Genetic and Cellular Analysis of Resistance to Vesicular Arbuscular (VA) Mycorrhizal Fungi in Pea Mutants

Screening of nodulation mutants of Pisum sativum has yielded mutants showing resistance to VA fungi (termed myc-). Most of these have aborted infections (myc-(1) phenotype) which are characterised by host cell reactions recalling those in certain pathogen infections. Mutants affected in later steps of mycorrhiza development (myc-(2) phenotype), with blocking of arbuscule formation, were less frequent. The myc-(1) character is recessive, segregates monogenically and occurs on at least five different, independently mutated loci, indicating that VA endomycorrhiza formation is under multiple gene control. Expression of the myc-(1) character is indissociable from that of nod’ in mutants and appears to be the result of pleiotropic effects of single genes. In late mutants where arbuscule formation is blocked, nodules develop but are inefficient (nod+,fix-). Coincidences between myc and nod characters may reflect common mechanisms in plant control over some step(s) in endomycorrhiza and nodule symbioses. Since chemical mutagenesis generally causes loss of gene function, inactivation of symbiosis-specific susceptibility genes in the mutants could affect production of signal molecules and somehow lead to stronger expression of plant resistance mechanisms to the symbionts.