Biological and Transcriptomic Characterization of Pre-haustorial Resistance to Sunflower Broomrape (Orobanche cumana W.)

Infestations with sunflower broomrape (Orobanche cumana Wallr.), an obligatory root parasite, constitute a major limitation to sunflower production in many regions around the world. Breeding for resistance is the most effective approach to reduce sunflower broomrape infestation, yet resistance mechanisms are often overcome by new races of the pathogen. Elucidating the mechanisms controlling the resistance to broomrape at the molecular level is thus the most desirable pathway to obtaining long-lasting resistance and reducing yield loss in sunflower. In this study, we investigated broomrape resistance in a confectionery sunflower hybrid with a robust and long-lasting resistance to sunflower broomrape. Visual screening and histological examination of sunflower roots revealed that penetration of the intrusive broomrape cells into the host root endodermis is blocked at the host cortex, indicating a pre-haustorial mechanism of resistance. A comparative RNA-Seq experiment conducted between roots obtained from the resistant cultivar, a bulk of five broomrape resistant lines and a bulk of five broomrape susceptible lines allowed the identification of genes that were significantly differentially expressed upon broomrape infestation. Among these differentially expressed genes, {beta}-1,3-endoglucanase, {beta}-glucanase and ethylene-responsive transcription factor4 (ERF4) genes were identified. These genes were previously reported to be pathogenesis-related genes in other plant species. This genetics investigation together with the histological examinations led us to conclude that the resistance mechanism involves the identification of the broomrape and the consequent formation of a physical barrier that prevents the penetration of the broomrape into the sunflower roots.