Coordination between endodermal barriers and arbuscular mycorrhizae for nutrient exchange
Résumé
During arbuscular mycorrhizal symbiosis, fungal structures called arbuscules develop inside of plant cortical cells. Arbuscules are a site of intense nutrient exchange between both partners. However, the way these nutrients are transported to and from the vascular tissues is not known. This transport could involve the apoplastic, transcellular and symplastic pathways. The endodermis, being located between the cortical and vascular tissues, can affect this transport through the deposition of lignin and suberin, which block the apoplastic and transcellular pathways respectively. To understand the role of the endodermis in the development and functioning of arbuscular mycorrhizae, we used the model legume Medicago truncatula and the arbuscular mycorrhizal fungus Rhizophagus irregularis. The role of the apoplastic pathway was studied using Casparian strip mutants showing reduced lignin deposition. We observed that the absence of this barrier does not prevent mycorrhizal colonisation, and are now investigating the impacts of these mutations on plant and fungal nutrition through mass spectrometry approaches. For the transcellular pathway, we developed image analysis protocols to determine if suberin deposition is correlated with arbuscule formation. Furthermore, we created transgenic lines showing an induced or repressed suberin deposition. Results point to a negative correlation existing between suberisation and mycorrhizal colonisation. Last, the symplastic pathway is being studied using diffusible reporters allowing for the quantification of symplastic connectivity between the vascular and cortical tissues. This work aims to determine the involvement of each transport pathway in plant-fungi nutrient exchange, and how the modification of these pathways can affect global plant nutrition.