Signalling and cellular mechanisms in rhizobial plant cell entry

Plenary talk

International audience

English. The formation of functional nitrogen-fixing nodules in legumes requires the coordinated integrationof nodule development and rhizobial infection symbiotic programs. In most legume species, therhizobial microsymbionts enter the roots of their host via de novo constructed tubular infectionthreads (IT), which guide rhizobia transcellularly from root hairs towards the newly formed noduleprimordium1. IT construction is tightly synchronized with nodule organogenesis, and during thisprocess, the plant promotes cell-to-cell communication for successful colonization2. The host plantalso anticipates microbial infection, by creating a transcellular column of cytoplasm connected to thenucleus, called the Pre-Infection Thread (PIT), which bridges cells for the future passage of thegrowing IT3. Although crucial for microbial entry, the regulatory mechanisms underlying the formationof this unique structure, dedicated to microbial accommodation and analogous to the PPA formedduring AM fungal colonization4, are still poorly determined. We combined live cell imaging andelectron microscopy approaches to better understand how this infection-related cellularreprogramming is regulated in Medicago truncatula. These analyses provided new insights into thespatio-temporal dynamics of these cellular rearrangements and showed their close association withcalcium-regulated signalling. To better understand the molecular players associated with thisprocess, we combined reverse genetics and transcription profiling of both infection-related cell typesand infection-defective plant mutants (ern1, ern1ern2)5. These analyses allowed us to identifyassociated-regulators and new candidate genes underlying IT development and cell reprogrammingin M. truncatula, as will be presented.References1. Fournier J et al (2015). Plant Physiol, 167, 1233-1242.2. Gaudioso-Pedraza et al (2018) Current Biology, 28, 1-16.3. Van Brussel et al (1992). Science, 257, 70-72.4. Genre et al (2008) Plant Cell, 20: 1407-1420.5. Cerri et al (2016). Plant Physiol, 171, 1037–1054.