SYMBIOTIC MUTANTS DEFICIENT IN NODULE ESTABLISHMENT IDENTIFIED AFTER T-DNA TRANSFORMATION OF LOTUS-JAPONICUS

Nitrogen-fixing root nodules develop on legumes as a result of an inte raction between host plants and soil bacteria collectively referred to as rhizobia. The organogenic process resulting in nodule development is triggered by the bacterial microsymbiont, but genetically controlle d by the host plant genome. Using T-DNA insertion as a tool to identif y novel plant genes that regulate nodule ontogeny, we have identified two putatively tagged symbiotic loci, Ljsym8 and Ljsym13, in the diplo id legume Lotus japonicus. The sym8 mutants are arrested during infect ion by the bacteria early in the developmental process. The sym13 muta nts are arrested in the final stages of infection, and ineffective nod ules are formed. These two plant mutant lines were identified in proge ny from 1112 primary transformants obtained after Agrobacterium tumefa ciens T-DNA-mediated transformation of L. japonicus and subsequent scr eening for defects in the symbiosis with Mesorhizobium loti. Additiona l nontagged mutants arrested at different developmental stages were al so identified and genetic complementation tests assigned all the mutat ions to IG monogenic symbiotic loci segregating recessive mutant allel es. In the screen reported here independent symbiotic loci thus appear ed with a frequency of similar to 1.5%, suggesting that a relatively l arge set of genes is required for the symbiotic interaction.