Acetylcholine as a shield: enhancing growth and salt tolerance in pepper through ionic homeostasis, hormonal...

Acetylcholine (ACh), a potential neurotransmitter, shows growth and stress responses in plants. Despite, the underlying mechanisms of how ACh mitigates salt toxicity in pepper plants remains to be fully elucidated. Here, pepper plants pre-treated with different concentrations of ACh (0, 50 and 100 µM) were subjected to salinity stress (150 mM NaCl) to investigate their morpho-physiological, biochemical, and molecular alterations. Salinity stress significantly arrested plant phenotypes, reduced leaf relative water content (LRWC), and altered essential nutrient elements and phytohormones. However, it led to tremendous acceleration of proline, sucrose, hydrogen peroxide (H₂O₂), malondialdehyde (MDA), electrolyte leakage (EL), and enzymatic antioxidants such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and glutathione reductase (GR) as compared to the non-stress control plant. ACh greatly improved the plant's phenotypes in non-stress control plants and significantly repaired the phenotype of salinity-affected plants. ACh improved LRWC and plant nutrient status by restricting Na⁺ and Cl^-. However, it stimulated synthesis of phytohormones (except ABA), and bolstered the functions of SOD, CAT, POD, and GR under salinity stress. While salinity stress upregulated CaHKT1;1 and CaHKT1;2, ACh treatment downregulated them. Compared to control, salinity and ACh exposure stimulated the expression of genes governing Photosystem II (PSII) and antioxidant activity, highlighting a coordinated genetic defence mechanism of plant while mitigating salinity stress. However, significant induction of CaPsbD by ACh under salinity stress suggests the protective role of this phytochemical in PS II recovery. Furthermore, multivariate analysis demonstrated that application of ACh (100 µM) improved most of the physiological and biochemical markers, effectively restoring plant growth despite salinity stress. The findings imply a crucial role for ACh in growth and mitigation of salt toxicity in the pepper plant.