Global changes in gene expression during compatible and incompatible interactions of cowpea (Vigna unguiculata L.) with the root parasitic angiosperm Striga gesnerioides

ABSTRACT: BACKGROUND: Cowpea, Vigna unguiculata L. Walp., is one of the most important food and forage legumesin the semi-arid tropics. While most domesticated forms of cowpea are susceptible to the rootparasitic weed Striga gesnerioides, several cultivars have been identified that show racespecificresistance. Cowpea cultivar B301 contains the RSG3-301 gene for resistance to S.gesnerioides race SG3, but is susceptible to race SG4z. When challenged by SG3, roots ofcultivar B301 develop a strong resistance response characterized by a hypersensitive reactionand cell death at the site of parasite attachment. In contrast, no visible response occurs inB301 roots parasitized by SG4z. RESULTS: Gene expression in the roots of the cowpea cultivar B301 during compatible (susceptible) andincompatible (resistant) interactions with S. gesnerioides races SG4z and SG3, respectively,were investigated at the early (6 days post-inoculation (dpi)) and late (13 dpi) stages of theresistance response using a Nimblegen custom design cowpea microarray. A total of 111genes were differentially expressed in B301 roots at 6 dpi; this number increased to 2102genes at 13 dpi. At 13 dpi, a total of 1944 genes were differentially expressed duringcompatible (susceptible) interactions of B301 with SG4z . Genes and pathways involved insignal transduction, programmed cell death and apoptosis, and defense response to biotic andbiotic stress were differentially expressed in the early resistance response; at the later timepoint, enrichment was primarily for defense-related gene expression, and genes encodingcomponents of lignifications and secondary wall formation. In compatible interactions (B301- SG4z), multiple defense pathways were repressed, including those involved in ligninbiosynthesis and secondary cell wall modifications, while cellular transport processes fornitrogen and sulfur were increased. CONCLUSION: Distinct changes in global gene expression profiles occur in host roots following successfuland unsuccessful attempted parasitism by Striga. Induction of specific defense related genesand pathways defines components of a unique resistance mechanism. Some genes andpathways up-regulated in the host resistance response to SG3 are repressed in the susceptibleinteractions, suggesting that the parasite is targeting specific components of the host'sdefense. These results add to our understanding of plant-parasite interactions and theevolution of resistance to parasitic weeds.