Biopriming with Schizophyllum commune Polysaccharides Modulates Antioxidant and Biochemical Responses in Pisum sativum L. Seedlings

This study investigated the biostimulant potential of endo- and exo-polysaccharides (PSHs) obtained from two Schizophyllum commune strains originating from Italy (ITA) and Serbia (SRB), with a primary focus on their ability to modulate the antioxidant defense and biochemical responses of Pisum sativum L. seedlings under optimal and drought conditions. The PSH fractions were structurally characterized by complementary spectroscopic and microscopic approaches, revealing strain- and drying-dependent differences in their structural features. The characterized PSHs were subsequently applied as seed biopriming agents, and pea seedlings were grown under two conditions: optimal growth and drought stress. Their effects were evaluated by analyzing enzymatic and non-enzymatic antioxidant responses, together with selected physiological and biochemical traits. Among the tested treatments, exo-PSHs differentially modulated antioxidant enzymes, with SRB primarily enhancing peroxidase and glutathione peroxidase and ITA increasing ascorbate peroxidase and catalase activity. SRB PSHs, particularly exo-PSH, showed the most pronounced effects on antioxidant-related responses, enhancing ABTS scavenging activity and altering DPPH capacity by approximately 35% compared with the control. In addition, endo-PSH from the ITA strain increased chlorophyll content by 26.12% and proline levels by 1.5-fold, indicating a distinct strain-dependent effect on plant physiological responses, particularly under stress conditions. LC–MS/MS profiling of plant MeOH extracts revealed a diverse range of secondary metabolites, including phenolic compounds, e.g., liquiritigenin, ferulic, and protocatechuic acids, which may contribute to the observed antioxidant responses. Multivariate analysis further supported clear biochemical differentiation among PSH treatments and growth conditions, confirming that the responses were dependent on both PSH origin and environmental conditions. Overall, these findings demonstrate that structurally distinct S. commune PSHs can differentially modulate antioxidant defense and key biochemical traits in pea seedlings, supporting their potential as sustainable fungus-derived biostimulants for enhancing plant resilience under both optimal and drought conditions.