SECONDARY METABOLITES AND PLANT DEFENSE

In Natural Habitats, plants are surrounded by an enormous number of potential enemies. Nearly all ecosystems contain a wide variety of bacteria, viruses, fungi, nematodes, mites, insects, mammals, and other herbivorous animals. By their nature, plants cannot avoid these herbivores and pathogens simply by moving away; they must protect themselves in other ways.

Structural DefensesCutin, Waxes and Suberin

Cutin

Is a macromolecule, a polymer consisting of many long-chain fatty acids that are attached to each other by ester linkages, creating a rigid three dimensional network.

Cutin is a principal constituent of the cuticle, a multilayered secreted structure that coats the outer cell walls of the epidermis on the aerial parts of all herbaceous plants

Waxes

are not macromolecules, but complex mixtures of long-chain acyl lipids that are extremely hydrophobic. The most common components of wax are straight-chain alkanes and alcohols of 25 to 35 carbon atoms

Suberin

Polymer whose structure is poorly understood. A principle component of the outer cell walls of all underground organs and is associated with the cork cells of the periderm, the tissue that forms the outer bark of stems and roots during secondary growth of woody plants. Suberin also forms at sites of leaf abscission and in areas damaged by disease or wounding

Second Metabolites

Plants produce a large, diverse array of organic compounds that appear to have no direct function in growth and development.

Secondary metabolites also differ from primary metabolites (amino acids, nucleotides, sugars, acyl lipids) in having a restricted distribution in the plant kingdom. That is, particular secondary metabolites are often found in only one plant species or related group of species, whereas primary metabolites are found throughout the plant kingdom.

Second metabolites have been suggested to the important ecological functions in plants:

- They protect plants against being eaten by herbivores and against being infected by microbial pathogens

- They serve as attractants for pollinators and seed dispersing animals and as agents of plant-plant competition

Secondary metabolites are divided into three Major Groups

Terpenes

Constitute the largest class of secondary products.

Generally insoluble in water

Two pathways for Terpene Biosynthesis

-Mevalonic acid pathwway

Three molecules of acetyl-CoA are joined together stepwise to form mevalonic acid

The six carbon intermediate is pyrophosphorylated, decarboxylated and dehydrated to yield isopentenyl diphosphate (IPP is the activated five-carbon building block of terpenes)

-Methylerythritol phosphate (MEP) pathway

it operates the chloroplasts and other plastids.

The glyceraldehyde-30phosphate and two carbon atoms derived from pyruvate will combine and generate eventually and converted to IPP

Terpenes has a characterized function in plant growth or development and it can be considered primary than secondary metabolites.

Plant hormones:

� Gibberellins, GA: diterpene

� Brassinoseteroids, BR: triterpene

� Abscisic acid, ABA: C15 terpene

� Sterols: triterpene derivatives

� Carotenoids: tetraterpenes

Terpenes against herbivores

Pyrethoids occur in the leaves and flowers of Chrysanthenum species show very striking insecticidal activity. This is used as a ingredient for commercial insecticides.

Essential oils contains volatile monoterpenes and sesquiterpenes that lend a characteristic odor to their foliage. Example of plants that contains essential oils are peppermint, lemon, basil and sage

Limnoids is well known as bitter substances in citrus fruits. This is the most powerful deterrent to insect feeding known is azadirachtin a complex limnoid from the Neem tree (Azadirachta indica).

Azadirachtin is a feeding deterrent to some insects and it exerts a variety toxic effects.

Phytoecdysones is first isolated from the common fern Polydium vulgare. This are group of plant steroids that have the same basic structure as insect molting hormones. Ingestion of Phytoecdysones will disrupt the molting and developmental processes

Cardenolides are glucosides that taste bitter and are extremely toxic to higher animals. Thus are extracted from species of foxglove (Digitalis) are prescribed to million patients for treatment of heart disease.

Saponins are steroid and glycosides, so named because of their soaplike properties.

Examples of plants that produces saponins are Sapindus (Soapberry or Soapnut) and Gymnostemma pentaphyllum

Phenolic Compounds

Plants produce a large variety of secondary products that contains a phenol group- a hydroxyl functional group on an aromatic ring

These plants are chemically heterogenous groups, some are water-soluble carboxylic acids and glycosides and some are only soluble in organic solvents.

Many serve as defense compounds against herbivores and pathogens.

Phenylalanine is an intermediate in the biosynthesis of most plant phenolics

Plant Phenolics are biosynthesized by several different routes and thus constitutes a heterogenous group from a metabolic point of view. The two basic pathways involved are shikimic acid pathway and malonic acid pathway

Shikimic acid pathway

Converts simple carbohydrate precursor derived from glycolysis and the pentose pathway to the aromatic amino acids.

Shikimic acid pathway is present in plants, fungi and bacteria but it is not found in animals because animals have no way to synthesize the three aromatic amino acids- Phenylalanine, Tyrosine and Tryptophan which are therefore essential nutrients in animal diet.

Some simple phenolics are activated by Ultraviolet light

Simple phenylpropanoids, such as trans-cinnamic acid, p-coumaric acid, and their derivatives, such as caffeic acid, which have a basi phenylpropanoid carbon skeleton

Phenylpropanoid lactones (cyclic esters) called coumarins, also with a phenylpropanoid skeleton

Lignin The most abundant organic substance and it has a high branched polymer of propanoid groups

The Lignin is found in the cell walls of various types of supporting and conducting tissue, notably the tracheids and vessel elements.

Lignin has significant protective functions in plants. It is physically tough and it is chemically durable that makes it relatively indigestible to herbivores.

Flavonoids

This are one of the largest classes of plant phenolics

This are classified into different groups

- Anthocyanin

-Flavones

-Flavonols

AnthocyaninIn addition to predator-prey interactions, there are mutualistic associations among plants and animals. Secondary metabolites are involved in these plant-animal interactions, helping to attract animals to flowers and fruit by providing visual and olfactory signals.

Anthocyanins are responsible for most of the red, pink, purple, and blue colors observed in plant parts. This are vitally important in attracting animals for pollination and seed dispersal.

Flavonoids may protect against damage by UV light

Flavones and flavonols generally absorb light at shorter wavelengths than do anthocyanins, so not visible by the human eyes, but by insects such as bees.

Protect cells from excessive UV-B radiation.

Arabidopsis thaliana chalcone synthase mutant produces no flavonoids, become more sensitive to UV-B radiation

Mutant grows poorly under normal conditions but

grow normally when shielding from UV light.

Isoflavonoids have antimicrobial activity

Mostly found in legumes Several different biological

activities Insecticidal activity Anti-estrogenic effect� Anticancer� Phytoalexins, antimicrobial

compounds synthesized in response to bacterial or fungal infection.

Tannins deter feeding by herbivores

Phenolic polymers with defensive properties

Toxins

Feeding repellents Polyphenols in red wine

reduce the risk of heart disease

Nitrogen-containing compounds

Most nitrogen-containing secondary metabolites are biosynthesized from common amino acids.

�Alkaloids

Cyanogenic glycosides�Glucosinolates�Nonprotein amino acids�

Alkaloids have dramatic physiological effects on animals

A large family, found in ~20% of the species of vascular plants.

Alkaline and generally water �soluble.

Synthesized from one of a few common amino acids.

Lysine, tyrosine, tryptophan Defenses against predators, �

especially mammals So alkaloids at lower doses �

are useful pharmacologically but poisoning agents at high doses.

Cyanogenic glycosides release the poison hydrogen cyanide

Cyanogenic glycosides are not normally broken down in the intact plant

Glycoside and degradative enzymes are spatially separated in different cellular �compartments or in different tissues

Widely distributed in the plant kingdom and are frequently encountered in legumes, grasses, and species of rose family.

Protective functions, deter feeding by insects or other herbivores; but some herbivores have adapted to feed on these plants and tolerate large doses of HCN.

Tubers of cassava a major staple good in many tropical countries, contain high cyanogenic glycosides.

Conventional breeding and genetic engineering approaches are underway to reduce the content of cyanogenic glycosides.

Glucosinolates release volatile toxins

� Found mainly in Brassicaceae and related

families The hydrolyzed products function in defense as

hervibore toxins and feeding repellents. Similar to cyanogenic glycosides, glucosinolates

are not normally broken down in the intact plant Efforts to develop varieties with low �

glucosinolate levels in seeds but high glucosinolate levels in leaves remain the resistance to pests and still provide a protein-rich seed residue for animal feedings

Non-protein amino acids defend against herbivores

Non-protein amino acids are not incorporated into proteins but are �present instead in the free form and act as protective substances.

Exert their toxicity in various ways:

Canavanine: mistakenly incorporated in to proteins and result in a �nonfunctional protein after ingestion by herbivores.

Plants that synthesize nonprotein amino acids are not susceptible to �the toxicity of these compounds.

Plant Defense StrategiesConstitutive defense responses

�-Always present

-Often species-specific and may exist as stored or conjugated �compounds

-Most secondary metabolites as defense are constitutive but �some are also induced

�Induced defense responses

�-Initiated only after actual damage occurs

-Require smaller investment of plant resources then �constitutive mechanisms, but they must be activated quickly

to be effective.

Plants can recognize specific components of insect saliva

Wound response

Insect-derived compounds—�elicitors

� Trigger signaling pathways systematically

Herbivore Damage Triggers a complex signaling Pathway

Proteinase inhibitors and certain other defenses are not continuously present in plants, but are synthesized only after initial herbivore or pathogen attack.

Jasmonic Acid Is a Plant Stress Hormone That Activates Many Defense Responses

Jasmonic acid level rise steeply in response to damage caused by variety of different herbivores and trigger the formation of many different kinds of plant defenses besides proteinase inhibitors, including terpenes and alkaloids. The structure and biosynthesis of jasmonic acid have intrigued plant biologists because of the parallels to some eicosanoids that are central to inflammatory responses and other physiological process to mammals.

Some JA-induced Plant proteins inhibit herbivore digestion

Alpha-amylase inhibitors

-Block the action of starch-digesting enzyme��Lectins

-Bind to carbohydrates or carbohydrate-containing proteins.�� -Bind to the epithelial cells lining the digestive tract and interfere

with nutrient absorption

�Proteinase inhibitors

� -Block herbivore proteolytic enzymes

� -Hinder protein digestion by binding tightly and specifically to the active site of proteinases of herbivores.

� -Defensive roles demonstrated by transgenic tobacco expressing high levels of proteinase inhibitors—suffer less damage by insect herbivores.

Wounded tomato leaves synthesize

prosystemin, a large precursor protein.

�Prosystemin is proteolytically processed to

produce the short polypeptide called

systemin.

�Systemin is released from damaged cells

into the apoplast.

�Systemin binds to its receptor on the

plasma membrane.

�The activated receptor becomes

phosphorylated and activates a

phospholipase A2 (PLA2)

� PLA2 generate the signal that initiates JA

biosynthesis.

�JA is transported through the phloem to

systemic parts of the plants (?) by

unknown mechanisms

�JA activates the expression of genes

encoding proteinase inhibitors

PhytoalexinChemically diverse group of secondary

metabolites with strong antimicrobial activity that accumulate around the site of infection.

Common mechanisms, but different plant �families employ different types of secondary metabolites as phytoalexins.

Isoflavonoids in legumes such as alfalfa and �soybean

Sesquiterpenes in solanaceous plants such �as potato, tomato, and tobacco

Generally not detectable in the plants before �infection, but rapidly synthesized after microbial attack, usually regulated at transcriptional activation of genes in the biosynthesis pathways

Roles of phytoalexins in defense are �demonstrated by mutant and Over expression plants.

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