Efficient Exploration of Silicon Derived Benefits to Combat Biotic and Abiotic Stresses in Fruit Crops


 Silicon (Si) is the most abundant element after oxygen on the earth crust surface. It plays essential role in crop production by stimulating the growth and development. Very substantial efforts have been performed to better explore Si derived benefits for horticulture crops. In the present review, molecular and physiological mechanisms explaining the observed beneficial effects plant derive from the Si supplementations, more particularly in horticultural species have been discussed. In general, horticulture crops need extensive management and higher crop protection measures compared to agronomical crops. Therefore, integrated approaches including Si supplementations will help to improve plant resilience under biotic and abiotic stresses. Application of Si to plants promotes cell walls strength and provides additional support through increased mechanical and biochemical support. Horticultural crop production is frequently subjected to the naturally occurring different biotic and abiotic stresses that can substantially reduce the absorption and translation of essential elements and ultimately decrease the crop yield. Fruit and vegetable production in Drought, salinity, high and low temperature, toxic metals and pest infection prone areas is the key to meet the world minimum nutrients demand. Here, molecular mechanism involved in the Si uptake by root and subsequent transport to areal tissues is also illustrated. However, Si uptake mechanism at molecular level poorly studied in horticulture crops. Here we described the role of Si and its transporters in mitigating abiotic stress condition in horticultural plants.


Introduction
Food security is one of the fundamental needs in present world that can never be ignored by society and police maker. The extensive increase in both human population and environmental damage due to unsustainable agriculture practices have the unlucky consequence to feed the world's population (Glick 2014). Climate change has similarly worsened the incidence and gravity of sundry abiotic stress with considerable reduction in economic yield in major cereals (Carmen and Roberto 2011). To increase the crop productivity, crops demand for fertilizer has caused rising production costs for farmers worldwide. So use of excess fertilizer in farming considered as one of the most important constraints to agriculture production in world. Various stresses instigated by complex environmental conditions have affected the production and cultivation of crops (Meena et al. 2017). Drought accounts for about 30% of the total worlds cultivable land area. Water stress has many similar features with salinity stress, but it is considered as more destructive to agricultural as compared to salt stress (Bodner et al. 2015). Now heavy metal pollution is rapidly increasing with modernization in farming system. It has major concern for ecosystem due to their prolonged toxic effect with long half-life in the environment (Etesami 2018).
Nutritional imbalance by abiotic stresses may hampers the normal physiological activities of plants (Paul and Lade 2014). In future, environmental stresses may increase with climate change. The cost associated with these stresses may impact heavily on agriculture, biodiversity and the environment (Acquaah 2007). Hence now feeding the worlds growing population with shrinking cultivable land area along with environmental stresses has given vital importance in research (Etesami and Bettie 2017).
Previous soil and plant researchers suggests that use of bene cial soil microorganisms and silicon (Si) in agriculture is a sustainable strategy for alleviation of various biotic and abiotic stresses in plants ; Wang et al. 2017). Earth crust comprises over 28% silicon (Si) and it is one of the most abundant element after oxygen (Spripanyakorn et al. 2009). Besides omnipresence, most of the soil types world-wide are de cient in plant available form of Si. Plant roots uptake it in the form of water-soluble silicic acid (H4SiO4) and follow the ascent of sap route to reach transpiration site via passive and active transport mechanism (Raven 2001). Earliest agronomist not considered Si as an essential element as N, P and K for plant growth because of non-availability of positive evidence to show Si involvement in any metabolic activities of plants (Datnoff et al. 2007). Plants are grouped into three categories based on Si uptake: low (rejecters), intermediate and high accumulators. Wide range of variation starting from less than 0.1 to as much as 10% of plant dry weight was observed in different plant species. Major Si accumulators in the plant kingdom are monocots (Ma and Tahkahashi 2002) whereas, dicots such as tomatoes and canola contain less than 0.1% Si in their biomass while some aquatic species have Si content over 5% (Deshmukh et al. 2015). Silica is taxonomically diverse biomineral (Knoll and Kotrc 2015) and its presence was recorded in almost all eukaryotes. Rhizarians, stramen opiles, opisthokonts, amoebae, animals, and land plants utilize silica for the formation of protective cell coverings, collagen, vertebrate bones, copepod mouthparts and other rigid structures (Henstock et al. 2015). It is necessary for coccolithophore calci cation (biomineralization) processes (Durak et al. 2016).
Horticulture crops, especially fruit and vegetables, acquire a key role in the food industry because of their higher demand for human consumption. They play an important role in commerce, employment and industrialization. On the other hand they are very vulnerable to the wide range of biotic and abiotic stresses, to overcome these hassles, they requires an optimum supply of balanced micro and macro nutrients. To meet the nutrition's demand under stress condition, Si commonly recommended in package of practices as per the plant need. Si can alleviate high temperature stress (Ashraf et al. 2010), reduce the heavy metal toxicity and protect from UB-radiation (Shen et al. 2009). Recently omics based were applied in different crops to gain a genomic level perception of the mechanisms by which SI application aids diameter (Prado and Natale 2005). After the application of Si at 0.05-0.1% in zaghloul date palms increased total chlorophyll content, leaf area, bunch weight, and nal yield (El-Kareem et al. 2014). In a study of post-harvest application of potassium silicate as a Si, supplier enhanced the fruit quality and reduced the ethylene evolution with increasing catalyze enzymatic activities in avocado fruit crops (Kaluwa et al. 2010). Several fruit crops are treated with different concentrations and formulation to know both positive and negative effects on fruit quality parameters, including nal economic yield.

Molecular mechanism involved in Si uptake and Transport
Plants have numerous transporter protein biomolecules which facilitate the uptake and accumulation of different metal ions and other mineral nutrition's from soil via aquaglyceroporins, ATP binding (ABC) cassette transporter, phosphate and nitrogen transporters (Bouain et al. 2014). These transporters helps to regulate the accumulation of unwanted elements beyond toxic level, as they play prominent role among all the crop plants.
Silicon transporters initially identi ed in diatoms and extended family of Si transporters reported in some non-silici ed living organisms. Based on the phylogenic study of SITs and Lsi2, they developed parallel by environmental selective pressure. SIT structure evolved multiple times via duplication and fusion of 5transmembrane-domain SIT-Ls. Later researchers identi ed the Lsi2 family similar to primary Si transporters, and they broadly distributed in siliceous and non-siliceous eukaryotes. These transporters involved in bio-silici cation. NPA motifs present in the aquaporins regulate the exclusion of H+ and various isoforms of nodulin26-like intrinsic proteins (NIPs) control the movement of water, boric acid, metabolites, and Si through different transporters (Wu and Beitz 2007). Identi cation of two Lsi Si transporters genes using rice mutants by Ma et al (2007) are the milestone for new research works on importance of Si and its transporters and impact of Si on aquaporins. Lsi1 (OsNIP2;1) is the rst Si transport protein identi ed in rice plant ) which is bidirectional transporter although its activity depends on the plant species but it mainly act as a Si in ux transporter. Whereas Lsi2 is an active Si e uxer which enable the ux of Si across the vascular system. The Lis2 proteins are H+ antiporters and similar to arsenic transporters (ArsB) of prokaryotes (Ma et al. 2007). Structurally Lsi1 has six transmembrane domains and two half helix harboring highly conserved Asn-Pro-Ala (NPA) motifs. The NIP's transports neutral solutes like glycerol, lactic acid, ammonia, boric acid, arsenite, selenite, silicic acid, and water also ). All Lsi1 present in plant species belongs to the NIP III group, which has a aromatic arginine (ar/R) selectivity lter (SF) comprising Gly (G), Ser (S), Gly (G), and Arg (R) popularly known as GSGR SF. Although Lsi1 acts as a bidirectional passive channel (Mitani et al. 2011).
Tomato plants, uptake of Si in the roots is mediated by Lsi1 (in ux transporter) and Lsi2 (e ux transporter). Sun H (2019) isolated and functionally characterized a SILsi1 expression in tomato, SILsi1 seems to be constitutively expressed in the root system but tomato plants accumulates very low Si (Deshmukh et al. 2015) as compared to cucumber (Mitani and Ma 2005). As the SILsi1 expression increased, the Ge (an analogue of Si) level increased but there is no signi cant variation observed in Si concentration in the root cells sap (Sun H 2019). Deshmukh et al (2015) proposed that, a precise distance of 108 amino acids required between the asparagine-proline-alanine (NPA) must require for Si permeability, while SILsi1 present in tomato plants has 109 amino acids between NPA domains. In potato, Lsi1 has expressivity at both roots and leaves, and its expression increased at slower rate after Si supply; while expression of Lsi2 decreased in tuber ush (Vulavala et al. 2016). Whereas in cucumber, the Lsi2 elevated expression observed in root system after 6 h of Si treatment (Sun et al. 2018).
Based on the past experiments, the expression of Si transport genes depends on the plant species, external stimulants and treatment periods although the exact mechanism in number of horticulture crops be remain to be investigated in future.

Importance Of Silicon Under Stress Condition
In recent years, the effect of Si on plants growth and involvement in biotic (blast, rust, downy mildew, powdery mildew, leaf spot, canker, leafhoppers, stem borer) and abiotic (metal toxicity, nutrient imbalance, lodging, drought, radiation, temperature, freezing, and wind) stress mitigating activities observed in a wide range of plant species (Ma et al. 2008). The concentration of Si varies within the genotypes belongs to the same species and even it can vary based on environmental conditions and other external stress conditions (Currie and Perry 2007). Silicon mostly deposited below the cuticle and epidermal layer of leaf and in the vascular system to strengthen cell wall and helps plants fend off biotic and abiotic stress (Debona et al. 2017;Etesami and Jeong 2018). It also gives better strength to control the transpiration rate and thus a greater resistance to both internal and external stress conditions (Kim et al. 2002). Silicon is known to participate in many cellular activities which can enhance the plant resilience under stress condition such as, it improves the acclimatization potential of the plants by cell wall lignin and hemicellulose deposition (Camargo et al. 2007), tolerance to UV stress and increased production of chlorophyll a and b (Yao et al. 2010).
Even increased phytohormones activity were observed in stressed plants after Si application to acclimatize to varying environmental condition (Kim et al., 2014). Number of studies reported that, Si decreases endogenous ethylene (Yin et al., 2016) (Wang et al. 2015b). Increased production of PAs may improve the antioxidant ability and modifying osmotic potential in stressed plant cells (Alcázar et al. 2011) and block inward and outward Na+ and K+ currents through non-selective cationic channels thereby control the Na+ concentration in intercellular space (Zhao et al. 2007).
Under stress condition, Si alleviate oxidative damage in plants by increasing the activity of key enzymes APX, CAT, SOD, POD, GR and GSH concentration (Kim et al. 2017) and prevent the membrane damage causes by the formation of malondialdehyde (MDA) (Zhu and Gong 2014). When Si applied to the plants rhizosphere zone, it increases the soil pH that ultimately enhances the soil phosphorus availability (Owino-Gerroh and Gascho, 2005; Eneji et al. 2008) and interact with iron and manganese metal ions (Ma and Takahashi 1990), and decreased metal uptake (Al, Cd, Fe, and Mn) to diminution the heavy metal stress on plants (Liang et al. 2005). Water and mineral uptake and transportation mainly controlled by the aquaporins present in the cell membrane (Maurel et al. 2015) and these aquaporins comes under membrane intrinsic proteins (MIP) family and further classi ed into nodulin26-like intrinsic proteins (NIP), plasma membrane intrinsic proteins (PIP), uncharacterized intrinsic proteins (XIP), tonoplast intrinsic proteins (TIP), small basic intrinsic proteins (SIP), and hybrid intrinsic proteins (HIP) based on the phylogenetic distribution, subcellular localization, substrate selection, length of the sequence, and function (Bienert and Chaumont et al. 2011). Apart from water uptake, these aquaporins involved in the mineral transportation. Numerous previous studies demonstrated the role of aquaporins in the regulation of solute transports such as ammonia, hydrogen peroxide, silica acid and lactic acid (Wu and Beitz 2007). The substrate selectivity of an aquaporins is primarily depends on the NPA motifs for the exclusion of H+ and a lter consisting of an aromatic/arginine region in the pore area . As compared to other NIP subfamily isoforms, NIP1 is highly permeable to water, whereas NIP2 helps transportation of metalloids and Si, and NIP3 regulate boric acid transportation (Wu and Beitz 2007). Absorption of Si from the soil by the plant depends on the Lsi1 and Lsi2 gene expression level, cellular localization, and polarity in the plant cell. Lsi1 activity extensively observed in roots, within a root, the Lsi1 expression is very low in root tip region and in root hairs and downregulated by ABA and dehydration stress (Ma and Yamaji 2007). Whereas Lsi2 localized on distal and proximal sides of the epidermis (Maa et al. 2007). Therefore, Lsi1 help to transport Si into the exodermis cells and later it released into the apoplast of a spoke-like structure across the aerenchyma by Lsi2. Fauteux et al. (2005) unveils that Si interact with several key components at different levels in plant signaling structures, thereby causing resistance in plants. Despite the above ndings, still very less is known about Si transporters in horticulture crops. All ndings concentrated only on con rmation of presence of Lsi1 and Lsi2 in horticulture crops. Except tomato, cucumber, strawberry and apple, very limited amount of works were done to identify special Si transporters present in horticulture crops. The list of different Si transporters identi ed in horticulture crops are studied by the abovementioned researchers, are listed in Table 1. Further study is still required to identify the Si transporters present in horticulture crops and recognize the role of these transporters on root morphology and Si-enhanced tolerance to biotic and abiotic stresses simultaneously ( Table 2).

Role of silicon in water stress condition
Drought is one of the key sources of environmental stress which severely affects the plant growth and development at any stage from germination to physiological maturity (Bodner et al. 2015). Water de cit during plant growth period adversely affects the normal physiological activities like photosynthesis, essential nutrient transport and production of excessive reactive oxygen species (ROS) leds to cell membrane damage ( Root system helps plant to absorb essential nutrients and water from soil, but under water stress conditions, reduced root growth and physiological activities observed (Gupta and Huang 2014). An increased root surface area is required to improve the water uptake ability to minimize the impact of these water stress conditions (Barber 1995); although Si did not trigger root progress under drought condition (Sonobe et al. 2010). It has been reported that Si improve In tomato plants, Si accumulate much less as compared to the monocotyledons such as rice and wheat (Nikolic et al. 2007).  analyzed the root proteomics in tomato plants under salt stress condition and found that, around, 17% stress responsive proteins, 11% to plant hormones, 11% to cellular biosynthesis, and transcriptional regulation, RNA binding, and secondary metabolisms related proteins were seen upregulated when Si applied to these stressed plants (Chakrabarti and Mukherji such as Calvin cycle, tricarboxylic cycle (TCA) cycle, and pentose phosphate cycle in salt stressed Rosa hybrid 'Rock re' and ensure photo-protection and physiological development. Increased expression of βglucosidases, β-galactosidases, and glucose-1-phosphate adenylyltransferase large subunit, acetyl-CoA carboxylase, and Glycerol-3-phosphate dehydrogenase (GPDH) (NAD+), observed after Si treatment and this leads to improvement in starch and sucrose metabolism, fatty acid biosynthesis and mentain the NADH yield.
During water limiting condition, increase in water absorption coincided with the increase in plasma membrane intrinsic protein (PIP) aquaporins expression (Liu et al. 2015). Silicon transport also mediated by cell aquaporins, especially members of the Nod26-like major intrinsic protein III subgroup, but they shows varying responses to Si application in different crops. For example, down regulated in soybean (Deshmukh et al. 2013) upregulated in cucumber after Si application (Ma and Yamaji 2015). Si enhances the shaker-like potassium channels (SKOR) activities and increases the potassium translocation into the xylem which result in increased hydraulic conductivity. It upregulate the expression of OsRDCP1, OsRAB16b, OsCMO ) and SbPIP ) genes in drought stressed plants. The homolog of both OsLsi1 and OsLsi2 downregulated upon Si supply ) in response to drought stress (Yamaji and Ma 2007). In barley, the expression of HvLsi1 gene was unaffected by external Si application ), while HvLsi2 expression was downregulated ). HvZEP1, plastid related pathway gene expression strongly increased with increasing Si concentration under stress condition, while expression of HvNCED1 and HvNCED2 increased in dosedependent matter (Kim et al. 2013). Several studies shows that Si directly or indirectly involve in physiological and biochemical activities during water stress condition. However, the mechanism of Simediated water stress tolerance in horticulture crops still need to be explored Role of silicon in salt stress condition Salinity stress also known as osmotic stress and ionic stress and it is one of the major constraint for agriculture including horticulture crops under surface irrigation system, thereby instigating damage and inhibition of crop growth and development. So now a days, salinity was considered as a one of major threat to worldwide crop production. Salt affected land increasing drastically with an increase in intensi ed cropping systems. Nearly 7% of the earth land and 20% of the cultivable land under salinity stress condition (Hu and Schmidhalter 2002). Unscienti c irrigation practices, excess fertilizer use, low precipitation, industrial pollution and high soil water evaporation are the various resions that can be associated with the emergence of salinity-affected land (Ouhibi et al. 2014). When EC level at root zone exceeds the s 4 dS m−1 at 25•C with exchangeable sodium of 15%, the land is said to be saline affected land and considered as un t for farming especially for horticulture crops (Munns et al. 2005). The presence of high salinity by deposition of excess salt around the root zone of soil causes osmotic, oxidative, and ionic stress on growing plants. Thus, breeding strategy to develop salinity tolerant horticulture crops are major challenge so as to surpass the reducing quality food production at surplus quantity. Where drainage system is not proper, salinity stress occurs due to excess accumulation of Na+ and Cl-ions in turn affecting the K+/Na+ ratio (Golldack et al. 2011). During salinity stress, the reduction in leaf area, stomatal conductance, chlorophyll content and function, stomatal conductance, decreased photosystem II e ciency (Netondo 2004). Along with osmotic stress and other nutrients de ciencies lead to the increased production of ROS (Hong et al. 2000).
Several attempts are made for effective salinity management, such as by changing farming system, introducing salt tolerant crops so as to include perennials in rotation with annual crops, in mixed farming (intercropping, alley cropping, etc.), or precision plantings (Munns 2002), . But the implementation is restricted by numerous factors such as non-availability of salt tolerant cultivars, good quality irrigation water, and cost. Other than agronomic methods, salt-tolerant varieties development through transgenic method, application of growth promoting hormones and using the micro irrigation techniques are the alternatives are present to minimize the salinity impact on total world production. Nevertheless little information present in public domain regarding the mineral status and plant dynamics towards tolerance to salinity stress (Manchanda et al. 2008). Therefore, a major challenge present in front of researchers to develop e cient, affordable, and easily adaptable mechanism to counter the salinity stress on plants. Si improves the photosynthesis rate (20%), water use e ciency (17%), turgor pressure (42%) and the ratio of plant dry matter to drought ratio (16%) in tomato (Romero-Aranda et al. 2006). Similar result found in sweet pepper (Tantawy et al. 2015) and squash (Siddiqui et al. 2014) in mitigating salinity induced hazardous effect. It has also been concluded that application of Si combination with Melia azadirchta phytoextract can effectively alleviate salinity stress in pea (Tantawy et al. 2015).
Plant transporters play a vital role in regulating mineral uptake and deposition in different tissues from growth medium. PIP regulate the water conductance under salinity stress condition (Liu et al. 2015) and is responsible for Si in ux into the root system even under moderate salinity stress condition ). Salt stress responsive (LeDREB-1, LeDREB-2 and LeDREB-3), antioxidants (LeAPX, LeSOD and LeCAT) and Si transport (leLsi-1, leLsi-2 and leLsi-3) genes are activated after application of Si as a fertilizer to tomato plants .
The mechanism of Si mediated salinity tolerance in horticulture crops is still not fully understand at physiological level. Some researchers assumes that Si alleviate salt-induced osmotic stress by enhancing root water uptake and upregulation of aquaporin speci c gene expression (Zhu et  Whereas some heavy metals like Zn, Cu, and Ni are essential micronutrients and they serve as cofactor in various enzymatic activities when they are present in trace. Interestingly, plants are natural non-selective bio-accumulators and they uptake heavy metals with other nutrition's (Ozturk et al. 2017). While Cd and Ld are supplied to the plants through pesticides as ingredients but they do not have any bene cial role. But they become more toxic when applied in excess amount with pesticides (Ali et al., 2017b). Heavy metal stress fuels superoxide radicals (O−2), hydroxyl radical (-OH), hydrogen peroxide (H2O2) and singlet oxygen (1O2) like ROS production, which have strong harmful in uence on photosynthesis, respiration, plasma membrane and fatty acids integrity (Stohs and Bagchi 1995). Plants have enzymatic antioxidants to protect the plant cell by ROS. Ascopate peroxide (APX), catalase (CAT), peroxidase (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR), as well as nonenzymatic constituents such as ascorbic acid (AsA), carotenoid (Car) and gluthione (GSH) are more prominent antioxidants which regulates the ROS level in plant cell (Tiryakioglu et al. 2006). The rate of heavy metal absorption, transportation and accumulation in various organs varies from species to species. Excess accumulation of heavy metals may shows toxic symptoms like stunted growth, chlorosis, root browning, decline and death (Ozturk et al. 2015).
At cellular level, heavy metals severely damages the intra-and inter-DNA and protein molecules cross linkage, DNA bases deletion, modi cation, rearrangements, strand breakage and depurination. Heavy metals produces promutagenic adduct 8-0xoG (7,8-dihydro8-oxoguanine) that miss pairs with adenine and leads to transversion of C to T (Cunningham 1997;Kasprzak 1995). Cation diffusion facilitator (CDF) and macrophage protein family (Nramp) family play important role in homeostasis but also draw stratagem for heavy metal tolerance (Williams et al. 2000). ZIP gene family transporters transport Cd, Fe, Mn and Zn, so regulation of ZIP genes expression may control the accumulation of these metals (Guerinot 2000). Usually, A ZIP family proteins has eight transmembrane domains and all domains has C and N terminus and they are exposed to apoplast. In vitro study of Si-mediated metal, precipitation provides the su cient information, in which Si decrease the availability of toxic metals to plants ) by precipitation . In some circumstances, Si reduces the bioavailability of metals to plants (Liang et al. 2005). In some circumstances, Si induces use of stored immobilized metals as a source of plant nutrients when Exogenous Si reduces the lipid peroxidation and fatty acid desaturation in plants and improves the plant physiological activities under heavy metal stress (Nagajyoti et al. 2010). Most widely accepted Si mediated heavy metal detoxi cation mechanism are metal immobilization in soil before absorption from plant roots and stimulation of enzymatic and non-enzymatic antioxidants, co-precipitation of metals, metal ion chelation, and compartmentation. The possible mechanism for reducing heavy metals can be explained that Si strongly form complexes with cell wall molecules and alter the structural integrity which leads to the blockage of the apoplastic transport and directly or indirectly restricts the entry of these heavy metals (Meharg and Meharg 2015). It was widely accepted that, Si has bene cial to plant growth and development by mitigating multifarious biotic and abiotic stresses including heavy metal stress ( gure 1.). Below, we discussed in detail about the mechanism of Si-induced mitigation of heavy metal toxicity in plants.
Arsenic (As) Arsenic (As) being considered as non-essential element and it may cause severe damage to soil health, plant growth and reproduction and ultimately affect human health after consumption of contaminated plant produce ). Due to excess accumulation of As in soil reduces the plants nutrient absorption ability which resulting in impaired plant physiological activities ). K being analogous to As and compete for some carriers present in the plasma lemma by providing same ionic strength. This result in reduction in As in ux (Hasanuzzaman et al. 2015). As is a semi-metallic element and form the organic and inorganic arsenicals by reacting with other elements. In these, inorganic As compounds are more lethal to plants as compared to the others. Inorganic As species largely occurs in arsenit Fe (As III ) in a reduced form and arsenate (As v ) an oxidized form (Finnegan et al. 2012). Nodulin 26-like intrinsic protein (NIP) is a major entry point for arsenite, whereas arsenate is taken by plants roots from rhizosphere via phosphate transporters (Wu et al. 2011). It is well known that As accumulation leds to oxidative stress by generating reactive oxygen species (ROS). To combat oxidative stress, plants produces enzymatic and non-enzymatic biomolecules. However, response of all the detoxi cation machineries in uenced by As bio-availibity, toxicity and mobility and presence of other ligands (Violante et al. 2010).
Silicon has potential to abate As toxicity and improves photosynthesis, carbohydrate accumulation. In some tomato cultivars, Si application signi cantly increases seed germination with inhibiting As accumulation in plants (Marmiroli et al. 2014). Under As stress condition, varioustransporters helps the movement of Si from root epidermis into root steel, and then shoot via xylem sap. Furthermore, study conducted on Lsi1 and Lsi2 transporter genes under arsenite treatment revals that these Si transporters effectively serve as major entry path for arsenite transport.Upregulation of OsLsi1, OsLsi2 and OsLsi6 recorded in As (III) + Si as compared to As (III) alone treated plants, indicating that the concentration of OsLsi family genes expression might not be su cient to accrue As in the presence of Si ). Increased activity of GPX and GST observed under As stress due to activation of GSHdependent peroxide scavenging mechanism, which helps in the reduction of oxidative damage, and prevent membrane damage with the help of Si. Till now Si-dependent amelioration of As toxicity from germination not studied in horticulture crops in detail.

Aluminum (Al)
Aluminum toxicity is the one of the limiting factor for crop growth in acidic soil (Von Uexkull and Mutert, 1995). Al normally form insoluble oxides and complex aluminosilicates (AS) at pH valve higher than 5.0,

Cadmium (Cd)
In agriculture, Cd content of soil is limited to 100mg/kg, if concentration crossed this limit, than the eld is declared as un t for forming (Salt et al. 1995). Cd toxicity results in chlorosis, retardation of plant growth and development, modi cation of enzyme activities, accumulation of excess ROS and protein denaturation ). Even it inhibit or modify the Fe+3 reductase activity, that leds to Fe+2 de ciency. Cd also interferes with the uptake of macro and micro nutrients mainly, Ca, P, K and Mg and reduces the absorption and translocation of nitrate by inhibiting nitrate reductase. In higher plants, excess Cd level can effectively inhibit photosynthesis by downregulating carotenoids function (Prasad 1995). At cellular level, it has potential to cause nucleolus damage leading to chromosome fragmentation and aberration, reduces the respiration by decomposition of mitochondria, affects the electron transport chain by interfering with redox reactions and it replace the Ca in calamodulin which is involved in cell signaling (Rivetta et al. 1997). In Brassica napus, even low concentration of Cd (5 μM To clarify the roles of Si in mitigating Cd toxicity, Nwugo and Huerta (2011) analyzed leaf proteome and they found 60 proteins ware responsible to minimize the Cd toxicity in plants when grown in Cd contaminated soil, among them over 50 proteins associated with photosynthesis, regulation of protein synthesis system, redox homeostasis, and pathogen response were differentially regulated by Si with upregulation of a class III peroxidase activities. Previously studies in Arabidopsis thaliana con rmed that, HMAs (AtHMA1, AtHMA2, AtHMA3, and AtHMA4) detoxify the Zn present in the chloroplast organs and regulate the Cd accumulation in various cell organelles especially in vacuoles and plasma membrane. Various researchers assessed the OsHMA2 and OsHMA3 genes expression in response to Cd stress in rice plants, and they found that the expression of OsHMA2 at 1-DAT was signi cantly increased with Si treatment (Courbot et al. 2007).
Addition of Si in rhizosphere zone contaminated by Cd, enhances the shoot and root biomass by 43-90% and 38-50% respectively. A similar detoxi cation mechanism has also been observed in peanut (Arachis hypogaea L.) (

Copper (Cu)
Copper (Cu) is an essential element and which can plays various key biochemical activities including photosynthesis, carbon assimilation and ATP synthesis. Cu is an important constituents of cytochrome and plastocyanins and cytochrome oxidase, both of which are vital components of both respiration and photosynthetic systems (Yadav 2010). In rhizosphere region, 1-20% of Cu present in an available form and remaining bond to organic matter. It is considered as nearly immobile in plants, so initially, fresh leaves and reproductive parts shows Cu de ciency symptoms. At the physiological level, symptoms of Cu de ciency include reduced respiration, impaired photosynthetic electron transport, and stunted growth symptoms that appeared after Cu de ciency (Marschner 1995). Neelima and Reddy (2002) studied the effect of Cu on Solanum melongena seeds and found that, excess Cu can adversely affect the germination and seedling establishment.
There are fewer studies conducted to know the Si-Cu interaction in plants (Frantz et al. 2011). Cu toxicity causes chlorosis on leaves and root biomass reduction in Arabidopsis thaliana (Khandekar and Leisner 2011) and Triticum aestivum (Nowakowski and Nowakowska 1997), after application of Si, these Cu toxic symptoms were diminished by Cu binding with deposited Si on cell wall (Rogalla and Ro¨mheld 2002) but the level of Cu content was not signi cantly changed in leaf even after external application of Si ).

Manganese (Mn)
At low concentration, Mn is essential for normal enzymatic activities. Plant species like peach, wheat and soybean are very susceptible to Mn de ciency whereas maize and rye are much less vulnerable (Reuter et al. 1988). Signi cantly yield reduction observed mainly in winter crops when Mn availability is below the critical level. Mn de ciency symptoms like decreased dry matter accumulation, decline in photosynthesis and chlorophyll content were normally observed in plants when grown in Mn de cient soil. (Papadakis et al. 2007). In dicotyledonous, interveinal chlorosis of younger leaves observed whereas, in cereals, a gray speck are major common symptoms. Observed in plants when grown in sandy and calcareous soils. Among all phenolic compounds, carotenoids present in vegetables and fruits crops in high amount (Racchi 2013). Carotenes includes the carbon and hydrogen atoms whereas, Xanthophylls of carotenoids contains oxygenated form of carotenes. The most important role of α-tocopherol is that regulate or eliminate the oxidative atoms or molecules, which are generated in the thylakoid membrane; thus, it can prevents lipid peroxidation (Kataria 2017). Si application under oxidative stress condition increases the activities of carotenoids along with SOD, GPX, APX, GR, and CAT actions (Liang et al. 2003).

High pH stress
High pH or Alkalinity is the major problem for plant growth and development (Adil Khan et al. 2019).
According to Jin et al. (2006), up to 831 × 10 6 ha world land is affected by saline. Out of which, nearly 434 × 10 6 ha is severely affected by saline-alkalinity, which causing severe reduction in world agriculture production and productivity. Under medium to high alkaline condition, high rhizosphere pH signi cantly affect the essential nutrients availability from by converting them into non-available form. High pH enhances the ROS level and hence plants need to produce antioxidants to neutralize the side effect of ROS (Peng et al. 2008). High pH stress activates many signal messengers such as, calcium, jasmonic acid (JA), salicyclic acid (SA) and ethylene (Klessig and Malamy 1994). Out of which, SA effectively regulate the ROS production with the help of SOD (Molina et al. 2002). Si been implicated similar bene ts on plant growth and development as SA during high pH stress conditions. Exogenous application of Si improves the chlorophyll content but reduces the MDA contents in the plants ( The second mechanism of Si that protects plants against pathogens is increased production of defensive biochemical like lignin, phenolic compounds, and phytoalexins like secondary metabolites (Ma and Yamaji 2006). Some studies reported that avonoid concentration increased in cucumber plants when powdery mildew infected (Fawe et al. 1998

Conclusion
Si treated plants shows increased environmental stress tolerance as compared to other micronutrients. Si improves the plant growth and development even under stress condition by improving photosynthesis, metabolism, and solute transportation. In this review, we intended to clarify the mechanism of Si and its transporters role in alleviation of different stresses in horticulture crops. Silicon has much more role in both biotic and abiotic stress conditions in agriculture as well as in horticulture crops. It can be used as a universal agent to minimize the effects of known and unknown factors. Based on the previous works, now Si considered an essential element and it also applied to horticultural crops along with the main fertilizer complexes. It has a positive effect on seed imbibition, germination, vegetative and reproductive stage of the plant. Nowadays, the salt-affected area increasing at a drastic rate leds to shrinking in cultivable land but the use of Si under this condition, some amount of salt effect can be minimized.     Figure 1 Role of silicon in plants Silicon mediated heavy metal stress regulation in fruit crops by modifying difeerent morphological, biochemical and molecular events (the downside arrow indicates downregulation and upside arrow indicates increased activities).