Molecular analyses on the mechanism of nonhost resistance of barley (Hordeum vulgare L.) to the wheat powdery mildew fungus (Blumeria graminis f.sp. tritici)
Every plant pathogen has only a limited range of host species on which it can cause disease. The remaining plants are nonhost plants to this pathogen and can resist the attacker due to a multitude of different mechanisms that collectively contribute to nonhost resistance. Due to its durable effectiveness, nonhost resistance has moved into the focus of scientific interest, since it promises to be of use for the generation of resistant crop plants. Considerable effort has been made to elucidate signal transduction processes of plant defense. In spite of a number of investigations, which examined the transcriptome of plants during the interaction with pathogens, particularly powdery mildew fungi, we are still far from understanding the nature of nonhost resistance especially in terms of its constancy. Hence, this study aimed at the advanced understanding of the mechanisms that underlie nonhost resistance and its counterpart, basic compatibility. For this purpose, the model system of barley (Hordeum vulare L.) interacting with appropriate or inappropriate formae speciales of the biotrophic fungal pathogen Blumeria graminis was used. Since colonization-success of the powdery mildew fungus depends upon the living host cell, it was also part of this work to investigate the role of a putative cell death inhibitor in cell death and defense regulation in barley.
A macroarray based approach was followed to comparatively analyze the expression of 1,536 barley gene transcripts in the early host interaction with Blumeria graminis f.sp. hordei (Bgh) and the nonhost pathogen Blumeria graminis f.sp. tritici (Bgt), respectively. The cDNA fragments on the macroarray mainly derived from epidermal peels of plants pre-treated with the chemical resistance activating compound acibenzolar-S-methyl, and were therefore expected to be enriched with defense-related transcripts. 102 spots corresponding to 94 genes repeatedly gave B. graminis-responsive signals on the macroarray at 12 and/or 24 hours after inoculation. In independent expression analyses, the differential expression of 18 arbitrarily selected genes could be confirmed. The temporal expression profile of the majority of the genes was similar in the compatible and the incompatible interaction. The data support the view that common genetic and mechanistic elements of plant defense underlie background resistance in compatible interactions and nonhost resistance.
BAX INHIBITOR-1 (BI-1) proteins are negative regulators of programmed cell death in mammals and plants. When overexpressed in epidermal cells of barley, BI-1 suppressed non-specific background resistance and mlo-mediated penetration resistance to the biotrophic fungal pathogen Bgh. It could be demonstrated that overexpression of BI-1 partially protected barley cells from cell death and breaks nonhost resistance of barley epidermal cells to the nonhost pathogen Bgt. The degree of transgene-induced accessibility was thereby similar to the effect achieved by overexpression of the defense suppressor gene MLO and could not be further enhanced by simultaneous expression of both BI-1 and MLO. Furthermore, results indicate that during defense suppression, BI-1 modulates defense-associated hydrogen peroxide accumulation underneath the site of attempted fungal penetration. In barley epidermal cells, a functional green fluorescing GFP-BI-1 fusion protein accumulated in endomembranes and the nuclear envelope and was found in the vicinity of the site of fungal attack and/or around intracellular fungal structures. Together, enhanced expression of barley BI-1 suppresses nonhost resistance to Bgt, linking barley nonhost penetration resistance with cell death regulation.
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