Transcriptome analysis oF a tropical maize inbred line reveals a NOVEL mechanism of tolerance to Striga hermonthica.

Maize is a major staple cereal in sub-Saharan Africa. Its cultivation is highly affected by the parasitic weed Striga hermonthica, an important biotic constraint to production in Africa. Considerable efforts have been invested in breeding for Striga resistance in cereals, however, the level of protection achieved to date is incomplete and the resistance is mainly determined by the coexistence of several mechanisms controlled by multigenic and quantitative systems. A maize inbred line (ZD05) showing significant  tolerance to the Striga parasite was previously identified at the International Institute of Tropical Agriculture (IITA) in Nigeria but the underlying mechanisms of resistance is yet unknown. To gain insights into the molecular basis of such resistance, we performed RNAseq profiling of the tolerant (ZD05) and a susceptible (5057) maize genotype, both infested (I) and un-infested (U) with Striga hermontica, and at three different time points. Transcriptome analysis revealed that the resistance reaction was characterized by differential expression of genes involved in cell wall rearrangement and in secondary metabolism. Unexpectedly, we also detected differential regulation of genes involved in the hydroxamic acids pathway (DIMBOA). To assess the allelophatic effects of hydroxamic acids against Striga, in the susceptible and the tolerant maize inbred lines, we performed biological quantitative tests of root extracts under different experimental conditions. The level of parasitization resulted correlate to the DIMBOA content in roots subject to different treatments, and the resistant line showed higher concentration of this compound compared to susceptible line. Furthemore, in vitro striga seed germination was depleted upon increasing concentrations of DIMBOA. Finally, the expression levels of candidate genes, was confirmed by quantitative real-time PCR in an independent experiment. These results suggest that ZD05 resists to Striga infection using multiple biochemical and physiological mechanisms implying that the resistance is likely polygenic. The importance of the DIMBOA pathway in the mechanism of resistance to Striga in maize is being confirmed by loss-of-function experiments.