Novel genes induced during an arbuscular mycorrhizal (AM) symbiosis formedbetween Medicago truncatula and Glomus versiforme

Many terrestrial plant species are able to form symbiotic associations with arbuscular mycorrhizal fungi. Here we have identified three cDNA clones re presenting genes whose expression is induced during the arbuscular mycorrhi zal symbiosis formed between Medicago truncatula and an arbuscular mycorrhi zal fungus, Glomus versiforme, The three clones represent M, truncatula gen es and encode novel proteins: a xyloglucan endotransglycosylase-related pro tein, a putative arabinogalactan protein (AGP), and a putative homologue of the mammalian p110 subunit of initiation factor 3 (eIF3), These genes show little or no expression in M, truncatula roots prior to formation of the s ymbiosis and are significantly induced following colonization by G, versifo rme. The genes are not induced in roots in response to increases in phospha te, This suggests that induction of expression during the symbiosis is due to the interaction with the fungus and is not a secondary effect of improve d phosphate nutrition. In situ hybridization revealed that the putative AGP is expressed specifically in cortical cells containing arbuscules. The ide ntification of two mycorrhiza-induced genes encoding proteins predicted to be involved in cell wall structure is consistent with previous electron mic roscopy data that indicated major alterations in the extracellular matrix o f the cortical cells following colonization by mycorrhizal fungi.