Uncovering the molecular mechanism underlying the virulence of Striga gesnerioides

Cowpea (Vigna unguiculata L.) is the most important food and forage legume in the African Sahel providing essential income and protein nutrition to millions of farmers. While most cowpea cultivars are susceptible to the root parasitic weed Striga gesnerioides, cultivar B301 is resistant to all known parasite races except SG4z. When challenged by races SG4 and SG3, the roots of B301 display a hypersensitive response (HR) at the site of parasite attachment followed by death of the invading parasite. In contrast, no visible response occurs in B301 roots parasitized by SG4z and the parasite successfully penetrates the host root cortex, forms vascular connections, and grows to maturity. This study seeks to clarify the molecular mechanism underlying the virulence of S. gesnerioides overcoming host resistance.

In order to identify potential parasite components that specifically suppress or elicit host defense responses, I have carried out transcriptome profiling using high throughput RNA-seq analysis on the tissues isolated from parasite haustoria during compatible and incompatible response of resistant (B301) and susceptible (Blackeye) cowpea cultivars with three races of S. gesnerioides (SG3, SG4 and SG4z). Comparative transcriptomics and in silico analysis revealed the first S. gesnerioides transcriptome and several candidate effectors that display differential expression between individual races, that may contribute to race-specific virulence.

To clarify the molecular mechanism underlying the hypervirulence of SG4z, I molecularly characterized a leucine-rich receptor (LRR)-protein kinase (PK) homolog dubbed SHR4z (Suppressor of Host Resistance 4z) that is highly expressed in SG4z haustoria and secreted into the host root. I found that overexpression of SHR4z in transgenic B301 roots leads to suppression of HR elicitation and loss of host innate immunity by targeting a host BTB-BACK domain containing ubiquitin E3 ligase homolog (VuPOB1). Subsequent silencing VuPOB1 expression in transgenic B301 roots lowers the frequency of hypersensitive response (HR) while overexpression of VuPOB1 results in decreased parasitism by SG4z suggesting VuPOB1 functions as a positive regulator of HR and plant innate immunity.

Finally, to explore host effect on Striga virulence, I examined global expression changes of the parasite during compatible and incompatible interactions with cowpea. I found that resistant hosts trend to induce the expression of disease resistant genes in SG3 and SG4 relative to susceptible hosts but suppress the expression of genes responsible for development. Comparative analysis on the expression profile of SG4z revealed that, even though SG4z triggers the same response from B301 and Blackeye, several candidate effectors still display differential expression at the interactions with different hosts. These candidates may contribute to the suppression of host resistance by SG4z.
Overall, this thesis study provides new insight into the role of secreted effectors as part of the strategy used by parasitic weeds to overcome host immunity and complete their life-cycles. It could potentially contribute to the development of novel strategies for controlling Striga and other parasitic weeds thereby enhancing crop productivity and food security globally.