Isolation of a premycorrhizal infection (pmi2) mutant of tomato, resistant to arbuscular mycorrhizal fungal colonization

Arbuscular mycorrhizae (AM) represent an ancient symbiosis between mycorrhizal fungi and plant roots which co-evolved to exhibit a finely tuned, multistage interaction that assists plant growth. Direct screening efforts for Myc^- plant mutants resulted in the identification of a tomato (Lycopersicon esculentum L. cv. Micro-Tom) mutant, M20, which was impaired in its ability to support the premycorrhizal infection (pmi) stages. The Myc^- phenotype of the M20 mutant was a single Mendelian recessive trait, stable for nine generations, and nonallelic to a previously identified M161 pmi mutant. The M20 mutant was resistant to infection by isolated AM spores and colonized roots. Formation of Glomus intraradices appressoria on M20 roots was normal, as on wild-type (WT) plants, but in significantly reduced numbers. A significant reduction in spore germination was observed in vitro in the presence of M20 exudates relative to WT. Our results indicate that this new mutant shares similar physiological characteristics with the M161 pmi mutant, but has a more suppressive Myc^- phenotype response.