Rhizode position as a functional trait in legumes. Study of trade-off for plant productivity and resilience

Prod ? EA BAP GEAPSI INRAInternational audienceRhizodeposition is the release of organic carbon (C) to the soil that connects the biotic and abiotic components of the C cycle. It can promote C storage to soil but also mediates plant-microbe interactions (Jones et al., 2009). These interactions are complexes as rhizodeposition will influence the composition and functioning of microbial populations which in return are able to increase the availability of nutrients in soil and provide protection against pathogens (Sasse et al., 2018). Despite their importance for current agriculture challenges, plant-soil microbes interactions remain poorly understood due to the methodological challenge they represent and the complexity of actors and processes involved (Oburger & Jones, 2018). Indeed in the context of climate change, our current agriculture needs to reconcile with ecology and mitigate these interactions to benefit mutually environment and food production while selecting more resilient ideotypes. As plants and soil health are highly dependent on trophic and signaling interactions in the rhizosphere, it is worth noting that plants from natural ecosystems tend to exhibit more rhizodeposition than in agrosystems (Pausch & Kuzyakov, 2018). Thus, it is crucial to evaluate why crop species exhibit less rhizodeposition and what are the plant physiological processes governing rhizodeposition in terms of quantity and type (exudation, mucilage, root cap and border cells loss) at the temporal and spatial scale. Identifying potential trade-off for carbon allocation would assist the selection of resilient ideotypes and development of sustainable farming practices. Because of their potential for agroecology, we study the relationship and potential trade-off of carbon allocation in legumes with focus on the specie Pisum sativum. Carbon budget for biomass, growth, storage, nodulation and rhizodeposition at the whole plant level is performed with state of the art phenotyping and isotope labelling plateforms. We show the different rhizodeposition processes and methods to assess them in order to evaluate their respective contribution to carbon budget and microbial population management. Root mucilage is a hydrogel composed mainly of polysaccharides actively secreted by exocytosis. It provides multiple benefits such as hydraulic continuity with soil, metal complexation and rhizosphere stabilization (Kroener et al., 2016). Border cells are cells that detach from root apex. They lower frictional stress during growth but also have a major role in mediating microbial populations in legumes through the release of secondary metabolites (Watson et al., 2015). Exudates are soluble plant derived primary and secondary metabolites that can be released through active and passive mechanisms (Jones et al., 2009). Second, we present our ecophysiological structure-function approaches at the root level focusing on sugars, taking into consideration carbon and water transport. The developmental gradient of roots is studied using anatomy in order to link it to functions such as growth, storage, respiration and exudation. Further the use of hydraulic methods informs on radial and axial resistance for water flow but also passive diffusion of sugars to decipher to which extent loss of organic carbon is an unfortunate necessity for water absorption through passive loss (Zwieniecki et al., 2002)