Der Einfluß des Mlo-Locus auf genetisch vermittelte und chemisch induzierte Resistenz der Gerste [Hordeum vulgare L.] gegen pilzliche Pathogene

In barley, mlo-specified resistance and chemically induced resistance (IR) to powdery mildew (Blumeria graminis f.sp. hordei; Bgh) share a very similar mode of action. The present study was aimed at analysing susceptible mutants of mlo-barley for their response to chemical induction of IR. A highly sensitive genotype (mlo5, ror1-2) was identified which, in contrast to wildtype plants (Mlo, Ror1), showed IR after application of a significantly lower concentration of inducer. Microscopic analyses indicated that challenge inoculation after applications of low amounts of inducer led predominantly to formation of papillae, associated with a hypersensitive response (HR), whereas higher amounts conditioned plants to form effective papillae without a HR. This revealed a direct correlation between the effective resistance reactions and the concentration of the chemical inducer applied. Transcripts specific for PR1b, a putative marker gene for IR in barley, and for the BCI3-gene, identified during the work presented here, accumulate in barley genotypes (Mlo, Ror1, mlo5, Ror1 and mlo5, ror1-2) to a similar degree after application of chemical inducers, although the level of resistance against Bgh varied. Thus, it is evident that neither Mlo- nor Ror1-locus have an impact on expression of genes under investigation after chemical induction of IR. It is tempting to speculate whether markergene expression is simply a consequence of inducer application rather than a cause for resistance. However, neither macroscopic, microscopic nor molecular analyses revealed whether the enhanced responsiveness of the mlo5, ror1-2-genotype to chemical inducers is due directly to the mutations in the Mlo and/or the Ror1-locus. Thus, this investigations do not reveal whether Mlo and/or Ror1 are components of the IR-network or of an independent resistance pathway. Since expression of PR1b was not correlated to the application of inducers, the identification of marker genes for IR was attempted by means of suppression subtractive hybridyzation (SSH). Three marker genes were identified which reliably indicated the status of IR. Whereas the genes coding for a thionin and a lipoxygenase were previously considered as marker genes, the BCI3-gene, coding for an acidic phosphatase-like protein, was unknown in barley so far. This gene shows a weak constitutive expression. However, application of different resistance-inducing chemicals led to a considerable accumulation of transcripts in a dose-dependent manner. Therefore, the BCI3-gene meets the requirements of a marker for chemically induced IR. Incompatible as well as compatible interactions of barley with Bgh reduced the constitutive and the inducer-dependent level of BCI3-expression. Homologs of the BCI3-gene were detected in wheat by northern analyses. A functional role of the BCI3-protein in defense against pathogens and in IR is conceivable in terms of a source/sink model. However, molecular analyses of gene expression in genotypes differing in their response to chemical inducers revealed no causal function. Moreover, the strong expression of the BCI3-gene in response to Bion® treatment is contrasted by the low degree of resistance gained. Additionally, treatments of barley with derivatives of jasmonic acid, leading to a strong accumulation of BCI-3 specific transcripts, did not provoke resistance to powdery mildew. Obviously, BCI3 is not solely involved in triggering a defense response. It seems reasonable that altered transcript levels are due to indirect metabolic changes after the application of resistance-inducing compounds. Due to the strong resistance phenotype of mlo-plants to powdery mildew infections, IR can not be investigated in this pathosystem. Therefore the interaction of barley with Magnaporthe grisea was examined. In this way, new insights regarding the impact of the Mlo-locus on the overall resistance status of barley were gained. Macroscopic analyses of plants revealed a slightly susceptible infection phenotype of Mlo-genotypes to M. grisea, whereas mlo-alleles conditioned hypersusceptibility. Microscopic analyses showed that in Mlo-plants the infection was resisted by the formation of effective papillae. In cases of successfully penetrated papillae, fungal growth was nevertheless retarded. In contrast, M. grisea was able to penetrate efficiently into epidermal cells and into subjacent mesophyll cells of mlo-plants – papilla formation was not capable of inhibiting fungal development. Host cell death in the mesophyll was rather a symptom of disease than a defense response. It seems that under high infection pressure the Ror-loci have no impact on this interaction. Thus, one might speculate as to whether the role of Ror1 is restricted to resistance against Bgh and non-host-resistance against other formae speciales of powdery mildew. Evaluation of disease symptoms on barley plants showed that treatment with biological, as well as chemical agents, was capable of inducing resistance against M. grisea independently of the Mlo- and Ror1-loci. The reduction of disease severity was relatively more pronounced than that to powdery mildew infections on barley plants showing IR. Neither the Mlo- nor the Ror1-proteins seem to orchestrate the signalling pathways of IR in barley. Therefore, analyses of the interaction barley/M.grisea led to the hypothesis that Mlo- and/or Ror1-proteins are not components of IR in barley.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2002

Author dc:creator

• Jarosch, Birgit

Contributors dc:contributor

• Kogel, Karl-Heinz

• info:eu-repo/classification/ddc/570
• Biowissenschaften, Biologie

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-119060%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:56988