Studien zur Residenz von Gerste gegen Magnaporthe oryzae

  • Studies on the resistance of barley against Magnaporthe oryzae

Jansen, Marcus; Slusarenko, Alan (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2007)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2007

Abstract

Barley is a host plant of Magnaporthe oryzae which is the causal agent of rice blast disease. Barley plants carrying a mlo-mutation have race non-specific resistance to powdery mildew but are contrariwise hypersusceptible to M. oryzae. MLO wild-type plants are moderately susceptible to M. oryzae. The mutants emr1 and emr2 were identified in a mutation analysis for enhanced resistance to M. oryzae in the genetic background of BCIngrid mlo5. The emr1 mutant had a quantitatively elevated resistance to M. oryzae in comparison to BCIngrid mlo5 plants which correlated with an enhanced cellular penetration resistance. The resistance was inherited by a single recessive gene and cosegregated with enhanced water retention of the leaves. The latter was caused by a diminished epicuticular wax layer whose chemical composition hinted at a mutation in a fatty acid elongase gene. Cosegregation of both traits indicated a mutation in one single gene or two closely neighbouring genes. Resistance mediated by emr1 was effective against M. oryzae but not against rust fungi or necrotrophic fungi, such as Drechslera teres or Rhynchosporium secalis. The resistance of emr1-plants to barley powdery mildew was due to the presence of the mlo5-gene. Peroxidase activity as biochemical marker and PR gene expression as molecular marker of pathogen defence comported in emr1 plants similar to mlo5 plants despite the difference in resistance to M. oryzae. Merely the accumulation of PR2-, PR3, and PR5b-specific transcripts was weaker in emr1 than in mlo5 plants, which is possibly due to the lower susceptibility of emr1. Acquired resistance to M. oryzae could be induced in emr1 plants by application of DCINA.• The emr1-mutation acted specifically in the interaction of barley with M. oryzae and strengthened the resistance of the plants against the fungus. The emr2-plants remained nearly free of blast symptoms after inoculation with M. oryzae. Resistance conferred by emr2, like emr1, was inherited by a single recessive gene but the mutations were not allelic. The emr2 plants were smaller and had lower grain yield per spike than BCIngrid mlo5 plants. Resistance was effective not only against M. oryzae but also against D. teres and R. secalis. However it was not effective against rust fungi. Additionally, an inhibition of growth of Pseudomonas syringae pv. syringae in emr2 intercellular spaces was contrasted by bacterial multiplication in intercellular spaces of MLO, mlo5, and emr1 plants. As in emr1, the mlo5-gene caused resistance to powdery mildew in emr2 plants. A constitutive enhancement of both POX-activity and PR-gene expression in emr2 hinted at constitutively activated resistance in the emr2 mutant. POX activity as well as PR gene expression showed an early enhancement after inoculation with M. oryzae and therefore could contribute to effective defence.• The emr2 mutant showed a constitutive resistance phenotype similar to the cpr- and cim- mutants of A. thaliana. Barley rom1 mutation restores resistance to powdery mildew in the Mla12/rar1-2 genetic background. Therefore a possible influence of the rom1 mutation on the interaction of barely with M. oryzae was analysed. A strong increase in the number of blasts per leaf was detected in rom1 plants compared to Mla12/rar1-2 plants. Effective resistance reactions of epidermal cells decreased concomitantly, as seen from microscopic analyses. At the same time in-planta growth of M. oryzae was limited which led to reduced blast diameters compared to the blasts on Mla12/rar1-2 plants. A possible impact of hordatines on the resistance of barley to M. oryzae was analysed by infection assays with hordatine-deficient mutants. These mutants were more susceptible to the fungus than wild type plants, indicating that hordatines can influence the interaction of barley with M. oryzae. Above-ground parts as well as roots can be colonised by fungi of the genus Magnaporthe. Leaves of barley and rice can exhibit resistance to the fungus. Infection assays with roots were carried out to analyse whether resistance acts in an organ-specific manner. Race-specific resistance of rice to M. oryzae and acquired resistance of rice and barley to M. oryzae were not as effective in roots as they were in leaves. Barley roots of MLO-plants were as susceptible to M. oryzae as roots of mlo5-plants. This result indicated an impaired efficiency of basal resistance in roots. Furthermore, barley roots were susceptible to an isolate of Magnaporthe, which shows a non-host interaction with barley leaves. Thus non-host resistance in roots was not as effective as in leaves.

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