Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/12/2003
Publication Date: 4/1/2004
Citation: Halterman, D.A., Wise, R.P. 2004. A single amino acid substitution in the sixth leucine-rich repeat of barley MLA6 and MLA13 alleviates dependence on RAR1 for disease resistance signaling. Plant Journal. 38:215-226. Interpretive Summary: Diseases caused by fungi, viruses, bacteria, insects, and nematodes impact agronomic and horticultural crops, in addition to commercial and recreational forests. Hence, major efforts have been devoted to understanding the mechanisms of genetic resistance and incorporating it into breeding programs to offset yield loss caused by pathogens. Many disease resistance responses of plants to pathogens are governed by resistance-protein mediated signaling via the SCF ubiquitin ligase complex. In barley, MLA-specified resistance to the obligate fungal pathogen, Blumeria graminis f. sp. hordei, may or may not require the zinc-binding protein, RAR1, as part of the SCF complex to activate downstream components. This article demonstrates that a single glycine to aspartate substitution in the 6th leucine-rich-repeat (LRR) of RAR1-dependent MLA proteins redirect downstream signaling, but retain the original resistance response. These results suggest that interactions that regulate resistance signal transduction between R proteins and RAR1 may be subject to subtle intra- or intermolecular folding between repeat motifs. The research is of high impact because plant diseases are among the greatest deterrents to crop production worldwide. Further understanding of the basic mechanisms of disease resistance will result in less damage to crops and higher yields, therefore increasing sustainability and profitability.
Technical Abstract: Interactions between barley and the powdery mildew pathogen, Blumeria graminis f. sp. hordei (Bgh), are determined by unique combinations of host resistance genes, designated Mildew-resistance locus (Ml), and cognate pathogen avirulence genes. These interactions occur both dependent and independent of Rar1 (required for Mla12 resistance) and Sgt1 (Suppressor of G-two allele of skp1), which are differentially required for diverse plant disease-resistance pathways. We have isolated two new functional Mla alleles, Rar1-independent Mla7 and Rar1-dependent Mla10, as well as the Mla paralogs, Mla6-2 and Mla13-2. Utilizing the inherent diversity amongst Mla-encoded proteins, we identified the only two amino acids exclusively conserved in RAR1-dependent MLA6, MLA10, MLA12, and MLA13 that differ at the corresponding position in RAR1-independent MLA1 and MLA7. Two- and three-dimensional modeling places these residues on a predicted surface of the sixth leucine-rich repeat (LRR) domain at positions distinct from those within the beta-sheets hypothesized to determine resistance specificity. Site-directed mutagenesis of these residues indicates that RAR1 independence requires the presence of an aspartate at position 721, as mutation of this residue to a structurally similar, but uncharged, asparagine did not alter RAR1 dependence. These results demonstrate that a single-amino acid substitution in the sixth MLA LRR can alter host signaling but not resistance specificity to B. graminis.