Transcriptional control of cellular reprogramming during the symbiotic interaction of Medicago truncatula with arbuscular mycorrhizal fungi

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Arbuscular mycorrhizae (AM) are the most widespread beneficial plant-microbe interactions on Earth. Due to the availability of comprehensive sequence information for the model legume Medicago truncatula and the model AM fungus Glomus intraradices, genome-wide expression profiling experiments are possible. To understand AM-related transcriptional reprogramming, we investigate three levels of signaling: rhizodermal cells perceiving diffusible Myc-factors, root cells infected by mycorrhizal fungi, and cortical cells establishing functional arbuscules. Since Myc-factors emerged as lipochitooligosaccharides similar to rhizobial Nod-factors, we will use symbiotic mutants to discriminate Nod- and Myc-factor signaling, in order to elucidate Myc-factor specific responses. Via laser microdissection, cell-specific gene expression during early and late AM stages is investigated. Based on these experiments, in situ markers for Myc-factor activity can be developed, and promoter motifs related to the control of gene expression in arbuscule-containing cells will be studied further. Reverse genetics continues to be performed for candidate genes selected by transcriptome profiling to investigate their role during early and late cellular reprogramming. Candidate genes identified so far are related to early AM signaling upstream and downstream of the calcium/calmodulin-dependent protein kinase DMI3, or are expressed in arbuscule-containing cells. In total, 12 Tnt1 insertion mutant lines are available that in addition to RNAi approaches can now be used for detailed functional studies.