Nar-1 and Nar-2, Two Loci Required for Mla 12 -Specified Race-Specific Resistance to Powdery Mildew in Barley

Abstract

Previously isolated susceptible host mutants were used in a genetic and functional study of the resistance response of barley specified by resistance gene Mla12 to the fungal pathogen Erysiphe graminis f sp hordei. Mutant M66 represents a defective allele of Mla12, whereas M22, M82, and M100 represent mutations in loci unlinked to Mla12. Intermutant crosses of the latter three show that susceptibility in M82 and M100 is caused by allelic, recessive mutations that define the Nar-1 gene (necessary for Mla12 resistance gene 1), whereas the semidominant mutation in M22 defines a second unlinked locus, Nar-2. We show that both genes are required for resistance specified by Mlal2 in different genetic backgrounds of barley. Nar-1 maps on barley chromosome 2 within an ~6-centimorgan restriction fragment length polymorphism interval: this is 0.5 centimorgans from the anthocyanin pigmentation gene Ant2. Quantitative cytological analysis showed that functional alleles of Mla12, Nar-1, and Nar-2 are required for triggering a cell death reaction of attacked host cells at early stages during infection. Functional alleles of all three genes were also required for high-level transcript accumulation of barley defense-related genes that encode chitinase, peroxidase, and pathogenesis-related protein-1. The data support the hypothesis that host cell death and high-level accumulation of defense-related gene transcripts, which are under common control of Mla12, Nar-1, and Nar-2, are crucial events of race-specific resistance to powdery mildew.

Resistance gene-dependent plant defense responses.

“…Mutations at the Rarl and Rar2 loci (originally named Narl and Nar2) significantly compromised Mlamediated resistance and caused reduced HR formation and defense-gene induction (Freialdenhoven et al, 1994). However, mutant rar alleles do not affect Mlg-mediated resistance (J. Jorgensen and P. Schulze-Lefert, unpublished data).…”

“…Quantitative cytological investigations indicate that putative R gene-mediated host cell defense responses can occur at several distinct stages of fungal ontogeny, depending on the R locus involved. Mla allele-mediated resistance involves predominantly the HR of cells containing haustoria, and the induction of numerous defense-related genes (Freialdenhoven et al, 1994;Boyd et al, 1995). Mlg allele-mediated resistance acts earlier and involves papillae formation ( Figure 1C); the HR occurs to0 late to be considered causal to fungal arrest (Gorg et al, 1993).…”

The hypersensitive response and the induction of cell death in plants

“…Large surveys for mutants in loci necessary for normal R gene function have been undertaken (Freialdenhoven et al, 1994(Freialdenhoven et al, , 1996. Using such a genetic approach, Salmeron et al (1996) identified a gene, Prf, necessary for Pto function.…”

“…In barley, gene interaction studies have shown that a mutant allele of a gene required for Mla-based resistance (rar1; Freialdenhoven et al, 1994) does not abolish Mlgbased resistance at the macroscopic level. Interestingly, at the microscopic level, the rar1 mutation modifies the Mlg resistance phenotype.…”

Overexpression of barley BAX inhibitor 1 induces breakdown of mlo -mediated penetration resistance to Blumeria graminis

“…The hypersensitive reaction (HR) of plants to avirulent pathogens restricts pathogen growth effectively and includes a characteristic PCD of one or a few cells at the site of pathogen invasion (6)(7)(8)(9). Interestingly, although plant BI-1 expression can inhibit BAX-induced PCD in yeast and Arabidopsis, overexpression of Arabidopsis BI-1 in resistant RPM1 plants appeared not to interfere directly with the hypersensitive cell death reaction induced by avirulent Pseudomonas syringae pv.…”