Seed Microbiota: A Key Factor in Plant Adaptation to Arsenic Stress ​

Aim: Arsenic contamination represents one of the most critical anthropogenic stressors compromising organism resilience in the context of Global Change. However, some plant species can complete their life cycle in soils contaminated with this metalloid. Studies on plant–soil microbiome symbiosis have emphasized horizontal microbiome transmission (from soil to roots), while underestimating the role of vertically transmitted seed borne microbiomes. This work examines the seed borne endophytic bacterium Acinetobacter radioresistens MC 14, isolated from arsenic hyper resistant plants Jasione montana and known for its arsenic tolerance and plant growth promoting traits. The study investigates its capacity to modulate plant phenotypic traits and enhance adaptation under arsenic stress. Methods: To this end, we evaluated the physiological responses of Arabidopsis thaliana exposed to As(III) concentrations following inoculation with A. radioresistens MC 14, which apoplastically colonizes roots and establishes a non invasive facultative symbiosis that improves plant survival under arsenic stress. Results: A. radioresistens MC 14 improves plant fitness and ecological success, with optimal inoculum levels maximizing the benefits of the interaction while minimizing symbiotic costs. A. radioresistens MC 14 mitigates arsenic induced phytohormonal imbalances in roots during early development. This bacterium–plant association promotes root growth and reduces As(III) triggered oxidative stress by activating cellular recovery mechanisms. As a result, plants produce more roots, flowers, and leaves even under toxic conditions. Conclusions: These findings indicate that plants exert selective pressure on their seed microbiome, driving co evolution and maintaining beneficial microbial reservoirs across generations, ultimately enhancing plant performance in stressful environments.