PLANT SECONDARY METABOLITES

METABOLITES

•  They are intermediates or products of metabolisms.
• Plant produces 2 types of metabolites.

1. Primary metabolites
2. Secondary metabolites

PRIMARY METABOLITES

• A primary  metabolite is a kind of metabolite that is  directly involved in plant growth, development and reproduction.Ex- photosynthesis,respiration,solute transport,translocation,protein synthesis,nutrients assimilation,differentiation,formation of carbohydrates proteins or lipids etc.

EXAMPLES OF PRIMARY METABOLITES

•  Aminoacids, Nucleotides, sugars, Acyl lipids.

SECONDARY METABOLITES

• Plants produce a large variety of organic compounds that  have no direct function in growth and development  processes known as Secondary metabolites.
• They defend plant against a variety of herbivores and pathogenic microbes
• They serve other important functions as well as structural support (e.g lignin) or pigments (e.g anthocyanins).
• They differ from primary metabolites in having a restricted distribution to plant kingdom. That is a particular secondary metabolite are found in only one plant species  or related group of species,whereas primary metabolites are found in whole plant kingdom.

SECONDARY METABOLITES DEFEND PLANT AGAINST HERBIVORES AND PATHOGEN

• For many years these were considered to be function less or simply the end products of metabolisms
• Later studies showed their importance  as medicinal drugs ,poisons, flavours and industrial materials.
• Recently many secondary metabolites suggested to have important ecological role in plants
• They protect plants against being eaten by herbivores and being infected by microbial pathogens.
• They serve as an attractant for pollinators and seed dispersing animals and as agents of plant plant competition.
• They increase the reproductive fitness of plant by warding off fungi,bacteria and herbivores.

SECONDARY METABOLITES ARE DIVIDED INTO THREE MAIN CLASSES

1. Terpenes
2. Phenolics
3. Nitrogen containing compounds

1)TERPENES

• Terpenes or Terpenoids constitutes the largest class of secondary products.
• The diverse substance of this class are generally insoluble in water.
• They are biosynthesised from Acetyl CoA or glycolysis intermediates.
• Terpenes structure is composed  by the fusion  of  5 carbon isoprene units . Occasionally terpenes are also called Isoprenoids.
• Isoprene unit-

Terpenes are classified by the numer of 5 carbon units

    10 C       2 C5 units       Monoterpenes

    15C        3 C5 units       Sequiterpenes

20C       4 C5 units        Diterpenes

  30C       6 C5 units        Triterpenes

      40C        8 C5 units        Tetraterpenes

                   n C5 units       Polyterpenes

PATHWAYS OF TERPENES BIOSYNTHESIS 

• There are two pathways-

1. Mevalonic Acid Pathway 
2. Methylerythritol Phosphate Pathway

TERPENES ROLE IN GROWTH AND DEVELOPMENT 

• Gibberellins an important class of plant hormones are diterpenes.
• Sterols are triterpenes derivatives that are essential components of cell membrane. 
• The Carotenoids ( red ,orange and yellow ) are Tetraterpenes that function as  accessory pigments. 
• The hormone Abscisic acid is a cis terpenes produced by degradation of carotenoids. 
• Long chain polyterpene alcohols known as Dolichols functions as carriers of sugars in cell wall and glycoprotein synthesis. 

TERPENES DEFEND AGAINST HERBIVORES IN MANY PLANTS 

•  There are Monoterpene ester called Pyrethroids that occurs in the leaves and flowers of chrysanthemum species show very striking insecticidal activity .
• Many plant contains mixtures of volatile monoterpenes and sequiterpenes called Essential oils that add a characteristic odor to their foliage. Ex- Peppermint,Lemon,Basil &Sage.
• Among non volatile terpenes anti herbivore  compounds such as Limonoids ,a group of triterpenes. Most powerful deterrent to insect feeding azadirachtin found in neem is a Limonoid.
• Triterpenes  that are active against vertebrates herbivores include Cardenolides and Saponins.

2)PHENOLIC COMPOUNDS 

• Plant produces a large variety of secondary products that contain a phenol group i.e a hydroxy functional group on an aromatic ring. These are classified as Phenolic compounds. 
• Plant phenols are a chemically heterogeneous group of nearly 10,000 individual compounds. 
• Some are soluble only in organic solvents,some are water soluble carboxylic acids and glycosides and other are insoluble polymers. 
• Example- Phenylpropanoids, Coumarins,Benzoic acid derivatives, Lignin ,Anthocyanins ,Isoflavins, Condensed tannins other flavonoids ,etc.

BIOSYNTHESIS OF PHENOLIC COMPOUNDS 

• There are two pathways-

1. Shikimic Acid Pathway (most plant phenolics).
2. Malvonic Acid Pathway ( for fungi and bacteria ,less significantly plants).

ROLE OF PHENOLIC COMPOUNDS 

• Certain Coumarins are phtotoxic called Furanocoumarins
• The release of phenolics into soil may limit the growth of other plants (e.g Caffeic acids and Ferulic acid)
• Lignin blocks the growth of pathogens ,protects against infection and wounding. Its physical toughness deters feeding by animals and its chemical durability makes it relative indigestible to herbivores. 
• Flavonoids like Anthocyanins attracts animals.
• Flavonoids can protect against damage by ultraviolet light (e.g flavones and flavonols).
• Isoflavanoids have anti bacterial activity. 
• Tannins deters feeding by herbivores. 

3)NITROGEN CONTAINING COMPOUNDS 

1. Alkaloids 
2. Cyanogenic Glycosides 

1-ALKALOIDS 

• The group ,Alkaloids are a large family of more than 15,000 nitrogen containing secondary metabolites.
• They are found in approximately 20% of the plant species of vascular plants.
• The nitrogen atom in these substance is usually part of heterocyclic ring ,a ring that contains both nitrogen and carbon. 
• These are best known for their striking pharmacological effects on vertebrate animals. 
• Most alkaloids are alkaline. At pH 7.2 values ,commonly found in cytosol or the vacuole. 
• The N atom is protonated ,hence alkaloids are positively charged and are generally water soluble. 
• Alkaloids are usually synthesized from one of a few common aminoacids -in particular Lysine, Tyrosine and Tryptophan.

• Most alkaloids function as defence against predators especially mammals because of their general toxicity and deterrence capability. 
• Nearly all alkaloids are toxic to humans when taken in sufficient quantity (e.g  Strychnine, Atropine and codeine are classical examples of alkaloid poisoning agents).
• However,at lower doses many are pharmacologically useful (e.g Morphine  ,
• Codeine and scopolamine).
• Others including Cocaine,Nicotine and Caffeine are used as stimulators or sedatives. 
• Alkaloids present in grass provides protection against insects but are not poisonous to livestock. 

2-CYANOGENIC GLYCOSIDES 

• This is the other class of nitrogen containing secondary metabolites .
• These are not toxic themselves but are readily broken down into volatile poison when crushed. 
• They releases a well known poisonous gas Hydrogen Cyanide (HCN).
• It is two steps enzymatic process-

• Cyanogenic glycosides are widely distributed  in plant kingdom and frequently encountered in legumes  ,grasses and the species of rose family .
• Cyanogenic glycosides have protective functions in plants by releasing HCN which inhibits Fe containing cytochrome oxidase which is involved in mitochondrial respiration. 
• It deters feeding by insects and other herbivores such as snails and slugs.