Document Not For Distribution or Use in the State of California
PRODUCT INFORMATION BULLETIN
Improving Abiotic Stress Tolerance in Plants with Silicon
CrossOverTM is a highly refined calcium and magnesium silicate soil amendment in pelletized form. Following its
application, CrossOver establishes a reservoir of water soluble calcium, magnesium and silicon in the soil that has
been proven to provide unique benefits to agronomic crops as well as improve the soil’s physical characteristics.
CrossOver is truly unique, inasmuch as it behaves as a functional hybrid. As silicon released from CrossOver is
absorbed by the plant, it “crosses over” from its involvement in soil geochemical reactions, becoming an active
participant in numerous plant biological processes that enhance the crop’s ability to resist and tolerate abiotic
stresses and regulate metal toxicity.
Abiotic stress occurs when non-living environmental factors create conditions
outside ranges favorable for growth and development of an organism. The
major plant environmental stresses of agronomic importance worldwide are
drought, cold (chilling and freezing), heat, salinity and soil mineral and metal
deficiency or toxicity.
Biotic Stress occurs as a result of damage to plants by other living
organisms. Fungi, bacteria, parasites, insects and weeds, are examples of
biotic stressors.
Abiotic Stress
ROS REACTIONS
O2
1
Singlet
Oxygen
O2
Oxygen
e-
Superoxide
Radical
O2•H
+
e-
Peroxide
ion
O22-
2H
+
HO2•
H2O2
Perhydroxyl
radical
Hydrogen
peroxide
Fe2+
Fenton reaction
•OH
Hydroxyl
radical
Toxic Metal Ions
Plants may experience physiological stress when an abiotic factor is
excessive or deficient (such as the lack of water of certain nutrients or the
lack of heat (referred to as an imbalance). Research shows that abiotic
stressors are most harmful when they occur in combination.
Agricultural crops, turfgrass and horticultural crops are frequently exposed
to combinations of stress including non-living (abiotic) stress and attack
from living organisms such as pathogens and insects (biotic stress).
How serious are abiotic stresses? It is estimated that annual crop genetic
potential for yield is reduced by as much as 75% due to suboptimal
climatic and soil conditions.
Transition metals are essential for plant cell development. At the same
time, they can be potentially toxic to plant metabolism.
In the presence of metals such as copper and iron, ROS reactions in
the plant continue to take place through the Haber-Weiss mechanism
or the Fenton Reaction – resulting in the production of highly toxic
hydroxyl radicals.
The Fenton reaction requires a catalytic free metal for the decomposition
of hydrogen peroxide (H2O2).
Metal2+ + H2O2
Reactive Oxygen Species
Reactive oxygen species (ROS) in plants are produced as normal byproducts of many metabolic pathways, including photosynthesis. The
production of ROS is also an unavoidable outcome of aerobic respiration.
It is estimated that 1-2% of atmospheric oxygen consumed by plants is
inevitably converted to ROS such as the superoxide radical, perhydroxyl
radical and hydroxyl radicals. Non-radical (molecular) forms, hydrogen
peroxide and singlet oxygen, are also produced and can participate in
destructive and toxic “chain reactions.”
Metal3+ + OH- + •OH
The hydroxyl radical can react with all biological molecules (DNA,
proteins, lipids and other constituents of cells) and because there is no
enzymatic mechanism to neutralize this radical, excessive production will
often conclude in cell death.
The hydroxyl radical is believed to be the major free radical responsible for
destructive modifications of membranes and cellular structures such as
lipid peroxidation that are capable of interrupting or damaging vital plant
processes.
CrossOver – from soil to plant
Lipid Peroxidation – ROS Damage at its Worst
The damage to cellular and organelle membranes by reactive oxygen
radicals is well illustrated by the lipid peroxidation process. The
peroxidation of lipids (LPO) is considered as the most damaging process
known to occur in any living organism.
The overall process of LPO illustrates the “chain reaction” that can occur
when ROS exist at above threshold levels.
LPO in a membrane is initiated by the hydroxyl radical (•OH) reacting with
a hydrogen atom on a membrane fatty acid molecule, producing a fatty
acid radical. When a radical reacts with a non-radical, it always produces
another radical, resulting in a “chain reaction.”
The fatty acid radical reacts with oxygen and produces yet another
radical – a peroxyl-fatty acid. These chain reactions continue only until
termination of the process is triggered by the reaction of two radicals that
produce a non-radical species. This requires high concentrations of ROS.
Membrane
A common result of abiotic stress in plants is
the overproduction of ROS, which frequently
results in oxidative stress. Oxidative stress
often occurs when the production of ROS
overwhelms the plant’s ability to maintain
ROS at steady state levels.
ABIOTIC STRESS
Drought • Heat • Cold • Salinity • UV
Plant Processes • Injury • Metal Toxicity
ROS
production:
Mitochondria
Chloroplasts
Cell Wall
Plasma Membrane
Peroxisomes
Many “symptoms” of stress such as
weakened plants and diminished crop
quality caused by abiotic factors (drought,
heat, salts, chilling, heavy metals, ultraviolet
rays and light) are for the most part, actually
indicators of oxidative stress.
Oxidative
Stress
Dual Role for ROS
Despite their harmful activity, ROS also play a dual role, particularly when
produced under abiotic stress. ROS has been shown to trigger a variety
of cellular processes, including expression of a number of genes and
mediation of signal pathways that activate and control a number of plant
defense responses.
Phospholipid
fatty acid group
Stress
Sensors
Stress signal
transduction
NAD(P)H
oxidase
Abiotic/Biotic
STRESS
H
H
H
C C C
H
H
C C C
C C C
H
H
Carbon
centered
fatty acid
radical
H H
H
Hydroxyl
radical
Oxygen
Peroxyl
radical
LPO ultimately undermines the structural integrity of membranes,
increasing the “leakiness” of the membrane and allowing substances to
bypass specific channels. It also damages membrane proteins causing
inactivation of receptors, enzymes, and ion channels.
Oxidative Stress
Under steady state conditions (normal metabolism), plant cells have
evolved a complex system of enzymatic and non-enzymatic antioxidants
which serve to maintain a delicate equilibrium between ROS production
and their conversion to non-harmful molecules (scavenging).
The components of the antioxidant defense system are enzymatic and
non-enzymatic antioxidants. Common enzymatic antioxidants include
superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione
peroxidase (GPX), glutathione-S-tranferase (GST), and catalase (CAT).
Examples of non-enzymatic low molecular metabolites include ascorbic
acid (ASH), glutathione (GSH), a-tocopherol, carotenoids and flavonoids.
(ROS-generating
reactions)
ROS
Metabolic
adjustments
Scavenging
(SOD, CAT,
GPx...
ROS
(ROS -generating
reactions)
ROS
Metabolic
adjustments
Scavenging
(SOD, CAT,
APx, GPx...
Sensors/
receptors
ROS signal
pathway
(MAPK)
ROS
Stress defense
proteins
Illustration of lipid peroxidation on a cell membrane fatty acid group and “chain reaction” process of
reactive oxygen species that cause damage to proteins, lipids, carbohydrates and DNA.
METABOLISM
METABOLISM
Sensors/
receptors
ROS signal
pathway
(MAPK)
Graphic showing increased production of ROS under influence of abiotic/biotic stress conditions.
Note that ROS also participates in plant defense mechanisms by triggering sensors/receptors in
signal pathways that produce plant defense responses.
However, in order for ROS to positively contribute to plant health and
survival under stress, its net accumulation must be kept within a tolerable
range. Unfortunately in many plants, their antioxidant systems, which
are tuned to maintain steady state levels, are insufficient to counteract
increased production of ROS due to abiotic stresses, particularly under
multiple stress conditions.
A major challenge to successful agricultural, turf and horticultural
management today is coping with oxidative stress associated with plant
abiotic stresses.
Silicon and Abiotic Stress
Silicon is not inert. It has been found to be a biologically active element,
participating in highly complex interactions with key components of the
plant’s defense response system.
The role of silicon in conferring tolerance in plants against abiotic
stresses is fully utilized with CrossOver. Plant available silicon, released
by CrossOver, plays a key role in activating processes that enhance and
improve the efficiency and effectiveness of defense response systems under
abiotic stress conditions.
It is well documented and recognized that silicon can play an important role
in increasing plants’ tolerance to environmental (abiotic) stress.
Silicon is known to effectively mitigate various abiotic stress factors such
as salinity, drought, heat, chilling and freezing stresses and manganese,
aluminum and other metal toxicities.
Plants have evolved protective mechanisms to minimize the destructive
effects of free radicals produced during metabolic processes. These antioxidant systems employ both enzymatic and non-enzymatic substances.
Plants have also developed a predisposition for the uptake and use of silicon
in conferring tolerance to abiotic stresses. Improved plant tolerance with
silicon can be seen as a quicker and more efficient response at the onset
of stress and enhanced recovery once the stress has abated.
Silicon Mediation of Abiotic Stress Factors
Silicon-mediated alleviation of metals in planta
Silicon-mediated alleviation of metal toxicity in the soil and in planta is
widely known.
Metals often move from the soil solution into the plant and can
accumulate in apoplastic intercellular spaces throughout the plant. Free
metals such as manganese and aluminum that are absorbed and move
to the the apoplast of plant tissues often serve as feedstock for Fenton
reactions resulting in the production of harmful hydroxyl radicals.
The key mechanisms of Si-mediated alleviation of abiotic stresses in
vascular plants include:
SYMPLASTIC ROUTE
• immobilization of toxic metal ions in soil
• c omplexation or co-precipitation of toxic metal ions with Si in or
ex planta
Mn
Mn
Mn
Mn
• a ltering the activity of enzymatic and non-enzymatic
antioxidants
Mn
Mn
Silicon-mediated alleviation of metals in soils
Monosilicic acid released from CrossOver into the soil is particularly
effective in establishing silicate processes that reduce toxic metals from
being absorbed by plants.
Complexing in soil solution. Monosilicic acids complex with metal
species in the soil solution to form less toxic compounds that are
removed from the rootzone as precipitates. This is similar to what
happens in planta when silicon complexes with metals in the apoplast.
2H4SiO4 + 2Al ↔ Al2Si2O5 + 2H + 3H2O
3+
+
SYMPLASTIC ROUTE
Graphic showing manganese ions in the apoplast (extracellular space) of leaf tissue.
Under stress conditons, plants often increase their absorption of silicon
(monosilicic acid) molecules. As the monosilicic molecules come in contact
with free metals, they form complexes with metals.
For example, when in contact with manganese, monosilicic acid will react
with the metal and form a manganese silicate molecule.
2Al3+ + 2H4SiO4 + H2O ↔ Al2Si2O5(OH)4 + 6H+
H
Mn
O
Si
O
H
O H
O
Mn
H
As monosilicic acid complexes with manganese molecules, the manganese
silicate product attaches to the extracellular walls of the apoplast or deposits
as a precipitate. In either case, these complexed metals are rendered inert,
and therefore do not engage in chemical reactions, that could otherwise
result in ROS production..
Si
H
Mn
O
H
Mn
H
O
Si
O
O
M
n
H
n
O
O
O
O
O
O
Mn
Si
O
O
H
Mn
O
O
M
O
O
Mn
APOPLASTIC ROUTE
Mn
Mn
O
H
O
Mn
Mn
O
O
Si
H
Mn
O H
SYMPLASTIC ROUTE
O
Sorption on exchange sites. Monosilicic acid can also adsorb to
hydroxides of aluminum, iron and other metals on soil surfaces. These
reactions confine these metals to the surface of the soil particles,
enhancing the management and correction of metal toxicity in the soil
as well as reducing the potential for these metals to be involved in the
formation of hydroxyl radicals in planta via the Fenton reaction.
O
H
Manganese Silicate
(Complexed molecule)
Monosilicic Acid
O
Illustraton of monosilicic acid complexing with exchangeable aluminum in the soil solution to form
non-toxic aluminum silicate and hydroxy aluminosilicate precipitates.
O
Si
O
Mn
H
Manganese
O
O
Mn
H
O
Si
O
H
SYMPLASTIC ROUTE
This co-deposition process effectively prevents “free” manganese ions
from entering the Fenton equation.
Metal2+ + H2O2
Monosilicic acid is shown adsorbing to hydroxides of aluminum and iron on the soil surface. This
renders these metals unavailable to enter the soil solution and be absorbed by plants.
Metal3+ + OH- + •OH
Dealing With Environmental Stresses
Altering the activity of enzymatic and non-enzymatic antioxidants
Silicon is a bioactive molecule that has preferential attraction to the
hydroxyl unit amino acids found in proteins, enzymes and hormones. This
may explain its bioactivity as a regulator of plant defense mechanisms.
It has also been suggested that the mode of action of Si in signal
transduction may derive from interactions with phosphorus.
ENZYME
MONOSILICIC ACID
C
O
C
O
H
R
O
+
Si
O
H
O
The body of research to date, confirms that the uptake, transport and
deposition of silicon enhances the ability of plants to overcome constraints
posed by abiotic stress conditions (including multiple stresses) as well as
improve the efficiency and effectiveness of plant defense response systems.
C
C
H 3N +
O H
H
O
H
While a wide variety of chemical options are available to control biological
stresses, options for the management of abiotic stresses have been limited –
until the beneficial effects of silicon became evident.
SILICON MODIFIED ENZYME
H
R
H 3N +
H
O
H
H
O H
Si
O
O
H
PROTEIN
MONOSILICIC ACID
SILICON MODIFIED PROTEIN
H
H 3N +
R
O
C
C
H
H
N
C
H
R2
O
C
O
O
+
Si
O
H
H 3N +
O H
O
C
C
H
H
O
H
R
N
C
H
R2
H
O
C
H
O
Si
O
H
O H
O
H
Graphic showing preferential hydroxyl attachment locations for monosilicic acid on enzyme and
protein models.
Enzymes, proteins and hormones are major constituents of relay
mechanisms, signaling pathways and cascades that drive defense and
response mechanisms in plants.
Silicon’s biochemical properties enable it to interact with a host of
enzymes, proteins and hormones and act as a modulator influencing the
amplitude, timing and duration of stress transmission signals and protein
activated plant defense response pathways.
H
O
H
H
H
O
O
O
Si
O H
H
O H
Si
O
H
O
O
O
H
H
H
The use of silicon amendments to enhance the ability of plants to tolerate or
resist abiotic stress is becoming more attractive as production demands for
higher yields and improved crop quality persist.
This is particularly true when the plant’s ability to source silicon from the
soil is limited or lost due to natural depletion or crop removal of silicon
from successive cropping of high yield varieties.
In turfgrasses, whic are high silicon accumulators, silicon is lost by mowing
and removal of clippings. Furthermore, and-based root zones found in
many greens, tees and surrounds, do not contain silicon in soluble forms
to meet the silicon losses due to clipping removal.
Much of the horticultural industry is now using “soilless mixes” to produce
their crops. Such mixes provide little plant available silicon for plant use.
Stress signal
transduction
NAD(P)H
oxidase
Stress
Sensors
Silicon (Si) is now designated as a “plant beneficial substance” by the
Association of American Plant Food Control Officials (AAPFCO). Silicon
formulations that meet AAPFCO’s standards for labels and testing can
now list Si on fertilizer labels with the new designation backed by an
established protocol for product quality, production, and accurate labeling
for commercialization of silicon fertilizers.
O H
Si
O
Dealing with abiotic stresses may be the biggest challenge facing worldwide
agricultural, turf and horticultural industries in the future. Massive losses
to crops and plants occur annually as the result of the inability of plants to
withstand abiotic stress. These losses are much greater than occur from
biological stresses.
H
Abiotic/Biotic
STRESS
O
H
O
H
METABOLISM
O
Si
Si
O H
O
H
H
O
O H
O
Sensors/
receptors
ROS
(ROS -generating
reactions)
H
ROS signal
pathway
(MAPK)
H
O
H
O
Si
O H
O
H
H
H
O
O
ROS
Metabolic
adjustments
Si
O H
H
O
O
Si
O H
O
H
O
O
O H
H
O
O H
Si
O
O
O
H
H
H
Silicon released by CrossOver plays a key role and actively participates
in plant and soil processes that boost the natural ability of plants to
withstand yield-robbing challenges from abiotic stresses.
H
H
Si
H
H
O
O
O
O H
O
H
H
H
Si
O
H
Scavenging
(SOD, CAT,
APx, GPx...
H
O
O H
Si
O
H
H
O
Stress defense
proteins
H
O
H
O
Si
H
O
Si
O
O
H
H
O H
H
O
Si
O H
H
O
Si
H
H
O H
H
O
Si
O
O
O
H
H
H
O
Si
O H
O
O
O
O
O
O H
H
H
H
O H
H
Illustraton of signal pathways being modulated by the presence of silicon, acting as a potentiator of
plant defense responses or as an activator of strategic signaling proteins.
Through its ability to interact with several key components of plant stress
signaling systems silicon increases the production of important enzymatic
and non-enzymatic antioxidants in plants (i.e., SOD, POD, CAT, GPX, APX,
MDHAR, GR, AsA, and GSH).
It is generally accepted that this over production of ROS scavenging
antioxidants (via upregulation of many ROS-scavenging genes) by silicon
improves plant tolerance and resistance to various abiotic stresses.
Investigate how CrossOver can enhance
abiotic stress tolerance in your production
management programs.
Purchase Information for CrossOver
is available at:
numerator
TECHNOLOGIES, INC.
Document Not For Distribution or Use in the State of California
©2013 Harsco Corporation. World Rights Reserved. CrossOver is a trademark of Harsco Technologies, LLC
P.O. Box 868
Sar asota, Fl or i da 3 4230
941.807.5333
www.nume r ator t e ch.com
CrossOver – from soil to plant
For technical advice, contact:
Harsco Metals and Minerals North
America, Inc.
359 North Pike Road
Sarver, PA 16055
Phone: 1-800-850-0527
Email: [email protected]
Web: www.crossover-soil.com