Multiple layers of incompatibility to the parasitic witchweed, Striga hermonthica.

• Witchweeds (Striga spp.) are major agricultural pests that infest important crops in sub-Saharan Africa. Striga hermonthica parasitizes gramineous plants including sorghum, maize and rice, but not dicots. To understand host recognition mechanisms of S. hermonthica, we investigated its interaction with nonhost dicots including Arabidopsis, cowpea, Lotus japonicus and Phtheirospermum japonicum, a hemiparasite. * Striga hermonthica seeds were pretreated with strigol, a germination stimulant, and allowed to germinate next to a potential host root. We characterized the histological phenotype of the interactions. Moreover, we monitored the infection of a host rice and the nonhost P. japonicum by S. hermonthica using time-lapse photography. * All nonhost dicots tested did not support S. hermonthica shoot growth beyond the six leaf-pair stage; however, the arrest of parasite development occurred at different stages. Striga hermonthica haustoria were able to reach the steles of Arabidopsis and cowpea, while L. japonicus blocked S. hermonthica infection in the root cortex. Striga hermonthica often failed to penetrate P. japonicum roots. * Our analysis indicates that there are at least four types of incompatible interaction to S. hermonthica. Combinations of these different incompatibility mechanisms contribute to the total resistance to S. hermonthica.