Presentation on

Various Morphological, Physiological & Anatomical

Adaptations In Plant For Insect-pest Resistance

SANDEEP KUMAR SATHUAPh.D Research Scholar

Entomology & Agril. ZoologyBanaras Hindu University,

Varanasi

CONSTITUTIVE

INDUCED

Present in plant species before

Produced and mobilized to site of injury

Plant Defense

CONCEPT OF DEFENSE & ITS TYPE

For million of years plants and insects have been interacting with each other as a food and feeder relationship. In progress of time, both have evolved strategies to avoid each other’s defense systems with adaptation of morphological, physiological, anatomical and behavioral mechanisms that have either toxic, repellent or anti-nutritional effects on herbivores.

Most of insects interact with plants for three basic reasons: 1- Food/ Nutrition

2- Shelter/ Colonization 3- Reproduction

PLANT HERBIVORE

NATURAL ENEMY

The plants can defense/ resist herbivores by following two ways:

I. Directly, by affecting host’s preference, survival and reproductive success (Intrinsic Plant Defense)

II. Indirectly, through attracting other species such as natural enemies of the insect (Extrinsic Plant Defense)

Extrinsic Plant Defense

Intrinsic Plant Defense

(Dhaliwal, G.S and Arora, R. (2006). ‘Integrated Pest Management’, Kalyani publisher: p. 119)

attack

attack

defense

defense

‘Resistance refers to the heritable qualities of a cultivar to counter act the activities of insects so as to cause minimum percentage reduction in yield as compared to other cultivars of same species under similar condition.’

- Dhaliwal et al. (1993) -

The mechanisms of resistance can be studied into 3 catagories, viz. antixenosis, antibiosis and tolerance.

The resistance mechanisms can be greatly affected by environmental factors like tempetarute, humidity, photoperiod, soil moisture, soil pH and also by plant and insect factors.

PLANT RESISTANCE & ITS MECHANISM

Tolerance:Ability of the host plant to withstand an insect population sufficient to damage severely the susceptible plants.

Antixenosis:Host plant characters responsible for non-preference of the insects for shelter, oviposition, feeding, etc.

Antibiosis:Adverse effect of the host plant on the biology (survival, development and reproduction) of the insects and their progeny due to the biochemical and biophysical factors present in it.

Various Defense Structures present in plant to resist against herbivore are listed as below:

a) Trichomesb) Thorns, spines and pricklesc) Silica contentd) Stem characters: solidness, toughness, thicknesse) Thickening of cell wall and rapid proliferation of plant tissuesf) Surface waxesg) Colorh) Plant cuticlei) Plant shape and size

MORPHOLOGICAL ADAPTATIONS IN PLANT RESISTANCE

Morphological or physical resistances constituted by interfering: locomotive function Feeding Reproductive functions

a) Trichomes (Pubescence) These are cellular, hair-like out-growths of the plant epidermis,

which may occur on leaves, shoots, or roots. In general functions of trichomes is water conservation but also

provide morphological defense against insects attack. 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.

The mechanical effects of trichomes depends on four main characteristics such as density, erectness, length, shape.

(Microscopic view of trichomes)

( Non-glandular vs glandular trichomes )

NGT

GT

I. Insect Mobility and Trichomes: Trichomes interfere insect mobility by Entrapping

Immobilizing or Impaling the insects pest from host. Few examples are as below : Hooked trichomes of bean Phaseolus vulgaris L. impale

aphid Aphis crassivora French bean small hooked epidermal hairs affects colonies of

A. Crassivora Hairiness in cotton confers resistance against Pectinophora

gossipiella restricting larval movement on leavs. Adults of white fly Bemisia tabaci were found trapped by glandular hairs

on tomato leaves

Interlocking of trichomes and tarsi of insect while walking on leaves of a plant

II. Insect Feeding Development and Trichomes:Trichomes affects insect feeding , development and survival.

Length and density of hair on lamina of cotton and soybean leaves prevent jassids Empoasca devastans/ fabae

Hairiness on pods of mung bean affect feeding of Callosobruchus chinensis

Rice varieties with hairy upper lamina were found less susceptible to Asiatic rice borer Chilo suppressalis

III. Oviposition and Trichomes: Trichome exudates of Lycopersicon, prevent the oviposition

of H. zea Presence of oxalic and malic acids in chickpea trichome

resistant to H. armigera Trichomes in okra prevent egg laying in A. bigutula bigutula

Thorns are modified branches that protect plants from grazing vertebrates, and include the honey locust tree (Gleditsia triacanthos). Many cacti produce thorn-like structures that are actually modified leaves or parts of leaves (e.g., stipules) called spines which serve similar purposes, such as in the barrel cactus (Ferocactus spp.). Botanically speaking, the “thorns” on the stem of rose plants (Rosaspp.) are neither true thorns nor spines: they are actually outgrowths of the epidermis called prickles.

b) Thorns and Spines (Spinescence)

THORNS

SPINES

PRIC

KLE

S

b) Silica Content: Silica deposited in plan parts. Character confined to Graminae, Cyperaceae and Palmaceae. Few examples are : Sorghum silica content in 4th and 6th leaf stage less incidence of

shoot fly Atherigonia soccata In paddy against asiatic rice borer Chilo suppressalis and

Scirpophaga incertulas

 c) Solidness and other stem characters: Mainly thickness and toughness. Few examples are : Rice varieties with small stem and rigid surface against Asiatic rice

borer In sugarcane rind hardness against internode borer Chilo

sacchariphagus indicus. Rind hardenss in combination with fibre conent against shoot borer

C. Infuscatellus Sorghum genotypes with thin and longer stem, fewer but long

internode, short peduncle resistant to stalk borer C. partellus

d) Thickness of cell wall: Thickness of wall , hardness of tissue and rapid proliferation of plant tissue affects penetration and feedingExamples: Thickness of all categories of veins has positive correlation

between the with egg laying by A. bigutulla bigutulla in okra, brinjal and cotton.

Sugarcane varieties with strong midrib are resistance to top borer Scirpophaga nivella

In chick pea resistance to bruchid, Calobruchus maculatus is associated with toughness of seed coat

Susceptible Resistance

Bruchid attack

e) Surface Waxes:

The first line of defense in plants is an intact slippery bark and a waxy cuticle.

Protect plant surface from desiccation, diseases, affect feeding behaviour of insect by lack of probing, phagostimulant or feeding deterrants. Few examples are :

Glossy leaved brassica oleriaceae not prefered by larvae of Plutella xylostella.

Glossy lines of Brassica species has low population of cabbage worm, cabbage aphid and diamond back moth .

Along with herbivore defense, wax helps to keep water in the plant and reduce transpiration.

Lettuce

Coconut

Lemon

f) Colour: Exist naturally in leaves, petals, fruits, stem etc and also

can be conferred by genetic modification. Few examples are :

Cotton cultivars with red leaves less attacked by boll weevil Red foliaged Brassica species less attacked by white flies

compared to green foliaged Mustard aphid prefer to alight to yellow colour of petals

Colours preferred by Insects:Yellow- Aphid, White fly, Leaf hoppersGreen & Bluish green- Cabbage butterflyDark green- Rice leaf hopper

R

R

R

S

S

S

R- ResistanceS- Susceptible

COTTON

MUSTARD

CABBAGE

g) Plant Cuticle: Consist wax , pectin, and cellulose. Affects feeding and oviposition behavior. Few examples are :i) Larve of Helicoverpa zea prefered to feed on older leaves of

soybean over younger one throughout seasonii) Potato leafhopper Emposca fabae preferred to oviposit on newest

part of the plant over elder tissue iii)Younger leaves of lemon are preferred for probing, oviposition and

survival by barberry whitefly, Parabemisia myricae over mature leaves.

h) Shape and size: Known to bring some behavioral changes Examples:iv)Sorghum cultivar with shorter glumes and shorter floral structure

resistance to earhead midge Contarinia sorghicolav) In chickpea pod damage due to H. armigera is positively correlated

with pod circumference, pod length, pod weight

Variations in plant structures also provide resistance to some extent. Examples:

Rice varieties with tight leaf sheath wrapping : Asiatic rice borer

Awned spring wheat: Sitobion avenea (Aphid) Corn silk balling : Corn earworm H. zea leaf glossiness, plumule and leaf sheath pigmentation :

shoot fly Atherigona soccata in sorghum  Trees such as coconut and other palms, may protect

their fruit by multiple layers of armor.

ANATOMICAL ADAPTATIONS

In corn, silk balling is associated with resistance to corn earworm, H. zea, as silk ball appears to present a physical barrier to ear penetration.

In sugarcane, low number of stomata per unit area has been associated with the resistance character of varieties to sugarcane scale Melanaspis glomerata (Green)

Clones with loose-fitting leaf sheaths are generally more damaged by the intemode borer, Chilo sacchariphagus indicus.

PHYSIO-CHEMICAL ADAPTATIONS AND PLANT RESISTANCE

Plant physio-chemicals can be divided into :

Primary metabolites are substances produced by all plant cells that are directly involved in growth, development, or reproduction (sugars, proteins, amino acids, and nucleic acids )

Secondary metabolites are organic compounds not directly involved in growth or reproduction but they are often involved with plant defense.

Secondary metabolites are often characterized as either qualitative or quantitative.

Secondary Metabolites

QUANTITATIVE METABOLITES

Most quantitative metabolites are

digestibility reducers that make plant cell walls indigestible to

herbivores.- present in high

concentration in plants(5 – 40% dry weight)

List of available secondary metabolites in plants :

Cyanogenic glycosides Glucosinolates Benzoxazinoids Terpenoids or isoprenoids- Monoterpenes, diterpenes,

triterpenoids. Phenolics- tannins, flavonoids, lignin, Furanocoumarins,

silymarin and cannabinoids etc. Nitrogen Compounds- Alkaloids (Caffeine, Nicotine, .

caffeine, morphine, cocaine, colchicine etc) Cyanogenic glycosides Plant defensive proteins Coumarins

Cyanogenic glycosides are stored in inactive forms in plant vacuoles. They become toxic when herbivores eat the plant and break cell membranes allowing the glycosides to come into contact with enzymes in the cytoplasm releasing hydrogen cyanide which blocks cellular respiration.

Glucosinolates are activated in much the same way as cyanogenic glucosides, and the products can cause gastroenteritis, salivation, diarrhea, and irritation of the mouth.

Benzoxazinoids, secondary metabolites, which are characteristic for grasses (Poaceae), are also stored as inactive glucosides in the plant vacuole. Upon tissue disruption they get into contact with β-glucosidases from the chloroplasts, which enzymatically release the toxins.

These are derived from various amino acids. Over 3000 known alkaloids exist; few examples are- include nicotine, caffeine, morphine, cocaine, colchicine,

ergolines, strychnine, and quinine.

Certain alkaloids bind to nucleic acids and can inhibit synthesis of proteins and affect DNA repair mechanisms.

Alkaloids can also affect cell membrane and cytoskeletal structure causing the cells to weaken, collapse, or leak, and can affect nerve transmission.

ALKALOIDS

PLANT PHENOLICS: They are produced primarily via the shikimic acid and

malonic acid pathways in plants, and include a wide variety of defense-related compounds including flavonoids, anthocyanins, phytoalexins, tannins, lignin, and furanocoumarins.

Flavonoids are one of the largest classes of phenolics. Mostly isoflavonoids protect the plant against insect pests by influencing the behavior, and growth and development of insects.

Tannins are water-soluble, produced by plants and stored in vacuoles. Tannins are toxic to insects because they bind to salivary proteins and digestive enzymes including trypsin and chymotrypsin resulting in protein inactivation.

PLANT DEFENSIVE PROTEINS AND ENZYMES

Many plants and seeds contain proteins that specifically inhibit pathogen and pest enzymes by forming complexes that block active sites or alter enzyme conformations, which ultimately reducing enzyme function.

They include defensins, amylase inhibitors, lectins, chitinases, lysozymes and proteinase inhibitors etc .

Insect attack induces various plant protiens due alteration of gene expression under stress, which play an important role in signal transduction, and oxidative defense.

Cry 1 protein or Bt- protein is best example of defensive proteins, but these come under induced genetic adaptations

INSECT HORMONE MIMICS AND ANTAGONISTS

Many insect hormones are known, two powerful hormones, the juvenile hormone (JH) and the ecdysone or moulting hormone (MH) are recognized to play a major role in these processes.

It is presumed that plants may have developed juvenoids and ecdysteroids (analogues of these hormones) as subtute for defenses against insect pests.

Farnesol, seasamin, juvabione, sterculic acid and thujic acid are some of the important juvenoids isolated from plants and are known to disturb normal metamorphosis, moulting and reproductive process of insects (Bowers W.S., 1991).

Natural products posessing a 5,6-benz-2-pyrone skeleton are called coumarins.

The coumarins may be variously hydroxylated, alkylated, alkoxylated or acylated, which exert a tremendous effects on the herbivorous insects.

Coumarins can deter feeding as well as interfere with development of insects.

e.g. The simple coumarin ‘bergamottin’ is ovicidal to Leptinotarsa decemlineata (Say) and ‘mammein’ toxic to mustard beetles.

COUMARINS

NUTRITIONAL ANTIBIOSIS

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.

DIMBOA in Corn: Prevent Southern corn borer

Less quantity of asparagine in Rice: Decrease BPH fecundity

Low amino acid and high sugar in Pea: Resistance to pea aphid

High gossypol content in Cotton: Resistance to bollworms

High saponin content in Potato: Resistance to cut worms

INDIRECT DEFENSES The mechanism by which the plant defenses itself by

enhancing the probability of attracting the natural enemies of herbivores.

Such an arrangement is known as mutualism. When a plant is attacked it releases allelochemicals and

volatile organic compounds (VOPs) may be allomones or kairomones, which attracted the predators towards the damaged plant, and feed on herbivores.

Plants also provide housing and food items for natural enemies of herbivores

For example: The tree Macaranga have adapted their thin stem walls to create ideal housing for an ant species. Most plant use of endophytic fungi in defense is well known.

Indirect defenses function by (a) attracting predators such as ants, wasps, and mites including food rewards or chemical signals advertising the presence of prey. (b) Once present, predators attack and/or remove herbivores that can damage the plant (c) direct defenses, do not require a mediator to negatively affect herbivores. (d) Decreased feeding by herbivores results in less damage to the plant.

CONCLUSION

Depending on the herbivore’s physical characteristics (i.e. size, feeding behaviour and defensive armor), structural defenses varies from plant to plant and insects to insects.

Plant adaptations to a particular insect pest may take long time and so the chemically induced resistance can be a better way to prevent the pests.

Instead of direct defense we can go for enhancing indirect defense of plant by releasing bio-control agents to crop ecosystem.

REFERENCES https://en.wikipedia.org http://

www.nature.com/scitable/knowledge/library/plant-resistance-against-herbivory

Juvik, J. A. and Babka, B. A. (1988). Journal of Chemical Ecology, Vol.-14, No. 4

Yoshida, M. et al. (1997). Journal of Chemical Ecology, Vol.-23, No. 4

Google Images Dhaliwal, G.S and Arora, R. (2006). ‘Integrated Pest

Management’, Kalyani publisher: p. 119 Panda, N. and Khush, G. S. (1995). Host plant resistance to Insect, CAB International, UK,.pp. 420. Ram, P. , Singh, R. and Dhaliwal, G. S. (2004). Biophysiacal

Bases of Resistance in plants to insects; Host Plant Resistance to Insect: Concepts and Applications eds., Panima Publishing corporation, New Delhi pp. 94-97. END…