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Barley Rom1 Reveals a Potential Link Between Race-Specific and Nonhost Resistance Responses to Powdery Mildew Fungi

¹Max-Planck Institut für Züchtungsforschung, Carl-von-Linné-Weg 10, D-50829 Köln, Germany; ²The Sainsbury Laboratory, John Innes Centre, Colney Lane, Norwich NR4 7UH, U.K.; ³Institut für Genetik, Martin-Luther-Universität Halle-Wittenberg, Weinbergweg 10, D-06120 Halle (Saale), Germany

The Rar1 gene, identified in the context of race-specific powdery mildew resistance mediated by the Hordeum vulgare (barley) resistance (R) gene Mla12, is required for the function of many R-mediated responses in monoand dicotyledonous plant species. Mla resistance is associated with an oxidative burst and a subsequent cell death reaction of attacked cells. Rar1 mutants are impaired in these responses and, to identify genetic elements which negatively regulate the Mla12-triggered response, we have screened mutagenized Mla12 rar1 mutant populations for restoration of the resistance response. Here we describe the restoration of Mla12-specified resistance (rom1) mutant that restores features of disease resistance to a Blumeria graminis f. sp. hordei isolate expressing the avirulence gene AvrMla12 and retains susceptibility to an isolate lacking AvrMla12. Histochemical analyses show that, in rom1 mutant plants, a whole-cell oxidative burst and cell death response in attacked epidermal cells is restored in the incompatible interaction. Defense responses against tested inappropriate powdery mildews, B. graminis f. sp. tritici and Golovinomyces orontii, were diminished in rar1 mutant plants and enhanced in rom1 mutant plants relative to the wild type. These findings indicate antagonistic activities of Rar1 and Rom1 and reveal their contribution to nonhost and race-specific resistance responses.

Additional keywords: negative regulator, pathogen resistance, suppressor screen.