The root extracellular trap: A checkpoint controlling root tip accessibility to microorganisms

The root tip and root-Associated Cap-Derived Cells (AC-DCs) release various organic compounds into the rhizosphere, forming the Root Extracellular Trap (RET), a network involved in root-soil microorganism interactions and root protection. This study investigates the role of soybean (Glycine max) and pea (Pisum sativum) RETs in interactions with soil rhizobacteria (Bacillus subtilis and Pseudomonas fluorescens) and the zoospores of the oomycete Phytophthora parasitica by examining their impact on microbial behavior. To this end, confrontation tests were performed, followed by imaging analyses of videos, in which the behavior of microorganisms (i.e., swimming speeds and trajectories) was characterized and quantified. The results show that the RET alters microbial access to the root tip and modifies significantly speeds and trajectories. Inside the RET, the speeds of B. subtilis and P. parasitica zoospores decreased three and nine times respectively. Outside the RET, the speed of the bacterium remains unchanged, while that of the zoospores decreases twice. These findings highlight the influence of the RET on microbial movement and its importance in plant-microorganism interactions in the rhizosphere.