Symbiosis in actinorhizal plants: Microbial-plant signaling and cellular mechanisms of root colonization – SYMActino

SYMActino project is based on cell imaging approach. This method uses confocal microscopy combined with plants expressing reporter genes such as Cameleon-Nup YC2.1 enabling measurement of the calcium oscillations or GFP (Green Fluorescent Protein) addressed to the endoplasmic reticulum allowing the observation of the cellular remodeling. Thus, this makes it possible to track responses of the actinorhizal plants during the early stages of the symbiotic interaction.

Confocal microscopy analyses were performed during the pre-infection stage on C. glauca and D. trinervis roots expressing the Prom35S::YC2.1-Nup construct. These analyses were focused on the plant response after treatments with Frankia exudates or CO4 molecule (synthetic chitooligosaccharide mimicking the response to the fungus). Promising calcium responses (spiking) were observed in root hairs of C. glauca (Frankia exudates treatment) or epidermal cells for the two species (CO4 treatment).

/

/

Actinorhizal plants grow in a wide variety of climatic zones ranging from alpine to tropical and play a vital role in enriching nutrient-poor soils with both nitrogen and organic material. This remarkable adaptability is due to the capacity of these trees and shrubs to form efficient root symbiotic associations with both nitrogen-fixing filamentous bacteria (Frankia) and mycorrhizal fungi. The principal goal of this project is to study the molecular and cellular processes associated with the earliest stages of these plant-microbe interactions and which are essential for successful recognition of the appropriate endosymbiont and subsequent root colonization. Two actinorhizal plants with quite different modes of Frankia infection have been selected for these studies - Casuarina glauca is colonized via intracellular infection through surface root hairs, whereas Discaria trinervis roots are penetrated via an ancestral intercellular strategy. For both species, host root responses to bacterial and fungal symbionts will be analyzed using a combination of confocal microscopy and in vivo cellular reporters in order to identify both the activation of symbiosis-specific signaling pathways and the plant cellular differentiation which precedes and accompanies the two types of root colonization. Furthermore, the host (Ca2+) signaling responses observed during the pre-infection and infection stages will be compared and contrasted to those already identified in leguminous plants. This original and ambitious project brings together a team at the forefront of the molecular analysis of actinorhizal nitrogen-fixing symbioses with a second team developing pioneering research in in vivo cellular dynamics and cell signaling in legumes.
Altogether, the SYMActino project will lead to the identification of key mechanisms by which actinorhizal plants accommodate Frankia and AMF. The comparison with Legumes should reveal common features to Fabids able to establish both AM and nitrogen-fixing endosymbioses. This also opens the way to identifying common and specific components for both plant–fungal and plant–bacterial endosymbioses between legumes and actinorhiza-forming Fagales.