Malhotra and Kapoor 2019). Within angiosperms, the monocots tend to accumulate
more silicon in their aerial parts due to the presence of silicon transporters (Henriet
et al. 2006; Malhotra and Kapoor 2019). For a long time, it is a noted fact that the
silicon levels in the soil is enhanced by fertilization. The agricultural wastes such as
silicate slag, bagasse furnace ash, lignite fly ash, and rice straw are considered as rich
silicon sources that are being employed mostly. The other sources of silicon
employed these days include wollastonite, calcium silicate, potassium silicate,
garnet, silica gel, diopside, calcium silicate hydrate, etc. (Kalra et al. 2003; Daniel
Maxim et al. 2008; Malhotra and Kapoor 2019; Zargar et al. 2019).
2.3
Silicon: Uptake, Transportation, and Accumulation
The majority of the investigations regarding silicon uptake and transportation has
been focused more on monocots; however, the speed for enhancing knowledge
regarding dicots has also increased in the last 5 years. As per literature, Si is absorbed
by lateral roots actively in the form of neutral, monomeric monosilicic acid, Si(OH) 4 ,
whose concentration ranges between 0.1–0.6 mM (Knight and Kinrade 2001; Rao
and Susmitha 2017). The ability of monosilicic acid to cross the plasma membrane
of lateral root depends highly on the physiological pH and water (Raven 2001). The
vehicle for its uptake and distribution is a simple molecule, i.e. water; however, both
molecules vary in size (Exley et al. 2020). Therefore, the pace of both water uptake
and Si adsorption has been classified into three possible situations, namely (1) active
(Si-uptake > water uptake), (2) passive (where Si-uptake ¼ water uptake), and lastly
(3) rejective (Si-uptake < water uptake) in higher plants (Cornelis et al. 2011; Zargar
et al. 2019). Ostensibly, upon the entry with water via the symplastic route, silicic
acid encounters a myriad of different enumerable water channels that control the
movement of silicic acid further. For example, in rice, a high silicon accumulating
plant, the silicon transportation is highly governed by majorly three low silicon rice
genes, i.e. OsLSi1, OsLSi2, and OsLSi6 (Ma et al. 2006, 2007; Yamaji and Ma
2009; Dhakate et al. 2019). Among these genes, LSi1 (influx transport activity) and
LSi2 (efflux transport activity) have been shown to be involved in silicon transport
from root cells to the apoplast (Ma and Yamaji 2008; Rao and Susmitha 2017)
(Fig. 2.1).
OsLSi1 gene belonging to the NIP-III (nodulin26-like intrinsic proteins) subfamily of aquaporin is primarily found to be constitutively located in the basal zones of
roots. Within this, the OsLSi1 gene is found to be localized exactly on the plasma
membrane of the distal side of both exodermis and endodermis cells where casparian
stripes are located (Yamaji and Ma 2007; Ma and Yamaji 2008; Dhakate et al. 2019).
On the other hand, expression patterns and cellular localization studies have revealed
that the OsLSi2 gene (efflux Si-transporter) is localized on the proximal side of the
same cells (Yamaji and Ma 2009; Yamaji and Ma 2011; Dhakate et al. 2019). This
rice OsLsi2 gene is found to be responsible for reloading and diffusing Si into the
vascular bundles (Yamaji and Ma 2011; Ma and Yamaji 2015). After reaching the
apoplast, monosilicic acid in xylem sap needs to be unloaded so as to prevent the Si
2 Silicon: A Plant Nutritional “Non-Entity” for Mitigating Abiotic
21
more silicon in their aerial parts due to the presence of silicon transporters (Henriet
et al. 2006; Malhotra and Kapoor 2019). For a long time, it is a noted fact that the
silicon levels in the soil is enhanced by fertilization. The agricultural wastes such as
silicate slag, bagasse furnace ash, lignite fly ash, and rice straw are considered as rich
silicon sources that are being employed mostly. The other sources of silicon
employed these days include wollastonite, calcium silicate, potassium silicate,
garnet, silica gel, diopside, calcium silicate hydrate, etc. (Kalra et al. 2003; Daniel
Maxim et al. 2008; Malhotra and Kapoor 2019; Zargar et al. 2019).
2.3
Silicon: Uptake, Transportation, and Accumulation
The majority of the investigations regarding silicon uptake and transportation has
been focused more on monocots; however, the speed for enhancing knowledge
regarding dicots has also increased in the last 5 years. As per literature, Si is absorbed
by lateral roots actively in the form of neutral, monomeric monosilicic acid, Si(OH) 4 ,
whose concentration ranges between 0.1–0.6 mM (Knight and Kinrade 2001; Rao
and Susmitha 2017). The ability of monosilicic acid to cross the plasma membrane
of lateral root depends highly on the physiological pH and water (Raven 2001). The
vehicle for its uptake and distribution is a simple molecule, i.e. water; however, both
molecules vary in size (Exley et al. 2020). Therefore, the pace of both water uptake
and Si adsorption has been classified into three possible situations, namely (1) active
(Si-uptake > water uptake), (2) passive (where Si-uptake ¼ water uptake), and lastly
(3) rejective (Si-uptake < water uptake) in higher plants (Cornelis et al. 2011; Zargar
et al. 2019). Ostensibly, upon the entry with water via the symplastic route, silicic
acid encounters a myriad of different enumerable water channels that control the
movement of silicic acid further. For example, in rice, a high silicon accumulating
plant, the silicon transportation is highly governed by majorly three low silicon rice
genes, i.e. OsLSi1, OsLSi2, and OsLSi6 (Ma et al. 2006, 2007; Yamaji and Ma
2009; Dhakate et al. 2019). Among these genes, LSi1 (influx transport activity) and
LSi2 (efflux transport activity) have been shown to be involved in silicon transport
from root cells to the apoplast (Ma and Yamaji 2008; Rao and Susmitha 2017)
(Fig. 2.1).
OsLSi1 gene belonging to the NIP-III (nodulin26-like intrinsic proteins) subfamily of aquaporin is primarily found to be constitutively located in the basal zones of
roots. Within this, the OsLSi1 gene is found to be localized exactly on the plasma
membrane of the distal side of both exodermis and endodermis cells where casparian
stripes are located (Yamaji and Ma 2007; Ma and Yamaji 2008; Dhakate et al. 2019).
On the other hand, expression patterns and cellular localization studies have revealed
that the OsLSi2 gene (efflux Si-transporter) is localized on the proximal side of the
same cells (Yamaji and Ma 2009; Yamaji and Ma 2011; Dhakate et al. 2019). This
rice OsLsi2 gene is found to be responsible for reloading and diffusing Si into the
vascular bundles (Yamaji and Ma 2011; Ma and Yamaji 2015). After reaching the
apoplast, monosilicic acid in xylem sap needs to be unloaded so as to prevent the Si
2 Silicon: A Plant Nutritional “Non-Entity” for Mitigating Abiotic
21