Large-Scale Analyses of Angiosperm Nucleotide-Binding Site-Leucine-Rich Repeat Genes Reveal Three Anciently Diverged Classes with Distinct Evolutionary Patterns

Nucleotide-binding site-leucine-rich repeat (NBS-LRR) genes make up the largest plant disease resistance gene family (R genes), with hundreds of copies occurring in individual angiosperm genomes. However, the expansion history of NBS-LRR genes during angiosperm evolution is largely unknown. By identifying more than 6,000 NBS-LRR genes in 22 representative angiosperms and reconstructing their phylogenies, we present a potential framework of NBS-LRR gene evolution in the angiosperm. Three anciently diverged NBS-LRR classes (TNLs, CNLs, and RNLs) were distinguished with unique exon-intron structures and DNA motif sequences. A total of seven ancient TNL, 14 CNL, and two RNL lineages were discovered in the ancestral angiosperm, from which all current NBS-LRR gene repertoires were evolved. A pattern of gradual expansion during the first 100 million years of evolution of the angiosperm clade was observed for CNLs. TNL numbers remained stable during this period but were eventually deleted in three divergent angiosperm lineages. We inferred that an intense expansion of both TNL and CNL genes started from the Cretaceous-Paleogene boundary. Because dramatic environmental changes and an explosion in fungal diversity occurred during this period, the observed expansions of R genes probably reflect convergent adaptive responses of various angiosperm families. An ancient whole-genome duplication event that occurred in an angiosperm ancestor resulted in two RNL lineages, which were conservatively evolved and acted as scaffold proteins for defense signal transduction. Overall, the reconstructed framework of angiosperm NBS-LRR gene evolution in this study may serve as a fundamental reference for better understanding angiosperm NBS-LRR genes.