The penetration of sunflower root tissues by the parasitic plantOrobanche cumanaWallr. is intracellular

Parasitic plants cause yield losses for important crops. Among these, Orobanche cumana Wallr, sunflower broomrape, is one of the major pests for sunflower. Previous studies stated that in most cases, the haustorium, a specific parasitic plant organ, penetrates host roots intercellularly. However, host cellular mechanisms involved during the parasitic cells penetration remained poorly described. We investigated sunflower root cellular behavior during haustorium penetration using various microscopy approaches including live cell imaging of inoculated transgenic fluorescent sunflower roots. We showed that the haustorium of O. cumana penetrated living sunflower root tissues, as a result of the degradation of the host cell wall and the formation of a new host trans-cellular apoplastic compartment for haustorium accommodation. Moreover, broomrape induced cell divisions in outer root tissues at very early stages of the interaction, leading to localized hypertrophy at the site of broomrape attachments. These findings are a change of paradigm in the research field of parasitic plants. They extend host root intracellular accommodation mechanisms initially shown for symbiotic and pathogenic biotrophic fungi to parasitic plants. It paves the way for future understanding and development of resistance to parasitic plants. Key message Combination of in vivo confocal, large field and transmission electron microscopy approaches revealed how intimate the relationship between the parasitic plant broomrape ( Orobanche cumana Wallr.) and its sunflower host ( Helianthus annus L.) is at very early stages of their interaction.