Striga hermonthica Del (Benth.) : biology of the host/parasite relationship

Striga hermonthica is a parasitic angiosperm that attaches to the roots of cereal hosts and causes severe reductions in crop yields m the semi-arid tropics of Africa. Striga seeds germinate in response to chemicals exuded by the host root. Further host derived chemicals on the root surface initiate the development of an haustorium (infection organ) in the parasite. The Striga seedling taps into the vascular system of the host via the haustorium, thereby obtaining all its water and nutrients. This thesis examines early parasite/host interactions. A method was developed to enable the production of gram quantities of germinated and haustorially induced S. hermonthica seedlings in vitro, thereby facilitating physiological, biochemical and molecular analysis of the haustorium. Ethylene was used to germinate the Striga seeds, and 2,6-dimethoxybenzoquinone (2,6-DMBQ) used to induce haustorial development. Induction of haustorial formation was rapid with only short exposures to 2,6-DMBQ being required, indicating the rapidity with which the haustorial inducer took effect. Water soluble proteins were extracted from seedlings at various stages of haustorial development and analysed by 2-D PAGE. This revealed that changes in protein synthesis occurred within 45 min exposure to 2,6-DMBQ. Further changes were observed as haustorial development continued. Analysis of cell-free translation products demonstrated a differential accumulation of translatable mRNA species from haustorially induced seedlings, indicating that increased or de novo gene transcription had occurred following exposure to 2,6-DMBQ. An investigation of host defence-related proteins revealed that peroxidase and phenylalanine ammonia lyase activities may be useful markers for host tolerance/resistance in maize and cowpea. Furthermore, pathogenesis-related proteins were found to be induced at low levels in Striga-infected maize cultivars and a novel mRNA was induced in sorghum roots following infection. These results indicate that Striga infection can stimulate responses similar to microbial pathogens in host plants. Finally, protocols were developed enabling nucleic acid extractions from Striga tissues, facilitating further molecular studies on S. hermonthica.