Abstract
Arbuscular mycorrhizal fungi (AMF) are of great ecological importance because of their effects on plant growth. Closely related genotypes of the same AMF species coexist in plant roots. However, almost nothing is known about the molecular interactions occurring during such coexistence. We compared in planta AMF gene transcription in single and coinoculation treatments with two genetically different isolates of Rhizophagus irregularis in symbiosis independently on three genetically different cassava genotypes. Remarkably few genes were specifically upregulated when the two fungi coexisted. Strikingly, almost all of the genes with an identifiable putative function were known to be involved in mating in other fungal species. Several genes were consistent across host plant genotypes but more upregulated genes involved in putative mating were observed in host genotype (COL2215) compared with the two other host genotypes. The AMF genes that we observed to be specifically upregulated during coexistence were either involved in the mating pheromone response, in meiosis, sexual sporulation or were homologs of MAT-locus genes known in other fungal species. We did not observe the upregulation of the expected homeodomain genes contained in a putative AMF MAT-locus, but observed upregulation of HMG-box genes similar to those known to be involved in mating in Mucoromycotina species. Finally, we demonstrated that coexistence between the two fungal genotypes in the coinoculation treatments explained the number of putative mating response genes activated in the different plant host genotypes. This study demonstrates experimentally the activation of genes involved in a putative mating response and represents an important step towards the understanding of coexistence and sexual reproduction in these important plant symbionts.
Highlights
Most terrestrial plants form symbioses with arbuscular mycorrhizal fungi (AMF), and this greatly impacts plant growth and diversity of plant communities [1, 2]
Six million reads did not map to the cassava genome and 3.45 million reads mapped to the R. irregularis genome (Supplementary Fig. 2 and Supplementary File 2)
We did not observe any statistically significant difference in the number of total reads, reads uniquely mapped to cassava, reads uniquely mapped to R. irregularis, and the number of genes detected between the coinoculation treatment compared with each single-inoculation treatment in plant genotype COL2215
Summary
Most terrestrial plants form symbioses with arbuscular mycorrhizal fungi (AMF), and this greatly impacts plant growth and diversity of plant communities [1, 2]. A study of R. irregularis isolates reported the identification of a putative MAT-locus, based on homology, that contained an HD1-like and HD2 HD gene in opposite transcriptional directions [8]. The presence of this locus was confirmed in the Glomerales and Diversiporales, with the exception of Gigasporaceae, where the locus presented no structural conservation [19]. No study has provided any experimental evidence of the involvement of this putative MAT-locus, or any other genes, in the interaction between genetically different AMF isolates of the same species. We investigated the molecular mechanisms of nonself-interactions between two genetically different, but closely related, R. irregularis isolates coexisting in planta. Coexistence of genetically different Rhizophagus irregularis isolates induces genes involved in a
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