Elucidating the Role of silicon in drought stress tolerance in Plants

Abstract

Predicted changes in climate, with more severe droughts and more extreme weather variability, are gaining considerable attention from stakeholders because of the already stressed and seriously challenging agricultural ecosystems of the contemporary world. One of the greatest challenges faced by these unique ecosystems due to climate change is drought stress, which affects plant growth, development and metabolic processes, thus reducing production, yield and quality of crop plants. Plants counter to this stress by employing complex mechanisms through a series of physiological, cellular, and molecular processes. Among the myriad of stress tolerance mechanisms, the positive effects of Si on water status of plants have been widely appreciated. Here, we review the potential of Si supplementation in alleviating drought stress and highlight the imported mechanisms involved in Si mediated reduction of drought stress in plants. Si fertilization not only enhances the photosynthetic pigments, growth, biomass, antioxidant enzymes, gene expression, osmolyte concentrations and nutrient uptake but also improves crop production, yield and grain quality during drought stress. In addition, it provides insights on important mechanisms involved in the modification of gas exchange attributes, gene modification, nutritional homeostasis, control synthesis of compatible solutes, osmotic adjustment and stimulation of phytohormone biosynthesis and antioxidant enzymes under drought stress. We also highlight knowledge gaps and future research prospects to understand Si mediated role in alleviating drought stress.

Full text

Plant Physiology and Biochemistry 165 (2021) 187–195
Available online 7 May 2021
0981-9428/© 2021 Elsevier Masson SAS. All rights reserved.
Research article
Elucidating the role of silicon in drought stress tolerance in plants
Mushtaq Ahmad Malik
a
, Abid Hussain Wani
a
, Showkat Hamid Mir
a
, Ishfaq Ul Rehman
a
,
Inayatullah Tahir
a
, Parvaiz Ahmad
b
,
c
,
*
, Irfan Rashid
a
,
**
a
Department of Botany, University of Kashmir Srinagar, 190006, Jammu and Kashmir, India
b
Botany and Microbiology Department, King Saud University, Riyadh, Saudi Arabia
c
Department of Botany, S.P. College, Srinagar, Jammu and Kashmir, India
ARTICLE INFO
Keywords:
Silicon
Drought stress
Physiology
Tolerance
ABSTRACT
Predicted changes in climate, with more severe droughts and more extreme weather variability, are gaining
considerable attention from stakeholders because of the already stressed and seriously challenging agricultural
ecosystems of the contemporary world. One of the greatest challenges faced by these unique ecosystems due to
climate change is drought stress, which affects plant growth, development and metabolic processes, thus
reducing production, yield, and quality of crop plants. Plants counter this stress by employing complex mech-
anisms through a series of physiological, cellular, and molecular processes. Among the myriad of stress tolerance
mechanisms, the positive effects of Si on water status of plants have been widely appreciated. Here, we review
the potential of Si supplementation in alleviating drought stress and highlight the imported mechanisms involved
in Si mediated reduction of drought stress in plants. Si fertilization not only enhances the photosynthetic pig-
ments, growth, biomass, antioxidant enzymes, gene expression, osmolyte concentrations and nutrient uptake but
also improves crop production, yield and grain quality during drought stress. In addition, it provides insights on
important mechanisms involved in the modication of gas exchange attributes, gene modication, nutritional
homeostasis, control synthesis of compatible solutes, osmotic adjustment and stimulation of phytohormone
biosynthesis and antioxidant enzymes under drought stress. We also highlight knowledge gaps and future
research prospects to understand Si mediated role in alleviating drought stress.
1. Introduction
Drought being one of the main concerns worldwide alters the plant
growth and development resulting in signicant losses to the crops in
terms of yield, productivity and quality, as such posing great threat to
food security (Kadam et al., 2014). Drought is a signicant abiotic stress
occurring almost in every ecosystem, but with changing frequency and
severity, limiting crop production around the world (Ault, 2020; Kim
et al., 2019). Drought stress causes shortage of water, nutrient imbal-
ances, ion toxicity, and oxidative stress (Thorne et al., 2020). Common
plant responses include a decline in growth that is expressed by the
down regulation of protein encoding genes involved in cell wall
expansion, DNA and protein synthesis (Skirycz and Inze. 2010), inhi-
bition of photosynthesis by down regulation of photosynthetic genes
encoding basic components of photosystem I and II (Chaves et al., 2009)
and also by closing of stomata and destroying photosynthetic apparatus
and chlorophyll material (Waraich et al., 2011).Water shortage being a
multifaceted stress, affects plants at different stages of development and
hinders morphological, physiological, biochemical and molecular ac-
tivities of cell (Du et al., 2020). Drought results in osmolyte accumula-
tion (Wang et al., 2019), elevated amounts of antioxidant enzymes
(Ahmad et al., 2019; Khan et al., 2020a,b; Verma et al., 2020), variance
in phytohormone levels (Khan et al., 2020a,b; Zlobin et al., 2020) and
expression of stress response genes (Parvathi et al., 2019).
Plants have developed mechanisms that include morphological,
physiological and molecular responses (Table 1) which can be specic to
particular stress like drought or shared response to more than one stress
(Atkinson et al., 2013; Hussain et al., 2019). These responses also differ
between genotypes and drought severity for example, Iqbal et al. (2019)
indicated that two contrasting Soyabean genotypes vary in responses to
various imbalanced water decit treatments using different PEG6000
concentrations. Similarly, Li et al. (2020) reported that increased
* Corresponding author. Botany and Microbiology Department, King Saud University, Riyadh, Saudi Arabia.
** Corresponding author. Department of Botany, University of Kashmir Srinagar, 190006, Jammu and Kashmir, India.
E-mail addresses: parvaizbot@yahoo.com (P. Ahmad), irfanrashid@uok.edu.in (I. Rashid).
Contents lists available at ScienceDirect
Plant Physiology and Biochemistry
journal homepage: www.elsevier.com/locate/plaphy
https://doi.org/10.1016/j.plaphy.2021.04.021
Received 14 November 2020; Accepted 13 April 2021

Plant Physiology and Biochemistry 165 (2021) 187–195
188
melatonin and ABA levels under drought function synergistically in
regulating response to limit non-stomatal water loss under severe but
not mild drought in watermelon.
Studies show that mineral nutrition plays a signicant role in the
alleviation of drought stress in plants. The use of macro-nutrients (N, K
and Ca) and micronutrients (Zn, Si and Mg) reduces the toxicity of ROS
by increasing antioxidant concentrations and enhancing drought resis-
tance in plants (Waraich et al., 2011). Micronutrients also enhance the
resilience of plants to stresses like cold and oxidative stress (Bradacova
et al., 2016). Over the last two decades silicon (Si) has gained consid-
erable attention as it has been shown to increase plant tolerance to many
biotic and abiotic stresses (Coskun et al., 2019). Silicon enhances
growth, yield as well as crop quality by affecting photosynthetic activity,
nitrogen assimilation and offering resistance against abiotic and biotic
stresses (Cooke and Leishman, 2011; Liang et al., 2015; Ahanger et al.,
2020). Si helps to alleviate low phosphorus induced plant growth by
enhancing photosynthetic activity, antioxidant defense system and
nutrient homeostasis (Zhang et al., 2019).
Silicon is the second largest element after oxygen present in suf-
cient amounts in the earth’s crust (Coskun et al., 2019).Recently Si has
been categorised as a benecial element by the International Plant
Nutrition Institute (IPNI, www.ipni.net/nutrifacts-northamerican) on
the basis of pragmatic role it plays in plants. Plants have different ca-
pacity to accumulate Si and the concentration varies from 0.1% to 10%
on the basis of dry weight (Epstein, 1994) but still it is not counted as an
essential element for proper plant growth and development.The differ-
ence in Si absorption capacity in plants is inuenced by several factors
like plant-available silicon (PAS) in soil, plant age, varying Si concen-
tration in the growing substratum and Si measurement methods
(Deshmukh et al., 2020) and environmental conditions (Chen et al.,
2018). The discovery of rst Si transporters in rice have laid the basis for
explaining the molecular mechanism of Si absorption in plants (Ma
et al., 2006). It has been described that Si enters the plant by specic Si
inux transporters (Lsi1) from the outside environment and is trans-
located by efux transporters (Lsi2) into xylem and then into the aerial
parts of the plants where it deposits as amorphous silica in various or-
gans and cells (Mandlik et al., 2020). These Si inux transporters are
part of large family of aquaporin’s (AQPs), a class of channel-forming
proteins that facilitates water transport and several other tiny solutes
throughout the cell membrane (Ma et al., 2006). Moreover, all the Si
inux transporters identied in crop plants have selectivity lter made
of glycine–serine–glycine–arginine (GSGR) conserved sequence and
these Si inux transporters are phylogenetically included in the aqua-
porin subfamily of nodulin 26-like intrinsic protein III (NIP-III) (Coskun
et al., 2019; Deshmukh and Belanger, 2016) and plants with unique
molecular signatures of NIP-III AQPs can accumulate more Si (Deshmuk
et al., 2020).
Si mitigates the drought stress in several plants such as rice, wheat,
maize, tomato, sorghum, sugarcane and broad bean, which is largely
due to improved water retention promoting photosynthesis (Desoky
et al., 2020; Gong and Chen, 2012; Khan et al., 2020a,b; Liu et al., 2014;
Verma et al., 2020). Several studies have reported that Si supplemen-
tation mitigates the adverse effects of biotic stresses such as bacterial,
fungal, and viral pathogens as well as infestation of insect pests (Liang
et al., 2015; Song et al., 2016) and abiotic stresses such as toxicity of
heavy metals, plant salinity and drought (Ahmed et al., 2014; Keller
et al., 2015; Ma et al., 2016; Maghsoudi et al., 2016).The use of organic
and inorganic silicon (Si) fertilizers is recognized as an important
eco-friendly and environmentally sustainable approach that can pro-
mote growth and enhance resistance of plants against various stresses,
due to the fact that Si is not hazardous, toxic or contaminating to plants
when present in surplus (Etesami and Jeong, 2018). Therefore, Si has
been considered as a "quasi-essential" element (because of its wide-
spread role in plants such as improved growth, yield and crop quality,
photosynthetic activity and nitrogen metabolism, primarily in response
to biotic and abiotic stresses ( Liang et al., 2015).
Previous literature suggests that the application of Si will be an
emergent eld in future crop cultivation process for enhancing plant
resistance against drought. In light of this, we analyzed recent advances
in identifying mechanisms of Si-mediated inuences in plant resistance
to drought.
1.1. Si-mediated drought tolerance mechanisms in plants
Drought is a key environmental stress in crop growing, with adverse
consequences on plant growth and key metabolic pathways like photo-
synthetic assimilation, water relations, mineral absorption, antioxidant
defense capacity resulting in considerable reduction of crop yield and
quality (de Vries et al., 2020; Fang et al., 2017) (Fig. 1). Plants have
several mechanisms to cope with drought stress, like activation of
antioxidant defense system, osmolyte accumulation and stomatal regu-
lation (Ahammed et al., 2020 “a”, “b”; Sohag et al., 2020). Several
Table 1
Effect of Si application on different physiological and biochemical parameters of
plant under drought stress.
Plant Physiological and
biochemical variables
Drought
effect
Si
effect
Reference
Rice Photosynthesis – +Chen et al.
(2011)
Transpiration ¡ þ
Basal quantum yield ¡ þ
Maximum quantum
efciency
¡ þ
Water potential ¡ þ
WUE ¡ þ
Mineral nutrient
absorption
¡ þ
Wheat Leaf membrane stability
Index
¡ þ Ahmad et al.,
2016a,b
Plant height ¡ þ
Spike length ¡ þ
Relative water content ¡ þ
Proline content ¡ þ
Grain yield ¡ þ
Biological yield ¡ þ
Wheat Growth ¡ þ Xu et al. .2017
Water potential ¡ þ
RWC and TWC ¡ þ
MDA and H2O2 þ ¡
CAT and SOD ¡ þ
APX and POD þ ¡
IAA and JA þ ¡
ABA ¡ þ
Sugarcane Growth parameters ¡ þ Verma et al.
(2019a)
Photosynthesis ¡ þ
Chlorophyll content ¡ þ
RWC (%) ¡ þ
Stomatal conductance ¡ þ
Leaf transpiration ¡ þ
CAT activity ¡ þ
POD activity ¡ þ
SOD activity ¡ þ
Si uptake ¡ þ
Plant hormones (ABA,
IAA and GA3)
¡ þ
MDA and proline content þ ¡
Total weight ¡ þ
Wheat Growth attributes ¡ þ Parveen et al.
(2019)
Chlorophyll contents ¡ þ
Proline and glycine
betaine
þ ¡
Phenolic content and
total sugars
þ ¡
MDA and H
2
O
2
þ ¡
CAT,SOD and POD
activity
þ þ
Whereas +means increase and – means decrease.
M.A. Malik et al.

Plant Physiology and Biochemistry 165 (2021) 187–195
189
studies have documented that Si addition increases plant drought
resistance by enhancing water status, osmotic adjustment, photosyn-
thetic activity, antioxidant defense system and balance of nutrient ab-
sorption thereby maintaining crop yield (Maghsoudi et al., 2016; Sattar
et al., 2020; Souri et al., 2020; Table 2). The mechanisms involved in
Si-mediated drought alleviation in crop plants at physiological,
biochemical and molecular level can be illustrated as follows:
1.2. Modication of gene expression
Secondary messengers such as Ca
2+
, ROS, ABA and phospholipids
send initial signals to drought-responsive genes via kinases (Johnson
et al., 2014; Table 2). These genes encode functional proteins that pro-
tect cellular proteins, maintain membrane integrity besides water ab-
sorption and transport (Hu and Xiong, 2014). Transcription factors (TFs)
regulate transcription activity either by activating or repressing genes
and ultimately leading to downstream plant responses, especially during
stressful conditions (Ahammed et al., 2020a). The WRKY transcription
factors (TFs) regulate the growth, development and stress responses of
plants (Jiang et al., 2017). The SlWKRY81 transcript has been found to
reduce drought tolerance in tomato by inhibiting proline biosynthesis
(Ahammed et al., 2020b) and stomatal closure (Ahammed et al., 2020c).
Preliminary studies have shown that Si supplementation to the plants
increases the expression level of several genes associated with the
mitigation of drought stress (Fig. 1). Liu et al.(2014) revealed that Si
application enhances plant drought resistance by controlling root hy-
draulic conductance via up-regulating the transcription of several root
aquaporin [SbPIP1;3/1;4(2), SbPIP1;6, SbPIP2;2, and SbPIP2;6] gene
expression under drought stress (Fig. 2). This upregulation of aquaporin
gene expression helps in rapid absorption of water that results in dilu-
tion of excess concentration of Na
+
ions which are otherwise toxic to the
plants (Gao et al., 2010). Thus, augmenting expression levels of aqua-
porin genes by Si application result in maintaining water status and ion
balance helping plants to recover from stress (Manivannan and Ahn,
2017). However, Shi et al. (2016) reported that Si didn’t substantially
regulate the expression of three aquaporin genes (SlPIP1; 3, SlPIP1; 5,
and SlPIP2; 6) in Tomato under drought stress. Moreover, Si supply
up-regulated the expression level of important polyamine biosynthesis
genes (ADC, CAP, ODC1, ODC2, ODC3, SAMDC04, SAMDC06 and SPDS)
and down regulated the expression level of the ACC synthesis-related
genes (ACS1 and ACS2) in Sorghum under drought stress (Yin et al.,
2014). These polyamines contribute to stress mitigation by affecting
Fig. 1. Responses of various physiological, biochemical and genetic attributes under drought stress.
Table 2
Mechanisms of Si-mediated drought resistance in crop plants.
Stress Crop
Plant
Si mediated Mechanism References
Drought Sorghum Enhanced water absorption
capacity
Hattori et al.
(2005)
Drought Tomato Reduced leaf transpiration,
increased water storage capacity
and improved plant water use
balance
Romer-Aranda
et al. (2006)
Drought Sunower Regulate absorption and
accumulation of essential and non-
essential mineral elements
Gunes et al. (2008)
Drought Potato Enhanced osmotic adjustment
during drought conditions
Crusciol et al.
(2009)
Drought Rice Improved photosynthesis, plant
water status, and mineral nutrient
uptake
Chen et al. (2011)
Drought Wheat Increased net photosynthesis,
transpiration and stomatal
conductance; Improved relative
water content and water potential
Gong and Chen.
(2012)
Drought Rice Enhanced expression of
transcription factors, NAC5,
DREB2A, as well as OsRDCP1 and
certain drought specic genes like
OsCMO coding for rice choline
monooxygenase and dehydrin
OsRAB16b
Khattab et al.,
2014
Drought Pistachio Promoted activation of antioxidant
enzymes, improved photochemical
efciency and photosynthetic gas
exchange features
Habibi and
Hajiboland (2013)
Drought Sorghum Increased transpiration rate, and
enhanced several physiological
processes
Yin et al., 2014
Drought Lentil Regulated the activity of
antioxidant and hydrolytic
enzymes and osmolytes.
Biju et al. (2017)
Drought Sugarcane Improved antioxidant responses,
photosynthetic and biochemical
responses.
Verma et al.,
(2019a),b
Drought Wheat Improved morpho-physiological
parameters, increased RWC and
antioxidant enzymes
Sattar et al. (2020)
Drought Tomato Si-mediated energy dissipation Cao et al. (2020)
Drought Wheat Improved photosynthetic pigments
and biochemical parameters
Bukhari et al.
(2020)
M.A. Malik et al.

Plant Physiology and Biochemistry 165 (2021) 187–195
190
different physiological and biochemical functions of plants (Milon et al.,
2020)
Si supplementation has been shown to increase the expression of
transcription factors such as OsNAC
5
and OsDREB
2
A in drought stressed
plants (Khattab et al., 2014), leading to increased expression levels of
drought stress genes such as OsRDCP1, OsRAB16 and OsCMO (Lenka
et al., 2011). OsRDCP1 plays a major role in physiological responses of
plants to neutralise dehydration stress (Bae et al., 2011). In addition, the
gene Choline Monooxygenase (CMO) encodes the essential
ferredoxin-dependent CMO enzyme which drives the synthesis of
glycine betaine resulting in much better stress tolerance to plants (Luo
et al., 2012). The overexpression of OsDREB2A gene might protect plant
cells against drought stress (Matsukura et al., 2010). Furthermore,
OsRAB16b is one of the important LEA encoding protein which help
plants to acclimatise to stressful conditions (Lenka et al., 2011).
Si application enhances OsNAC5 transcripts which prevent lipid
peroxidation by reducing malondialdehyde (MDA) content and gener-
ating excess hydrogen peroxide (H
2
O
2
) helping to preserve plasma
membrane integrity and reduced membrane permeability. (Liang et al.,
2015; Manivannan and Ahn, 2017; Xu et al., 2017).Si application has
been found to upregulate the expression of four ASH-GSH cycle genes
such as TaDHAR, TaGR, TaGS and TaMDHAR in plants under drought
stress, which ultimately lead to the decrease in MDA levels and H
2
O
2
concentrations (Kim et al., 2017; Ma et al., 2016). Si application also
upregulated the expression level of genes such as TaCHS, TaPAL, TaDFR,
TaF3H, TaCI, and TaANS involved in avonoid biosynthesis under
drought stress (Ma et al. 2014, 2016) thus protecting plants from
drought. These results suggests that avonoids have a protective role
during drought stress through their special structures which may enable
plants to combat stress induced oxidative damage by preventing ROS
generation (Agati and Tattini, 2010) and ROS scavenging (Jaakola and
Hohtola, 2010). Ma et al. (2016) found that Si application enhanced the
expression of antioxidant enzyme coding genes like TaSOD, TaCAT in
plants during drought stress which in turn help to reduce accumulation
of H
2
O
2
(Fig. 2).
1.3. Osmotic adjustment and osmolytes
Plants require optimal water levels especially during drought con-
ditions which is critical for their survival as well as growth. Drought
stress leads to a substantial reduction in water content and water po-
tential in plants which negatively impacts growth and development and
severe reduction may threaten plant survival (Gong and Chen, 2012)
(Fig. 1). Responding to drought stress, plants maintain optimum water
content mainly by osmotic adjustment (Osakabe et al., 2014).
Several studies have reported that Si application increases water
content of many plant species under drought stress (Ahmed et al., 2014;
Saud et al., 2014). Si application increases water potential as well as
osmotic potential in plants under drought stress while maintaining
higher turgor pressure (Amin et al., 2014) (Fig. 2); while as Sonobe et al.
(2010) found that Si supplementation reduces the osmotic potential of
plant roots during drought without affecting water content, suggesting
the role of soluble sugars and amino acids viz. alanine and glutamic
acids in the osmotic adjustment. Similarly, Yin et al. (2014) documented
that Si supplementation reduced leaf osmotic potential under water
decit. Contrarily, Wang et al. (2015) stated that Si addition in
drought-stressed plants did not reduce leaf and root osmotic potential.
The improvement in water content and water potential of leaf by
supplying Si during water decit conditions could be attributed to the
leaf thickness (Gong et al., 2003) as well as to the accumulation of Si in
leaves, which minimizes transpiration as water molecules may not have
sufcient free energy to escape easily from Si deposited leaf surfaces
(Ahmed et al., 2014; Keller et al., 2015). Lux et al. (2002) proposed that
root endodermal silicication might well be associated with higher
tolerance to drought in plants. The osmotic adjustment due to Si
application during drought could also be attributed to Si deposition in
the cytoplasm which results in forming high molecular weight Si com-
plexes within the vacuoles of plant cells (Gunes et al., 2008).
The foliar application of Si increases plant tolerance to drought stress
by modication of osmolytes in several plant species (Gong et al., 2005;
Kaya et al., 2006). For example, Si supplementation enhanced proline
content but decreased soluble proteins and total sugar content in leaves
Fig. 2. Possible underlying mechanisms involved in Si-mediated mitigation of drought stress in plants.
M.A. Malik et al.

Plant Physiology and Biochemistry 165 (2021) 187–195
191
of Solanum tuberosum (Crusciol et al., 2009).This increase in the level of
proline content may have been caused by several factors such as
increased expression of genes encoding key enzymes for proline syn-
thesis, reduction of proline oxidation and reduction in the use of proline
for protein synthesis and improvement of protein turnover (Sabbagh
et al., 2014).On the other hand, Yin et al. (2014) described that Si
application decreased the plant proline content while increasing soluble
sugar under drought stress in Sorghum plants. The reduced proline level
may be due to the stimulating effect of Si on the vegetative growth of
plants grown under water stress (Kaya et al., 2006). Similarly, Pei et al.
(2010) found that addition of Si to the plants decreases proline levels,
suggesting that proline accumulation is an indication of stress-related
injury. In addition, Si application decreased the osmolyte content
(glycine betaine, proline and total soluble sugars) of Lentil genotypes
during drought (Biju et al., 2017).
Recently Bukhari et al. (2020) also reported that application of Si
improves plant biochemical attributes like total proline, soluble sugars
except soluble protein content. Similarly, nano-silicon treatment effec-
tively elevated glycine betaine, sucrose and soluble sugar content in
Sugar beet, but reduced proline under drought stress (Namjoyan et al.,
2020). However, Si seed priming in maize decreased the concentration
of osmolytes such as proline, soluble sugars, and glycine betaine under
drought stress (Parveen et al., 2019) and the reduction in proline content
by Si priming could be a symptom of stress relief and mitigation of stress
damage. The increase in the levels of osmoprotectants like proline and
glycine betaine could be controlled by Si-mediated augmentation of
transcriptional factors such as DREB2A and NAC5 expressions in plants
under drought and subsequently enhance drought tolerance (Khattab
et al., 2014).
From the above discussion it can be suggested that alleviation of
drought stress in plants by Si amendment occurs mainly via osmotic
adjustment along with other ways like reducing transpiration.
1.4. Regulation of phytohormone biosynthesis
Phytohormones such as abscisic acid, salicylic acid, jasmonic acid,
etc. are known to play an essential role in improving tolerance to mul-
tiple stresses by mediating different aspects of growth and physiological
responses in plants (Ahmad et al., 2016a,b; Kurepin et al., 2017; Khan
et al., 2020a,b).These phytohormones act as chemical messengers,
which help plants in sensing and responding to the drought stress and
other stress. For example, exogenous application of salicylic acid (SA)
and H
2
O
2
enhances relative water content (RWC) and maintain opti-
mum water status in plants under drought (Sohag et al., 2020).
Si application has been shown to differentially regulate the endog-
enous phytohormone levels such as indole acetic acid (IAA), abscisic
acid (ABA), and jasmonic acid (JA) in plants (Jang et al., 2018; Xu et al.,
2017).This regulation of phytohormone levels by Si addition results in
increased tolerance to drought stress in plants (Yin et al., 2016).
Hamayun et al. (2010) found that Si supplementation under drought
reduces Jasmonate (JA) levels while enhancing the level of Salicylic acid
in plant shoots. The reduced levels of JA might be due to Si mediated
down-regulation of enzymes necessary for JA synthesis such as allene
oxide synthase 1, lipoxygenase, allene oxide synthase 2, allene oxide
cyclase, and 12-oxophytodienoate reductase 3 (Kim et al., 2014a,b).
Similarly, Pei et al. (2010) observed that Si addition marginally
increased ABA levels in drought stressed plant leaves, which may be
attributed to Si mediated increase in the OsZEP gene expression (Kim
et al., 2014a). As a consequence, ABA synthesis induces ABA-inducible
gene expression and triggers stomatal closure, thereby minimizing
water loss by transpiration and eventually restricting cell growth
(Yoshida et al., 2019). Si application enhanced IAA (Indoleacetic acid),
GA (Gibberellic acid), and CK (Cytokinin) but reduced ABA (Abscisic
acid) in drought stressed plants (Helaly et al., 2017). Furthermore, it has
been reported that Si application increased ABA, IAA and GA levels in
Sugarcane (Verma et al., 2019b) and Maize (Merhij et al., 2019) during
drought stress. These ndings suggested that Si could increase plant
tolerance to drought by regulating phytohormone synthesis.
1.5. Reduction in oxidative stress
One of the instant responses that plants show when subjected to
drought stress is the excessive synthesis of Reactive Oxygen Species
(ROS) e.g., superoxide (O
2
˙
−
), hydrogen peroxide (H
2
O
2
), hydroxyl
radical (OH˙) and singlet oxygen (
1
O
2
) by activation of NADPH oxidase
and photosynthetic electron transport. This excessive ROS production
results in oxidative stress which contributes to the disintegration of
cellular structure and cell organelles caused by lipid peroxidation,
reduced membrane stability and increased electrolyte leakage (Ahmad
et al., 2010; Kohli et al., 2019; Cao et al., 2020; Thorne et al., 2020)
(Fig. 1). However, plants have developed a complex antioxidant mech-
anism to detoxify and scavenge excessive ROS thus help in maintaining
cellular homeostasis. This system comprises of enzymatic antioxidants,
such as catalase (CAT) superoxide dismutase (SOD), peroxidase (POD),
catalase (CAT), and ascorbate peroxidase (APX), glutathione reductase
(GR),dehydroascorbate reductase (DAR) and guaiacol peroxidase
(GPOX) and non-enzymatic antioxidants such as ascorbate, carotenoids,
tocopherols, non-protein amino acids, phenolic compounds, and gluta-
thione (GSH) (Alzahrani et al., 2018; Kim et al., 2017).
Si application has been documented to decrease oxidative damage in
plants by improving the activities of essential plant antioxidant enzymes
such as SOD, CAT, POD, GR and APX, as well as the concentration of
GSH which helps to scavenge ROS (Flores et al., 2019; Kim et al., 2017;
Ma et al., 2016; Sattar et al., 2020; Thorne et al., 2020). Si supple-
mentation increased the activity of antioxidant enzymes such as SOD,
CAT and POD in drought stressed Maize plants, thus reducing the
accumulation of MDA and H
2
O
2
, leading to the recovery of plant growth
and improved yield (Ning et al., 2020; Parveen et al., 2019). Similarly,
Namjoyan et al. (2020) reported that nano-silicon addition improved the
activities of antioxidant enzymes like CAT, GPX and SOD but decreased
MAD and H
2
O
2
levels in Sugar beet under drought. Several studies have
revealed that Si supply affects the concentration of certain
non-enzymatic antioxidants such as glutathione reductase and ascorbic
acid in many plant species (Gunes et al., 2008; Pei et al., 2010) which are
also involved in the removal of ROS.
The exogenous application of silicon and selenium nanoparticles
increased the activity of antioxidant enzymes such as CAT, SOD, APX
and GPX and reduced the MDA and H
2
O
2
levels in Strawberry under
drought stress (Zahedi et al., 2020). In addition, the Si application has
been documented to enhance the activities of CAT, SOD and POD in
Sugarcane during drought stress (Verma et al., 2019 “a”, “b”). Helaly
et al. (2017), however, described that Si supply reduced the activity of
antioxidant enzymes such as CAT, SOD and POD in Mango under
drought. This decrease in antioxidant enzyme activity could be direct
scavenging of toxic free radicals and their enhancing effects on the
synthesis of internal protective antioxidants (Pei et al., 2010).
Reports suggest that Si application reduces H
2
O
2
and lipid peroxi-
dation in drought stressed plant leaves (Biju et al., 2017; Ma et al., 2016;
Shi et al., 2014) by Si-mediated dissipation of energy (Cao et al., 2020).
However, Sohag et al. (2020) reported that exogenous application of
H
2
O
2
successfully safeguards the photosynthetic pigments and mitigated
oxidative damage by upregulating antioxidant enzymes (CAT, APX and
GPOX), that neutralized ROS in drought stressed rice seedlings. Further,
Si application has been shown to minimize leaf electrolyte leakage in
rice and wheat plants during drought (Gharineh and Karmollachaab,
2013), which could be attributed to positive effects of Si on the struc-
tural composition and integrity of plasma membranes by affecting the
stress-dependent peroxidation of membrane lipids (Ashraf et al., 2010).
Since membranes are main targets of abiotic stresses, preservation of
membrane integrity and stability is a key factor in drought stress
tolerance in plants.
M.A. Malik et al.

Plant Physiology and Biochemistry 165 (2021) 187–195
192
1.6. Increase in absorption and assimilation of mineral nutrients
Plants require sufcient amounts of essential mineral nutrients for
proper growth and development. Ionic hoemostasis mechanisms are
important in controlling downstream events which determine the fate of
plants (Tripathi et al., 2020). Water deciency (drought) not only in-
terrupts, but also inhibits the absorption of nutrients by the roots, as well
as transport to the shoots, thereby restricting the supply of nutrients and
subsequently affecting growth and yield of agricultural crop plants thus
affecting plant growth and yield of agricultural crops (; Ratnakumar
et al., 2016; Xu et al., 2017). It has been shown that the use of Si can play
a key role in stabilizing the absorption, transportation and distribution
of different mineral nutrients in plants under drought stress thereby
helping plants in stress elevation (Etesami and Jeong, 2018).
Many reports suggested that, Si application enhanced the uptake of
several macronutrients like Mg, K, Ca, N, and P level (Emam et al., 2014;
Gong and Chen, 2012; Helaly et al., 2017) and micronutrients such as
Cu, Fe, and Mn, etc. (Gunes et al., 2008) in drought stressed plants.
Foliar application of Si raised the concentration of potassium (K) and
total phosphorus (P) in straw and seeds of Wheat plants thus affecting
growth and development of plants (Ratnakumar et al., 2016). Xu et al.
(2017) observed that the exogenous application of Si signicantly
stimulated growth of wheat plants by optimizing absorption of nutrients
(Na, Si and Mg) under water-restricting conditions. Bukhari et al. (2015)
found that Si supplementation enhanced the levels of K, P, Zn, Mg, and Si
in leaves of two contrast Wheat varieties relative to drought stressed
plants. Addition of Si to the Lentil seedlings improved their drought by
increasing Si deposition in the cell (Biju et al., 2017). In addition, Si
application has increased the K
+
level in plant grains and shoots, which
helped to maintain water potential in plants, even as the moisture
content of plants and soil was reduced (Ahmad et al., 2016a,b). There-
fore, Si addition improves nutrient absorption by enhancing root activity
(Chen et al., 2011) and root hydraulic conductance (Hattori et al.,
2008). Si supplementation improved the levels of certain nutrients, like
K
+
and Ca
2+
in Maize (Kaya et al., 2006) and Wheat plants (Maghsoudi
et al., 2019) under water stress. This increase in Ca
2+
and K
+
levels
could be due to decrease in membrane permeability and increase in the
activity of membrane H- ATPase (Kaya et al., 2006). The higher Ca levels
in plants may improve growth and survival of crop under stress condi-
tions (Cachorro et al., 1994) and also enhance expression of stress
induced genes that plays role in plant protection (Knight et al., 1997).
Similarily, greater K level boost growth and expansion of root for
effective water absorption from soil solution and also minimize the rate
of transpiration by regulating the opening and closing of stomatal, thus
saving plant water content (Umar and Moinuddin, 2002). Furthermore,
Cakmak (2005) has reported that adequate K level in plants reduces
oxidation of NADPH 8 times in contrast to low K levels by declining the
activity of NADPH oxidase under drought stress.
Several studies have shown that silicon supplementation during
drought stress can signicantly increase root growth (Ahmed et al.,
2011; Etesami and Jeong, 2018; Hameed et al., 2013). Hattori et al.
(2005) revealed that in comparison to non-silicon-treated plants, Si
application dramatically decreases the shoot/root ratio while still
retaining higher root dry mass accumulation in drought-stressed plants,
indicating the possible role of silicon in promoting root growth during
drought resistance.Similarily, Wang et al. (2015) also reported increase
in root/shoot ratio in Si-treated plants indicating that Si-mediated root
morphology modications can account for Si-treated plants’ increased
water uptake ability. Si application has been shown to enhance root
growth by promoting root elongation and stimulated by an increase in
the extensibility of cell wall in the growing regions of the root (Etesami
and Jeong, 2018; Hattori et al., 2003). However, Si application during
drought stress did not promote root growth but actually improved water
absorption thus helping to stimulate nutrient uptake (Sonobe et al.,
2010).
These results suggest that Si application helps to improve ion
homeostasis by increasing either root growth or enhancing water uptake
under drought stress. But further studies are still needed to establish the
relationship between Si and changes in the root morphology under
water decit.
1.7. Modication of gas exchange attributes
Gaseous exchange measurements provide useful insights about plant
mechanisms during drought stress (Chen et al., 2011; Liu et al., 2014;
Ma et al., 2016). Many studies have reported that Si supplementation
controls gaseous exchange attributes of plants during drought stress
(Ahmed et al., 2014; Sattar et al., 2019, 2020; Verma et al., 2019 “a”;
Table 1).
Reports suggest that Si addition signicantly increases photosyn-
thetic rate, stomatal conductance and transpiration rate in drought
stressed plants in contrast to plants not supplied with Si (Pereira et al.,
2013; Sattar et al., 2020; Verma et al., 2019 “a”; Zhang et al., 2020;
Verma et al., 2020). Zahedi et al. (2020) has reported that silicon
nano-particles (SiNPs) improve the concentration of photosynthetic
pigments and chlorophyll uorescence in drought stressed strawberry
plants.Similarly, nano-silicon application has been shown to enhance
photosynthesis, chlorophyll content, stomatal conductance and tran-
spiration in sugar beet under water decit condition (Namjoyan et al.,
2020). Parveen et al. (2019) described that Si priming signicantly
enhanced photoynthetic pigments in maize cultivars under drought
stress. In addition, Si increased the net photosynthetic rate, stomatal
conductance and intercellular CO
2
concentration in drought stressed
tomato (Cao et al., 2020). Several studies have also shown that Si
application reduced transpiration, but enhanced photosynthetic ef-
ciency and stomatal conductance (Chen et al., 2011) and water use ef-
ciency (WUE) (Gong and Chen, 2012; Ma et al., 2016; Gomaa et al.,
2021) in drought stressed plants. Some reported that Si supply can
improve photosynthesis and nutrient absorption under drought condi-
tions attributing to increased gas exchange and decreased Na
+
absorp-
tion by minimizing transpiration rate (Etesami and Jeong, 2018). The
improvement in photosynthetic rate by Si application in plants under
drought could be attributed to several factors such as improvement in
photosynthetic pigments, gaseous exchange features, enhancement in
water potential, reduction in oxidative stress (Tayyab et al., 2018), and
mitigation of chloroplast damage (Xu et al., 2017), stomatal limitations
(Zhang et al., 2020), chlorophyll stability index of plants (Ma et al.,
2016; Maghsoudi et al., 2016; Sattar et al., 2017), increase in RuBisCo
activity (Romero-Aranda et al., 2006), PEPCase activity and the inor-
ganic phosphorus concentration in leaves (Gong and Chen, 2012).The
improvement in photosynthesis could also be due to Si mediated
maintenance of plant leaf erectness and vigorous stem, thereby reducing
self-shading (Salman et al., 2012) and raising the photosynthetic canopy
(Verma et al., 2019a), increase in net CO
2
assimilation rate of leaves
(Maghsoudi et al., 2016), enhancement in water use efciency (Gong
and Chen, 2012; Ma et al., 2016), improvement in maximum quantum
efciency of photosystem II and basal quantum yield (Chen et al., 2011)
and also due to increase in photochemical efciency and activity of
photosynthetic enzymes (Khan et al., 2017; Zhang et al., 2017). Si
fertilization has been found to improve stomatal conductance by
different mechanisms like signal perception (Liu et al., 2014), movement
of ions contributing to osmotic pressure changes (Sonobe et al., 2010)
and by hydraulic adjustments related to stomatal movements (Liu et al.,
2014). The improvement in transpiration and stomatal conductance
under drought by Si application may be attributed to Si-mediated in-
crease in water absorption and transport in response to water loss and
maintenance of water balance in plants under stress (Zhang et al., 2020).
The Si-mediated reduction in transpiration rate could be due to the
development of a double layer of silica-cuticle on the leaf epidermis,
resulting in increased rigidity of the cell wall (Avestan et al., 2019;
Flores et al., 2019; Luyckx et al., 2017). It has also been observed that Si
reduces stomatal transpiration during drought stress by inuencing
M.A. Malik et al.

Plant Physiology and Biochemistry 165 (2021) 187–195
193
stomatal movement (Gao et al., 2005) and also by deposition of Si as
phytoliths along plant cell walls thereby modifying cell wall features and
thus causing decrease in stomatal conductance in relation to loss of
turgor in guard cells (Luyckx et al., 2017). In addition, the reduction in
transpiration rate of Si- supplied plants may also be attributed to sto-
matal pore regulation (Gao et al., 2006) which is primarily due to the
lack of turgor in guard cells.
2. Conclusions
Here we conclude that Si supplementation can successfully improve
plant growth, biomass, photosynthesis, nutrient uptake and antioxidant
enzymes during drought stress conditions, which may be triggered by
various biochemical mechanisms, physiological and molecular mecha-
nisms. Si addition can be useful in boosting crop growth, quality and
yield under drought stress. Furthermore, using molecular tools to
elucidate the plant gene cascade involved in alleviating drought toler-
ance during Si application could be very important for agriculture
sector. The focus of this review is to make broad audience understand
the underlying mechanisms involved in Si mediated alleviation of
drought tolerance which will help researchers to plan for developing a
sustainable cropping system by harnessing Si derived benets. Extensive
work is required to identify the underlying molecular mechanisms
involved in alleviating Si-mediated drought tolerance in plants. There-
fore comprehensive models need to be developed for improved Si rec-
ommendations based on soil types, plant species and variations in the
environment.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
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