A switch in Ca2+ spiking signature is concomitant with endosymbiotic microbe entry into cortical root cells of Medicago truncatula.
Ca(2+)  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 Ca(2+)  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 Ca(2+)  oscillatory profiles correlate with specific stages of transcellular cortical infection by both rhizobia and arbuscular mycorrhizal fungi. Outer cortical cells exhibit low-frequency Ca(2+)  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 Ca(2+)  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 Ca(2+)  signature during this particular stage of host infection in the outer cortex. Together, these findings lead to the proposal that tightly regulated Ca(2+) -mediated signal transduction is a key player in reprogramming root cell development at the critical stage of commitment to endosymbiotic infection.
Laboratory of Plant-Microbe Interactions, Institut National de Recherche Agronomique (UMR 441), Centre Nationale de Recherche Scientifique (UMR 2594), Castanet-Tolosan, France.
Björn J
BJ
Sieberer
Mireille
M
Chabaud
Joëlle
J
Fournier
Antonius C J
AC
Timmers
David G
DG
Barker
D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling; D046913:Medicago truncatula / Q000382:microbiology* / Q000502:physiology; D038821:Mycorrhizae / Q000502:physiology; D018517:Plant Roots / Q000166:cytology / Q000382:microbiology / Q000502:physiology*; D016962:Sinorhizobium meliloti / Q000502:physiology; D013559:Symbiosis / Q000502:physiology*
D002118:Calcium