The effects of three film-forming polymers, with and without a polyamine biosynthesis inhibitor, on powdery mildew infection of barley seedlings

Powdery mildew infection of barley seedlings was significantly reduced using either pre-inoculation or post-inoculation sprays of the film-forming polymers Nu-Film P, Emerald or Vapor Gard. The greatest reduction in mildew infection was obtained with a pre-inoculation spray of the polymer Vapor Gard. Although the addition of 1 mm α-difluoromethylornithine (DFMO) to a 2% solution of Emerald or Vapor Gard resulted in better control of powdery mildew infection than that obtained using the polymer or DFMO alone, the reductions in infection were not significant. Reduction in mildew infection was greatest when the Vapor Gard + DFMO mixture was applied 1 day prior to inoculation with the fungus. Although substantial reduction in mildew infection was achieved using as little as 0.1 mm DFMO with 2% Vapor Gard, or reducing the polymer concentration to 1% in combination with 1 mm DFMO, best control was achieved using 2% Vapor Gard plus 1 mm DFMO.     

Sources of powdery mildew resistance in barley landraces from morocco

A total of forty eight accessions of barley landraces from Morocco were screened for resistance to powdery mildew. Twenty two (46%) of tested landraces showed resistance reactions and thirty four single plant lines were selected. Eleven of these lines were tested in seedling stage with seventeen and another twenty three lines with twenty three isolates of powdery mildew respectively. The isolates were chosen according to the virulence spectra observed on the ‘Pallas’ isolines differential set. Line 229–2–2 was identified with resistance to all prevalent in Europe powdery mildew virulence genes. Lines 230–1–1, 248–1–3 showed susceptible reaction for only one and lines 221–3–2, 227–1–1, 244–3–4 for only two isolates respectively. Three different resistance alleles (Mlat, Mla6, and MLA14) were postulated to be present in tested lines alone or in combination. In thirty (88%) tested lines it was impossible to determine which specific gene or genes for resistance were present. Most probably these lines possessed alleles not represented in the ‘Pallas’ isolines differential set. The distribution of reaction type indicated that about 71% of all reaction types observed were classified as powdery mildew resistance (scores 0, 1 and 2). Majority (79%) of resistance reaction types observed in tested lines was intermediate resistance reaction type two and twenty three lines (68%) showed this reaction for inoculation with more than 50% isolates used. The use of new effective sources of resistance from Moroccan barley landraces for diversification of resistance genes for powdery mildew in barley cultivars was discussed.     

Differences in powdery mildew epidemics in spring and winter barley based on 30‐year variety trials

Information on disease severity assists greatly with making decisions about crop protection systems. However, there are no published long‐term field trials in different locations to determine the severity of individual diseases. National trials that relate to the agronomic traits of varieties, including disease responses, are available. The current article describes the use of such data to determine disease severity based on official Czech variety trials. In total, 838 and 334 field trials of spring and winter barley, respectively, conducted from 1976 to 2005, were analysed. To compare year to year powdery mildew severity, five parameters were used. Annual disease severity in both crops differed substantially, whereas similarity of the disease severity was rare. During the last 15 years, disease severity in spring barley was stable despite a rapid increase in the area of fully resistant spring barley varieties, particularly those with the gene mlo. During the period 1976–2005, the mean disease severity in winter barley accounted for 64.1% of the mean disease severity in spring barley. The effect of deployment of the resistance genes and growing areas of winter barley on the determined disease severity was analysed. The method described here allows easy comparisons to be made of disease severity among years, locations and different crops and diseases if similar scoring scales for disease infection evaluation are used.     

Genetics of Powdery Mildew Resistance in Barley

A comprehensive, critical review on the present knowledge regarding the genetics of resistance of barley to the powdery mildew fungus is presented. The review deals with six kinds of resistance: Race-specific resistance; Mlo resistance; partial resistance; induced resistance; passive resistance; and non-host resistance. Most of the sections are subdivided into: phenotype of the interaction; resistance mechanisms; and genetics. A distinction is made between three groups of genes involved in the defense of plants to diseases: those that serve exclusively to mediate resistance; those that are mobilized to strengthen the plants' defense; and those that serve exclusively functions other than disease defense, but may bring about resistance. The more than 200 gene symbols assigned to race-specific mildew resistance genes over time are summarized and revised to 85 symbols that may be considered valid.     

Comparative effects of selenite and selenate on nitrate assimilation in barley seedlings

Abstract. The effect of SeO₃ and SeO₄ on NO₃ assimilation in 8-d-old barley (Hordeum vulgare L.) seedlings was studied over a 24-h period. Selenite at 0.1 mol. m^? in the uptake solutions severely inhibited the induction of NO₃ uptake and active nitrate reductases. Selenate, at 1.0 mol m^?3 in the nutrient solution, had little effect on induction of activities of these systems until after 12 h; however, when the seedlings were pretreated with 1.0 mol m^?3 SeO₄ for 24 h, subsequent NO₃ uptake from SeO₄-free solutions was inhibited about 60%. Sulphate partially alleviated the inhibitory effect of SeO₃ when supplied together in the ambient solutions, but had no effect in seedlings pretreated with SeO₃. By contrast, SO₄ partially alleviated the inhibitory effect of SeO₄ even in seedlings pretreated with SeO₄. Since uptake of NO₃ by intact seedlings was also inhibited by SeO₃, the percentage of the absorbed NO₃ that was reduced was not affected. By contrast, SeO₄, which affected NO₃ uptake much less, inhibited the percentage reduced of that absorbed. However, when supplied to detached leaves, both SeO₃ and SeO₄ inhibited the in vivo reduction of NO₃ as well as the induction of nitrate reductase and nitrite reductase activities. Selenite was more inhibitory than SeO₄; approximately a five to 10 times higher concentration of SeO₄ than SeO₃ was required to achieve similar inhibition. In detached leaves, the inhibitory effect of both SeO₃ and SeO₄ on in vivo NO₃ reduction as well as on the induction of nitrate reductase activity was partially alleviated by SO₄. The inhibitory effects of Se salts on the induction of nitrite reductase were, however, completely alleviated by SO₄. The results show that in barley seedlings SeO₃ is more toxic than SeO₄. The reduction of SeO₄ to SeO₃ may be a rate limiting step in causing Se toxicity.     

The effects of host carbon dioxide, nitrogen and water supply on the infection of wheat by powdery mildew and aphids

In two experiments, winter wheat (Triticum aestivum cv. Cerco) was grown in 350 (ambient) and 700 μmol mol^-1 (elevated) atmospheric CO₂ concentrations. In the first experiment, plants were grown at five levels of nitrogen fertilization, and in the second experiment, plants were grown at three levels of water supply. All plants were infected with powdery mildew, caused by the fungus Erysiphe graminis. Plants grown in elevated atmospheric CO₂ concentrations had significantly reduced % shoot nitrogen contents and significantly increased % shoot water contents. At elevated atmospheric CO₂ concentrations, where plant nitrogen content was significantly reduced, the severity of mildew infection was significantly reduced, and where host water content was significantly increased, the severity of mildew infection was significantly increased. In a moderate water supply treatment, the plants grown in elevated atmospheric CO₂ concentrations had significantly reduced nitrogen contents (9·9%) and significantly increased water content (4%), the amount of mildew infection was unchanged. The severity of mildew infection appeared to be more sensitive to host water content than to host nitrogen content.     

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.     

A barley SKP1‐like protein controls abundance of the susceptibility factor RACB and influences the interaction of barley with the barley powdery mildew fungus

In an increasing number of plant–microbe interactions, it has become evident that the abundance of immunity‐related proteins is controlled by the ubiquitin–26S proteasome system. In the interaction of barley with the biotrophic barley powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh), the RAC/ROP [RAT SARCOMA‐related C3 botulinum toxin substrate/RAT SARCOMA HOMOLOGUE (RHO) of plants] guanosine triphosphatase (GTPase) HvRACB supports the fungus in a compatible interaction. By contrast, barley HvRBK1, a ROP‐binding receptor‐like cytoplasmic kinase that interacts with and can be activated by constitutively activated HvRACB, limits fungal infection success. We have identified a barley type II S‐phase kinase 1‐associated (SKP1)‐like protein (HvSKP1‐like) as a molecular interactor of HvRBK1. SKP1 proteins are subunits of the SKP1‐cullin 1‐F‐box (SCF)–E3 ubiquitin ligase complex that acts in the specific recognition and ubiquitination of protein substrates for subsequent proteasomal degradation. Transient induced gene silencing of either HvSKP1‐like or HvRBK1 increased protein abundance of constitutively activated HvRACB in barley epidermal cells, whereas abundance of dominant negative RACB only weakly increased. In addition, silencing of HvSKP1‐like enhanced the susceptibility of barley to haustorium establishment by Bgh. In summary, our results suggest that HvSKP1‐like, together with HvRBK1, controls the abundance of HvRACB and, at the same time, modulates the outcome of the barley–Bgh interaction. A possible feedback mechanism from RAC/ROP‐activated HvRBK1 on the susceptibility factor HvRACB is discussed.     

In vitro effects of the polyamine biosynthesis inhibitor, α-difluoromethylornithine, on ornithine decarboxylase activities from three plant pathogenic fungi

The effects of α-difluoromethylornithine (DFMO) on in vitro ornithine decarboxylase (ODC) activities from three plant pathogenic fungi, Pyrenophora avenae, Pyricularia oryzae and Uromyces viciae-fabae , were studied. DFMO concentrations from 0·01 to 1·0 mmol/l produced no significant effects on ODC activities from the three fungi. However, increasing the DFMO concentration to 5 mmol/l produced a substantial reduction in in vitro ODC activity from Pyre, avenae. The ODC inhibitor, α-monofluoromethylornithine (2 mmol/l), significantly reduced in vitro ODC activity from Pyre. avenae , whereas RR-methyl acetylenic putrescine, an ODC inhibitor based on putrescine, produced no significant effect on the fungal enzyme.     

Differential expression of phenylalanine ammonia-lyase genes in barley induced by fungal infection or elicitors

Gene-specific probes were used to assess the expression patterns of four different phenylalanine ammonia-lyase ( pal ) genes in infected or elicitor-treated leaves and suspension-cultured cells of barley. Genes corresponding to hpal2 , hpal3 , hpal4 , and hpal6 were all induced by mercuric chloride and fungal infection by Bipolaris sorokiniana Sacc. (Shoem.) in barley ( Hordeum vulgare L. cv. Pokko) leaves, but with considerable variation in their expression level and timing. The expression patterns of hpal2 and hpal6 were similar, both showing a rapid, strong induction after treatment with mercuric chloride and a slower induction after fungal inoculation, whereas the more divergent hpal3 was induced at a later time and at a lower level after both treatments. Hpal4 was expressed with timing like that of hpal2 and hpal6 in infected or mercuric chloride-treated leaves but its expression was much weaker. Hpal2 and hpal4 were induced in elicitor-treated, suspension-cultured barley cells, whereas the expression of hpal3 was nearly undetectable, and hpal6 was strongly and constitutively present. All pal genes except hpal4 were developmentally regulated, but differentially expressed in various barley tissues. The results suggest that the four pal genes studied here might be responsible for the activation of different branches in the phenylpropanoid biosynthesis of barley.     

The effects of light on induction, time courses, and kinetic patterns of net nitrate uptake in barley

Barley seedlings ( Hordeum vulgare L.) were grown hydroponically with (induced) or without (uninduced) nitrate in a light/dark cycle with high photon flux density to determine the effects of light on time courses, induction and kinetics of net nitrate uptake. Nitrate uptake was induced by external nitrate in both light and dark and was prevented by 1 mol m^–3 p-fluorophenylalanine. In high light, nitrate uptake was about 2-fold higher than in low light. During time course experiments the uptake rates oscillated due to daily light–dark changes. Rates of nitrate uptake also increased at about 2200 h during continuous darkness. This increase coincided approximately with the time at which the dark period started during the previous culture of the plants, indicating that it was due to a mechanism associated with an endogenous diurnal rhythm. When calculating the kinetics of nitrate uptake, a model with two saturable systems, including a high-affinity system (HATS) and a low-affinity system (LATS), gave the best fit to data in all treatments. The apparent affinity of the HATS ranged from 7·7 to 12·2 mmol m^–3 in induced plants in all light conditions. The effect of light on the HATS was mainly an increase of apparent V _max in the step from low to high light. In uninduced plants the HATS operated at a very low activity which was strongly enhanced during induction. Interpretation of the calculated kinetics of the LATS was much more difficult on the basis of net uptake data. The apparent affinity of the LATS increased from 24·3 mol m^–3 in low light up to 0·17 mol m^–3 after acceleration in high light. These extreme changes in apparent affinity of the LATS could not be explained satisfactorily, and the nature of this system is also discussed with respect to the method used.     

Effects of fungal infection and wounding on the expression of chitinases and β-1,3 glucanases in near-isogenic lines of barley

Chitinases (EC 3.2.1.14) and β -1.3 glucanases (EC 3.2.1.39) have been known to play a vital role in the defense of plants against fungal pathogens. The pattern of induction of these two enzymes subsequent to infection by powdery mildew was studied in 10 pairs of near-isogenic lines of barley ( Hordeum vulgare L.) which possess powdery mildew resistance genes. These isogenic lines have been grotiped according to their reaction to the fungus. The induction patterns varied between the resistant and the susceptible cultivars within each group and between different groups. More tsozymcs were induced in susceptible varieties of highly resistant groups and the overall levels and the number of isozymes of chitinases and β -1.3 glucanases were lower in groups with low resistance. The effect of powdery mildew infection and mechanical wounding on the cellular localization of chitinases and β -1.3 glucanases in barley leaves has also been studied. The 31 kDa leaf chitinase, L-CH2, and trace amounts of a 25 kDa chitinase. L-CH3. were present in healthy leaves. Wounding increased the levels of L-CH3 within I ft h. Powdery mildew infection increased the levels of L-CH3 both in intercellular fluid and in intracellular extract of leaves. A /3-I.3 glucanase. GH, also increased after infection and wounding. In infected barley leaves, GL-1 was present both in intercellular space and intracellular extract. It is concluded that powdery mildew resistance genes exhibit qualitative and quantitative differences in the expression of chitinases and β -1.3 glucanases. Further, chitinases and β -1.3 glucanases appear to be a response to active infection rather than the factors responsible for disease resistance.     

Correlation of gibberellin-induced growth, polyamine levels and amine oxidases in epicotyl, root and leaf blade of barley seedlings

The potential role of diamine oxidase (DAO) and polyamine oxidase (PAO) in relation to polyamines was investigated in epicotyls, roots and leaf blades at 3 and 6 days after gibberellic acid (GA) application in barley (Hordeum vulgare L.) seedlings of cvs. Maythorpe (non-mutant parent) and Golden Promise (semi-dwarf mutant). There was a significant increase in epicotyl and leaf-blade elongation rates in GA-treated seedlings of cv. Maythorpe as compared to cv. Golden Promise. DAO and PAO were detectable in all segments of the leaf blade, but the highest activities were present in basal segments. These enzymes, which are thought to have a role in the elimination of cellular polyamines, increased in activity following GA application compared to controls. Application of 10⁻⁶ M GA to the first leaf, significantly increased endogenous bound putrescine (Put) levels in both the epicotyl and leaf blade of cv. Maythorpe. In contrast, there was only a slight increase in cv. Golden Promise. Levels of soluble Put increased in roots and leaf blades of both cultivars following GA treatment but the effect was greatest in leaves of cv. Maythorpe. It is suggested that polyamines may play a role in GA-induced epicotyl and leaf-blade elongation in barley.