A Specific Resistance Gene to Leptosphaeria maculans, Rlm11, Has a Limited Impact on Brassica napus Mycobiota Compared with Plant Compartment or Cropping Season Effects

One important ecological question regarding the use of plant resistance genes against fungal pathogens concerns whether and how such resistance genes may modify pathogenic or beneficial members of the plant-associated microbiota. We studied the impact of a plant resistance gene by analyzing the mycobiota associated with Brassica napus organs over two cropping seasons. Sampling dates coincided with key stages of the life cycle of the B. napus pathogen Leptosphaeria maculans. Leaf samples were collected at three time points in autumn and spring, and stem base samples were collected at two time points a few weeks before and at harvest. Stem residues, where L. maculans survives in the intercropping season and develops sexual reproduction, were also analyzed at four time points between the two cropping seasons. The sampling was performed on two plant genotypes, Darmor and Darmor- Rlm11, only differing by the effective resistance gene against L. maculans, Rlm11. Altogether, 419 samples were analyzed using two barcode: internal transcribed spacer (ITS) and Actin. The plant organ was shown to be the main mycobiota structuring factor, as clear-cut alternation of the species suggested that each plant organ represented a specific ecological niche. The cropping season and plant genotype also significantly influenced the community structure in lower proportions. The resistance gene contributed differently to the community structure depending on the year and the organ concerned. A significant but low impact of Rlm11 on other B. napus fungal pathogens was detected. The ITS and Actin barcodes showed similar results, but the species assignation was limited for the latter.