Defense Mechanisms of Plants to Insect Pests: From Morphological to Biochemical Approach - Juniper publishers
Journal of Trends in Technical and Scientific Research
Abstract
The plants defend themselves against insect
herbivores through utilizing the combination of direct defense traits
and indirect defense approaches. Direct defense is involved both
physical and chemical barriers which synergistically obstruct insect
herbivore's growth, development, reproduction, etc. The indirect defense
approach has no direct impact on insect herbivores but suppress pests
by releasing volatile compounds that attract natural enemies of the
herbivores. Plant defense against insect herbivores is just one of
multiple layers of interactions. Together with plants, these players are
involved in complex interaction networks. To elucidate these
fascinating interactions biochemical, ecological as well as molecular
studies, and combinations thereof, are required.
Keywords: Insects; Defense mechanisms; Biochemical defense; Secondary metabolites; Induced defense mechanisms
Abbreviations: PIs:
Proteinase Inhibitors; BXs: Benzoxazinoids; HIPVs: Herbivore Induced
Plant Volatiles; CDPK: Calcium-Dependent Protein Kinases
Introduction
Insects are one of the dominant forms of life in
terms of the number of species and of individuals. Plants can have
different types of interactions with insects, such as antagonistic
interactions with herbivores and mutualistic interactions with
carnivorous and pollinating insects. Plants can defend themselves
against insects by employing a 'do-it- yourself' strategy and/or by
enlisting 'bodyguards' that attack herbivores. These plant strategies
can be present constitutively or they can be induced by herbivory.
Inducible defenses result in variable plant phenotypes and consequently
in variable types of interactions with insects [1].
Plants respond to herbivore through various
morphological, biochemical, and molecular mechanisms to counter/offset
the effects of herbivore attack. According to Oerke [2],
each year there is a huge crop yield loss by different insect pests
around the world. Therefore, understanding the defensive systems or
mechanisms of plants enables development of resistant crops or pest
management systems reducing the need of hazardous pesticides and
supporting safer crop production. Another positive effect would be a
reduction of the development of pesticide resistant pest strains.
Insect herbivores have traditionally been divided
into generalists (polyphagous) that feed on several hosts from different
plant families, or specialists (monophagous and oligophagous), which
feed on one or a few plant types from the same family. The generalists
tolerate a wide array of defenses present in most plants, while they
cannot feed on certain plants that have evolved more unique defense
mechanisms. Specialists, on the other hand, use a specific range of host
plants releasing defense compounds that at the same time may function
as feeding stimulants and provide ovipositioning cues [3].
Pegadaraju et al [4]
stated that, the defensive mechanisms in plants operate at different
levels. They vary from external defenses like thorns to complicated
chemical responses leading to poisoning of the attacker. To overcome the
insect attack, plants produce specialized morphological structures or
secondary metabolites and proteins that have toxic, repellent, and/or
anti nutritional effects on the insect pests. In addition, plants also
release volatile organic compounds that attract the natural enemies of
the herbivores [5].
Resistance factors for direct plant defense against
herbivorous insects comprise plant traits that negatively affect insect
preference (host plant selection, oviposition, feeding behavior) or
performance (growth rate, development, reproductive success) resulting
in increased plant fitness in a hostile environment. Such traits include
morphological features for physical defense, like thorns, spines, and
trichomes, epicuticular wax films and wax crystals, tissue toughness, as
well as secretory structures and conduits for latices or resins. They
also include compounds for chemical defense, like secondary metabolites,
digestibility reducing proteins, and anti nutritive enzymes. All these
traits may be expressed constitutively as preformed resistance factors,
or they may be inducible and deployed only after attack by insect
herbivores. The induction of defensive traits is not restricted to the
site of attack but extends to non-infested healthy parts of the plants.
The systemic nature of plant responses to herbivore attack necessitates a
long-distance signaling system capable of generating, transporting, and
interpreting alarm signals produced at the plant-herbivore interface.
Much of the research on the signaling events triggered by herbivore has
focused on tomato and other solanaceous plants. In this model system,
the peptide system in acts at or near the wound site to amplify the
production of jasmonic acid. Jasmonic acid or its metabolites serve as
phloem-mobile long-distance signals, and induce the expression of
defense genes in distal parts of the plant [6].
Host Plant Defenses against Insects
Plants respond to insect attack through an intricate
and dynamic defense system that includes structural barriers, toxic
chemicals, and attraction of natural enemies of the target pests. Both
defense mechanisms (direct and indirect) may be present constitutively
or induced after damage by the insects. Induced response in plants is
one of the important components of insect pest control in agriculture,
and has been exploited for regulation of insect herbivore population.
Induced defenses make the plants phenotypically
plastic, and thereby, decrease the chances of the attacking insects to
adapt to the induced chemicals. Changes in defensive constituents of a
plant on account of insect attack develop unpredictability in the plant
environment for insect herbivores, which in turn, affects the fitness
and behavior of the insects. If induced response occurs very early, it
is of great benefit to the plant, and reduces the subsequent herbivore
and pathogen attack, besides improving overall fitness of the plant.
Plants with high variability in defensive chemicals exhibit a better
defense compared with those with moderate variability [7].
Direct defenses
Plant structural traits such as leaf surface wax,
thorns or trichomes, and cell wall thickness/and lignification form the
first physical barrier to feeding by the insects, and the secondary
metabolites such act as toxins and also affect growth, development, and
digestibility reducers form the next barriers that defend the plant from
subsequent attack. Moreover, synergistic effect among different
defensive components enhances the defensive system of plants against the
insects' invaders. For example, in tomato, alkaloids, phenolics,
Proteinase Inhibitors (PIs), and the oxidative enzymes when ingested
separately result in a reduced affect, but act together in a synergistic
manner, affecting the insect during ingestion, digestion and
metabolism. In a wild tobacco (Nicotiana attenuate), trypsin
proteinase inhibitors and nicotine expression, contributed
synergistically to the defensive response against beet armyworm (Spodoptera exigua) [7].
Morphological features for physical defense
Insect herbivores from all feeding guilds must make
contact with the plant surface in order to establish themselves on the
host plant. It is therefore not surprising that physical and chemical
features of the plant surface are important determinants of resistance.
All plant parts offer some sort of resistance against herbivory. They
range from tissue hardness to highly complex glandular trichomes and
spines. Epicuticular wax films and crystals cover the cuticle of most
vascular plants. In addition to their important role in desiccation
tolerance, they also increase slipperiness, which impedes the ability of
many non-specialized insects to populate leaf surfaces. The physical
properties of the wax layer as well as its chemical composition are
important factors of preformed resistance [6].
Based on different findings, plant structures are the
first line of defense against insect pests, and play an important role
in host plant resistance to insects. The first line of plant defense
against insect pests is the erection of a physical barrier either
through the formation of a waxy cuticle, and/or the development of
spines, setae, and trichomes. Structural defenses includes morphological
and anatomical traits that confer a fitness advantage to the plant by
directly deterring the herbivores/ insects from feeding, and range from
prominent protrubances on a plant to microscopic changes in cell wall
thickness as a result of lignification and suberization. Structural
traits such as spines and thorns (spinescence), trichomes (pubescence),
toughened or hardened leaves (sclerophylly), incorporation of granular
minerals into plant tissues, and divaricated branching (shoots with wiry
stems produced at wide axillary angles) play a leading role in plant
protection against insect pests. Sclerophylly refers to the hardened
leaves, and plays an active role in plant defense against herbivores by
reducing the palatability and digestibility of the tissues, thereby,
reducing the herbivore damage.
Spinescence includes plant structures such as spines,
thorns and prickles. It has been reported to defend the plants against
many insects. Pubescence consists of the layer of hairs (trichomes)
extending from the epidermis of the above ground plant parts including
stem, leaves, and even fruits, and occur in several forms such as
straight, spiral, stellate, hooked, and glandular. Chamarthi et al. [8] reported that leaf glossiness, plumule and leaf sheath pigmentation were responsible for shoot fly (Atherigona soccata resistance in sorghum Sorghum bicolor (L. Moench).
Trichomes
The plant epidermis is often covered by outgrowths
called trichomes. They are found in all major groups of terrestrial
plants. They originate from epidermal tissue and then develop and
differentiate to produce hair-like structures [9].
Trichomes play an imperative role in plant defense against many insect
pests and involve both toxic and deterrent effects. Trichomes density
negatively affects the ovipositional behavior, feeding and larval
nutrition of insect pests. In addition, dense trichomes affect the
insect mechanically, and interfere with the movement of insects and
other arthropods on the plant surface, thereby, reducing their access to
leaf epidermis. These can be, straight, spiral, hooked, branched, or
un-branched and can be glandular or non-glandular. Glandular trichomes
secrete secondary metabolites including flavonoids, terpenoids, and
alkaloids that can be poisonous, repellent, or trap insects and other
organisms, thus forming a combination of structural and chemical
defense.
Induction of trichomes in response to insect damage has been reported in many plants [10].
This increase in trichomes density in response to damage can only be
observed in leaves developing during or subsequent to insect attack,
since the density of trichomes of existing leaves does not change. A
given authors reported that damage by adult leaf beetles (Phratora vulgatissima) in Salix cinerea plant induced higher trichome density in the new leaves developing thereafter. Likewise, increase in trichome density in S. cinera
in response to coleopteran damage has also been reported. Increase in
trichome density after insect damage has also been reported in Lepidium virginicum L. and Raphanus raphanistrum L. In black mustard, trichomes density and glucosinolate levels were elevated after feeding by small white butterfly (Pieris rapae). Furthermore, change in relative proportion of glandular and non-glandular trichomes is also induced by insect.
Leaf and root toughness and quantity
Leaf toughness interferes with the penetration of
plant tissues by mouthparts of piercing-sucking insects and increase
mandibular wear in biting-chewing herbivores [11].
The cell walls of leaves are also reinforced during feeding through the
use of different macromolecules, such as lignin, cellulose, suberin and
callose, together with small organic molecules, such as phenolics, and
even inorganic silica particles. Roots eaten by insect herbivores
exhibit extensive regrowth, both in density, as seen in Trifolium repens
eaten by Sitona lepidus (clover root weevil), and in quantity, as
observed in Medicago sativa (alfalfa) attacked by clover weevil (Sitona hispidulus).
The former might be caused by additional lignification that could
increase the toughness of the roots. In addition, genotypes with long
fine roots suffered less from herbivory compared to genotypes with short
and thick roots.
Secondary metabolites for chemical defense of plants
Plants produce a large and diverse array of organic
compounds that appear to have no direct functions in growth and
development i.e. they have no generally recognized roles in the process
of photosynthesis, respiration, solute transport, translocation,
nutrient assimilation and differentiation. These compounds or chemicals
play a significant role in direct defense impair herbivore performance
by one of two general mechanisms: these chemicals may reduce the
nutritional value of plant food, or they may act as feeding deterrents
or toxins. There has been considerable debate as to which of these two
strategies is more important for host plant selection and insect
resistance. An important part of this debate concerns the extent to
which variation in the levels of primary and secondary metabolites has
evolved as a plant defense [12].
Plant primary metabolism, which is shared with insects and other living
organisms, provides carbohydrates, amino acids, and lipids as essential
nutrients for the insect.
Secondary metabolites are the compounds that do not
affect the normal growth and development of a plant, but reduce the
palatability of the plant tissues in which they are produced. The
defensive (secondary) metabolites can be either constitutive stored as
inactive forms or induced in response to the insect or microbe attack.
The former are known as phytoanticipins and the latter as phytoalexins
(antimicrobial compounds synthesized by plants that accumulate rapidly
at areas of pathogen infection). The phytoanticipins are mainly
activated by p-glucosidase during herbivory, which in turn mediate the
release of various biocidal aglycone metabolites. The classic examples
of phytoanticipins are glucosinolates that are hydrolyzed by myrosinases
(endogenous p-thioglucoside glucohydrolases) during tissue disruption.
Other phytoanticipins include Benzoxazinoids (BXs), which are widely
distributed among Gramineae. Hydrolyzation of BX-glucosides by
plastid-targeted p-glucosidases during tissue damage leads to the
production of biocidal aglycone BXs, which play an important role in
plant defense against insects. Phytoalexins include isoflavonoids,
terpenoids, alkaloids, etc., that influence the performance and survival
of the insects. The secondary metabolites not only defend the plants
from different stresses, but also increase the fitness of the plants. It
has been reported that maize to corn earworm, Helicoverpa zea is
mainly due to the presence of the secondary metabolites C-glycosyl
flavone maysin and the phenylpropanoid product, chlorogenic acid.
Compound, 4, 4- dimethyl cyclooctene has been found to be responsible
for shoot fly resistance in sorghum [8].
Study on secondary metabolites could lead to the
identification of new signaling molecules involved in plant resistance
against insect pests. Ultimately genes and enzymes involved in the
biosynthesis of these metabolites could be identified. Some of the
secondary metabolites in plant defense will be the following.
Plant phenolic compounds
Among the secondary metabolites, plant phenols
constitute one of the most common and widespread group of defensive
compounds, which play a major role in host plant resistance against
insects. Phenols act as a defensive mechanism not only against insects,
but also against microorganisms and competing plants.
Lignin, a phenolic heteropolymer plays a central role
in plant defense against insects and pathogens. It limits the entry of
pathogens by blocking physically or increasing the leaf toughness that
reduces the feeding by insects, and also decreases the nutritional
content of the leaf. Lignin synthesis has been found to be induced by
insect or pathogen attack and its rapid deposition reduce further growth
of the pathogen or insect fecundity.
Plant defensive proteins
Ecologically, in insect-plant interaction,
interrelationship between two is important for the survival of the both.
Insects always look for a true and healthy host plant that can provide
them proper food and could be suitable for mating, oviposition and also
provides food for the offspring's. The nutritional requirements of
insects are similar to other animals, and any imbalance in digestion and
utilization of plant proteins by the insects' results in drastic
effects on insect physiology. Alteration of gene expression under stress
including insect attack leads to qualitative and quantitative changes
in proteins, which in turn play an important role in signal
transduction, and oxidative defense. Many plant proteins ingested by
insects are stable, and remain intact in the mid gut, and also move
across the gut wall into the hemolymph. An alteration in the protein's
amino acid content or sequence influences the function of that protein.
Likewise, anti-insect activity of a proteolysis- susceptible toxic
protein can be improved by administration of protease inhibitors (PIs),
which prevent degradation of the toxic proteins, and allows them to
exert their defensive function. Better understanding of protein
structure and post- translational modifications contributing to
stability in the insect gut would assist in predicting toxicity and
mechanism of plant resistance proteins. Recent advances in microarray
and proteomic approaches have revealed that a wide spectrum of plant
resistance protiens is involved in plant defense against insects.
Plant lectins
Lectins are carbohydrate-binding (glyco) proteins,
have protective function against a range of pests. The insecticidal
activities of different plant lectins have been utilized as naturally
occurring insecticides against insect pests. One of the most important
properties of lectins is their survival in the digestive system of
insects that gives them a strong insecticidal potential. They act as
antinutritive and/or toxic substances by binding to membrane glycosyl
groups lining the digestive tract, leading to an array of harmful
systemic reactions. Lectins are stable over a large range of pH and
damage the luminal epithelial membranes, thereby interfere with the
nutrient digestion and absorption (Table 1).
Flavonoids
Flavonoids are cytotoxic and interact with different
enzymes through complexation. Both flavonoids and isoflavonoids protect
the plant against insect pests by influencing the behavior, and growth
and development of insects.
Tannins
Tannins have a strong deleterious effect on
phytophagous insects and affect the insect growth and development by
binding to the proteins, reduce nutrient absorption efficiency, and
cause mid gut lesions. Tannins are astringent (mouth puckering) bitter
polyphenols and act as feeding deterrents to many insect pests. They
precipitate proteins nonspecifically (including the digestive enzymes of
insects), by hydrogen bonding or covalent bonding of protein NH2
groups. In addition, tannins also chelate the metal ions, thereby
reducing their bioavailability to insects. When ingested, tannins reduce
the digestibility of the proteins thereby decrease the nutritive value
of plants and plant parts to insects. Role of tannins in plant defense
against various stresses and their induction in response to insect
damage has been studied in many plants.
Proteinase inhibitors
Proteinase inhibitors (PIs) cover one of the most
abundant defensive classes of proteins in plants. Higher concentration
of PIs occurs in storage organs such as seeds and tubers, and 1 to 10%
of their total proteins comprise of PIs, which inhibit different types
of enzymes and play an important role in plant defense against insect.
PIs bind to the digestive enzymes in the insect gut and inhibit their
activity, thereby reduce protein digestion, resulting in the shortage of
amino acids, and slow development and/or starvation of the insects. The
defensive function of many PIs against insect pests, directly or by
expression in transgenic plants to improve plant resistance against
insects has been studied against many lepidopteran and hemipteran
insects. The success of transgenic crops in expressing PIs against
insect pests has accentuated the need to understand the mechanisms, and
interactions of multiple PIs with other defenses, and the adaptive
responses of the insects (Table 2).
Enzymes
Enzymes also one of the important aspects of host
plant resistance against insects is the disruption of insect's
nutrition. The enzymes that impair the nutrient uptake by insects
through formation of electrophiles includes peroxidases, polyphenol
oxidases, ascorbate peroxidases, and other peroxidases by oxidizing mono
or dihydroxyphenols.
Indirect Defenses Methods
The defensive response in plants to attract natural
enemies of insects plays a pivotal role in protecting the plants against
insect attack. Indirect defenses can be constitutive or induced as a
result of combined action of mechanical damage and elicitors from the
attacking herbivore. Production of volatiles and the secretion of extra
floral nectar mediate interactions of plants with natural enemies of the
insect pests (i.e., parasitoids
or predators), which actively reduce the numbers of
feeding herbivores. Induced indirect defenses have received increasing
attention recently and have been studied on the genetic, biochemical,
physiological, and ecological levels.
Herbivore induced plant volatiles (HIPVs)
In this case plants indirectly defend themselves from
insect feeding by emitting a blend of volatiles and non-volatile
compounds. Insect induced plant volatiles (HIPVs) play an important role
in plant defense by either attracting the natural enemies ofthe insects
or by acting as feeding and/or oviposition deterrent. HIPVs are the
lipophilic compounds with higher vapor pressure which are released from
the leaves, flowers, and fruits into the atmosphere, and into the soil
from the roots by plants in response insect attack. The HIPV's produced
vary according to the plant and insect species, the developmental stage
and condition of the plants and the insects. An optimum quantity of
volatile compounds is normally released by the plants into the
atmosphere, whereas a different blend of volatiles is produced in
response to insect. The volatile blend released by plants in response to
insect attack is specific for a particular insect-plant system,
including natural enemies and the neighboring plants. The HIPVs mediate
the interactions between plants and arthropods, microorganisms,
undamaged neighboring plants, or intraplant signaling that warns
undamaged sites within the plant (Figure 1).
Depending upon the modes of feeding of insect pests, different defense
signaling pathways are activated, which induce the production of
specific volatile compounds [13].
Defense elicitors (insect oral secretion)
Plants undergo a dynamic change in transcriptomes,
proteomes, and metabolomes in response to herbivore- induced physical
and chemical cues such as insect oral secretions and compounds in the
oviposition fluids. It is generally believed that insect-induced plant
responses are mediated by oral secretions and regurgitates of the
herbivore. The defenses generated by various elicitors differ based on
the type of the elicitor and the biological processes involved. A
potential elicitor of herbivore-induced plant volatiles from the
regurgitate of Pieris brassicae L. larvae has been identified as
β-glucosidase which results in emission of a volatile blend from
mechanically wounded cabbage leaves that attract the parasitic wasp, Cotesiaglomerata (L.) [14].
Role of phytohormones in induced resistance in plants
Plant defense against insect attack involves many
signal transduction pathways that are mediated by a network of
phytohormones. Plant hormones play a critical role in regulating plant
growth, development, and defense mechanisms. A number of plant hormones
have been implicated in intra- and interplant communication in plants
damaged by insects. Most of the plant defense responses against insects
are activated by signal- transduction pathways mediated by jasmonic
acid, salicylic acid, and ethylene. Specific sets of defense related
genes are activated by these pathways upon wounding or by insect
feeding. These hormones may act individually, synergistically or
antagonistically, depending upon the attacker (Figure 2).
Jasmonic acid is the most important phytohormone
linked to plant defense against insects and activates the expression of
both direct and indirect defenses. Jasmonic acid is derived from
linolenic acid through octadecanoid pathway and accumulates upon
wounding and herbivory in plant tissues. Chewing of plant parts by
insects causes the dioxygenation of linoleic acid and linolenic acid.
Jasmonic acid has also been reported to affect
Calcium- Dependent Protein Kinases (CDPK) transcript, and activity in
potato plants. CDPKs comprise of a large family of serine/ threonine
kinases in plants (34 members in Arabidopsis) and play an important role
in plant defense against a variety of biotic and abiotic stresses
through signal transduction [15].
In addition to the role played by jasmonic acid in direct resistance
against insect pests through the induction of various defensive
compounds, its role in indirect resistance has also been well
established.
Ethylene is an important phytohormone, which plays an
active role in plant defense against many insects. Ethylene signaling
pathway plays an important role in induced plant defense against insects
and pathogens both directly and indirectly. Ethylene signaling pathway
works either synergistically or antagonistically, with jasmonic acid in
expression of plant defense responses against pathogens and herbivorous
insects. It has been reported that Ethylene and jasmonic acid work
together in tomato in proteinase inhibitors expression.
Generally, different plant elicitors induced in
plants upon herbivory undergo different signal transduction pathways.
For example, Calcium ions (Ca2+), reactive oxygen species, etc.
Conclusion and Future Perspective
In all natural habitats, plants are surrounded by an
enormous number of potential enemies (biotic) and various kinds of a
biotic environmental stress. Nearly all ecosystems contain a wide
variety of bacteria, viruses, fungi, nematodes, mites, insects, mammals
and other herbivorous animals, greatly responsible for heavy reduction
in crop productivity. By their nature, plants protect themselves
directly by developing different morphological structure and by
producing some compounds called as secondary metabolites. Plant
mechanical defenses act negatively on herbivores insects, diminishing
their larval and adult performance. Generally, a plant character may
present two or more roles at least in some phase of a plant's life
history. I discussed a few cases where pubescence, tissue texture,
crystals, latex, waxes and resins are effective against insect
herbivores. Secondary metabolites, including terpenes, phenolics and
nitrogen (N) and sulphur (S) containing compounds, defend plants against
a variety of herbivores and pathogenic microorganisms as well as
various kinds of abiotic stresses.
An understanding of induced resistance in plants can
be utilized for interpreting the ecological interactions between plants
and herbivores and for exploiting in pest management in crops. Since the
biochemical pathways that lead to induced resistance are highly
conserved among the plants, the elicitors of these pathways could be
used as inducers in many crops. The future challenge is to exploit the
elicitors of induced defense in plants for pest management, and identify
the genes encoding proteins that are up and/or down regulated during
plant response to the herbivore attack, which can be deployed for
conferring resistance to the herbivores through genetic transformation.
However, before using an elicitor effectively in agricultural systems,
it is important to understand the chemical changes they induce in the
plant, the effect of these chemicals on the herbivores especially in the
field, and to see if there is any alteration in plant growth and yield.
The Eco- genomic approach which includes association and correlation
studies, natural selection mapping, and population genomics enables the
estimation of variable selection at loci, and differentiates this from
processes acting on the whole genome, such as migration and genetic
drift.
From a biotechnological, food-developmental, and
breeding point of view, understanding the defense systems of plants and
learning how to apply the knowledge is of course of huge interest. For
instance, modifications of the jasmonic acid pathway have been proposed [16].
However, due to the extensive crosstalk with other hormone signaling
pathways, increased resistance against one certain insect herbivore
might result in susceptibility towards another. Furthermore, some
defense responses might have negative effects on the environment and
humanity as well, as they involve toxic bioactive natural products and
proteins reducing digestibility of plant material. Still, reducing the
need for synthetic insecticides, by developing crop plants resistant to
insect herbivores, would be of significant gain for the food and
production industry, both at an economical and environmental level.
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