Ancient diversification of the Pto kinase family preceded speciation in Solanum

Recent phylogenetic analyses of the nucleotide binding sites (NBS)-leucine- rich repeats (LRR) class of plant disease resistance (R) genes suggest that these genes are ancient and coexist next to susceptibility alleles at resi stance loci. Another class of R genes encodes serine-threonine protein kina ses related to Pto that were originally identified from wild relatives of t omato, In this study, we exploit the highly diverse genus Solanum to identi fy Pto-like sequences and test various evolutionary scenarios for Pto-like genes. Polymerase chain reaction amplifications with the use of primers tha t were developed on the basis of conserved and variable regions of Pto reve aled an extensive Pto gene family and yielded 32 intact Pto-like sequences from six Solanum species. Furthermore, Pto-like transcripts were detected i n the leaf tissue of all tested plants. The kinase consensus and autophosph orylation sites were highly conserved, in contrast to the kinase activation domain, which is involved in ligand recognition in Pto, Phylogenetic analy ses distinguished nine classes of Pto-like genes and revealed that ortholog s were more similar than paralogs, suggesting that the Pto gene family evol ved through a series of ancient gene duplication events prior to speciation in Solanum, Thus, like the NBS-LRR class, the kinase class of R genes is h ighly diverse and ancient.