As drought events intensify due to climate change, understanding plant adaptation mechanisms is essential to preserving biodiversity and developing resilient agroecosystems. This PhD thesis is part of the TRASR project (Root Traits and Microbial Signals for Adaptation and Resilience to Water Stress), led by CEFE and LIPME, which aims to combine functional ecology, molecular biology, and genetics to better understand how herbaceous plants respond to water stress.

The project focuses on the central role of roots in drought tolerance and their interaction with natural microbial signals. The objectives are to:

• Identify root traits associated with the water-use niche of a broad range of herbaceous species from contrasting habitats;
• Test whether plant responsiveness to microbial signals depends on root functional strategies and modulates tolerance to water deficit;
• Assess root trait plasticity under controlled conditions, by varying the intensity and frequency of water stress, with or without microbial signals such as LCOs.

A panel of approximately 50 species (Fabaceae, Lamiaceae, Poaceae), sampled across moisture availability gradients (peat bogs, mesic grasslands, Mediterranean drylands), will be studied. For each species, adult root samples and propagules will be collected. One or two widely distributed species will also be analyzed at the intraspecific level to capture population-level variability.

Root traits will be analyzed across three main dimensions:

• Morphological: specific root length, root density, diameter;
• Architectural: branching angle and intensity, spatial organization;
• Anatomical: cortex thickness, tissue differentiation, mycorrhizal colonization structures.

Microscopy and image analysis will enable precise and standardized quantification. Controlled-condition experiments will be conducted using collected propagules, subjected to different water regimes (optimal, moderate, severe) and microbial signal treatments. This will help assess the plasticity of root traits in response to both environmental stress and microbial cues.

Species responses will be interpreted in light of their ecological strategies (e.g., RES) and their hydric niche. The aim is to identify robust trait combinations predictive of microbial signal sensitivity and adaptive capacity under drought.

Expected outcomes:

• A functional reference framework of root traits (morphological, architectural, anatomical) linked to species’ hydrological niches and microbial responsiveness;
• Improved understanding of how microbial signals modulate root development depending on species strategies and ecological context;
• An evaluation of the transferability of findings to intraspecific levels and to agronomic species (especially forage plants), to support varietal selection or biostimulant development.