SENESCENCE AND PROGRAMMED CELL DEATH IN PLANTS

SENESCENCE

•  Every autumn plants changes their colour, after that the loss of leaves can be seen  from deciduous trees. The reason behind this is , because of change in  day length and cooling temperature which triggers developmental process that lead to leaf senescence and death.
• Senescence and necrosis are two different processes,  although both of them  lead to death of the cells.
• Necrosis is death brought out by physical damage, poison or any external injury. In contrast,  Senescence is a normal ,energy dependent developmental process, that is controlled by plant's own genetic program.
• Senescence of plant organs is generally associated with abscission, a process where specific cells in petiole differentiate to form an abscission layer,allowing the senescent organ to separate from the plant. Ethylene have a role in controlling abscission.
• Cytokinins and a Ethylene can act as signaling agents that regulate plant senescence.

IMPORTANCE OF SENESCENCE IN PLANTS

• Leaves are genetically programmed to die and their senescence can be initiated by environmental factors.  New leaves are initiated from the shoot apical meristem, older leaves often are shaded and loose the ability to function efficiently in photosynthesis.
• Senescence helps in recovering of some valuable resources that the plant had invested in leaf formation.
• Senescence cause the production of   many hydrolytic enzymes , that breakdown many  proteins ,carbohydrates and nucleic acids in the cell  ,are then transported back into the plant via the phloem,  where they will be reused for synthetic processes.
• After senescence, many minerals\nutrients  are also transported back to the plant for use from the dead cells .

VARIOUS TYPES OF SENESCENCE IN PLANTS

• Senescence occurs in various of organs of plant and is induced by  many different environmental factors.
• Many annual plants, including major crop plants such as wheat, maize and soyabeans ,get yellow and die following fruit production even under optimal growing conditions.
• Senescence of an entire plant after a single reproductive cycle is called Monocarpic senescence.
• Other types of senescence include the following-

1. Senescence of aerial shoot in herbaceous perennials
2. Seasonal leaf senescence (as in deciduous trees)
3. Sequential leaf senescence (in which the leaves die when they reach a certain stage)
4. Senescence is storage cotyledons & floer organs
5. Senescence (ripening)of fleshy fruits ;senescence of dry fruits
6. Senescence of specialized cell types (e.g. trichomes ,tracheids &vessels elements)etc.

• The triggers for the various types of senescence are different and can be internal, as in monocarpic senescence, or external such as day length and temperature in autumnal leaf senescence of deciduous trees.
• Regardless of initial stimulus, the different senescence patterns may share common integral programmes in which a regulatory senescence gene initiates a cascade of secondary gene expression that eventually brings about senescence and death.

CYTOLOGICAL AND BIOCHEMICAL EVENTS IN SENESCENCE

• On the cytological level some  organs are destroyed while others remain active.
•  The chloroplast is the first organ to deteriorate during the onset of leaf senescence, with destruction of thylakoid protein component and stromal enzymes. Whereas , the nucleus remains structurally and functionally intact until the late stages of senescence.
• Senescence tissues carryout  processes that requires the synthesis of various kind of hydrolytic enzymes, such as proteases, nucleases, lipases and chlorophyll degrading enzymes.
• These senescence specific enzymes are synthesised after  the activation of specific genes.
• In leaf  most mRNA levels decline significantly during senescence phase, but the abundance of certain specific mRNA transcripts increases.
• Genes whose expression decreases during the senescence phase are called Senescence Down-regulated Genes (SDGs).SDGs includes genes that encode proteins involved in photosynthesis. However senescence involves much more than the simple switching off to photosynthetic genes.
• Genes whose expression is increased during senescence are called Senescence Associated Genes (SAGs).SAGs include genes that encode hydrolytic enzymes such as proteases, ribonucleases and lipases, as well as enzymes involved in ethylene synthesis.

PROGRAMMED CELL DEATH IS SPECIAL TYPE OF SENESCENCE

• Senescence can occur at the level of whole plant (as in monocarpic senescence);at the organ level (as in leaf senescence) ;and at the cellular level (as in tracheary elements differentiation).
• The process whereby individual cells activate an intrinsic senescence program is called Programmed Cell Death (PCD).
• PCD can be initiated by specific signals, such as errors in DNA replication during divisions and involves the expression of a characteristic set of genes. These gene expression  results in cell death.
• In animals PCD is studied extensively in comparison to plants because of less available knowledge about PCD in plants.
• In animals,PCD is followed by changes in morphology and biochemical processes  in the body called Apoptosis. During apoptosis cell nucleus condenses and nuclear DNA fragments in a specific pattern caused by DNA degradation.
• Some plants cells, particularly in senescing tissues, exhibit similar cytological changes.
• One of the important functions of PCD in plants is protecting against pathogens. When a pathogenic organism, infects a plant, signals from the pathogen cause the plant cells at the site of infection to quickly accumulate high concentration of toxic phenolic compounds and die. The dead cells form a circular island of cell death called necrotic lesions.
• The necrotic lesions isolates and prevents the infection from spreading to surrounding healthy tissues, by surrounding the pathogen with toxic and nutritionally depleting environment. This rapid, localized cell death due to pathogen attack is called Hypersensitive response.
• It is studied  Arabidopsis mutants that shows to have mimic the effect of infection and trigger the entire cascade of events leading to the formation of necrotic lesions, even when there is no  pathogen , its existence has shown that the hypersensitive response in plants  is a genetically programmed thing not a simple lesions.

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GLYCOLYSIS

Glycolysis

• Glycolysis  derives its name  from the Greek words "glycos"- sugar and "lysis" - splitting.
• In this process , Glucose  is converted into two molecules of Pyruvate or pyruvic acid, an organic acid.
• Apart from preparing substrate for Citric acid cycle, the other function of this is to yield chemical energy in the form of ATP and NADH.
• Glycolysis occurs in all living organisms (prokaryotes and eukaryotes).
• Generally ,the commom type of glycolysis studied is the EMBDEN MEYERHOF PARNAS PATHWAY (EMP).
• The word ,Fermentation is generally used for anaerobic degradation of glucose or other organic nutrients to obtain energy, conserved as ATP.
• In the early steps of glycolysis, sucrose is broken down into two monosaccharides Glucose  and Fructose  which can readily enter the glycolytic pathway.
• The reaction of glycolysis takes place in cytosol. All of the enzymes of glycolysis are found in cytosol.
• Certain trypanosomes carried out the first seven reactions of glycolysis in an organized cytoplasmic organelle called the Glycosomes .Three reactions are inconvertible  and are catalyzed  by hexokinase , phosphofructokinase and pyruvate kinase .

GLYCOLYSIS HAS TWO PHASES

1-PREPARATORY PHASE

• The breakdown of the 6 carbon glucose molecule into two molecules of 3 carbon pyruvate occurs in ten steps. The first five reactions  constitutes the Preparatory Phase. In this -
• Step 1- At first ,Glucose is phosphorylated at hydroxyl group on Carbon -6.
• Step-2-The D Glucose-6-phosphate ,then converted into D Fructose-6-phosphate.
• Step-3- It is again phosphorylated ,this time at Carbon-1position  to yield D Fructose-1,6-bisphosphate
•  (for both of the above phosphorylation ATP is phosphorylated group donor)
• Step-4- The Fructose- 1,6-bisphosphate is split to yield 3 carbon molecules Dihydroxyacetone phosphate and glyceraldehyde -3-phosphate
•  (this is the lysis step that gives its name)
• Step-5- the dihydroxyacetone Phosphate is isomerized to a second molecule  of glycerol aldehyde-3-phosphate.
• (First phase ends here 2 molecules of ATP are invested in the cleavage of glucose)

2-PAYOFF PHASE

• The next five steps contributes the Payoff phase. The energy gain comes from this phase.
• Step-6- Each molecule of glyceraldehyde 3-phosphate is oxidized and phosphorylated by inorganic phosphate (not by ATP) to form 1,3-biphosphoglycerate.
• Step-7- 1,3-biphosphoglycerate are converted to 3-phosphoglycerate by releasing energy (2ADP changes to 2ATP).
• Step -8- 3-phosphogylceratechanges to 2 -phosphoglycerate
• Step-9- 2-phosphoglycerate changes to phosphoenol pyruvate
• Step-10- The phosphoenol pyruvate converted to pyruvate or the pyruvic acid by releasing energy (2ADP changes to 2ATP).
• The energy released in the step 7 and 10  is conserved by coupl phosphorylation of 4ADP molecules to 4ATP molecules.
• The net yield in glycolysis is two ATP molecules per molecule of glucose used ,because two molecules of ATP were invested in the Preparatory phase.
• Energy is also conserved in two molecules of NADH per molecule of glucose.

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SEED DEVELOPMENT AND GERMINATION

• The life of an individual plant begins when an egg nucleus in maternal organ of a flower is fertilized by a sperm nucleus to form a zygote.
• Growth and differentiation of the zygote produces an Embryo, contained in a protective structure called Seed.
• When the seed gets appropriate conditions, the embryo within the seed will renew its growth and  will continue to develop into a mature plant.

SEED DEVELOPMENT

• Seed is a mature embryo surrounded by nutritive tissues and encased in protective seed coat.
• The development of seed begins with the fertilized ovule or zygote. The early stage of seed development is characterized by excessive cell divisions that form the embryo and ,in endospermic seeds the tissue that store nutrients that will support the eventual germination of the seed and seedling development.
• The first division of the zygote is usually transverse and immediately establishes polarity of the embryo. The upper cell  becomes the embryo while the lower produces a stalk like suspensor that anchors  the embryo at the base of the embryo sac.
• During the earlier stages of embryo development, cell divisions occurs throughout the entire cell mass but at the heart shaped embryo stage both root and shoot apical meristem begins to organize as centres of cell divisions.
• Through out the development of embryo, there is a continuous flow of nutrients from the parent plant into the endosperm or Cotyledons.
• In some  cereal grains and many monocots, the endosperm is retained until maturity and may comprise the bulk of the seed. These are called Endospermic seeds.
• The endosperm of mature endospermic seeds consists of cells filled with starch along with proteins and some amounts of lipids.
• In some monocot seeds ,most notably the cereal grains such as Tritium (wheat),Hordeum(barley) and Avena (oats),the endosperm is surrounded by one or more distinctive layers called Aleurone. Aleurone cells are observed to have , presence of numerous protein bodies and are the source of enzymes needed to mobilize nutrients during germination.
• In endospermic dicot seeds, such as Castor bean (Ricinus  communis) have retained a significant amount of endosperm and at maturity the cotyledons are thin leaf like structures.
• In non endospermic dicot seeds ,such as Pisum (pea) &Phaseolus (Bean), the cotyledons enlarge at the expense of the endosperm which occupy 90% of seed volume at maturity .
• Both endosperm and cotyledons contain a large quantity of stored carbon (in form of carbohydrates,  proteins and lipids ),mineral elements and hormones that support the growth and development of the seedlings until it can establish itself as a photosynthetically competent plant.
•  The maturation of seed development is characterized by cessation of embryo growth and development of desiccation resistance. Maturation is terminated by a dramatic desiccation in which the water content of the seed is reduced from  80 or 90 percent to approximately 5 percent.
• Surrounding the mature seed is a hard coat derived from maternal tissue (the Integuments) which surround the seed during its development in ovary .
• Comprised of heavy walled cells and covered with a thick ,waxy cuticle ,the Seed coat often presents a significant barrier to the uptake of both water and oxygen by seed.

SEED GERMINATION

• Seeds are quiescent or resting organs because seeds are severely dehydrated, any metabolic activity takes place so slowly that it is scarcely detected.
• Resumption of embryo growth is called Germination.
• Germination is based on a number of factors but three are specifically important-1)adequate water to rehydrate the tissue 2)the presence of oxygen to support aerobic respiration and 3)a physiological temperature.
• Although many seeds will germinate over a wide range of temperatures, the optimum range for most of the seeds is 25°C -45°C.
• The initial step in germination is the uptake of water and regulation of seed tissues by the process Imbibition. Imbibition is the movement of water down a water potential gradient. Unlike osmosis it doesn't require differentially permeable membrane and is carried out  primarily by surface acting or Matric forces.
• Hydration causes swelling of the imbibing material ,which may generate substantial force (called Imbibition pressure). Imbibition pressure developed by a germinating seed will cause the seed coat to rupture, thus permitting the embryo to emerge.
•  After Imbibition of water the next step is the general activation of seed metabolism within minutes of water entering the cells, initially utilizing a few mitochondria and respiratory enzymes that had been considered in the dehydrated state.
• Renewed protein synthesis is also an early event, utilizing previously existing RNA transcripts and ribosomes, as existing organelles are repaired and new organelles are formed.
• This is events are followed by -1)the release of hydrolytic enzymes that digest and mobilize the stored reserves 2)renewed cell divisions and cell enlargement in the embryonic axis.
• In non endospermic dicot seeds such as the legumes (peas,beans) the initial stages of radicle elongation appear to depend on reserved stored in the tissue of radicle itself. Later carbon reserves are mobilized from the cotyledons and transported to elongating axis.
• Generally germination is considered to be complete when the radicle emerges from the seed coat. Radicle emergence occurs through a combination of cell enlargement within radicle itself and imbibition pressures developed within the seed. The  seed coat is ruptured and  the radicle is protruded which allows it to make direct contact with water and nutrient salts required to support further growth of the young seedling.

Cross section of a mature Maize kernel showing the principal structure of an Embryo

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DOUBLE FERTILIZATION

Structure of the flower

• Flowers vary enormously in their structures,yet all flowers follow the same basic plan .
• A generic flower consists of 4 whorls or circles.
• The two outermost- the Sepals and the Petals , are vegetative structures.
• The two innermost- the Stamens and the Pistil ,are the male and female reproductive structures.
• At the base of pistil or female structure is the ovary ,which contains one or more ovules.

DOUBLE FERTILIZATION
STAGE 1
FORMATION OF FEMALE GAMETE/EGG

• Within each ovule ,a single large diploid cell called the Megaspore mother cell is present.
• Which undergoes the mitosis to produce four Megaspore cells.
• Three out of these cells degenerate and only one survives . This cell undergoes three  meiotic division to produce an Embryo sac with eight haploid nuclei.
• Subsequent cell divisions produces a mature embryo sac in which eight haploid nuclei are segregated in seven cells.
• One of these cell is the Egg (n), another is the large Central cell consists of two polar nuclei (2n), the rest cells are the haploid synergids(n) and antipodal cells(n).

                            

FORMATION OF MALE GAMETE /POLLEN GRAIN

• The male structure or stamens ,are surrounded by the pistil and consists of Anther perched on a stalk called filament.
• The anther contains a large number of microspore mother cells, each of which undergoes meiotic divisions to form uninucleated, single celled Microspore.
• The microspore are then  srrounded in heavy resistant, outer walls and the nucleus divides mitotically, forming two cells-a Tube cell and a Generative cell-within the original spore wall. This is called as mature Pollen grain.

A mature Pollen grain showing tube cell and generative cell. The diploid generative nucleus divides meiotically to produce two haploid sperm nuclei.

STAGE 2

• Mature Pollen grain are shed from the anthers and carried to stigmatic surface of the pistil by insects, wind or some other vector.
• Once the pollen grain lands on the stigmatic surface by an event called Pollination.
• The pollen grain lands takes up the water and sends out Pollen tube that grows down the style of pistil towards ovule.
• The tube nucleus start moving down the Pollen tube and appears to direct its growth.
• The cell wall of the generative cell breaksdown and the generative nucleus divides into two sperm nuclei that follow the tube nucleus down the tube as it elongates.
• Pollen tube growth towards the embryo sac requires a calcium gradient, but the precise signaling mechanism remains uncharacterized .

            A germinating pollen grain

STAGE 3

• In the final stage ,the elongating pollen tube enters the ovule by growing through micropyle (the space between two integuments) and releases two sperm nuclei into the embryo sac.
• Ultimately, one of the sperm nuclei enters the egg cell and fertilize  the egg cell nucleus to form a diploid cell called ZYGOTE (2n). This process is called as SYNGAMY.
• The second sperm nuclei enters the large central cell and fuses with the two polar nuclei to form a triploid PRIMARY ENDOSPERM NUCLEUS (3n).This process is called as TRIPLE FUSION.
• This process of fusion of two sperm nuclei with egg cell and large central cell to give zygote and primary endosperm nucleus  is called as DOUBLE FERTILIZATION.
• This is a characteristic feature of flowering plants or angiosperms. 

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TRANSPIRATION

TRANSPIRATION 

• Plant absorbs a large quantity of water from soil,and it is translocated through the plant and eventually lost to the surrounding atmosphere.
• The process by which water is lost from the plant into the surrounding atmosphere in the form of water vapor,is called TRANSPIRATION.
• All of the water absorbed by the plants,less than 5% actually retained for growth and even less is used biochemically.
• Although a small amount of water vapor may be lost through small openings  in the bark of young twigs and branches,called Lenticles , the largest portion by far (>90%) escapes through Stomata (sing -Stoma).
• Stomata are small pores occassionally present between the epidermis and overlying cuticle.
• Each   of the stomatal pore is surrounded by a pair of Guard cells . These guard cells function as hydraulically operated valves that control the size of pore.
• Stomata are located such that,when they open they provide a route for exchange of gases (carbondioxide, oxygen & water vapor) between the internal air space and the bulk atmosphere surrounding leaf.

TYPES OF TRANSPIRATION 

1. Cuticular transpiration -Cuticle is a multilayered  waxy deposit  covering on the epidermis of leaves and herbaceous stems. It is meant to check transpiration . However some water may be lost through it. The loss of water in the form of water vapor through the cuticle is called as cuticular transpiration. It accounts for about 5-10% of total transpiration by plants.
2. Lenticular transpiration- Loss of water in the form of water vapor through the lenticles is called lenticular transpiration. It accounts for only 1-5% of the total water loss by the plants. 
3. Stomatal transpiration- The primary function of stomata is gas exchange between plant's internal tissues and the atmosphere. When water is lost through stomata in the form of water vapor,  it is called stomata transpiration. It accounts for ~90% of total transpiration by the plant.

TRANSPIRATION IS A TWO STAGE PROCESS 

1. 1)Diffusion  of water from the moist cell walls into the substomatal air space

• It is assumed that water is pulled out from the xylem and evaporation occurs primarily at the surfaces of those mesophyll cells that borders substomatal air  spaces. 
• However some investigators have suggested that most of the water evaporates from the inner surface of epidermal cells in the vicinity of stomata known as Peristomal evaporation.

1. 2) Diffusion of water vapor from substomatal space into air

• It is relatively straightforward. Once the water vapor has left the cell's surfaces, it diffuses through the substomatal space and exits the leaf through the stomatal pore.

THE DRIVING FORCE FOR TRANSPIRATION IS THE DIFFERENCE IN WATER VAPOR CONCENTRATION

• The water vapor concentration is the driving force behind transpiration ,the  water vapor concentration difference  is expressed as Cwv (leaf) -Cwv (air).
• The water vapor concentration of bulk air (Cwv(air)) can easily be measured  ,but that of the leaf (Cwv(leaf)) is more difficult to assess.
• The air space in leaf is close to water potential equilibrium with the cell wall surfaces from which water is evaporating.
• Within the range of water potentials experienced by transpiring leaves (generally <2.0 MPa) the equilibrium water vapor concentration is within few percentage points of the saturation water vapor concentration. This allows to calculate water vapor concentration within a  leaf.

• The concentration of water vapor (Cwv)changes at various points along the transpiration pathway.

• From figure, we see that Cwv changes at each step of the pathway from the cell wall surfaces to the bulk air outside leaf.
• The important points to remember 

1. That the driving force for water loss from the leaf is the absolute concentration difference 
2. That this difference depends on leaf temperature. 

WATER LOSS IS ALSO REGULATED BY THE PATHWAY RESISTANCE 

• The second important factor governing water loss from the leaf is the Diffusional Resistance of the transpiration Pathway, which consists of two varying components-

1. The resistance associated with diffusion through the stomatal pore, the Leaf stomatal resistance (rs).
2. The resistance due to the layer of unstirred air next to the leaf surface through which  water vapor must diffuse to reach the turbulent air of the atmosphere. This second resistance (rb)is called the Leaf boundary layer resistance. 

GUARD CELLS ACTION MECHANISM 

• Guard cells are found in leaves of all vascular plants, and they are also present in organs from primitive plants such as liverworts and mosses 
• The cell wall of  guard  cells have specialized features. They are substantially thickened at portions. 
• Morphologically guard cells can be distinguished into two main types-

1. Dumbbell shaped guard cells- They are found in grasses and other monocots, such  as palms. They have a characteristic dumbbell shape with bulbous ends. These guard cells are always flanked by a pair of differential epidermal cells called Subsidiary cells, which helps the guard cell control stomatal pore. The guard cell, the subsidiary cells and pore are collectively called Stomatal complex. 
2. Kidney shaped guard cells- They are found in dicot plant  and non grass monocots.  They have an elliptical contour with the pore at its center. Subsidiary cells are not uncommon, but mostly absent. In that case the guard cells are surrounded by ordinary epidermal cells. 

Figure_(A)-Kidney shaped guard cells 

                  (B)Dumbbell shaped guard cells 

AN INCREASE IN GUARD CELLS TURGOR PRESSURE OPENS THE STOMATA

• Guard cells functions as multisensory hydraulic valves. 
• Environmental factors such as light intensity, temperature, relative humidity and intracellular CO2 concentrations are sensed by guard cells and these signals are integrated into well defined stomatal responses. 
• The early processes are ion uptake and other metabolic changes in the guard cells. Here we note that decrease in Osmotic pressure resulting from ion uptake and from biosynthesis of organic molecules in the guard cells. 
• As Osmotic pressure decreases, the water potential decreases and water subsequently moves to the guard cells. 
• As water enters the guard cells, Turgor pressure increases. Because of the elastic properties of their walls, guard cells can reversibly increase in their volume by 40-100% depending on  the species. 
• Because of the differential thickening of guard cell walls, such changes in volume lead to opening and closing of stomata.

FACTORS INFLUENCING TRANSPIRATION

• Several factors influence transpiration these factors can be external as well as internal.
• External factors that affect transpiration are-

1. Light -plant transpire  more in light than in the dark. Because  light stimulates the opening of stomata.
2. Temperature-plant transpire more rapidly at higher temperatures
3. Humidity- Increased humidity decreases transpiration because air is already saturated with water vapor.
4. Soil water - A plant cannot continue transpiration , if its water loss is not made up by replacement from the soil. Decrease in soil water slows down the transpiration.

• Internal factors-Internal factors that affect the transpiration are number of stomata ,thickness of cuticle ,leaf area .distribution of stomata etc.

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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.

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VERNALISATION

• Plants have evolve the ability to alter their developmental programme in response to environmental stimuli. 
• A major switch in the development programme is the transition to flowering. Photoperiod and temperature play an important role in determining the  correct time to flower.
• Vernalisation is also called  as Chilling treatment/ Cold treatment.
• Many flowers don't come to flower before they experience a low temperature. 
• These plants remain vegetative during warm season experience low temperature during winter grow further  and  bear flowers and fruits
•              VERNALISATION
• Promotion of flowering by giving cold treatment  is called VERNALISATION. 
• The effective temperature range for vernalisation is from just below freezing to >10°C ,with  a broad optimum usually  between about 1-5°C.
• Vernalisation is the process in plants  where the  flowering is promoted by a cold treatment given to a fully hydrated seed (i.e a seed that has imbibed water )or to a growing plant. 
• Photoperiodism and vernalisation are to most important mechanisms underlying seasonal responses. Photoperiodism -is response to the length of the day whereas Vernalisation-is the promotion of flowering. 
• Without the cold treatment  plant that requires vernalisation show delayed flowering or remain vegetative. In many cases these plants grow as russettes(with no elongation of stem).
• Competence of the meristem to undergo the floral transition.
• After vernalisation,plants do not necessarily initiate flowering but acquires the competence to do so.
• It is reversible and can be lost as a result to de-vernalisation with conditions such as high temperatures. 
• According to studies show , the apical meristem is the site of cold perception during vernalisation and that vernalisation causes the meristem to become competent to flower. 
• Later there comes a hypothesis that the VERNALIN compound  is responsible for vernalisation stimulus which produced in plants after vernalisation process. 
• Vernalin got activated at low temperature. 
• "VERNALIN has not been isolated yet ".

• Hence vernalisation is a process in plants where juvenile or vegetative phase is shortened and flowering is induced by a cold treatment. 
• An example of  Winter rye  may be quoted here. When the seeds of this variety of rye  were germinated at 1°C for 4 weeks, the plant flowered 11 weeks after planting,but at the same time seed germinated at 18°C did not have  flowering  shoot in the same duration. 
• Vernalisation  occurs in dry seeds . The seed must be germinated so that they contain an active embryo . Due to this reason , the seeds are moistened before exposing them to low temperature. For a complete plant an active meristem is required. 
• The stimulus recieved by the actively dividing cells of shoot or embryo travels to all parts of the plant and prepare it to flower. This stimulus has been named VERNALIN. 
• Attempts to isolate and chemically identify Vernalin haven't been succeeded 
• However Anton lang have demonstrated that treatment with Gibberllic acid ,a plant hormone substitute for cold treatment in same species plants.

•       MECHANISMS OF FLORAL INDUCTION IN VERNALISED PLANT (Genes involved)
• This process  have been identified in a Arabidopsis Thaliana ,some flowering genes are involved in this process. 
• Some stable changes  are found in the pattern of gene expression in the meristem after cold treatment. 
• FLC gene( Flower  locus C ) -A gene that acts as a repressor of flowering. FLC is highly expressed in non vernalised shoot apical meristem.  In Vernalisation this gene is epigenetically switched off by an unknown mechanism for the remainder of plant's  life cycle,permitting  flowering in response 
• FT gene (flower locus T ) - A genes in leaves,Florigenic factor (flowering gene).
• SOC 1(Supressor of over expression of constan S1)-A gene in shoot apical meristem(flowering  gene)
• FD (Flower locus)-A gene in shoot apical meristem (flowering gene).
• Vernalisation 1&2(VRN 1 AND VRN2)=Activated by cold temperatures 
• Vernalisation insensitive (VRN3)=Activated after VRN 1&2 or after cold temperature. 

If FLC activated- NO FLOWERING 

If FLC is inactive- FLOWERING 

            ●  NO VERNALISATION 

                             |

                 FLC ACTIVATED                                                           
                              |

                       INHIBIT

                  1.FT gene (in leaves)

                  2.FD gene(in apical meristem)

                  3.SOC 1 gene ,flowering  gene (in apical meristem)

                              |

                   NO FLOWERING 

            ●    VERNALISATION 

                               |

                    VRN 1&2ACTIVE

                               |

                     VRN3 ACTIVE

                               |

                      FLC INACTIVE 

                                |

                       FLOWERING

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