Immunocytochemistry and Density Functional Theory evidence the competition of aluminum and calcium for pectin binding in Urochloa decumbens roots

Abstract

Root growth is reduced in soils with low pH [H⁺] and abundant soluble aluminum [Al³⁺], which can be a consequence of the interaction between Al³⁺ and cell wall composition. The competition between Al³⁺ and Ca²⁺ toward binding to pectin molecules was evaluated in roots of Urochloa decumbens, an African grass highly adapted to acidic Al-rich soils. Variations in the composition and distribution of pectins can change the extensibility, rigidity, porosity, and adhesive properties of plant cell walls, which were tested in seedlings of U. decumbens exposed to pH 3.5, 4.5 and 5.8 and to 0, 80, 160 and 320 μM of Al³⁺ for 80h. Root growth corroborated that U. decumbens is very tolerant to soil acidity, with effective reduction of root growth only at pH 3.5. Immunocytochemical approaches demonstrated variations in pectin composition induced both by Al³⁺ and by H⁺ in root tissues and zones. Based on the usual linkage between Ca²⁺ and pectins, Density Functional Theory (DFT) analyses indicated that Al³⁺ bound easier to pectins than Ca²⁺ did, leading to the formation of more Al³⁺-pectate complexes than Ca²⁺-pectate complexes, which resulted in higher rigidity of cell walls, and hampered cell extension.

Keywords: Acidity tolerance; Aluminum tolerance; Cell wall; Monoclonal antibodies; Quantum physics principles.