Nuclear Ca 2+ Signaling Reveals Active Bacterial‐Host Communication Throughout Rhizobial Infection in Root Hairs of Medicago truncatula

Infection threads (ITs) are specialized apoplastic compartments formed within the epidermal and outer cortical root cells of leguminous host plants, whose role is to provide a confined transcellular route for the delivery of nitrogen-fixing symbiotic rhizobia to differentiating nodule tissues. IT construction is a progressive and tightly regulated process requiring major plant cell reprogramming. In order to investigate the relationship between rhizobial signaling and the host cell remodeling which precedes and accompanies IT formation, we have exploited the calcium cameleon reporter Nup:YC2.1 to monitor nuclear Ca2+ signaling responses (spiking) throughout the different stages of Medicago truncatula root hair infection. Live-tissue confocal imaging has revealed that once tip-curling root hairs have encircled the attached rhizobia to create a closed infection chamber there is a severe attenuation in host cell Ca2+ spiking. However, subsequent rhizobial entry from the chamber into the newly formed IT invagination is accompanied by a re-activation of sustained nuclear calcium spiking. In contrast to previous findings for cortical cell infection, pronounced spiking appears to be maintained throughout the period of active IT growth within the root hair. This implies that there must be differences in the requirements for rhizobial-host signaling during IT progression across root hair and cortical tissues, and this is discussed in the light of the respective modes of infection which characterize these two cell types.