Regulation of thermodynamics and kinetics of silica nucleation during the silicification process in higher plants

• Published in: Plant physiology and biochemistry Vol. 198; p. 107674
• Main Authors: Sheng, Huachun, Li, Ying, Feng, Jingqiu, Liu, Yuan
• Format: Journal Article
• Language: English
• Published: France Elsevier Masson SAS 01.05.2023
• Subjects: Cell wall components; Drivers; Kinetic barrier; Kinetics; Oryza - metabolism; Plant silicification; Silica nucleation; Silicic Acid; Silicon Dioxide - metabolism; Thermodynamic barrier; Thermodynamics; Cell wall components; Thermodynamic barrier; Plant silicification; Kinetic barrier; Drivers; Silica nucleation
• ISSN: 0981-9428; 1873-2690
• DOI: 10.1016/j.plaphy.2023.107674

The formation mechanism of SiO2 aggregates is controversial because two contrasting hypotheses are often proposed to explain plant silicification. In this review, we summarize the physicochemical fundamentals of amorphous silica nucleation and discuss how plants regulate the process of silicification by influencing the thermodynamics and kinetics of silica nucleation. At silicification positions, plants overcome the thermodynamic barrier by establishing the supersaturation of the H4SiO4 solution and reducing the interfacial free energy. Among the thermodynamic-drivers, the establishment of supersaturation of H4SiO4 solution mainly depends on the expression of Si transporters for H4SiO4 supply, evapotranspiration for concentrating Si, and the other solutes in H4SiO4 solution for influencing the dissolution equilibrium of SiO2; while the interfacial free energy was reduced seemingly by the overexpression Na+/H+ antiporter SOS1 in high NaCl-stressed rice. Moreover, some kinetic-drivers, such as silicification-related proteins (Slp1 and PRP1) and new cell wall components, are actively expressed or synthesized by plants to interact with silicic acid, thereby reducing the kinetic barrier. According to classical nucleation theory, when the thermodynamic barrier is overcome, the super-saturated silicic acid solution (such as H4SiO4 in xylem sap) does not necessarily have to precipitate, just has the potential ability to precipitation. Thus, based on the mediators of SiO2 deposition at the thermodynamic-driven stage, it is difficult to evaluate whether the process of plant silicification is active or passive. We conclude that the characteristics of kinetic-drivers determine the mechanism of plant silicification. •Plant silicification is a process that should overcome the thermodynamic- and kinetic-barriers.•The main thermodynamic-drivers for plant silicification are Si transporters and evapotranspiration.•Silicification-related proteins and new cell wall components are the main kinetic-drivers of silica nucleation.•The interactions between kinetic-drivers and silicic acid promote silica nucleation in higher plants.•The characteristics of kinetic-drivers rather than thermodynamic-drivers, determine the mechanism of plant silicification.