Influence of Nitrogen Supply on the Susceptibility of Wheat to Septoria tritici

Nitrogen (N) fertilization is required for achieving high yields in wheat (Triticum aestivum L.) but may enhance the development of Septoria tritici Rob. ex Desm. A study was undertaken to investigate how N supply influences the area under disease progress curve (AUDPC). Two field experiments were carried out in a typical argiudol in 1996 and 1997. Six cultivars were grown at two N treatments (0 and 100–150 kg N/ha) in a split plot design. Percentage of necrosis (severity) of the two upper leaves of each treatment was evaluated three times during the growing season. Height and heading date were also scored and related to the resistance level. In 1996, with weather conditions conducive to the disease, AUDPC values were higher in the fertilized treatment. In 1997, with insufficient rain immediately after inoculation, the disease only progressed faster under N fertilization in the flag leaf, which was exposed to conducive environmental conditions from its appearance. The effect of N fertilization was influenced by the cultivar characteristics, climatic and agronomic conditions. Knowledge that N fertilization promotes the development of S. tritici in conducive conditions will be useful for deciding management strategies of the cultivars and for optimizing conditions for the selection in breeding programmes.     

Virulence Spectrum of Mycosphaerella graminicola Isolates on Wheat Genotypes Carrying Known Resistance Genes to Septoria tritici Blotch

The virulence spectrum of 23 monopycnidiospore isolates of Mycosphaerella graminicola was determined using wheat genotypes that carried different resistance genes (Stb1–Stb8 and Stb15). Disease severity was measured as the percentage of necrotic leaf area. The isolates used in the experiments were of diverse origin: eight from Poland, seven from Germany, and eight from other countries around the world. Analysis of variance revealed significant differences in the virulence of the isolates. Using multiple regression and Cook’s D statistic, 26 significant cultivar × isolate interactions were detected. The Israeli isolate IPO86036 showed the widest spectrum of specific reactions. It expressed specific virulence on at least four cultivars and specific avirulence on at least three. The other isolates showed specific interactions with 1–6 different cultivars. Despite the limited number of isolates that were tested, we recommend that a number of resistant lines, namely cultivars Veranopolis (Stb2), Cs/Synthetic 7D (Stb5), Arina (Stb15, Stb6 and partial resistance), and Liwilla (unknown resistance factors), could be incorporated into central European wheat breeding programmes that are aimed at developing resistance against septoria tritici blotch. In contrast, resistance gene Stb7, which is carried by cultivar Estanzuela Federal, was ineffective against most of the isolates that were used. These results on the virulence spectrum of M. graminicola isolates provide valuable information for effective wheat breeding programmes to develop resistance to the pathogen.     

Stability of Adult‐plant Resistance to Septoria tritici blotch in 24 European Winter Wheat Varieties Across Nine Field Environments

Septoria tritici blotch (STB) is one of the most important leaf diseases in wheat worldwide. Objectives of this study were (i) to compare inoculation and natural infection; (ii) to evaluate the level of adult‐plant resistance to STB using four isolates; and (iii) to analyse environmental stability of 24 winter wheat (Triticum aestivum L.) varieties in inoculated vs. non‐inoculated field trials across 3 years including nine environments (location × year combinations). Field trials were sown in split‐plot design inoculated with four aggressive isolates of S. tritici plus one non‐inoculated variant as main factor and 24 wheat varieties as subfactor. Septoria tritici blotch severity was visually scored as percentage flag leaves covered with lesions bearing pycnidia. Overall STB rating ranged from 8% (Solitär) to 63% (Rubens) flag leaf area affected, resulting in significant (P < 0.01) genotypic variance. Variance of genotype × environment interaction amounted to approximately 50% of the genotypic variance. Genotype × isolate interaction variance was significant too (P < 0.01) but of minor importance. Therefore, environmental stability of varieties should be a major breeding goal. The varieties Solitär, History and Florett were most resistant and stable as revealed by a regression approach, and the susceptible varieties were generally unstable. Hence, STB resistance and stability are correlated (P < 0.01), but there were some exceptions (Tuareg, Ambition). Promising candidates for an environmentally stable, effective adult‐plant resistance have been identified.     

Role of hydrogen peroxide during the interaction between the hemibiotrophic fungal pathogen Septoria tritici and wheat

Hydrogen peroxide (H(2)O(2)) is reported to inhibit biotrophic but benefit necrotrophic pathogens. Infection by necrotrophs can result in a massive accumulation of H(2)O(2) in hosts. Little is known of how pathogens with both growth types are affected (hemibiotrophs). The hemibiotroph, Septoria tritici, infecting wheat (Triticum aestivum) is inhibited by H(2)O(2) during the biotrophic phase, but a large H(2)O(2) accumulation occurs in the host during reproduction. Here, we infiltrated catalase, H(2)O(2) or water into wheat during the biotrophic or the necrotrophic phase of S. tritici and studied the effect of infection on host physiology to get an understanding of the survival strategy of the pathogen. H(2)O(2) removal by catalase at both early and late stages made plants more susceptible, whereas H(2)O(2) made them more resistant. H(2)O(2) is harmful to S. tritici throughout its life cycle, but it can be tolerated. The late accumulation of H(2)O(2) is unlikely to result from down-regulation of photosynthesis, but probably originates from damage to the peroxisomes during the general tissue collapse, which is accompanied by release of soluble sugars in a susceptible cultivar.     

Histopathology of Leaf Rust Infection and Development in Wheat Genotypes containing Lr12 and Lr13

Evidence exists that certain genes for resistance to leaf rust in wheat, e.g. Lr13 and Lr34 , may interact with other genes to condition higher levels of resistance than that conferred by each gene individually. In this study, the hypothesis that Lr12 and Lr13 , both genes for adult plant resistance to Puccinia recondita Roberge ex. Desmaz f. sp. tritici Eriks. and Henn., interact to confer an improved level of resistance, was investigated using fluorescence and phase-contrast microscopy. Flag leaf segments of monogenic and digenic Thatcher lines, sampled 64 and 240 h post-inoculation, were stained with Uvitex 2B and screened, using fluorescence microscopy, for development of infection structures or host response. To study cell wall appositions, specimens were stained with trypan blue and a solution of picric acid in methyl salicylate. Aborted penetration, consisting of nonpenetrating appressoria and aborted substomatal vesicles, showed that inhibition of fungal growth in wheat lines containing Lr12 and/or Lr13 was activated, to a certain degree, before haustoria were formed. At 240 h after inoculation colony size indicated that fungal colonies in the Lr gene combination lines were generally smaller than in the parents, but not necessarily smaller than those in a line with Lr13 only. Host cell necrosis was more frequently associated with infection sites, specifically of pathotype UVPrt2, in the combination lines than in the parents. The morphology of cell wall appositions varied considerably from a narrow, luminous zone slightly wider in the centre, to a thick central part opposite the haustorium mother cell, sharply decreasing towards both ends. Histological assessments could, however, not conclusively prove pronounced resistance enhancement or unconventional resistance mechanisms due to combining the genes Lr12 and Lr13 .     

Trichoderma spp. as elicitors of wheat plant defense responses against Septoria tritici

Leaf blotch of wheat is a widespread and highly active disease that affects wheat production. In addition to the use of chemicals and proper cultivation methods, microbial antagonists are used to control plant pathogens. Trichoderma spp. stimulate a systemic induced response in plants. Therefore, the efficacy of Trichoderma spp. against wheat leaf blotch was evaluated under greenhouse conditions. The susceptible plants were sprayed with Septoria tritici conidiospores. In order to select an efficient method of pretreatment with Trichoderma spp., leaf spraying and seed coating with 14 isolates were tested in 2003 and 2004. The extent of leaf necrosis area and pycnidial coverage was estimated. Antagonism was assessed by the capacity of each Trichoderma spp. isolate to restrict the progress of leaf blotch, 21 days after inoculation. Of the two methods, seed coating was more efficacious against leaf blotch than leaf spraying. Amongst the 14 isolates tested, the isolate prepared from T. harzianum (Th5) produced the highest level of protection. None of the treatments caused changes in plant stem diameter or dry weight. Trichoderma spp. did not get into leaves while S. tritici was present, even in asymptomatic leaf extracts. In addition, the leaf apoplast antifungal proteolytic activity was measured in plants 7, 15, and 22 days after sowing. This antifungal action decreased in plants only inoculated with S. tritici, but increased in those grown from seeds coated with the T. harzianum (Th5) isolate. This increase conferred resistance to the susceptible wheat cultivar. The endogenous germin-like protease inhibitor coordinated the proteolytic action. These results suggest that T. harzianum stimulates a biochemical systemic induced response against leaf blotch.     

Remarkable recent changes in the genetic diversity of the avirulence gene AvrStb6 in global populations of the wheat pathogen Zymoseptoria tritici

Septoria tritici blotch (STB), caused by the fungus Zymoseptoria tritici, is one of the most economically important diseases of wheat. Recently, both factors of a gene-for-gene interaction between Z. tritici and wheat, the wheat receptor-like kinase Stb6 and the Z. tritici secreted effector protein AvrStb6, have been identified. Previous analyses revealed a high diversity of AvrStb6 haplotypes present in earlier Z. tritici isolate collections, with up to c.18% of analysed isolates possessing the avirulence isoform of AvrStb6 identical to that originally identified in the reference isolate IPO323. With Stb6 present in many commercial wheat cultivars globally, we aimed to assess potential changes in AvrStb6 genetic diversity and the incidence of haplotypes allowing evasion of Stb6-mediated resistance in more recent Z. tritici populations. Here we show, using targeted resequencing of AvrStb6, that this gene is universally present in field isolates sampled from major wheat-growing regions of the world in 2013–2017. However, in contrast to the data from previous AvrStb6 population studies, we report a complete absence of the originally described avirulence isoform of AvrStb6 amongst modern Z. tritici isolates. Moreover, a remarkably small number of haplotypes, each encoding AvrStb6 protein isoforms conditioning virulence on Stb6-containing wheat, were found to predominate among modern Z. tritici isolates. A single virulence isoform of AvrStb6 was found to be particularly abundant throughout the global population. These findings indicate that, despite the ability of Z. tritici to sexually reproduce on resistant hosts, AvrStb6 avirulence haplotypes tend to be eliminated in subsequent populations.     

The activity of peroxidase in various cell fractions of wheat plants infected with Septoria nodorum berk.

The activities of peroxidase isoforms and hydrogen peroxide content in leaf cuttings of wheat (Triticum aestivum L., cv. Diamant) resistant to Septoria blotch were studied during aging and following the infection with Septoria nodorum Berk. The differential activation of peroxidase isoforms was regulated by hydrogen peroxide level in the tissue. At early stages of fungus development in plant tissues, the decrease in the activities of soluble, membrane and ion-bound fractions of peroxidase elevated the level of hydrogen peroxide in infected tissues and rapidly activated peroxidase isoforms in infected tissues as compared to the aging ones even before disease symptoms appeared. The anionic peroxidases, which were first to respond to the pathogen, seem to stand for wheat resistance to fungal infections and the protection of leaf tissues from oxidative stress.     

Morphophysiological Indices of the Source Leaf in Wheat Plants Acclimated to Conditions of Nitrogen Nutrition

Growth, leaf and cell morphology, and the chemical composition of the second leaf were studied in wheat (Triticum aestivumL., cv. Inna) plants grown on the medium containing nitrate, ammonium, or no nitrogen at all. Independent of the nitrogen nutrition, the second leaf of the 21-day-old plants matures and functions as a source of assimilates. Both ammonium nutrition and nitrogen deficiency decreased the fresh weight, area, and cell size in the leaf; however, the conditions of nitrogen nutrition did not affect the dry weight of the leaf. Nitrogen starvation increased and ammonium nutrition decreased the relative content of the cell walls in the dry weight. In the nitrate-fed plants, the leaf content of sucrose increased, and the contents of reduced nitrogen (N_red) and protein were lower than in the ammonium treatment. Reciprocally, the contents of reduced nitrogen and protein were highest in the ammonium treatment, the content of sucrose was lowest, with starch practically absent from the leaf. The nitrogen-starved leaf accumulated a large amount of starch, the N_redcontent was two times lower than in the ammonium-fed plants, and the protein content was similar to that in the nitrate-fed plants. Thus, leaf and cell morphology and the content of N_red, protein, and carbohydrate changes in different ways during wheat acclimation to the condition of nitrogen nutrition. By assessing the cell wall weight, the authors established that, depending on nitrogen nutrition, this cell compartment accepts a variable flow of carbon.     

Study of Genetic Variability Among Monopycnidial and Monopycnidiospore Isolates Derived from Single Pycnidia of Stagonospora ssp. and Septoria tritici with the use of RAPD-PCR, MP-PCR and rep-PCR Techniques

Thirty-six isolates of Stagonospora avenae f. sp. triticea, S. nodorum and Septoria tritici recovered from asexual fruiting bodies - pycnidia and their spores were assessed for DNA polymorphism with the use of three molecular techniques: microsatellite-primed polymerase chain reaction (MP-PCR), primers correspond to dispersed repetitive elements (rep-PCR) and random amplified polymorphic DNA (RAPD-PCR). These polymerase chain reaction (PCR)-based techniques were simultaneously evaluated for their capacity to detect genetic variation at DNA level. The most polymorphic DNA profiles of monopycnidial and monopycnidiospore isolates were detected with diverse microsatellite motifs used for PCR priming. The lowest similarity values 0.86, 0.76 and 0.84 were identified among monopycnidiospore isolates derived from the same pycnidium of S. avenae f. sp. triticea, S. nodorum and S. tritici, respectively. The above, rather low similarities, found for isolates recovered from single pycnidia, supported a hypothesis that heterokaryosis resulted from high mutation rate of microsatellites and transposons activity. This would have fundamental consequences for the genetic status of asexual populations of Stagonospora spp. and S. tritici. The data produced by this study indicate that more attention should be paid to asexual reproduction as a possible source of genetic variability among populations of the pathogens.     

Genetic architecture is more complex for resistance to Septoria tritici blotch than to Fusarium head blight in Central European winter wheat

Background

Fusarium head blight (FHB) and Septoria tritici blotch (STB) severely impair wheat production. With the aim to further elucidate the genetic architecture underlying FHB and STB resistance, we phenotyped 1604 European wheat hybrids and their 135 parental lines for FHB and STB disease severities and determined genotypes at 17,372 single-nucleotide polymorphic loci.

Results

Cross-validated association mapping revealed the absence of large effect QTL for both traits. Genomic selection showed a three times higher prediction accuracy for FHB than STB disease severity for test sets largely unrelated to the training sets.

Conclusions

Our findings suggest that the genetic architecture is less complex and, hence, can be more properly tackled to perform accurate prediction for FHB than STB disease severity. Consequently, FHB disease severity is an interesting model trait to fine-tune genomic selection models exploiting beyond relatedness also knowledge of the genetic architecture.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1628-8) contains supplementary material, which is available to authorized users.     

In vitro screening for resistance against Septoria nodorum blotch in wheat

This study was carried out to develop an in vitro test for the identification of genotypes resistant to Septoria nodorum blotch. The basis for this project was a previous study in which a crude extract of S. nodorum was used as a selective agent (Keller et al. 1994). It was possible to distinguish resistant and susceptible cultivars in an in vitro test with zygotic embryos. In our project we wanted to test whether this in vitro test can also be used to detect resistant and susceptible genotypes in early segregating populations. Specific crosses between eight winter wheat lines showing contrasting resistance reaction for S. nodorum blotch on leaves and ears were made. The resistance level of both leaf and ear was evaluated after artificial inoculation in the field for the parental lines, the F₁ progenies, as well as for segregating F₃ and F₄ populations. In addition, this plant material was tested in vitro using methods similar to those described by Keller et al. (1994), i.e. culturing immature zygotic embryos and mature seeds on selective media. A good agreement between in vitro screening and field resistance on the ear was found for the parental lines, the F₁ and F₄ generation but not for the F₃ generations. This leads to the conclusion that the in vitro screening might be integrated into wheat breeding programs. Populations showing a high susceptibility to the pathogen metabolites in vitro could be discarded. Another promising implementation for wheat breeding would be the screening of advanced breeding material or candidate partners in a crossing program for resistance on the ear. However, the in vitro screening is not precise enough to select single plants in early segregating populations. Received: 18 January 1999 / Accepted: 30 April 1999     

Bacteria associated with Stagonospora (Septoria) nodorum increase pathogenicity of the fungus

In studies with a laboratory isolate of the fungal pathogen Stagonospora ( Septoria ) nodorum three different isolates of bacteria were closely associated with the fungus. Bacteria were also closely associated with fresh isolates of S. nodorum obtained from artificially and naturally infected field material. Although a range of bacteria was isolated, only one type of bacterium was found to be associated with each isolate of S. nodorum . In co-inoculation studies with pycnidiospores of the fungus on detached leaves, some of the bacterial isolates significantly increased the pathogenicity of the fungus, particularly Xanthomonas maltophilia , Sphingobacterium multivorum , Enterobacter agglomerans and Erwinia amylovora . Evidence is presented indicating that one of the ways that the 'helper bacteria' may assist in the establishment of infections is by the production of lipases that were not detected in germinating fungal spores.     

Inheritance of resistance to leaf and glume blotch caused by Septoria nodorum Berk. in winter wheat

Sixteen crosses between eight winter wheat cultivars were screened for resistance to Septoria nodorum leaf and glume blotch in the F₁ and F₄ generations using artificial inoculation in the field. The F₁ of most crosses showed dominance for susceptibility on both ear and leaf. The effects of general combining ability were of similar magnitude as the effects for specific combining ability. On the basis of the phenotypic difference of the parents, no prediction was possible about the amount and the direction of genetic variance in the segregating populations. The variation observed in this study both within and among the segregating populations suggests a quantitative inheritance pattern influencing the expression of the two traits. The components of variance between F₂ families within a population were as high as (for S. nodorum blotch on the ear) or higher (for S. nodorum blotch on the leaf) than those between populations. Therefore, strong selection within a few populations may be as effective to obtain new resistant genotypes as selection in a large number of populations. In almost all crosses, progenies were found that were more resistant than the better parent. Thus transgression breeding may be a tool to breed for higher levels of resistance to S. nodorum blotch. Highly resistant genotypes were found even in combination with two susceptible parents. The genetic source for Septoria resistance is probably broader than is generally assumed and could be used to improve S. nodorum resistance by combination breeding followed by strong selection in large populations. Received: 18 January / Accepted: 30 April 1999     

小麦叶绿体蛋白质编码基因RNA编辑位点的测定及与返白现象的关系

RNA编辑是一种转录后基因加工修饰现象,广泛存在于高等植物细胞器中。已有研究表明,RNA编辑与植物发生白化或者黄化有关。通过PCR、RT-PCR及测序的方法,对具有阶段性白化特性的小麦（Triticum aestivum）返白系FA85及其野生型矮变一号（Aibian 1）的叶绿体蛋白质编码基因RNA编辑位点进行了测定,在14个基因上发现了26个编辑位点。有5个编辑位点在2个株系之间存在编辑效率的差异,且这些差异的位点均位于编码叶绿体RNA聚合酶的基因上,其中3个位点编辑前后对应的蛋白质二级结构可能有差异。对2个株系叶绿体中PEP、NEP及PEP、NEP共同依赖基因转录水平的检测显示,除psbA和clpP外,其它基因在小麦返白系中的转录水平均有不同程度的下降。这种转录水平的显著下降及叶绿体RNA聚合酶基因上RNA编辑位点编辑效率的改变,可能与小麦返白系叶片的返白有关。     

Superoxide Production and the Activity of Extracellular Peroxidase in Plant Tissues under Stress Conditions

The production of reactive oxygen species on the plant-cell surface is considered. Along with the plasmalemmal redox systems, cell-wall peroxidase is involved in the production of superoxide and hydrogen peroxide. Under stress conditions, some soluble peroxidase isoforms are easily secreted into the apoplast. Various membranotropic compounds, salicylic acid in particular, can also induce this process. Mobile peroxidase forms are supposed to induce the plant defense response.     

Modelling wheat growth and yield losses from late epidemics of foliar diseases using loss of green leaf area per layer and pre-anthesis reserves

BACKGROUND AND AIMS: Crop protection strategies, based on preventing quantitative crop losses rather than pest outbreaks, are being developed as a promising way to reduce fungicide use. The Bastiaans' model was applied to winter wheat crops (Triticum aestivum) affected by leaf rust (Puccinia triticina) and Septoria tritici blotch (STB; Mycosphaerella graminicola) under a range of crop management conditions. This study examined (a) whether green leaf area per layer accurately accounts for growth loss; and (b) whether from growth loss it is possible to derive yield loss accurately and simply. Methods Over 5 years of field experiments, numerous green leaf area dynamics were analysed during the post-anthesis period on wheat crops using natural aerial epidemics of leaf rust and STB. Key Results When radiation use efficiency (RUE) was derived from bulk green leaf area index (GLAI), RUE(bulk) was hardly accurate and exhibited large variations among diseased wheat crops, thus extending outside the biological range. In contrast, when RUE was derived from GLAI loss per layer, RUE(layer) was a more accurate calculation and fell within the biological range. In one situation out of 13, no significant shift in the RUE(layer) of diseased crops vs. healthy crops was observed. A single linear relationship linked yield to post-anthesis accumulated growth for all treatments. Its slope, not different from 1, suggests that the allocation of post-anthesis photosynthates to grains was not affected by the late occurring diseases under study. The mobilization of pre-anthesis reserves completely accounted for the intercept value. Conclusions The results strongly suggest that a simple model based on green leaf area per layer and pre-anthesis reserves can predict both growth and yield of wheat suffering from late epidemics of foliar diseases over a range of crop practices. It could help in better understanding how crop structure and reserve management contribute to tolerance of wheat genotypes to leaf diseases.     

The Ultracytochemical Localzation of Acid Phosphatase Activity in Wheat During Fertilization

No acid phosphatase activity was observed in the mature embryo sac of wheat (Triticum aestivum) except the chalazal cytoplasm Of the central cell before fertilization. During fertilization, acid phosphataseactivity was observed in the following loci: part of chromatin of the egg nucleus and most of the mitochondria in the egg cytoplasm; the perinuclear spaces of the egg and sperm nuclei at the fusion of the egg and sperm nuclei; the chalazal cytoplasm and some vacuoles of the degenerated synergid; two sperm nuclei within the cytoplasm of female cells; the cell wall of each cell of the embryo sac and that of the nucellar cells surrounding the embryo sac. No acid phosphatase was observed in the two-celled proembryo. Dense enzyme reaction product was localized in the chromatin of the free nuclei at early stage of the endosperm. The characteristic of acid phosphatase distribution during fertilization may be associated with the physiological change of the egg Cell, the reorganization of mitochondria in the egg cell cytoplasm, the degeneration of one of the two synergids, the physiological state of the sperm nuclei and the nuclear membrane fusion of the egg and sperm nuclei.     

Changes in the Levels of IAA,ABA, and Cytokinins in Wheat Seedlings Infected with Tilletia caries

The seedling growth and the content of endogenous phytohormones in wheat seedlings were estimated 3, 6, and 9 days after infection with the bunt pathogen (Tilletia caries) (DC.)TUL. The infection of a pathogen-susceptible species Triticum aestivum L. and a resistant species T. timopheevii Zhuk. resulted, respectively, in an increase and a decrease in the seedling growth and the IAA content as compared to the control. The cytokinin content increased in both species, and the increase in T. timopheevii was more rapid. The pathogen-induced increase in auxin content is suggested to enhance fungal invasion of plants. In the susceptible species, a high ABA concentration was retained for a longer period of time and could act as a factor of virulence. At the same time, in the resistant species, an increase in ABA content was transient and seems to trigger plant defense mechanisms.