Isolation of two different phenotypes of mycorrhizal mutants in the model legume plant Lotus japonicus after EMS-treatment

Lotus japonicus has been proposed as a model plant for the molecular geneti c study of plant-microbe interaction including Mesorhizobium loti and arbus cular mycorrhizal (AM) fungi. Non-mycorrhizal mutants of Lotus japonicus we re screened from a collection of 12 mutants showing non-nodulating (Nod(-)) , ineffectively nodulating (Fix(-)) and hypernodulating (Nod(++)) phenotype s with monogenic recessive inheritance induced by EMS (ethylmethane sulfona te) mutagenesis, Three mycorrhizal mutant lines showing highly reduced arbu scular mycorrhizal colonization were obtained. All of them were derived fro m Nod- phenotypes. In Ljsym72, the root colonization by Glomus sp. R-10 is characterized by poor development of the external mycelium, formation of ex tremely branched appressoria, and the blocking of hyphal penetration at the root epidermis. Neither arbuscules nor vesicles were formed in Ljsym72 roo ts. Fungal recognition on the Foot surface was strongly affected by the mut ation in the LjSym72 gene. Unique characteristics in mutant lines Ljsym71-1 and Ljsym71-2 were the overproduction of deformed appressoria and arrested hyphal penetration of the exodermis, Small amounts of internal colonizatio n including degenerated arbuscule formation occurred infrequently in these types of mutants. Not only fungal development on the root surface but also that in the root exodermis and cortex was affected by the mutation in LjSym 71 gene. These mutants represent a key advance in molecular research on the AM symbiosis.