Systemic Activation of the Antioxidant System by Root Priming With Non‐Pathogenic <i>Fusarium oxysporum</i> in Flax Infected With Pathogenic <i>Fusarium oxysporum</i>

Plants rely on specialised adaptive mechanisms to enhance resistance against environmental stress. One such mechanism, priming, enables faster and stronger defence responses upon subsequent stress exposure. This study examines whether the non-pathogenic Fusarium oxysporum Fo47 primes flax by colonising roots and activating antioxidant defences. Flax plants primed with Fo47 and those treated with both Fo47 and the pathogenic strain F. oxysporum Foln were analysed for fungal colonisation, PR genes expression and antioxidant systems: enzymatic (ROS metabolism-related genes expression, catalase and superoxide dismutase activity, hydrogen peroxide and superoxide anion levels) and non-enzymatic (phenolic compound content and antioxidant potential). The results demonstrate that Fo47 colonises host tissues, significantly reducing Foln penetration and colonisation, particularly in primed plants. Root-specific suppression of Foln by Fo47 was stronger than systemic suppression in shoots. Fo47 induced early chitinase and NADPH oxidases D transcript accumulation and reduced superoxide anion level in roots, likely triggering defence activation. Notably, Fo47 also activated both enzymatic and non-enzymatic antioxidant systems in shoots, suggesting a systemic priming effect. These findings underscore the potential of non-pathogenic F. oxysporum strains in sustainable plant protection strategies.