Extrachromosomal plasmid-like DNA in the obligate parasitic fungus Erysiphe graminis f. sp. hordei

Summary The obligate parasitic fungus, Erysiphe graminis f. sp. hordei, was found to harbour plasmid-like extrachromosomal DNA. A 1.35-kb fragment of this 9kb plasmid was cloned into the pUC12 vector. No homology was detected to nuclear or mitochondrial DNA. As only about half of the 27 isolates examined contained plasmid-like DNA, this appears to be inessential for fungal survival. The plasmid is frequent in European isolates and is found in both newly collected isolates and in isolates kept under laboratory conditions for many years. No correlation between presence of plasmid and specific avirulence/virulence genes was found. The plasmid appear to be located in the mitochondria.     

Interaction of a Blumeria graminis f. sp. hordei effector candidate with a barley ARF‐GAP suggests that host vesicle trafficking is a fungal pathogenicity target

Filamentous phytopathogens, such as fungi and oomycetes, secrete effector proteins to establish successful interactions with their plant hosts. In contrast with oomycetes, little is known about effector functions in true fungi. We used a bioinformatics pipeline to identify Blumeria effector candidates (BECs) from the obligate biotrophic barley powdery mildew pathogen, Blumeria graminis f. sp. hordei (Bgh). BEC1–BEC5 are expressed at different time points during barley infection. BEC1, BEC2 and BEC4 have orthologues in the Arabidopsis thaliana‐infecting powdery mildew fungus Golovinomyces orontii. Arabidopsis lines stably expressing the G. orontii BEC2 orthologue, GoEC2, are more susceptible to infection with the non‐adapted fungus Erysiphe pisi, suggesting that GoEC2 contributes to powdery mildew virulence. For BEC3 and BEC4, we identified thiopurine methyltransferase, a ubiquitin‐conjugating enzyme, and an ADP ribosylation factor‐GTPase‐activating protein (ARF‐GAP) as potential host targets. Arabidopsis knockout lines of the respective HvARF‐GAP orthologue (AtAGD5) allowed higher entry levels of E. pisi, but exhibited elevated resistance to the oomycete Hyaloperonospora arabidopsidis. We hypothesize that ARF‐GAP proteins are conserved targets of powdery and downy mildew effectors, and we speculate that BEC4 might interfere with defence‐associated host vesicle trafficking.     

Pathogenic divergence of Central European and Australian populations of Blumeria graminis f. sp. hordei

Hosts and pathogens have adapted their response to each other through genetic changes that have arisen during the course of their co‐evolution. In developed countries the longevity of varieties is often short; new varieties frequently possess novel genes with specific resistance to pathogens. The latter must adapt to the resistance genes to maintain pathogenicity. To study this adaptation, 50 Central European and 50 Australian isolates of Blumeria graminis f. sp. hordei (Bgh) were tested on 50 barley differential varieties with different specific resistance genes. All the Central European isolates differed from each other in their virulence combinations and belonged to 50 various pathotypes, whereas Australian isolates comprised 37 pathotypes. None of the pathotypes detected in Central Europe was identical or similar to any of those in Australia. This can be attributed to the much higher number of virulences in Central European isolates that developed over a long period of contact with a range of host varieties containing specific resistance genes. This has led to a gradual divergence of the Australian and the European Bgh populations. In Europe, unlike Australia, new specific resistance genes are still widely used in breeding barley varieties and the divergence of both populations will continue.     

Polyamine metabolism in barley reacting hypersensitively to the powdery mildew fungus Blumeria graminis f. sp. hordei

Polyamine levels and activities of enzymes of polyamine biosynthesis and catabolism were examined in the barley cultivar Delibes (Ml1al + Ml(Ab)) reacting hypersensitively to the powdery mildew fungus, Blumeria graminis f. sp. hordei (race CC220). Levels of free putrescine and spermine and of conjugated forms of putrescine, spermidine and spermine were greatly increased 1–4 d following inoculation of barley with the powdery mildew. These changes in polyamine levels were accompanied by elevated activities of the polyamine biosynthetic enzymes ornithine decarboxylase (ODC), arginine decarboxylase (ADC) and S‐adenosylmethionine decarboxylase (AdoMetDC) and the polyamine catabolic enzymes diamine oxidase (DAO) and polyamine oxidase (PAO). Activities of two enzymes involved in conjugating polyamines to hydroxycinnamic acids, putrescine hydroxycinnamoyl transferase (PHT) and tyramine feruloyl‐CoA transferase (TFT) were also examined and were found to increase significantly 1–4 d after inoculation. The possibility that the increased levels of free spermine, increased polyamine conjugates, and increased DAO and PAO activities are involved in development of the hypersensitive response of Delibes to powdery mildew infection is discussed.     

Effects of sowing date and vernalisation on the growth of winter barley and its resistance to powdery mildew (Erysiphe graminis f.sp. hordei)

Detailed studies on the production of individual leaves, and the development of powdery mildew on them, were made in field plots of winter barley sown on different dates. The greater severity of the disease on early-sown than on later-sown seedlings during the autumn and winter can probably be explained mainly by changes in the abundance of inoculum and the suitability of the weather for infection. Results from glasshouse experiments suggest that the differences may be reinforced by direct effects of vernalisation on the susceptibility of seedlings to the disease. Contrary effects of sowing date on mildew severity during summer are probably due to the progressively greater resistance to mildew of the later-formed than of seedling leaves, and the earlier appearance of corresponding leaves on early-sown than on later-sown plants. Early sowing can also increase the total number of leaves produced per stem. Therefore, because resistance of the leaves increases progressively, the maximum degree of resistance expressed by the later-formed (e.g. flag) leaves will often be greater on early-sown than on later-sown plants.     

小麦TCTP基因的克隆及白粉菌诱导下的表达

翻译控制肿瘤蛋白(translationally controlled tumor protein,TCTP)最初在鼠肿瘤细胞中被发现,研究表明TCTP广泛存在于动植物细胞中,并具有多种生物学功能。本研究用RT PCR和RACE技术在抗白粉病栽培小麦Brock中克隆了一个TCTP基因,该基因全长766 bp,推测编码一个168个氨基酸的多肽。ScanProsite分析表明,该多肽链具有2个TCTP特征结构区（TCTP1和TCTP2）和7个可能的功能位点。表达半定量分析发现,该基因受华北地区流行的优势小种15号白粉菌诱导,且随着诱导时间的增加其表达量增加。本研究将可能在小麦白粉病抗性研究领域开辟新的研究思路。     

Seasonal changes in infection of barley seedlings by Erysiphe graminis f.sp. hordei

Infection of artificially inoculated barley seedlings, exposed out of doors, was variable but there was a seasonal trend. As temperature affects both incubation time and the proportion of successful infections, these two factors will have additive effects on disease development. Infections developed only slowly in winter but, in both 1972 and 1973, conditions became much more favourable to infection in late April or early May.     

Ultrastructural and Histochemical Studies on Mildew of Barley (Erysiphe graminis DC. f. sp. hordei Marchal)

Plants of the mildew susceptible barley cultivar Peruvian and the adult plant resistant cultivar Osiris were inoculated with Erysiphe graminis f. sp. hordei at the first and fifth leaf stages. Samples taken at 32 hr after inoculation were examined by electron microscopy to compare papillae associated either with penetration failure or with successful penetration of the fungus into the epidermal cell and haustorium formation. Four types of papillae with ultrastructural differences could, be classified. Although their definite association with fungal ingress or failure is not possible, our data suggest that papillae with larger, more compacted and amorphous or globular structures may be more effective as penetration barriers than others, with more or less uniform distribution of irregular, smaller electrondense structures.     

Half sib selection for resistance to powdery mildew (Erysiphe graminis DC. f.sp. secalis Marchal) in rye populations

Summary Three rye populations originating from different cultivars were subjected to half sib progeny selection for three cycles to improve their resistance to powdery mildew (Erysiphe graminis DC. f.sp. secalis Marchal). Selection was carried out in the greenhouse (measurement of spore production at the three-leaf stage), in the field (scoring of mildew infections at ear emergence) or at both locations. All selections resulted in significant responses, the level of which varied depending on the genetic base of mildew resistance in the populations. Greenhouse selection was most effective when the frequency of dominant genes could be increased, whereas field selection was less effective. Estimates for heritability in the narrow sense ranged from 0.42–0.94 indicating that mildew resistance can be increased effectively. Inbred lines should be selected in base populations with a high level of quantitative resistance and/or a high frequency of dominant genes. For an effective resistance strategy in hybrid breeding such dominant genes are most valuable, and can be selected in young growth stages. In Strain CG the increase of dominant resistance genes by early selection resulted in a significant improvement of kernel yield.     

Identification of powdery mildew (Erysiphe graminis sp. tritici) and take-all disease (Gaeumannomyces graminis sp. tritici) in wheat (Triticum aestivum L.) by means of leaf reflectance measurements

The ability to identify diseases in an early infection stage and to accurately quantify the severity of infection is crucial in plant disease assessment and management. A greenhouse study was conducted to assess changes in leaf spectral reflectance of wheat plants during infection by powdery mildew and take-all disease to evaluate leaf reflectance measurements as a tool to identify and quantify disease severity and to discriminate between different diseases. Wheat plants were inoculated under controlled conditions in different intensities either with powdery mildew or take-all. Leaf reflectance was measured with a digital imager (Leica S1 Pro, Leica, Germany) under controlled light conditions in various wavelength ranges covering the visible and the near-infrared spectra (380–1300 nm). Leaf scans were evaluated by means of L*a*b*-color system. Visual estimates of disease severity were made for each of the epidemics daily from the onset of visible symptoms to maximum disease severity. Reflectance within the ranges of 490₇₈₀ nm (r² = 0.69), 510₇₈₀nm (r² = 0.74), 516₁₃₀₀nm (r² = 0.62) and 540₁₃₀₀ nm (r² = 0.60) exhibited the strongest relationship with infection levels of both powdery mildew and take-all disease. Among the evaluated spectra the range of 490₇₈₀nm showed most sensitive response to damage caused by powdery mildew and take-all infestation. The results of this study indicated that disease detection and discrimination by means of reflectance measurements may be realized by the use of specific wavelength ranges. Further studies have to be carried out, to discriminate powdery mildew and take-all infection from other plant stress factors in order to develop suitable decision support systems for site-specific fungicide application.