Detailed list of references are provided in the Supplementary Text 2.

#### **4. Silicon: A Multifaceted Element for Alleviating Heavy Metal Toxicity in Crops**

Silicon derived enhancement in plant tolerance to heavy metal toxicity is well documented, and the beneficial role of Si in detoxification can be ascribed to both external (growth media) and internal plant mechanisms [21,22]. The external mechanism of elevating heavy metal tolerance is mainly due to the increased pH by silicate application resulting in metal silicate precipitates that decrease the metal phyto-availability [22]. In plants, Si affects the translocation and distribution of metals in various plant parts and allows them to survive under higher metal stress [23]. Given that plants vary in their ability to accumulate Si, higher accumulators such as monocots will usually obtain greater benefits, even though metal toxicity in both monocots and dicots can be alleviated by Si [24,25]. In rice, Si-rich amendments showed a reduction of heavy metal accumulation, as well as increased growth in multi-metal (Cd, Zn, Cu, and Pb), contaminated acidic soil [26]. Silicon is also reported to increase seedling biomass and decrease Zn concentration in both roots and shoots of rice seedlings, and the xylem sap flow. In addition, Zn accumulation was significantly suppressed by Si supply in different plant parts such as roots and leaves of cotton and maize [27,28]. Shi et al. [29] reported that Si minimizes Cd metal toxicity by reducing ion absorption and translocation from root to shoot in rice seedlings. Silicon application was also found to reduce lipid peroxidation and fatty acid desaturation in plant tissues and improve the growth and biomass of plants under heavy metal stress [4].

Silicon can also be effective in alleviating Al toxicity in barley plants [30]. Similarly, decreased Al content with Si application was observed in the stem, roots, and leaves of peanut and rice seedlings [31,32]. Silicon amendments as an *alternative* detoxification method for Al toxicity have been reported in sorghum, tomato, soybean and maize [21]. Considerable reduction of Si pools in agricultural soils results from regular removal of Si-rich litter during the crop harvest [33]. Hence, external application of Si in agriculture will become a trend in the near future to compensate its depletion in soils, simultaneously reaping its benefits of improving plant growth and alleviating heavy metal toxicity.

#### **5. Silicon-Mediated Mechanisms of Metal Detoxification**

Several mechanisms explaining Si-derived benefits towards metal detoxification have been proposed. The widely accepted mechanisms include toxic metal immobilization in the soil (at soil level), and stimulation of enzymatic and non-enzymatic antioxidants, co-precipitation of metals, metal ions chelation, compartmentation, and structural alterations of plant tissues and alteration in molecular responses (at plant level) (Figure 2; Table 2).



**Table 2.** *Cont.*

