Lead stress triggers the synthesis of protecting salicylic acid through polyamine accumulation in the Pb-resistant species Piptatherum miliaceum (L.) Cosson

To assess the impact of polyamines and salicylic acid on the response of Piptatherum miliaceum to lead toxicity, plants were cultivated in nutrient solution containing 500 μM Pb(NO₃)₂ in the presence or absence of polyamine synthesis inhibitors (d-Arginine 5 mM, cyclohexylammonium 5 mM, methylglyoxal-bis-guanyl-hydrazone 1 mM), ethylene synthesis inhibitor (2-aminoethoxyvinyl-glycine 2 μM) and/or exogenous supplementation of 100 μM putrescine, spermidine, spermine or 10 μM salicylic acid. Although all plants remained alive until the end of the treatment, Pb toxicity induced a decrease in growth and cell viability and an increase in electrolyte leakage. Plants accumulated up to 84.2 mg kg⁻¹ DW Pb in the shoots and 901.6 mg kg⁻¹ DW in the roots. Lead increased endogenous concentration of polyamines and salicylic acid. It increased phenylalanine ammonia lyase activity (PAL) in roots and shoots and isochorismate synthase activity (ICS) in shoots only. Glutathione increased in response to Pb, as well as root ascorbate and phytochelatins. All inhibitors of polyamines aggravated the deleterious impact of Pb on recorded parameters while simultaneous application of polyamines allowed to compensate this effect. Spermidine restricted root-to-shoot Pb translocation while ethylene increased Pb absorption and accumulation. Spermine stimulated ICS activity in the shoots but not in the roots, leading to high salicylic acid (SA) concentrations in photosynthetic tissues. Salicylic acid stimulated glutathione and phytochelatin synthesis in the roots. Hence, polyamines and SA play a role in Pb tolerance of P. miliaceum but act on distinct targets. These compounds exhibit specific interaction in Pb-stressed organs which differ between roots and shoots.