Invisible to arbuscular mycorrhizal fungi: Positional cloning and characterisation of a novel arbuscular mycorrhizal mutant in Zea mays

Arbuscular mycorrhizal fungi from the Glomeromycotina form a beneficial symbiotic relationship with the root systems of a majority of plant species, enabling plants to profit from increased phosphate uptake efficiency. Further understanding of this interaction may prove vital in meeting the challenge of an increasing demand for phosphate supply in agriculture. Establishment of this symbiotic interaction relies on molecular crosstalk between the fungus and the plant roots in the rhizosphere, even before physical contact has occurred. A forward genetics screen has identified invisible to arbuscular mycorrhizal fungi (ina), a Zea mays mutant phenotype that displays a complete block in colonisation when inoculated with the arbuscular mycorrhizal fungus Rhizophagus irregularis. Extensive phenotyping has established that this maize mutant lacks the ability to signal to its symbiotic partner during the pre-symbiotic stage, abolishing the ability to initiate their beneficial relationship. To identify the impact of the removal of this signal, a transcriptomics analysis of Rhizophagus irregularis has found distinct transcriptomic signatures when spores are exposed to WT and mutant plant exudates. Positional cloning methods have identified a deleted region in the ina genome containing three candidate genes that may be responsible for this phenotype. Although further research is required to confirm the gene behind the ina phenotype, it was discovered that among these three candidate genes is the ortholog of a gene with a known role in the arbuscular mycorrhizal symbiosis, STR2.