Soil microbiota promotes early developmental stages of Phelipanche ramosa L. Pomel during plant parasitism on Brassica napus L.

The root holoparasitic plant Phelipanche ramosa has become a major constraint for rapeseed cultivation in western France for the last decades and its control remains challenging. To date, few studies have considered soil microbiota as a third partner of the parasitic plant-plant interaction. Therefore, we here addressed the question of how soil microbiota interferes with host-derived signal metabolites required for host plant recognition by the parasitic plant. Using a branched broomrape infested soil (genetic group 1) from a rapeseed field, we first provided soil physicochemical and microbiological descriptions by metabarcoding, followed by P. ramosa seed germination and prehaustorium formation bioassays, and by in vitro co-cultivation with Brassica napus. Co-cultivation in presence of soil microorganisms promoted parasitic plant seed germination and attachments to host's roots. Seed germination assays showed that only the combination of gluconasturtiin (main rapeseed glucosinolate) with soil extracts stimulated broomrape germination. This suggests a microbial conversion of gluconasturtiin into germination stimulants via soil microbial myrosinase enzymes. Furthermore, soil bacteria Arthrobacter, Ralstonia, Actinobacterium, Proteobacterium spp. and fungus Penicillium spp. were isolated and screened for myrosinase activity. Pre-germinated seeds treated with soil extracts or differentially filtrated soil extracts also promoted the formation of P. ramosa prehaustorium and led to more parasitic attachments on rapeseed roots in co-cultivation assays. This thus suggests that this enhancement of parasitic attachments could also be partly attributed to soil microbial production of haustorium inducing factors. Soil microbiota influences B. napus - P. ramosa interaction by altering direct and indirect recognition signals.