lines expressed race-nonspecific (RNS) broad-spectrum resistance (BSR) to Pst. Moreover, no significant difference was found in agronomic yield traits between Tacipk14 mutant plants and Fielder control plants under greenhouse and field conditions. These results demonstrate that TaCIPK14 acts as an important susceptibility factor in wheat response to Pst, and knockout of TaCIPK14 represents a powerful strategy for generating new disease-resistant wheat varieties with BSR to Pst.

Sensitivity of Puccinia triticina f. sp. tritici from China to Triadimefon and Resistance Risk Assessment.

Wheat leaf rust, caused by Puccinia triticina f. sp. tritici (Pt), is distributed widely in wheat-producing areas and results in serious yield losses worldwide. In China, leaf rust has been largely controlled with a demethylation inhibitor (DMI) fungicide, triadimefon. Although high levels of fungicide resistance in pathogens have been reported, no field failure of wheat leaf rust to DMI fungicides has been reported in China. A resistance risk assessment of triadimefon to Pt was investigated in the present study. The sensitivity of 197 Pt isolates across the country to triadimefon was determined, and the density distribution of EC50 values (concentration at which mycelial growth is inhibited by 50%) showed a continuous multimodal curve because of the extensive use of this fungicide in wheat production, with a mean value of 0.46 µg/ml. The majority of the tested Pt isolates were sensitive to triadimefon, whereas 10.2% developed varying degrees of resistance. Characterization of parasitic fitness revealed that the triadimefon-resistant isolates exhibited strong adaptive traits in urediniospore germination rate, latent period, sporulation intensity, and lesion expansion rate. No correlation was observed between triadimefon and tebuconazole and hexaconazole, which have the similar mode of action, or pyraclostrobin and flubeneteram, which have different modes of action. Overexpression of the target gene Cyp51 led to the triadimefon resistance of Pt. The risk of resistance to triadimefon in Pt may be low to moderate. This study provided important data for fungicide resistance risk management against wheat leaf rust.

Transcription factor BZR2 activates chitinase Cht20.2 transcription to confer resistance to wheat stripe rust.

The brassinosteroid pathway promotes a variety of physiological processes in plants and the brassinosteroid insensitive1-ethylmethane sulfonate suppressor (BES)/brassinazole-resistant (BZR) functions as one of its key regulators. We previously showed that the BES/BZR-type transcription factor TaBZR2 mediates the drought stress response in wheat (Triticum aestivum) by directly upregulating the transcriptional activity of glutathione S-transferase 1. However, the function of TaBZR2 in plants under biotic stresses is unknown. In this study, we found that transcript levels of TaBZR2 were upregulated in response to inoculation with wheat stripe rust fungus (Puccinia striiformis f. sp. tritici, Pst) and treatment with flg22 or an elicitor-like protein of Pst, Pst322. Wheat lines overexpressing TaBZR2 conferred increased resistance, whereas TaBZR2-RNAi lines exhibited decreased resistance to multiple races of Pst. TaBZR2 targeted the promoter of the chitinase gene TaCht20.2, activating its transcription. Knockdown of TaCht20.2 in wheat resulted in enhanced susceptibility to Pst, indicating the positive role of TaCht20.2 in wheat resistance. Upon Pst infection in vivo, the overexpression of TaBZR2 increased total chitinase activity, whereas RNAi-mediated silencing of TaBZR2 reduced total chitinase activity. Taken together, our results suggest that TaBZR2 confers broad-spectrum resistance to the stripe rust fungus by increasing total chitinase activity in wheat.

Identification of Mahonia Species as Alternate Hosts for Puccinia striiformis f. sp. tritici and Determination of Existence of Sexual Propagation of the Rust Pathogen on Mahonia Under Natural Conditions in China.

Many Berberis species have been identified as alternate hosts for Puccinia striiformis f. sp. tritici. Importantly, susceptible Berberis species are determined to play an important role in the occurrence of sexual reproduction, generation of new races of the rust pathogen. However, little is known about Mahonia serving as alternate hosts for P. striiformis f. sp. tritici and their role to commence sexual reproduction of the rust fungus under natural conditions. Herein, three Mahonia species or subspecies, Mahonia fortunei, M. eurybracteata subsp. ganpinensis, and M. sheridaniana, were identified as alternate hosts for P. striiformis f. sp. tritici, and seven Mahonia species were highly resistant to the rust pathogen. We recovered seven samples of P. striiformis f. sp. tritici from naturally rusted Mahonia cardiophylla plants. Totally, 54 single uredinium (SU) isolates, derived from the seven samples, generated 20 different race types, including one known race type, and 19 new race types. SNP markers analysis showed that all SU isolates displayed high phenotype diversity (H = 0.32) with a high Shannon's information index (I = 0.49). Analysis of linkage disequilibrium indicated an insignificant rbarD value (rbarD = 0.003, P < 0.1). As a result, all SU isolates are sexually produced, suggesting that P. striiformis f. sp. tritici parasitizes susceptible Mahonia to complete sexual reproduction under natural conditions. The role of Mahonia in occurrence of wheat stripe rust are needed to study for management of the disease.

Role of Sexual Reproduction in the Evolution of the Wheat Stripe Rust Fungus Races in China.

Experimental and population genetic approaches have reshaped our view of how fungal pathogens reproduce, with consequences for our understanding of fungal invasions. Puccinia striiformis f. sp. tritici, the causal agent of stripe rust, poses a severe threat to wheat production worldwide. The sexual stage of P. striiformis f. sp. tritici was discovered >10 years ago, but how it affects the evolution of the pathogen, especially the emergence of the new virulent races, remains largely unknown. Here, using population genetic analyses, we demonstrate that sexual reproduction plays an important role in the evolution of P. striiformis f. sp. tritici races in China, specifically the newly emerged and devastating race virulent to resistance gene Yr26, which is widely used in China and exerts strong selective pressure on the pathogen population. Association analysis identified six genes encoding secreted proteins as candidates for virulence on wheat cultivars carrying the Yr26 resistance gene. Our results highlight the important role of sexual reproduction and selection exerted by hosts in the emergence of new virulent races in China.

Populations of Puccinia striiformis f. sp. tritici in Winter Spore Production Regions Spread from Southwestern Oversummering Areas in China.

Stripe rust, caused by Puccinia striifomis f. sp. tritici (Pst), is one of the most destructive wheat diseases in China. Understanding the interregional dispersal of Pst inoculum is important for controlling the disease. In the present study, wheat stripe rust samples collected from the winter spore production and oversummering regions in November 2018 to March 2019 were studied through virulence testing and molecular characterization. From 296 isolates, 96 races were identified using a set of 19 Chinese wheat cultivars and 111 races were identified using 18 Yr single-gene lines as differentials. The isolates from Hubei province in the winter spore production area had the highest similarity in virulence with those from eastern Yunnan in the oversummering area. Molecular characterization using 13 simple-sequence repeat and 43 Kompetitive allele specific PCR-single nucleotide polymorphism markers supported the conclusion that the Pst populations in the winter spore production regions were from Guizhou and eastern Yunnan, key oversummering areas in the southwest. Furthermore, an analysis of wind movement at the 700-hPa high altitude also supported the conclusion of spore dispersal from the southwestern oversummering region to the south-central winter spore production region. The results of this study provide an epidemiological basis for deploying various effective resistance genes in different regions to control stripe rust.

Sensitivity and Resistance Risk Assessment of Puccinia striiformis f. sp. tritici to Triadimefon in China.

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a destructive disease of wheat that seriously threatens production safety in wheat-producing areas worldwide. In China, the disease has been largely controlled with the fungicide triadimefon. Although high levels of fungicide resistance in other fungal pathogens have been reported, failure to control Pst with any fungicides has seldomly been reported, and fungicide sensitivity of Pst has not been evaluated in China. The distribution of triadimefon-resistant Pst isolates was investigated in the present study. The baseline sensitivity of 446 Pst isolates across the country to triadimefon was determined, and the concentration for 50% of maximal effect showed a unimodal distribution curve, with a mean value of 0.19 µg ml-1. The results indicated a wide range of sensitivity to triadimefon, with more insensitive isolates collected from Pst winter-increasing areas and northwest oversummering areas, whereas more sensitive isolates were collected from southwest oversummering areas and epidemic areas of Xinjiang and Tibet. The majority of the tested Pst isolates were sensitive to triadimefon; only 6.79% had developed varying degrees of resistance. Characterization of parasitic fitness revealed that the triadimefon-resistant isolates exhibited strong adaptive traits in the urediniospore germination rate, latent period, sporulation intensity, and lesion expansion rate. Positive cross-resistance was observed between triadimefon and tebuconazole or hexaconazole, but not between pyraclostrobin or flubeneteram. The point mutation Y134F in the 14α-demethylase enzyme (CYP51) was detected in triadimefon-resistant isolates. A molecular method (kompetitive allele-specific PCR) was established for the rapid detection of Y134F mutants in the Pst population. Two genotypes with one point mutation Y134F conferred resistance to triadimefon in Pst. The risk of resistance to triadimefon in Pst may be low to moderate. This study provided important data for establishment of high throughput molecular detection methods, fungicide resistance risk management, and the development of new target fungicides.

Distinct Transcriptomic Reprogramming in the Wheat Stripe Rust Fungus During the Initial Infection of Wheat and Barberry.

Puccinia striiformis f. sp. tritici is the causal agent of wheat stripe rust that causes severe yield losses all over the world. As a macrocyclic heteroecious rust fungus, it is able to infect two unrelated host plants, wheat and barberry. Its urediniospores infect wheat and cause disease epidemic, while its basidiospores parasitize barberry to fulfill the sexual reproduction. This complex life cycle poses interesting questions on the different mechanisms of pathogenesis underlying the infection of the two different hosts. In the present study, transcriptomes of P. striiformis f. sp. tritici during the initial infection of wheat and barberry leaves were qualitatively and quantitatively compared. As a result, 142 wheat-specifically expressed genes (WEGs) were identified, which was far less than the 2,677 barberry-specifically expressed genes (BEGs). A larger proportion of evolutionarily conserved genes were observed in BEGs than that in WEGs, implying a longer history of the interaction between P. striiformis f. sp. tritici and barberry. Additionally, P. striiformis f. sp. tritici differentially expressed genes (DEGs) between wheat at 1 and 2 days postinoculation (dpi) and barberry at 3 and 4 dpi were identified by quantitative analysis. Gene Ontology analysis of these DEGs and expression patterns of P. striiformis f. sp. tritici pathogenic genes, including those encoding candidate secreted effectors, cell wall-degrading enzymes, and nutrient transporters, demonstrated that urediniospores and basidiospores exploited distinct strategies to overcome host defense systems. These results represent the first analysis of the P. striiformis f. sp. tritici transcriptome in barberry and contribute to a better understanding of the evolutionary processes and strategies of different types of rust spores during the infection process on different hosts.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Identification of Berberis Species Collected from the Himalayan Region of Pakistan Susceptible to Puccinia striiformis f. sp. tritici.

Puccinia striiformis f. sp. tritici (Pst), the stripe rust pathogen infecting cereal crops and grasses, was believed to have a hemicyclic life cycle consisting of uredinial and telial stages before the recent discovery of barberry (Berberis spp.) as an alternate (aecial) host for the fungus. This discovery has improved the understanding of the biology of the stripe rust pathogen. The Himalayan and near-Himalayan regions of Pakistan, China, and Nepal are considered as the center of diversity for Pst pathogen. High genetic diversity has been reported in these areas, probably resulting from the sexual reproduction of the stripe rust fungus. To determine if Berberis species growing in Pakistan are susceptible to Pst, we collected seeds of five species and two subspecies from the Himalayan region in 2016 and inoculated the seedlings with germinated teliospores of a Pakistani Pst isolate under controlled conditions. Pycnia and aecia were produced on all inoculated plants of these species and subspecies, and were demonstrated as Pst by successful infection of wheat plants with aeciospores. This study showed that the tested Pakistani Berberis species and subspecies are susceptible to Pst under controlled conditions.

Inheritance of Virulence and Linkages of Virulence Genes in an Ethiopian Isolate of the Wheat Stripe Rust Pathogen (Puccinia striiformis f. sp. tritici) Determined Through Sexual Recombination on Berberis holstii.

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most devastating wheat diseases in Ethiopia. To study virulence genetics of the pathogen, 117 progeny isolates were produced through sexual reproduction of an Ethiopian isolate of the stripe rust pathogen on Berberis holstii plants under controlled conditions. The parental and progeny isolates were characterized by phenotyping on wheat lines carrying single Yr genes for resistance and genotyped using 10 polymorphic simple sequence repeated (SSR) markers. The progeny isolates were classified into 37 virulence phenotypes and 75 multilocus genotypes. The parental isolate and progeny isolates were all avirulent to resistance genes Yr5, Yr10, Yr15, Yr24, Yr32, YrTr1, YrSP, and Yr76 but virulent to Yr1 and Yr2, indicating that the parental isolate was homozygous avirulent or homozygous virulent at these loci. The progeny isolates segregated for virulence to 12 Yr genes. Virulence phenotypes to Yr6, Yr28, Yr43, and Yr44 were controlled by a single dominant gene; those to Yr7, Yr9, Yr17, Yr27, Yr25, Yr31, and YrExp2 were each controlled by two dominant genes; and the virulence phenotype to Yr8 was controlled by two complementary dominant genes. A linkage map was constructed with seven SSR markers, and 16 virulence loci corresponding to 11 Yr resistance genes were mapped with some loci linked to each other. These results are useful in understanding host-pathogen interactions and selecting resistance genes to develop wheat cultivars with highly effective resistance to stripe rust.

TaMDAR6 acts as a negative regulator of plant cell death and participates indirectly in stomatal regulation during the wheat stripe rust-fungus interaction.

We identified a new monodehydroascorbate reductase (MDAR) gene from wheat, designated TaMDAR6, which is differentially affected by wheat-Puccinia striiformis f. sp. tritici (Pst) interactions. TaMDAR6 is a negative regulator of plant cell death (PCD) triggered by the Bax gene and Pst. Transcript levels of TaMDAR6 are significantly upregulated during a compatible wheat-Pst interaction, indicating that TaMDAR6 may contribute to plant susceptibility. In addition, H2 O2 production and PCD are significantly induced and initial pathogen development is significantly reduced in the TaMDAR6 knocked-down plants upon Pst infection. Thus, the suppression of TaMDAR6 enhances wheat resistance to Pst. Besides, the suppression of TaMDAR6 during an incompatible interaction induces a change in the morphology of stomata, which leads to poor stoma recognition and as a consequence to reduced infection efficiency. The percentage of infection sites that develop substomatal vesicles decreases in the TaMDAR6 knocked-down plants during the incompatible interaction presumably due to the increase in ROS accumulation, which is likely to activate other resistance mechanisms that have a negative effect on substomatal vesicle formation. TaMDAR6 can therefore be considered a negative regulator of PCD and of wheat defense to Pst.

Virulence and Simple Sequence Repeat Marker Segregation in a Puccinia striiformis f. sp. tritici Population Produced by Selfing a Chinese Isolate on Berberis shensiana.

Puccinia striiformis f. sp. tritici, the causal agent of wheat stripe rust, frequently produces new races overcoming resistance in wheat cultivars. A recently identified race, V26 with virulence to Yr26 and many other stripe rust resistance genes, has a high potential to cause epidemics in China. In this study, teliospores from a single-urediniospore isolate of V26 (Pinglan 17-7) produced on the wheat line 92R137 (Yr26) were used to produce a sexual population through selfing by infecting Berberis shensiana plants under controlled conditions. One hundred and eighteen progeny isolates and the parental isolate were phenotyped for virulence/avirulence on 24 Yr gene lines of wheat. These progeny isolates were all avirulent to Yr5, Yr8, Yr15, and YrTr1 and virulent to Yr1, Yr2, Yr7, Yr9, Yr10, Yr17, Yr24, Yr25, Yr26, YrA, YrExp2, and YrV23, indicating that the parental isolate is homozygous avirulent or homozygous virulent at these loci. The progeny population segregated for avirulence to Yr6, Yr43, and YrSP at one locus (3 avirulent:1 virulent ratio); for virulence to Yr27 and Yr28 at one locus (3 virulent:1 avirulent); and for Yr4, Yr32, and Yr44 at two loci (15 virulent:1 avirulent). Among the eight segregating avirulence/virulence loci, association was found between virulence to Yr4 and Yr32, as well as between virulence to Yr6 and Yr43 based on χ(2) tests. From 82 genotypically different progeny isolates, 24 pathotypes and 82 multilocus genotypes were identified. The results show that a highly diverse population can be produced from a single isolate by selfing on a barberry plant and sexually produced population can be used to genetically characterize virulence of the stripe rust pathogen.

Virulence and Molecular Diversity of the Puccinia striiformis f. sp. tritici Population in Xinjiang in Relation to Other Regions of Western China.

In recent years, wheat stripe rust caused severe yield losses in western China, especially the Xinjiang Autonomous Region. The population of the stripe rust fungus Puccinia striiformis f. sp. tritici in the vast region had not been well studied. To determine the population structure and compare it with the populations in the neighboring provinces or autonomous regions, P. striiformis f. sp. tritici isolates from Xinjiang, Qinghai, Gansu, Ningxia, and Tibet in western China were characterized by virulence tests with 19 wheat genotypes that are used to differentiate races of P. striiformis f. sp. tritici in China and by genotyping tests with 15 simple-sequence repeat (SSR) markers. In total, 56 races, including 39 previously known and 17 new races, were identified from 308 isolates obtained from the three epidemiological regions covering five provinces, of which 27 previously known and 8 unknown races were detected in Xinjiang, higher than the numbers in either of the other two regions. The races in Xinjiang consisted of those historically and recently predominant races in other regions of China. The P. striiformis f. sp. tritici population in Xinjiang had a higher genetic diversity than populations in other epidemiological regions. Molecular variation among subpopulations within Xinjiang was higher than in other regions. Both virulence and molecular data indicate that the P. striiformis f. sp. tritici population in Xinjiang is related to but more diverse than those in other epidemiological regions. The results show that Xinjiang is an important stripe rust epidemiological region in China, and the information should be useful for control of the disease in the region as well as in other regions.

Different non-host resistance responses of two rice subspecies, japonica and indica, to Puccinia striiformis f. sp. tritici.

KEY MESSAGE: Japonica and indica have different non-host resistance (NHR) abilities to Puccinia striiformis f. sp. tritici ( Pst ), and hydrogen peroxide (H 2 O 2 ) has a positive function in NHR to japonica against Pst. ABSTRACT: Non-host interactions between Puccinia striiformis f. sp. tritici (Pst) and two rice subspecies were characterized using 23 rice varieties, including 11 japonica and 12 indica. Results showed that the infected fungal structures were easily produced in the leaves of indica, whereas only several substomatal vesicles and primary infection hyphae were observed in the leaves of japonica. This result indicated that indica is less resistant or more susceptible to Pst than japonica. Hydrogen peroxide accumulated in the initial phase of japonica-Pst interaction but not in indica-Pst interaction. A set of reactive oxygen species (ROS)-related genes was also induced in response to Pst infection, suggesting that ROS activation is one of the major mechanisms of non-host resistance of rice to Pst.

Identification of eighteen Berberis species as alternate hosts of Puccinia striiformis f. sp. tritici and virulence variation in the pathogen isolates from natural infection of barberry plants in China.

ABSTRACT The wheat stripe rust pathogen (Puccinia striiformis f. sp. tritici) population in China has been reported to be a distinct genetic group with higher diversity than those in many other countries. Genetic recombination in the P. striiformis f. sp. tritici population has been identified with molecular markers but whether sexual reproduction occurs in China is unknown. In this study, we surveyed barberry plants for infection by rust fungi in the stripe rust "hotspot" regions in Gansu, Sichuan, and Shaanxi provinces; collected barberry plants and inoculated plants of 20 Berberis spp. with germinated teliospores under controlled greenhouse conditions for susceptibility to P. striiformis f. sp. tritici; and tested P. striiformis f. sp. tritici isolates obtained from aecia on naturally infected barberry plants on the wheat genotypes used to differentiate Chinese P. striiformis f. sp. tritici races to determine virulence variations. Different Berberis spp. were widely distributed and most surveyed plants had pycnia and aecia of rust fungi throughout the surveyed regions. In total, 28 Berberis spp. were identified during our study. From 20 Berberis spp. tested with teliospores of P. striiformis f. sp. tritici from wheat plants, 18 species were susceptible under greenhouse conditions. Among 3,703 aecia sampled from barberry plants of three species (Berberis shensiana, B. brachypoda, and B. soulieana) under natural infections in Gansu and Shaanxi provinces, four produced P. striiformis f. sp. tritici uredinia on susceptible wheat 'Mingxian 169'. Sequence of the internal transcribed spacer (ITS) regions of the four isolates from barberry shared 99% identity with the P. striiformis f. sp. tritici sequences in the National Center for Biotechnology Information database. The four isolates had virulence patterns different from all previously reported races collected from wheat plants. Furthermore, 82 single-uredinium isolates obtained from the four barberry isolates had high virulence diversity rates of 9.0 to 28.1%, indicating that the diverse isolates were produced through sexual reproduction on barberry plants under natural conditions. In addition to P. striiformis f. sp. tritici, sequence analysis of polymerase chain reaction products of the ITS regions and inoculation tests on wheat identified P. graminis (the stem rust pathogen). Our results indicated that P. striiformis f. sp. tritici can infect some Berberis spp. under natural conditions, and the sexual cycle of the fungus may contribute to the diversity of P. striiformis f. sp. tritici in China.

Population genetic diversity of Puccinia striiformis f. sp. tritici on different wheat varieties in Tianshui, Gansu Province.

Population genetic diversity in Tianshui city was analyzed with SSR markers in 605 single-pustule isolates of the stripe rust pathogen, Puccinia striiformis f. sp. tritici (Pst), obtained from 19 varieties of wheat. Significant differences in genetic diversity among populations were defected. Genetic diversity was highest in population on Tian 863-13, a highly resistant variety, whereas genetic diversity was lowest in population on Huixianhong, a highly susceptible variety. Seven populations from seven varieties that carried the common Yr18 resistance gene were clustered as one sub-group at 0.88 similarity coefficient, which showed that resistance gene selection had close relation with pathogen's component. The results of present study can provide a theoretical basis for integrated management of wheat stripe rust and effective deployment of resistance genes in Pst over-summering zones in China.

High-throughput RNA sequencing reveals differences between the transcriptomes of the five spore forms of Puccinia striiformis f. sp. tritici, the wheat stripe rust pathogen.

The devastating wheat stripe (yellow) rust pathogen, Puccinia striiformis f. sp. tritici (Pst), is a macrocyclic and heteroecious fungus. Pst produces urediniospores and teliospores on its primary host, wheat, and pycniospores and aeciospores are produced on its alternate hosts, barberry (Berberis spp.) or mahonia (Mahonia spp.). Basidiospores are developed from teliospores and infect alternate hosts. These five spore forms play distinct roles in Pst infection, disease development, and fungal survival, etc. However, the specific genes and mechanisms underlying these functional differences are largely unknown. In this study, we performed, for the first time in rust fungi, the deep RNA sequencing to examine the transcriptomic shift among all five Pst spore forms. Among a total of 29,591 identified transcripts, 951 were specifically expressed in basidiospores, whereas 920, 761, 266, and 110 were specific for teliospores, pycniospores, aeciospores, and urediniospores, respectively. Additionally, transcriptomes of sexual spores, namely pycniospores and basidiospores, showed significant differences from those of asexual spores (urediniospores, teliospores, and aeciospores), and transcriptomes of urediniospores and aeciospores were more similar to each other than to the three other spore forms. Especially, the basidiospores and pycniospores which infected the berberis shows wide differences in the cell wall degrading-enzymes and mating and pheromone response genes. Besides, we also found that there are 6234 differential expressed genes between the urediniospores and pycniospores, while only have 3 genes have alternative splicing enents, suggesting that differential genes expression may make more contribution than AS. This comprehensive transcriptome profiling can substantially improve our understanding of the developmental biology of the wheat stripe rust fungus.