Role of Silicon in Counteracting Cadmium Stress in Pea Plants (Pisum sativum L.): Insights Into Cadmium Binding Mechanisms and Pectin Methylesterase Activity

Purpose: The aim of this study was to investigate the role of silicon (Si) in counteracting a cadmium (Cd) stress to pea plants (Pisum sativum L.) and to identify the mechanism by which Cd is bound within pea roots. Methods: These goals were achieved through (i) a histochemical study of Cd localization in pea roots, (ii) spectrophotometric determination of pectin content and the activity of pectin methylesterase (PME), (iii) speciation of Cd extracted from pea roots conducted through size exclusion chromatography (SEC) and inductively coupled plasma mass spectrometry (ICP/MS). Results: Cd was found mainly in the root stele of the Cd-stressed plants. The pectin content and PME activity were lower in the Cd-stressed plants, but Si supplementation reversed these effects. Selectivity was noticed in Cd extraction efficiency with water being the least effective and enzymatic-assisted extraction proving to be the most effective. Speciation analysis revealed significant heterogeneity in molar mass, ranging from approximately 295 to 95 kDa. Galacturonic acid was identified the dominant species responsible for Cd binding. The choice of solvent for extraction markedly influenced the Cd binding profile, indicating shifts in the distribution of species’ molar mass and their relative concentrations in extracts. Conclusions: Si alleviates Cd toxicity in pea plants, and one of the mechanisms through which it operates involves increasing pectin levels and PME activity. Pectin plays an active role in Cd detoxification in the root cell walls, forming electrostatic bonds with Cd cations through its carboxyl groups.