Maize mutants affected at distinct stages of the arbuscular mycorrhizal symbiosis.

Maize mutants affected in the symbiotic interaction with the arbuscular mycorrhizal fungus Glomus mosseae have been found by a visual, macroscopic screen in a Mutator-tagged population of maize. Seven mutants have been identified, falling into three phenotypic classes. For each class one mutant has been characterized in more detail. The nope1 (no perception 1) mutant does not support appressoria formation of G. mosseae, suggesting the absence of a plant-encoded function necessary for early recognition prior to contact. The phenotype segregated as a monogenic recessive trait, indicating that a mutation in a single locus abolished compatibility of maize to G. mosseae. On a second mutant termed taci1 (taciturn 1), appressoria form at reduced frequency but their morphology is normal and leads to penetration of the rhizodermis. However, intraradically, the majority of hyphae are septate, resulting in terminated fungal spreading. This phenotype suggests that the mutation of taci1 has an effect on recognition and on cortex invasion. Segregation analysis indicates taci1 to carry a recessive mutation. In contrast, wild-type fungal morphology has been recorded in the Pram1 (Precocious arbuscular mycorrhiza 1) mutant, which displays enhanced and earlier fungal invasion. This trait segregates in a dominant fashion indicative of a gain-of-function mutation affecting the plant's control over restricting fungal colonization.