Molecular Basis of Cowpea (Vigna unguiculata) Resistance to the Root Parasitic Angiosperm, Striga Gesnerioides.

Mellor, Karolina Ewa, Department of Biology, University of Virginia
Timko, Michael, Department of Biology, University of Virginia
Li, Lei, Department of Biology, University of Virginia
Henriksen, Melissa, Applied Research Institute, University of Virginia
Bekiranov, Stefan, Department of Biochemistry and Molecular Genetics, University of Virginia
Kozminski, Keith, Department of Biology, University of Virginia

Cowpea (Vigna unguiculata L. Walp.) is the most important grain legume grown in sub-Saharan Africa. Approximately 12.5 million tons of cowpea grains are produced worldwide each year, with the majority of the production (over 64%) taking place in low-input, subsistence farms in west and central Africa. In these regions, cowpea is often referred to as "poor man's meat" because of its high protein content (20-25%) and good nutritional value. Two characteristics add to the plant's agronomic importance: it is generally drought tolerant and it fixes nitrogen symbiotically, thereby enhancing soil fertility, especially when used in rotation with cereals. Like most crops, cowpea growth and grain yields are greatly reduced by a variety of biotic and abiotic stresses. Among the major biotic constraints is parasitism by Striga gesnerioides (Orobanchaceae) commonly referred to as witchweed. While most cowpea plants are susceptible to Striga parasitism, some local cowpea landraces and wild accessions have been identified to be resistant to the parasite. Here I report the molecular basis for cowpea resistance to Striga gesnerioides. To do this, I looked at global in gene expression in the resistant cowpea-Striga gesnerioides interaction, which take place in the early and late infection stages. The study was also completed for the susceptible cowpea-Striga interaction. I showed that a range of processes occur as a consequence of a Striga attack. Among the most highly induced genes are those involved in response to biotic and abiotic stimuli, wounding, oxidative stress and hypersensitive response, and components of the jasmonic and ethylene signaling pathways. Further I report that accompanying the visible hypersensitive response around the infection site is the up-regulation of genes involved in signal transduction and biosynthetic processes associated with formation barriers to 2 prevent parasite ingress. These include activation of gene expression associated with cell wall biogenesis and strengthening (e.g., lignification), as well as processes leading to programmed cell death at the host-parasite interface. I also explored the role of individual resistance components using a novel screening technique called a composite plants. I showed that a hypersensitive response which is present in the resistant cowpea-Striga interaction may be mediated through the jasmonic acid pathway and can be reduced by altering transcript levels of genes such as peroxidase, cationic peroxidase, narbonin, small heat shock protein and transcriptional activator - GRF. I also showed that reduction of aquaporin NIP1 and lipoxygenase transcripts levels in cowpea lead to enhanced susceptibility to Striga infection. Overall my data gives a better insight of the molecular processes that underlie the resistance of cowpea to Striga gesnerioides. My work explores host-pathogen interactions which were reported in other plant-parasite systems, but also gives some novel ideas about how the defense mechanism in cowpea may be operated.

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PHD (Doctor of Philosophy)
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