A switch in Ca2+ spiking signature is concomitant with endosymbiotic microbe entry into cortical root cells of Medicago truncatula

Ca2+ spiking is a central component of a common signaling pathway that is activated in the host epidermis during initial recognition of endosymbiotic microbes. However, it is not known to what extent Ca2+ signaling also plays a role during subsequent root colonization involving apoplastic transcellular infection. Live-tissue imaging using calcium cameleon reporters expressed in Medicago truncatula roots has revealed that distinct Ca2+ oscillatory profiles correlate with specific stages of transcellular cortical infection by both rhizobia and arbuscular mycorrhizal fungi. Outer cortical cells exhibit low-frequency Ca2+ spiking during the extensive intracellular remodeling that precedes infection. This appears to be a prerequisite for the formation of either pre-infection threads or the pre-penetration apparatus, both of which are fully reversible processes. A transition from low- to high-frequency spiking is concomitant with the initial stages of apoplastic cell entry by both microbes. This high-frequency spiking is of limited duration in the case of rhizobial infection and is completely switched off by the time transcellular infection by both microsymbionts is completed. The Ca2+ spiking profiles associated with both rhizobial and arbuscular mycorrhizal cell entry are remarkably similar in terms of periodicity, suggesting that microbe specificity is unlikely to be encoded by the Ca2+ signature during this particular stage of host infection in the outer cortex. Together, these findings lead to the proposal that tightly regulated Ca2+-mediated signal transduction is a key player in reprogramming root cell development at the critical stage of commitment to endosymbiotic infection.