Study of early microbe-host signaling in D. trinervis (Rosales), a South American actinorhizal shrub which uses a primitive intercellular infection mechanism

IMANISHI L; BROTTIER L; PIRROLLES E; BOURRILLONS J; VAYSSAYRE V; BOGUSZ D; WALL LG; GHERBI H; CHABAUD M; FOURNIER J; BARKER DG; SVISTOONOFF S

Montpellier

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

IRD

Nitrogen is one of the major limiting factors of plant growth and development in the soil and therefore symbiotic nitrogen fixation plays an essential role in sustainable agriculture and natural ecosystems. However, only legumes, Parasponia and actinorhizal plants have the ability to form nitrogen fixing nodules on their roots through symbiotic interactions with either rhizobia or Frankia. The establishment of root nodule symbioses (RNS) involves the successful recognition between the host plant and its symbiotic bacteria and the subsequent intracellular accommodation of the bacteria within nodular tissues. The molecular mechanisms leading to the formation of the RNS have been primarily described in model legumes which make use of root hair intracellular infection. (Downie, 2014). A signaling pathway is initiated in root hairs following the specific recognition of rhizobial Nod factors through LysM receptor-like kinases in the plasma membrane that then leads to the triggering of calcium oscillations in the nucleus and the activation of a calcium and calmodulin-dependent protein kinase (CCaMK).Unlike model legumes, Discaria trinervis, an actinorhizal shrub belonging to the Rosales order, establishes RNS via intercellular infection of the outer root tissues (Valverde and Wall, 1999). Such intercellular infection without root hair deformation and without the formation of infection threads can be considered as a ?basic? or ?primitive? mode of infection (Madsen et al., 2010). Very little is known about the molecular events involved in the establishment of this type of symbiosis, apart from the lack of canonical nod genes in the sequenced genomes of most Frankia (Tisa et al., 2013). With the objective of studying this important question we first showed that the expression of an auto-active version of CCaMK in D. trinervis results in the formation of nodules in the absence of Frankia, similar to what was previously observed in legumes (REF SVISTOONOFF 2013). We now describe here the characterization of DtNFP and DtCCaMK, the orthologs of NFP/NFR5 and CCaMK in D. trinervis. Whilst both nodulation and arbuscular mycorrhization were impaired when RNAi was used to knockdown DtCCaMK, no negative effect was observed for either endosymbiotic association when knocking down DtNFP expression. We also generated transgenic plants expressing a nuclear-localized cameleon reporter (REF SIEBERER) and showed that calcium oscillations were not induced in epidermal cells of the root region susceptible for infection following treatment with culture supernatants of the Frankia strain BCUXX that nodulates Discaria. These results suggest that there are major differences regarding early microbe-host signaling beween D. trinervis and model legumes.