The bioavailability of Mn in the rhizosphere mainly depends on the soil pH, root exudates, and microorganisms (Alejandro et al., 2020). In addition, the fate of Mn is closely interwoven with aluminum (Al) which is abundant in acidic soils (Shao et al., 2017). Traditional silicon fertilizers produced from wollastonite are the Lewis base, whose supply can consume the proton (H⁺) for the formation of H₄SiO₄ and interact with Al in the root apoplast (Hodson and Evans, 2020), leading to an increased pH of rhizosphere soil in theory. It has been reported that the uptake of Si by rice plants can significantly reduce the degree of root acidification (Ma et al., 2021; Pang et al., 2023) by inhibiting the expressions of proton pump and organic acid secretion genes (Pang et al., 2023), and the elevated soil pH decreases the Mn²⁺ availability.

Root exudation is a dynamic behavior that mediates interactions between plant roots and soil matrix. Interestingly, the leaf Mn concentrations can be used as a proxy for rhizosphere carboxylate concentrations because the root exudates mobilize the micronutrient Mn in the rhizosphere (Lambers et al., 2015, 2021; Yan et al., 2024). Under Mn-excessive conditions, root exudates of total phenolics are increased in Citrus sinensis, while the secretion of root total free amino acids, total soluble sugars, malate, and citrate are not altered (Zheng et al., 2024). Furthermore, a great number of researches do show that the application of Si fertilizer could reduce the secretion of organic acids by plant roots (e.g., Wu et al., 2015; Fan et al., 2018; Javed et al., 2020; Pang et al., 2023), which are observed under the normal and stress conditions. Based on these discoveries, it can be speculated that Si may immobilize the excessive micronutrient Mn by mediating the secretion of metabolites in the rhizosphere.

Moreover, root exudates have multiple effects on bacterial community composition and microbiome assembly (Vives-Peris et al., 2020; Qu et al., 2021; Li et al., 2022; Kabir et al., 2024; Shi et al., 2024), so does Si by altering root-released carboxylates and phenolics. It was proposed that Si fertilization influences microbial assemblages of rice roots in a five-season in-situ remediation field study (Gao et al., 2022a). In this study, Si improvement of microbial diversity and richness in the rhizosphere was detected after the third fertilization, suggesting a prolonged effect of Si fertilization on the microbiome in roots (Gao et al., 2022a). Besides working in the rhizosphere, Si enhances the abundance of reducing microbes in the rhizoplane as well (Gao et al., 2022b). Very recently, it has been reported that Si could regulate the reassembled microbial communities in soil (Gao et al., 2024).

Despite these results being involved in the Si-mediated arsenic uptake by rice plants, we still believe that Si may decrease the soil Mn²⁺ availability with microbial mechanisms. There are two reasons: (1) Mn functions as an electron donor and acceptor for microorganisms, and the anaerobic redox transition between Mn²⁺ and MnO_x accelerates a dynamic biogeochemical cycle coupled to microorganisms (Wang et al., 2022); (2) Anaeromyxobacter is closely linked to the Si-regulated microbial interactions (Gao et al., 2024), which might be involved in Mn oxidation and is capable of bioremediation in the Mn-contaminated soil (Liu et al., 2024).

In mycotrophic plants, arbuscular mycorrhizas also modify rhizosphere chemistry and influence Mn bioavailability and uptake (Brito et al., 2014; Nazeri et al., 2014). According to reports, inoculating legumes with arbuscular mycorrhizal fungi (AMF) increased the pH of the rhizosphere by around 0.2–0.7 pH units, reduced the total amount of carboxylates in the rhizosphere by 52%, and reduced the concentration of Mn in the shoots by 38% (Nazeri et al., 2014). In wheat, AMF colonization that begins with the intact extraradical mycelium improves bio-protection by decreasing Mn uptake in roots (Brito et al., 2014). Recently, it was proposed that arbuscular mycorrhizal symbiosis alleviates Mn toxicity by downregulating Mn transporter genes in Eucalyptus tereticornis (De Oliveira et al., 2023). Moreover, the relationships between Si and arbuscular mycorrhizas were established: AMF improve Si uptake and translocation, meanwhile Si increases mycorrhizal effectiveness in plants (Etesami et al., 2022). These results suggest an AMF-related mechanism of Si-mitigating Mn toxicity.

In roots, Casparian bands intercept the apoplastic flow, resulting in the requirement of transporters for nutrient uptake and translocation. Thus, Mn uptake and translocation are governed by the expression of transporter genes and the development of apoplastic barriers. There are many Mn transporters in plants, including members of the natural resistance-associated macrophage protein (NRAMP) family, the ZRT/IRT-related protein (ZIP) family, the yellow stripe-like (YSL) family, the cation exchanger (CAX) family, the cation diffusion facilitator/metal tolerance protein (CDF/MTP) family, the vacuolar iron transporter (VIT) family and others (Shao et al., 2017; Alejandro et al., 2020).

Among the Mn transporters, OsNramp5 and OsMTP9 make up the Mn uptake system in rice roots, and OsNramp5 acts as a metal/H⁺ symporter and facilitates the Mn permeation into the root epidermis (Sasaki et al., 2012), while OsMTP9 is an efflux Mn transporter and mediates the export of Mn²⁺ into the stele (Ueno et al., 2015). Che et al. (2016) proposed that Si inhibits the Mn uptake in rice roots by down-regulating the expression of OsNramp5 gene after a relatively long-term exposure to Si. Furthermore, the Mn concentration can be decreased in the shoots but increased in the roots due to the Si effects, suggesting that Si can reduce the root-to-shoot translocation of Mn in rice plants (Che et al., 2016). The probable mechanism for Si-reduced translocation is the formation of the Mn-Si complex in root cells.

The development of apoplastic barriers, including Casparian bands and the suberin lamellae, has a role in controlling the radial fluxes of water and nutrients and preventing the uptake of toxicants (Kreszies et al., 2020). It is widely known that Si can enhance the formation of Casparian bands and suberin lamellae by forming phenol complexes (Vaculík et al., 2012; Wu et al., 2019; Kreszies et al., 2020). Moreover, Si-modified phenols have an excellent ability to bind metals (Sheng and Chen, 2020), thereby accomplishing the Mn retention in roots. Our previous study also found that Casparian bands and Si-lignin interactions promote the silica deposition in the inner tangential cell walls of endodermis (Sheng et al., 2023). Root silica gels may serve as a pool that can store the excess Mn through the formation of the Mn-Si complex.

In rice plants, most of the total Mn taken up by roots was translocated to the shoots, regardless of the Si presence in roots (Che et al., 2016). Additionally, Si tends to raise leaf tissue tolerance rather than induce root Mn exclusion in cucumbers (Dragišić Maksimović et al., 2007, 2012). Considering that symptoms of Mn excess mainly occur in leaves, Si alleviation of foliar Mn stress is a top priority. At the cellular level, the first strategy is that Si alters the cell wall chemistry to enhance the Mn-binding to the cell wall. For example, the lignin synthesis in cucumber leaves is altered by Si supply in response to Mn stress (Dragišić Maksimović et al., 2007), as well as the callose synthesis in maize (Doncheva et al., 2009). These results are in agreement with our assumption that Si crosslinks with different cell wall components in various plants (Sheng and Chen, 2020). In Si-accumulating plants, leaf apoplast is one of the main locations for silica deposition, relevant or irrelevant to the cell wall (Hodson, 2019). It is likely that the Mn-Si complex can form in the leaf apoplast of Si-accumulating plants to prevent the uptake of Mn into the cytoplasm. Taken together, cell wall-bound Si and silica mediate the Mn accumulation in leaf apoplast for reducing the Mn toxicity.

Once the excess Mn²⁺ ions enter the cytoplasm, the intracellular reactions will be triggered. To avoid metal toxicity, Mn compartmentation into vacuole is one of the most important ways (Sharma et al., 2016; He et al., 2021), a process which depends on the tonoplast stability and tonoplast-localized transporter. To our knowledge, there is no evidence to show that some tonoplast-localized Mn transporters are regulated by Si. Furthermore, Si significantly strengthens the capacity of antioxidant system to scavenge reactive oxygen species (ROS), which can harm the membrane system. It means that Si increases the stability of membrane system, including the tonoplast, under stress conditions (Sheng et al., 2024). This, in turn, enhances the activity of tonoplast-localized Mn transporters (Sheng et al., 2018, 2024). Thus, Si has the potential ability to mediate the Mn compartmentation into the vacuole, but this should still be elucidated. Similar results have been reported in Cd-stressed rice cells: In the case of Cd, Si addition can reduce the Cd toxicity by compartmentation of Cd into vacuoles (Ma et al., 2016). Overall, it can be concluded that Si optimizes the subcellular distribution of Mn in shoots.