An in vitro technique for studying specific Striga resistance mechanisms in sorghum

Witchweed (Striga sp.) is a noxious parasitic weed of many cereals, that causes considerable crop damage in the semi-arid tropics. Although, a number of control measures have been suggested, breeding crops which are resistant to the attack is the most feasible and effective way of control. However, breeding efforts have been hampered by the lack of adequate laboratory techniques that uncover critical host-parasite interactions, that occur naturally beneath the soil. Germination stimulant production is the sole Striga resistance mechanism in sorghum [Sorghum bicolor (L) Moench] that has been extensively studied and exploited for breeding purposes. Other Striga resistance mechanisms have not been effectively characterized and used. The purpose of this study was to develop an in vitro screening technique for evaluation of sorghum germplasm for specific Striga resistance mechanisms. We hereby report the development of a reliable screening technique, the Extended Agar Gel Assay (EAGA). Using the technique, we screened seven sorghum genotypes with known reactions to Striga parasitsm, SRN39, Framida, IS9830, 555, N13, Dobbs, Serena , CK60B, Shanqui Red, IS-4225, and two wild sorghum accessions, P78, and P47121, and we were able to characterize specific host defense reactions (mechanisms). These reactions indicate the potential existence of at least four separate mechanisms of Striga resistance in sorghum: 1) low production of Striga seed germination stimulants; 2) evidence of germination inhibitors; 3) low production of the signal required for haustoria initiation and 4) a hypersensitive response (characterized by a distinct necrotic area on the host root at the attachment site that discourage parasitic establishment. The development of this laboratory assay enabled us to identify and characterize distinct resistance mechanisms. These mechanisms can be exploited through conventional plant breeding programs. Additionally, the resistance genes can be pyramided into one background for more durable Striga resistance.