A. thaliana seems to specifically recognize the AM fungi in initial stages of the interaction:
During the pre-symbiotic stage of an AM symbiosis both partners communicate through the exchange of diffusible molecules. Plant strigolactones have been identified as major contributors during host plant-AM fungi communication in pre-symbiotic stages. Upon perception of the AM fungal partner, strigolactone biosynthesis is induced in the host root. Interestingly, the strigolactone biosynthesis pathway is among the elements from the ‘symbiotic toolkit’ that are still present in the non-host A. thaliana. In order to study whether a strigolactone-based pre-symbiotic dialog is present in this AM incompatible interaction, we analyzed the activation of the A. thaliana strigolactone biosynthesis genes CCD7 and CCD8. We found that similar to AM-compatible interactions, the expression of both genes was induced in the non-host roots, after the perception of the AM fungus. These results suggest that the non-host plant A. thaliana might recognize actively the presence of the AM fungus Rhizophagus, increasing the expression of genes involved in the strigolactone biosynthesis pathway (CCD7 and CCD8). In order to further characterize the specificity of the R. irregularis-induced expression of CCD7 and CCD8, we tested the expression of these genes in roots in response to the pathogenic Fusarium oxysporum and the beneficial Trichoderma harzianum fungi. Our results show that neither the pathogen nor the beneficial fungus induced expression of CCD7 and CCD8. Our results suggest that the early induction of the strigolactone biosynthesis is still conserved in A. thaliana.
R. irregularis colonizes the non-host plant A. thaliana endophytically, inducing plant defense responses:
By using similar set-up as Veiga et al. (2103) we explored whether the non-host A. thaliana was able to establish a functional symbiosis when an active mycelium network was provided. We found that R. irregularis was able to colonize the root cortex of the non-host plant A. thaliana. However, arbuscules, which are the most characteristic structures in the AM symbiosis were not observed. Importantly, fungal colonization was only observed when R. irregularis was supported by a host plant, indicating the requirement of an active AM fungal network for A. thaliana colonization. These findings were further supported by the observation that transcripts of the constitutively expressed R. irregularis gene GintrRNA were expressed in A. thaliana roots grown in soil with an active mycorrhizal mycelium network. The absence of arbuscules in the cortex of A. thaliana indicates that the interaction between AM fungi and A. thaliana may not be functional. In order to verify this, we checked the expression of the fungal P and N transporter genes GintPT and GintAMT2, which are well-characterized markers for a functional AM symbiosis. The molecular analysis revealed that these genes were not up-regulated in R. irregularis when interacting with A. thaliana, further corroborating the absence of AM functionality. Collectively these results showing the ability of R. irregularis to colonize A. thaliana roots endophytically, and further demonstrating the absence of a functional AM symbiosis in this interaction.
In addition, we observed a strong growth reduction in shoot biomass of A. thaliana plants that were colonized by the AM fungus R. irregularis. This plant growth depression observed could be related to the non-compatible nature of the A. thaliana-R. irregularis interaction, which might induce costly defenses that in turn might compromise plant growth. To unravel the molecular mechanisms that underlie this plant growth reduction, a RNA-seq analysis was performed on A. thaliana roots after R. irregularis root colonization. To analyze the biological process triggered in the non-host plant A. thaliana after R. irregularis root colonization, a gene ontology (GO) terms analysis was performed. The analysis of the most significant GO terms over-represented in A. thaliana roots after R. irregularis colonization, showed the up-regulation of sulfur compounds related plant defense responses (i.e. glucosinolates and glycosinolates biosynthesis) and various categories associated to systemic acquired resistance. These results suggest that despite to the fact that the AM fungus R. irregularis can colonize A. thaliana roots in specific conditions, this colonization triggered plant defense responses in the non-host plant. This plant defense induction could explain the growth reduction observed in A. thaliana after R. irregularis root colonization.
