Journal of Alumni Association of Biotechnology (2022) 4(2):1-7
Biostimulants: An Eco-friendly Approach in Reducing Heavy
Chhattisgarh Institute of Medical
Sciences, Bilaspur, Chhattisgarh, India
*Corresponding Author Email- firstname.lastname@example.org
25 August 2022
Received in revised form
18 October 2022
One of the most serious issues
in the nation is heavy metal toxicity. It severely affects both plant and
animal kingdom as well as disrupts the soil ecosystem. The consequence of
heavy metal toxicity is the generation of osmotic and oxidative stress,
membrane dysfunctioning, metabolic imbalances and cellular toxicity. To
combat the prevailing issue, seeking eco-friendly ways to enhance and
stimulate plant tolerance against stress, is the vital step. Hence, the
application of biostimulants can be the best and effective alternative.
Biostimulant is a non nutrient preparation of microorganism and/ or
substances which enhance the crop quality enrich nutrient uptake efficiency
and show tolerance against abiotic stresses. The super dynamic substances
utilized in such arrangements are humic and fulvic acids, compounds
containing nitrogen, protein hydrolysates, silicon, melatonin, seaweed
extracts, and valuable microorganisms. The review briefly articulates the
definition of biostimulants, its various types as well as its role towards
reducing the heavy metal toxicity. Apart from these, the review also takes in
consideration the ameliorating effects of silicon against heavy metal
toxicity. Hence, biostimulants play an important role as reactive oxygen
scavengers, stimulate antioxidant system of the plant, and induce gene
expression of stress responsive gene thereby fostering plant tolerance to
heavy metal toxicity.
One of the
major agronomic challenges that have seriously threatened food safety is heavy
metal accumulation. This is the main reason behind raised concerns over heavy
metal pollution, which unpleasantly disturbing agro ecosystems and crop
production. Once the toxic heavy metals deposited beyond certain permissible
limits, they badly affects the soil fertility, density, presence and
physiological activities of microbiota. Therefore, the metal induced
phytotoxicity problems warrant urgent and immediate attention (Jägermeyr,
All abiotic stresses conditions such as low/ high temperature, nutrient
deficiency, drought, metal toxicity, UV exposure and photo-inhibition are
periodic in nature .Plants under stress start various physiological, cell, and
sub-atomic changes to adjust and react to such sort of stresses. Distinctive
abiotic stress prompts diverse physiological or metabolic changes in the plant.
The noticed changes might be inordinate reactive oxygen species (ROS) creation,
membrane damage, lower water potential, accumulation of osmolyte, nutritional
disorder, rolling of leaves, hormonal imbalances, membrane dysfunction,
increased photo oxidation, reduced photosynthesis and protein synthesis etc .
Among all the stresses, heavy metal contamination is one of the concern
environmental issues which severely affect the productivity of crop as well as
has the negative impact on human health (Hu et al. 2017; Singh et al. 2020).
metal toxicity has many major effects on the plants. Heavy metals are generally
accumulated in the root cells as Casparian strips blocks the further movement
of heavy metals into the vascular system. This causes physiological,
biochemical, and morphological changes in plants affecting crop productivity
(Shahid et al. 2015; Guo et al. 2016). Protein oxidation and the generation of
reactive oxygen species (ROS) such hydroxyl radicals (OH), superoxide radicals
(O-2), and hydrogen peroxide (H2O2) are caused by the phytotoxic effects of
heavy metals. Accumulation of heavy metals are also responsible for production
of methylglyoxal, a cytotoxic compound which leads to peroxidation of membrane
lipids thereby damaging cellular membranes causing membrane leakage as well as
fragmentation of DNA causing oxidative stress. These overall mechanism harm
cell and subcellular organelles (Hayat et al. 2012; Ali et al. 2013; Qadir et
al. 2014; Ghori et al. 2019).
mitigate the magnitude of metal induced changes a novel and eco-friendly
approach is the practice of plant biostimulants, having potential which can
improve nutrient uptake and efficiency, abiotic stress tolerance as well as
securing yield stability under unfavorable environmental and edaphic conditions
(Colla and Rouphael, 2015; Colla et al., 2014). The aim of this review article
is to highlight recent advances on the use of biostimulants and discuss about
the major silicon-related mechanisms to reduce the toxicity of heavy metals in
plants to improve plant nutrition and organic farming.
and their benefits
on new agricultural technologies and approaches is taking a tremendous leap and
aimed to improve crop quality and productivity under existing environmental
conditions by preserving soil quality. The application of bio based material
such as ‘Biostimulant’ is one such approach. It is considered as one of the
efficient and sustainable way as well as a better alternative to agrochemicals
to promote plant growth (Rouphael and Colla 2020). As per the the European
Biostimulants Industry Council lists the benefits of biostimulants are as
follows (Fig. 1).
•Refining the efficacy of the plant’s metabolism to increase the yield
and crop quality.
•Increasing tolerance to and recovery from abiotic stresses in plants.
•Enabling nutrient assimilation and their translocation.
•Enhancing quality attributes of produce, including sugar content, color,
fruit seeding, etc.
•Rendering water use more efficient.
Categories of Biostimulants and their role in reduction of Heavy metal toxicity
are classified on the basis of different properties. One includes the
classification on the basis of their origin into biological and chemical or
physical type (Rafiee et al. 2016). Another order approach separates
biostimulants in microbial, which are acquired from plant growth promoting
bacteria and arbuscular mycorrhizal organisms, and non-microbial biostimulants
which incorporate, humic substances, micro-algae extracts and biopolymers, for
example, chitosan (Ahemad and Kibret 2014; Rouphael and Colla 2020). Thus both
microbial and non-microbial substances are further categorized into different
protein hydrolysates have the capacity to accumulate proline in many plants,
subjected to heavy metal stress. Proline performs a variety of crucial tasks,
including osmoregulation, scavenging free radicals during the intake of heavy
metals, and chelation of metal ions in plant cells and xylem sap. It has been
reported that amino acids including
cysteine, asparagine, glutamine and glutathione peptides as well as
phytochelatins are potentially involved in
chelation of Zinc (Zn), Nickle (Ni), Copper (Cu), Arsenic (As) and
Cadmium (Cd) (Sharma and Dietz 2006; Sytar et al. 2013).
and fulvic acids
microbial activity like decomposition of microbial, plant and animal residues
results in the formation of Humic substances (HS). It is naturally found in
soil. On the basis of their solubility and molecular mass, HS are classified
into humic acids, humins and fulvic acids. HS have been recognized for their
major contribution in increasing soil fertility. HS also enhance the plasma
membrane H+-ATPase activity which results in cell wall loosening, enlargement
and organ growth etc., (Jindo et al. 2012). HS also activate the antioxidant
property and induce the synthesis of secondary metabolites (Jindo et al. 2020).
extracts & botanicals
has been used for hundreds of years as a fertilizer and to improve soil
structure. Polysaccharides laminarin
(From brown algae Laminaria), alginates, carrageenans (originate from red
seaweeds) and their breakdown products are comes under this criteria. They
include micro- and macronutrients, nitrogen-containing compounds like betaines,
sterols and hormones (Khan et al. 2009; Craigie 2011). In soils,
polysaccharides obtained from seaweed leads to water retention, gel formation
and soil aeration. ‘Botanicals’ are the substances taken out from plants and
are used in pharmaceutical and cosmetic products. They are used as food
ingredients and also in plant protection products (Seiber et al. 2014). Further-more
botanicals includes allelochemicals i.e. plant active compounds involved in
plant interactions with ecosystems are receiving more attention in the context
of sustainable crop management (Ziosi et al. 2012; Ertani et al. 2013).
& other biopolymers
is a biopolymer, deacetylated form of the chitin, produced naturally and
industrially. Chitin and chitosan apparently use different signaling pathways.
Agricultural applications of chitosan have been focusing on plant protection
against fungal pathogens, tolerance for abiotic stress like drought, salinity,
cold stress. Chitosan induces stomatal closure via an ABA-dependent mechanism
to down the environmental stress (Iriti et al. 2009). Laminarin is a storage
glucan of brown algae used in agricultural applications (Gozzo and Faoro 2013).
Figure: 1 Role of biostimulants in plant growth
criterion mineral like cobalt (Co), silica (Si), and selenium (Se) are present
which promote plant growth, plant products and tolerance to abiotic stress.
Insoluble forms like amorphous silica (SiO2.nH2O) are present in Graminaceaous
species. Function of inorganic compounds includes rigidification of cell wall,
reduction in transpiration by crystal deposits, osmoregulation, enzyme activity
by co-factors, antioxidant protection, symbiotic associations, protection
against heavy metals toxicity and plant hormone synthesis and signaling
(Pilon-Smits et al. 2009; Hoque et al. 2021).
bacteria and fungi
many beneficial bacterial-plant connections:
(i) As for
fungi there is a continuum between mutualism and parasitism;
niches extend from the soil to the interior of cells, with intermediate
locations called the rhizosphere and the rhizoplane;
or permanent associations, some bacteria being even vertically transmitted via
influencing plant life cover participation to the bio-geochemical cycles,
supply of nutrients, and increase in nutrient use efficiency, induction of
disease resistance, enhancement of abiotic stress tolerance, modulation of
morphogenesis by plant growth regulators. With regard to the agricultural
practices of biostimulants, mutualistic and rhizospheric plantgrowth-promoting
rhizobacteria (PGPRs) are used as microbial inoculants facilitating nutrients
acquisition by plants (Ahmad et al. 2019).Apart from bacteria, Arbusculer
Forming Mycorrhiza (AMF) are well-known endomycorrhiza associated with crop and
horticultural plants. This association involves fungal hyphae of Glomeromycota
species to penetrate root cortical cells and form branched structures called
arbuscules (Bonfante and Genre 2010; Behie and Bidochka 2014).
alleviation of metal toxicity in plants
crust of the planet, silicon is the second most prevalent element. Silicon is a helpful element that is
important in reducing abiotic stress, like the toxicity of heavy metals to
plants. Si can alleviate and decrease the heavy metal uptake and its
transportation in plants. During plant growth enhancement, silicon could
increase malic and formic acid and they consequently diminish the uptake of
Aluminum (Al). Silicon role has been reported as barrier because can expand the
epidermal layer of maize and causes accumulation of Mn in non-photosynthetic
tissue (Kaya et al. 2009; Santiago et al. 2021). Root anatomy plays an important
role in reducing heavy metal toxicity (Mn and Cd) because roots are the first
line exposed to heavy metals. By decreasing the apoplastic bypass flow, silicon
promotes the binding of metal ions and reduces the transfer of hazardous metals
from roots to shoots. (Ma and Yamaji 2006; Ye et al. 2012). Most of the plants
use silicon to augment their defenses against the entrance of toxic heavy
metals via the root apoplast. This process is achieved by Sodium ions, which
cross the symplast through the apoplastic pathway which is also called a bypass
flow. Casparian bands and suberin lamellae are the main constituents of
apoplastic barriers, poor development of this barrier leads to the generation
of bypass flow. Silicon also alleviates metal ion toxicity. In cucumber plants
formation of hydroxy-aluminum silicates in the apoplast of the root apex may be
the reason behind the reduction in apoplastic Aluminium (Al) mobility and the
binding of excess Magnese (Mn) and Copper (Cu) to cell walls (Horst et al.
2010; Bosni et al. 2019; Cristofano et al. 2021).
and Heavy metal Tolerance
In plants, silicon frequently enters interior
structures (cell walls and lumens) as mono silicic acid and precipitates. In
addition, it is deposited as amorphous silica (Sio2 ⋅ 𝑛H2o) in
intercellular sites and space between the plant cells like phytoliths (Greek:
stone of plant) Silicon’s defense mechanisms appear throughout plants parts. In
leaves silicon forms epidermal trichomes and hair like structure while in
spines it is present as amorphous silica (SiO2) and phytoliths (Tripathi et
al., 2014; Hartley and De Gabriel, 2016). There are various channels and
methods through which silicon reduces heavy metals in plants and soil. Silicon detoxification mechanisms can be
grouped into chemical or physical mechanisms. Co-precipitation of heavy metals
along with silicon comes under Chemical mechanism, while physical mechanisms
involves changes in apoplastic barrier which stimulates silicon to reduce the
translocation of toxic metals to shoot and different aerial parts (Gu et al., 2011). Generally, Si
mediated toxic metals alleviation process in plants includes (a) stimulation of
antioxidant enzyme action to increase ROS scavenging, (b) complexation and
immobilization of heavy metal in plants (Liang et al., 2007) (c) accumulation
and deposition in plant tissue to enrich the rigidity and stability in leaves,
(d) mobilization of water, and (e) providing plant nutrient and
co-precipitation of metal toxicity (Savvas and
Silicon detoxification in plants
Mechanism One: Reduction in Heavy
Metal uptake in Plants
Silicon can potentially reduce the heavy metal
uptake and its transportation in plants. Free metals in plant organs decrease
apoplasmic metal uptake and translocation activity. Silicon has been found in
the cell walls of plant roots, stems, leaves, and hulls, which increases the
durability of plant tissue as a physical barrier. Transportation of metal ions
from roots to aerial organs is regulated by silicon accumulation inside
the cell wall (Ma and Yamaji, 2006; Ye et al., 2012). In rice plants extension
of apoplastic barriers, such as endodermis development under metal stresses, is
an important factor to prevent the translocation of cadmium to aerial parts
(Pontigo et al., 2017). Most prominent role of silicon can be reported in the
cell walls of root, stems, leaves, and hulls, which improve the stability of
plant tissue as a physical barrier. In Roots, reduction of toxic metal
translocation from roots to shoots achieved by binding of metal ions through
decreasing the apoplastic bypass flow. Moreover, a buildup of silicon in the
stem, leaf, and hull cell walls restricts cuticle transpiration and strengthens
the plant's tolerance to heavy metal stress. In shoots, silicon decreases
uptake of cadmium and its translocation. During the silicon precipitation,
silica precipitates as phytoliths in the water evaporation sites of plant shoot
and simultaneously shifts Mn to the leaf blade that leads to decrease in Mn
uptake (Ma J.F. 2004; Rizwaan et al., 2012).
Mechanism Two: Changes in pH Value in Soil and Plant Culture
The mobility and bioavailability of heavy metals in
soil are significantly influenced by pH. Biosolids, a Si compound raises the pH
value and improves the absorption of silicon. Further it can lead to
an immobilization of cadmium in plants. Amelioration of Al by silicon in
soybeans includes soil sodium metasilicate or alkaline pH action,
playing an important role in the reduction of metal availability in soil
and consequently amelioration of metal toxicity to plants. Additionally,
silicon can facilitate the transportation of aluminium from the root to the
shoot by causing the formation of hydroxyl aluminium silicate complexes in
shoots, which lowers the pH level to below 4.0 and promotes aluminium
detoxification (Zhao et al., 2007; Violante et al., 2010; . Caporale and
Mechanism Three: Formation of
Si-Heavy Metal Complex
of heavy metals in plants with Silicon includes solution chemistry mechanism
(heavy metals complex formation) and plant mechanism i.e., stimulation of
organic acid exudate in plants to chelate metals ions. In first mechanism,
silicon creates a complex with heavy metals by making silicates and oxides,
unavailability of Si in plants results in complex formation. For example, Al-Si
complex decreases the toxicity of Al3+. Similarly in Minuartia
verna, Zn silicate precipitated in the leaf epidermis and acts as an
essential pathway for Zn detoxification (Wu et al., 2013; Lu et al., 2017).
Gunes et al. (2007) revealed that silicon limited translocation of baron from
root to shoot in spinach. In Cardaminopsis
sp., during the silicate formation Zn precipitates in to the cytoplasm
along with silica. Then Zn-silicate degrades to SiO2 and Zn immediately. After
that, Zn is transferred to the vacuole, and in the cytoplasm, SiO2
precipitates. The compartmentation method in plants involves the formation of
pinocytotic vesicles by the plasma membrane and tonoplast, which directly
transferred Zn from extracellular portions to vacuoles. (Adrees et al., 2015;
Tubana and Heckman, 2015).
Mechanism Four: Antioxidant Defense
Heavy metal toxicity induces an excess formation of
reactive oxygen species (ROS),
which results in some metabolic disorders in crop plants (Adrees et al., 2015;
Ahmad et al., 2019). To overcome these issues Si induced enzymatic and non-enzymatic
antioxidant system helps to lower oxidative stress by decreasing the production
of ROS. In cucumber, mitigation of
Mn toxicity by Si was attributed to a significant drop in lipid peroxidation
(LPO) caused by excess Mn. Si also helps in significant increase in enzymatic
antioxidants i.e. Superoxide dismutase, glutathione reductase and ascorbate
peroxidase as well as non-enzymatic antioxidants such as ascorbate and
glutathione (Shi et al., 2005). Similarly, under Cd stress, application of Si
reduced the H2O2 and electrolytic leakage (EL) in Solanum
nigrum (Liu et al., 2013). Thiobarbituric acid reactive substances (TBARS)
is widely used marker for reactive oxygen contents, it has been found that with
Si supplementation in rice and maize plants grown under Cd stress condition,
TBARS reduced significantly Si-mediated detoxification through stimulating
enzymatic and non-enzymatic antioxidants has also been seen under Zn, Pb, Mn
and Cu stress (Bhat et al., 2019).
agriculture always emphasizes on preserving the soil quality and enhancing the
crop yield. It also focuses on the well-functioning of soil ecosystem. However,
plants have to face multiple environmental challenges (variety of stresses)
everyday which affects its overall physiology, growth and development thereby
reducing the yield and productivity. The production of reactive oxygen species,
disturbing the plant nutrient homeostasis, affecting photosynthetic processes
and other metabolic cycles are the common outcomes of any stress. Heavy metal
contamination is becoming one of the important stressor against which the plant
should initiate the defensive mechanism. And the application of biostimulants
is one such support to enhance the defensive approach to regulate and control
the overall processes of growth and development. Biostimulants fight various
biotic and abiotic stresses through a combination of an array of mechanisms.
Thus, the purpose of this review is to briefly explain the biostimulant, its
types and the role of few important biostimulant to ameliorate the toxic effect
of heavy metals. This will provide a basis to understand the types of
biostimulants and open the area to study their detail mechanism in response to
heavy metal toxicity. The overall impact of this review will be to set a
scientific frame to identify how the plant biostimulants treatments (substances
and/or microorganisms) have the potential to enhance plant resilience to
nutrient limitation typical of organic farming, and consequently reducing the
gap between organic and conventional yields.
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