Dynamic Hormonal Networks in Flax During Fusarium oxysporum Infection and Their Regulation by Spermidine

<b>Background:</b> Flax (<i>Linum usitatissimum</i> L.) is an economically important crop that is highly susceptible to <i>Fusarium oxysporum</i> f. sp. <i>lini</i> (Foln). While phytohormones are key regulators of defence, their interaction with polyamines during infection remains poorly understood. This study aimed to characterise hormonal dynamics in flax under Foln infection and the modulatory role of spermidine (Spd). <b>Methods:</b> Targeted UPLC-MS/MS profiling quantified over 30 hormone-related compounds, including auxins, cytokinins, gibberellins, jasmonates, salicylic acid, and abscisic acid, in shoots and roots of healthy, infected, and Spd-treated plants. Two Spd concentrations (10 and 100 mM) were applied under controlled in vitro conditions. <b>Results:</b> Foln infection triggered tissue- and time-specific hormonal shifts, with early activation of jasmonate and auxin metabolism in shoots and later accumulation of salicylic acid and gibberellins in roots. Spd, particularly at 10 mM, reshaped these responses by reinforcing cytokinin and salicylic acid responses, stabilising auxin homeostasis, and enhancing jasmonate and abscisic acid responses. <b>Conclusions:</b> Spermidine coordinates hormone crosstalk, enabling balanced and efficient defence activation. The results highlight its potential as a priming agent enhancing flax resilience to <i>F. oxysporum</i>.