Genome-Wide Analysis Of NBS Resistance Genes And Evolution In Medicago Truncatula

During the growth and development process, most plants are susceptible to infection by a variety of pathogens, and in the long progress of evolution,plants have developed a variety of mechanisms to detect pathogens and pests. The study of plant disease resistance genes is one of the key subjects of plant genetics and plant pathology. As the largest class of disease resistance genes, the genes encoding nucleotide-binding-site (NBS) play a critical role in defending plants from a multitude of pathogens and pests. In the present study, a full set of disease resistance (R) candidate genes encoding (NBS) from the complete genome of Medicago was identified and characterized using structural diversity and evolutional relationship, and also compared with monocots and dicots plants, represented by Sorghum and Arabidopsis. These results provided an important reference for the selection of candidate NBS resistence genes for cloning, which can contribute to disease-resistant breeding in plants. The main results are as follows:1. By BLAST (Basic Local Alignment Search Tool) and HMM (Hidden Markov Model) analysis, 446 regular and 23 non-regular NBS resistence genes were identified from Medicago complete genome by BLAST search. According to sequences analysis, 446 regular resistence genes could be divided into 10 categories. 75 NBS resistence genes contained the TIR domain, and 173 resistence genes had two CC structure, which consisted of 20-50 amino acid residues in the N terminal of NBS domain.2. Gene duplications were analyzed for the 446 regular resistence genes of Medicago, and compared with rice and Arabidopsis. The results showed that, in Medicago, the multi-genes, the ratio of multi-genes and single-genes, average members of per cluster and gene family numbers were distinctively higher in rice and Arabidopsis during the 446 NBS resistence genes. It indicated that gene duplications play an important role in Medicago's NBS resistence genes expansion.3. By analyses of chromosomal locations of NBS resistence genes, we found that NBS resistence genes appeared on each chromosome was non-random. the largest numbers of NBS resistence genes are located on chromosome 3 (125 genes), chromosome 5 and 6 wase less than chromosome 3, respectively. The resistence genes showed the remarkable clusted phenomenon on the chromosomes, but there was not distinct rule between the different types NBS genes'location on the chromosomes.4. Based on the phylogenetic analysis of Medicago NBS resistence genes, we found that the major clades was obviously. Most of the clades likely contained the same types NBS resistence genes, but some clades were also consisted of different types genes.5. Comparative analysis of gene structure, phylogenetic evolution and chromosomal locations were performed among Sorghum, Medicago and Arabidopsis, and some obvious differences were found. In Sorghum, the NBS resistence genes contained the TIR domains were not identified, which confirmed that the TIR domain contained genes were dicots-specific proteins again. In addition, the phylogenetic tree of the Medicago NBS resistence genes was similar to that of Arabidopsis, but different from Sorghum, which also contained different types NBS genes in the same clades. The chromosomal locations of the Medicago NBS resistence genes were also similar to that of Arabidopsis, and the majority of Sorghum NBS resistence genes were distributed on the distal end of each chromosome.