Molecular genetics of race-specific resistance of cowpea to Striga gesnerioides (Willd.)

Cowpea, Vigna unguiculata (L.) Walp., is an important warm-season legume grown primarily in the semi-arid tropics. The majority of cowpea is grown by subsistence farmers in West and Central sub-Saharan Africa, where its grain and leaves are valued as human food and its stover is used for animal forage. Like all crop plants, cowpea is subject to yield losses resulting from a variety of biotic and abiotic stresses. Among the important biotic constraints to cowpea production is parasitism by the root hemiparasitic weeds Striga gesnerioides (Willd.) [witchweed] and Alectra vogelii (Benth.). At least seven races of S. gesnerioides have been identified within the cowpea-growing regions of West Africa, based on host differential response and genetic diversity analysis. Several race-specific resistance genes have been identified and located to one of two linkage groups (LG1 and LG6) of the current cowpea genetic map. Molecular markers associated with race-specific resistance genes have been identified, and several sequence-confirmed amplified regions (SCARs) have been developed for use in marker-assisted selection and breeding strategies for rapid cowpea improvement. The mechanism of race-specific resistance in the cowpea-Striga interaction has also been examined, with several genes involved in phytohormone and general disease resistance signaling transduction observed to be differentially expressed in resistant and susceptible phenotypes. Results suggest that PR5 expression may be a useful marker of Striga infection, and that salicylic acid signaling appears to play a role in the cowpea-Striga interaction.