Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/19/2007
Publication Date: 1/2/2008
Citation: Ameline-Torregrosa, C., Wang, B., O'Bleness, M., Deshpande, S., Zhu, H., Roe, B., Young, N.D., Cannon, S.B. 2008. Identification and characterization of NBS-LRR encoded genes in the model plant medicago truncatula. Plant Physiology. 146:5-21. Interpretive Summary: One of the first lines of defense that plants have against pathogens and insect pests is a group of proteins that serve as sentinels against particular threats, much like the immune system antibody protein in mammals. These sentinal plant "disease resistance" proteins are produced from a group of genes called "NBS-LRRs," or "NBS genes." The NBS genes are among the most numerous classes of genes in any plant, usually with several hundred varieties of the gene coded in any plant's DNA. This study describes all known NBS genes in the biological model plant Medicago truncatula. The paper describes 333 known genes, of the 400-500 estimated to exist. An important feature of this group of genes is that it is organized in such a way that it can evolve rapidly, enabling it to respond over relatively short evolutionary time periods to new varieties of pathogens and pests. Information in this study will help plant researchers more quickly identify sources of resistance to important plant diseases.
Technical Abstract: The NBS-LRR gene family accounts for the largest number of known disease resistance genes, and is one of the largest gene families in plant genomes. We describe this gene family in the model legume genome Medicago truncatula (“Medicago”). The current public draft genome sequence (Mt1.0) is estimated to contain approximately two thirds of the euchromatic space of the genome, and a similar proportion of genes. Thus, the current set of 333 non-redundant NBS-LRRs in Mt1.0 likely represents 4-500 NBS-LRRs in the full genome. Many characteristics of the gene family are similar to those described on other plant genomes, including organization in clusters, rapid gene turnover, and diverse domain arrangements. The degree of clustering is even more extensive in Medicago than in other genomes described to date, with two extraordinarily large superclusters. One arm of chromosome 6 is dominated by TIR-NBS-LRRs (TNL) (with approximately 34% of all TNLs), and one arm of chromosome 3 is dominated byCC-NBS-LRRs (CNL) (with approximately 40% of all CNLs). Other noteworthy features include evidence of significant numbers of ectopic translocations from clusters to other parts of the genome, a small number of more evolutionarily stable NBS-LRRs, and numerous truncations and fusions leading to novel domain compositions. Almost all atypical domain combinations are in the TIR-NBS-LRR subfamily, and many occur within one genomic cluster. Expression patterns are highly variable across the gene family, and composition of upstream regulatory elements shows few systematic differences between clades or genomic locations.