STUDYING THE SIGNALING AND CELLULAR DYNAMICS OF INTRA- AND INTER-CELLULAR FRANKIA INFECTION IN ACTINORHIZAL SPECIES

FOURNIER J; CHABAUD M; PIRROLLES E; BRICHET L; ABDOU-BEN-ABDOU I; BROTTIER L; MOUKOUANGA D; IMANISHI L; WALL LG; FRANCHE C; BOGUSZ D; GHERBI H; SVISTOONOFF S; BARKER DG

Montpellier

Congreso; 18 th International Meeting on Frankia and Actinorhizal Plants; 2015

IRD

The establishment of the N2-fixing symbioses between Frankia sp. and their actinorhizal host plants and between rhizobia and their legume hosts share a number of common mechanisms based on the more ancient and widespread plant endosymbiosis involving arbuscular mycorhizal (AM) fungi (1). Perhaps the most striking example of this is the highly conserved Common Symbiotic Signaling Pathway (CSSP), which is activated in host cells in response to microbial signals and is characterized by the generation and transduction of nuclear-associated calcium oscillations known as spiking (2). Furthermore, when entry of either AM fungi, rhizobia or Frankia into the outer root tissues of their respective hosts occurs intracellularly, transcellular infection compartments (known as infection threads in the case of bacterial microsymbionts) are formed in host cells (3). The development of in vivo confocal microscopy approaches in the model legume Medicago truncatula has allowed us to perform dynamic and comparative studies of the cellular mechanisms associated with the formation of these transcellular infection compartments for both AM and rhizobial infection, as well as the associated host-microbe communication monitored via calcium signaling. These studies have detailed the pro-active role of root epidermal cells in controlling and directing microsymbiont entry as well as novel features of rhizobial infection thread construction (4, 5, 6). Our objective here is to investigate the cellular mechanisms and calcium signaling associated with the initial penetration of Frankia species across the outer root tissues of their respective actinorhizal hosts. Studies are focused on the two actinorhizal hosts Casuarina glauca and Discaria trinervis, which display different modes of Frankia entry. C. glauca is colonized intracellularly via the formation of infection threads in root hairs (3), whereas D. trinervis is infected intercellularly without the formation of infection threads (7). We aim to compare and contrast the respective roles of the host root tissues throughout these two quite different modes of root entry and evaluate the microbe-host signaling that accompanies intra- and intercellular infection processes. The tools and approaches that will be used to investigate these various questions as well as the mode of AM fungal infection in the two actinorhizal species will be presented.