Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/13/2000
Publication Date: N/A
Citation: Interpretive Summary: Fungal diseases cause widespread damage to crop plants. The most efficient way to control these diseases in by the incorporation of genetic resistance to the pathogens that cause these diseases. In barley, a large number of genes, which confer resistance to the powdery mildew fungus, Blumeria (=Erysiphe) graminis f. sp. hordei, are clustered together at a locus designated Mla. We are interested in understanding how these genes trigge an active defense response. In order to do this, it is necessary to molecularly isolate the different variants of the Mla gene. By doing so, we can begin to understand the signaling processes involved to confer resistance to disease. In this report, we identified cDNA and genomic copies of the Mla6 allele. Two single-cell expression assays were employed to demonstrate functionality of the cloned Mla6 allele in barley and wheat. The second of these assays was used to substantiate that Mla6 function is dependent on the Mla-signaling gene, Rar1. Despite their differences in downstream signaling requirements, DNA sequence comparison of Mla6 and the recently cloned Mla1 allele has revealed that the two predicted proteins share highly similar architecture, thus, providing a unique situation from which to explore the subtle features of recognition and signaling specificity. This is one of the first two descriptions (together with the companion paper on Mla1 submitted to The Plant Cell) of the positional cloning of a functional Mla allele that confers resistance to an obligate fungal pathogen in barley. The results described in this manuscript will impact scientists that work in the area of resistance gene signaling and molecular pathology as well as the field of plant pathology and molecular breeding.
Technical Abstract: The barley Mla locus confers multiple resistance specificities to the obligate fungal biotroph, Blumeria (=Erysiphe) graminis f. sp. hordei. Interspersed within the 240-kb Mla complex are three families of resistance gene homologs (RGHs). Probes from the Mla-RGH1 family were used to identify three classes of cDNAs. The first class is predicted to encode a full-length CC-NBS-LRR protein and the other two classes contain alternatively spliced, truncated variants. Utilizing a cosmid that contains a gene corresponding to the full-length candidate cDNA, a single- cell expression assay was used to demonstrate complementation of AvrMla6- dependent resistance to B. graminis in barley and wheat. This assay was also used to substantiate previous genetic data that the Mla6 allele requires the signaling pathway component, Rar1, for function. Computational analysis of MLA6 and the Rar1-independent, MLA1 protein reveals 91.2% sequence identity and shows that the LRR domain is subject t diversifying selection. Our findings demonstrate that highly related CC- NBS-LRR proteins encoded by alleles of the Mla locus can dictate similar powdery mildew resistance phenotypes yet still require distinct downstream signaling components.