The largest group of plant disease resistance (R) genes that share similar structures
contains a predicted nucleotide-binding site (NBS) domain. NBS domains of this class of
R genes show highly conserved amino acid motifs, which makes it possible to isolate
resistance gene analogs (RGAs) by PCR with degenerate primers and homology
searches from public databases. Multiple combinations of degenerate primers were
designed from three conserved motifs (one motif was used for a subgroup-specific
primer design) in the NBS regions of R genes of various plants. All combinations of
primer pairs were used to amplify genomic DNA from sorghum. TIR-specific primer
combinations showed no PCR amplification in sorghum. Homology searches identified
many NBS-encoding sequences among the expressed or genomic molecular database
entries for sorghum. Motif analysis of the sorghum NBS sequences that were identified
in this study revealed eight major conserved motifs plus two additional highly conserved
motifs, but no TIR-specific motifs. Phylogenetic analysis of sorghum NBS sequences
showed tree topology typical of NBS-LRR genes, including clustered nodes and longbranch
lengths. Eleven distinct families of NBS sequences, representing a highly diverse
sample, were isolated from Sorghum bicolor. With two exceptions, sorghum RGA
families appeared to be closely related in sequence to at least one R-gene cloned from
other species. In addition, deduced amino acid sequences of sorghum RGAs showed
strong sequence similarity to almost all known non-TIR (Toll/Interleukin 1 Receptor)-
type R-genes. Mapping with sorghum RGA markers revealed one linkage group
containing four out of ten randomly selected markers, suggesting non-random
distribution of NBS sequences in the sorghum genome. Rice sequences homologous to
sorghum NBS sequences were found from two-way BLAST searches. Some of them
were shown to be orthologs, when determined by using phylogenetic approaches which
combined five different evolution models and tree-building methods.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/2375 |
| Date | 29 August 2005 |
| Creators | Cho, Jae-Min |
| Contributors | Magill, Clint W. |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | 1800632 bytes, electronic, application/pdf, born digital |
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