Molecular characterization of plant mutants defective in mycorrhiza formation

Arbuscular mycorrhizas play a key role in plant growth by supporting the acquisition of mineral nutrients, especially phosphorus. Around 80 % of vascular plants exhibit this association. The major aim of research work was to isolate plant genes that are related to mycorrhizal association by using a mutant screening approach. Several plant genes involved in the arbuscular mycorrhizal association have been identified in the past decade. However these genes are also involved in the association of another type of symbiosis that is nodulation. In order to identify genes that are specifically involved in mycorrhiza formation, independent of nodulation, a transposon mutagenized population of Medicago truncatula was screened for reduced mycorrhizal colonization in the presence of normal nodulation. Two mutants showing reduced colonization were identified. In the presence of colonised host plants in the same growth compartment the number of intraradical hyphae and vesicles was normal, but arbuscules did not fully develop. This suggests that the block in colonization is not absolute. The genes affected in both mutant lines have been isolated. One mutant is affected in a GRAS transcription factor that is likely to act in the mycorrhiza-specific transduction of fungal signals. The other is affected in a mycorrhiza induced H+-ATPase that is likely to be involved in generating an electrochemical gradient at the periarbuscular membrane to facilitate nutrient uptake at the fungus/plant interphace.