Antagonistic interactions between the two phytopathogens Leptosphaeria maculans and Leptosphaeria biglobosa : only a question of location and timing
Leptosphaeria maculans 'brassicae' (Lmb) and Leptosphaeria biglobosa 'brassicae' (Lbb) are two fungi infecting rapeseed (Brassica napus) and belong to a species complex. They are found together, coinfecting rapeseed throughout the whole plant life, from the early stage of infection on leaves to the final necrotic stage at the stem base and survival on plant residues. Lmb has a hemibiotrophic lifestyle on leaves and causes the basal stem canker. Lbb is considered as a strict necrotroph on leaves and stems and causes less severe stem lesions than Lmb. Previous studies have suggested that Lbb could limit the early colonisation of Lmb during coinfection and consequently, decrease the incidence and damages caused by Lmb. However, this potential interaction is still poorly understood at the molecular level. Here we took advantage of RNA-Sequencing technology to decipher the role of the three partners (including the plant host) during a two-week kinetics of coinfections on cotyledons in controlled experiments. Fungal biomass quantifications and fungal development monitoring provided complementary approaches to get an integrative overview of this tri-partite interaction. We demonstrated that during simultaneous co-infection by both species, Lmb was strongly impacted in its transcriptomic dynamics and its development in planta: in the presence of Lbb, Lmb cannot initiate the biotrophic part of its cycle required for massive mycelium increase prior to sporulation and systemic colonization of tissues In contrast, Lbb was unaffected by the presence of Lmb. The response of the host to Lmb or Lbb individually confirmed the hemibiotrophic or necrotrophic behaviour of each species while, during coinfection, the plant responded to the mixed inoculum identically to the response to Lbb alone. However, the drastic inhibition of Lmb growth by the presence of Lbb strongly depended on the Lbb inoculation parameters. Whether due to competition for trophic resources, to the generation by Lbb of an environment unsuitable for the lifecycle of Lmb or/and to the effect of plant defense responses induced by Lbb on Lmb, the final result is that Lmb is blocked early in the setting up of its pathogenicity program.