Phylogenetic Characterization of the Eubacterial lcrD Gene Family: Molecular Evolutionary Aspects of Pathogen-induced Hypersensitivity in Plants

Hrp (hypersensitivity response and pathogenicity) genes encode signal-peptide independent transporter molecules that function in the Type III secretion pathway and are present in a number of plant pathogenic bacterial species. These Hrp transporter molecules largely export harpin and other virulence factors across the bacterial membrane and onto the plant surface where they aid in disease induction on host and hypersensitivity on non-host. IIrp loci are part of a larger transporter gene family known as the lcrD family which encode the low calcium response proteins. Members of this family serve to transport a number of diverse virulence factors in a variety of enteric and other purple bacteria species both pathogenic to animals and plants. In order to determine whether the trait of hrp-induced pathogenicity by different plant pathogenic bacterial species is the result of a single evolutionary event or evolved independently, cladistic analyses were performed on both the nucleotide and the amino acid coding sequences of the complete lcrD gene family. The results of these studies supported the existence of three distinct phylogenetic clades. The plant pathogenic bacterial lcrD or hrp genes formed a single group separate and distinct from the animal lcrD homologues which comprised the other two clades. These findings indicate that the hrp transporter genes do not capture the phylogenetic history of their host bacterial species, but rather that during evolution, the same lcrD gene was horizontally introduced into each of four different plant pathogenic species which may have resulted from four independent transfer events. This monophyletic partitioning of hrp genes precludes their use as reliable taxonomic markers while further supporting the current notion that hrp transport molecules function under a single conserved molecular mechanism.