The root hemiparasite witchweed (Striga spp.) is a devastating agricultural pest that causes losses of up to $1 billion US annually in sub-Saharan Africa. Development of resistant crops is one of the cost-effective ways to address this problem. However, the molecular mechanisms underlying resistance are not well understood. To understand molecular events upon Striga spp. infection, we conducted genome-scale RNA sequencing expression analysis using Striga hermonthica-infected rice (Oryza sativa) roots. We found that transcripts grouped under the Gene Ontology term defense response were significantly enriched in up-regulated differentially expressed genes. In particular, we found that both jasmonic acid (JA) and salicylic acid (SA) pathways were induced, but the induction of the JA pathway preceded that of the SA pathway. Foliar application of JA resulted in higher resistance. The hebiba mutant plants, which lack the JA biosynthesis gene allene oxide cyclase, exhibited severe S. hermonthica susceptibility. The resistant phenotype was recovered by application of JA. By contrast, the SA-deficient NahG rice plants were resistant against S. hermonthica, indicating that endogenous SA is not required for resistance. However, knocking down WRKY45, a regulator of the SA/benzothiadiazole pathway, resulted in enhanced susceptibility. Interestingly, NahG plants induced the JA pathway, which was down-regulated in WRKY45-knockdown plants, linking the resistant and susceptible phenotypes to the JA pathway. Consistently, the susceptibility phenotype in the WRKY45-knockdown plants was recovered by foliar JA application. These results point to a model in which WRKY45 modulates a cross talk in resistance against S. hermonthica by positively regulating both SA/benzothiadiazole and JA pathways.
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