Genome- Wide Analysis of the Nucleotide Binding Site Leucine-Rich Repeat Genes of Four Orchids Revealed Extremely Low Numbers of Disease Resistance Genes.

Orchids are one of the most diverse flowering plant families, yet possibly maintain the smallest number of the nucleotide-binding site-leucine-rich repeat () type plant resistance () genes among the angiosperms. In this study, a genome-wide search in four orchid taxa identified 186 genes. Furthermore, 214 genes were identified from seven orchid transcriptomes. A phylogenetic analysis recovered 30 ancestral lineages (29 and one ), far fewer than other angiosperm families. From the genetics aspect, the relatively low number of ancestral genes is unlikely to explain the low number of genes in orchids alone, as historical gene loss and scarce gene duplication has continuously occurred, which also contributes to the low number of genes. Due to recent sharp expansions, and having 52 and 115 genes, respectively, and exhibited an "early shrinking to recent expanding" evolutionary pattern, while and both exhibit a "consistently shrinking" evolutionary pattern and have retained only five and 14 genes, respectively. genes remain in extremely low numbers with only one or two copies per genome. Notably, all of the orchid genes belong to the ADR1 lineage. A separate lineage, NRG1, was entirely absent and was likely lost in the common ancestor of all monocots. All of the genes were absent as well, coincident with the NRG1 lineage, which supports the previously proposed notion that a potential functional association between the and NRG1 genes.