Plant Growth Regulators Plant Harmone Phytoharmone. PHARMACOGNOSY & Phytochemistry-I (BP405T)Unit-IIPart-3 PLANT HORMONES AND GROWTH REGULATORS Five major classes of plant hormones are mentioned: 1. auxins, 2. cytokinins, 3. gibbereilins, 4. abscisic acid 5. ethylene.

1. PHARMACOGNOSY & Phytochemistry-I
(BP405T)
Unit-II
Part-3
Plant Growth Regulators.
Name: Mrs. Pooja Deepak Bhandare
Assistant Professor
G H RAISONI UNIVERSITY
SCHOOL OF PHARMACY

2.  PLANT HORMONES AND GROWTH REGULATORS
• Plant hormones (phytohormones) are physiological inter-cellular messengers
that control the complete plant lifecycle, including germination, rooting,
growth, flowering, fruit ripening, foliage and death. In addition, plant
hormones are secreted in response to environmental factors such as excess of
nutrients, drought conditions, light, temperature and chemical or physical
stress. So, levels of hormones will change over the lifespan of a plant and are
dependent upon season and environment.

3. • The term ‘plant growth factor’ is usually employed for plant hormones or
substances of similar effect that are administered to plants. Growth factors are
widely used in industrialized agriculture to improve productivity.
• The application of growth factors allows synchronization of plant development
to occur. For instance, ripening fruits can be controlled by setting desired
atmospheric ethylene levels.
• Using this method, fruits that are separated from their parent plant will still
respond to growth factors; allowing commercial plants to be ripened in storage
during and after transportation.
• This way the process of harvesting can be run much more efficiently and
effectively. Other applica-tions include rooting of seedlings or the suppression
of rooting with the simultaneous promotion of cell division as required by plant
cell cultures.
• Just like with animal hormones, plant growth factors come in a wide variety,
producing different and often antagonistic effects.

4. • In short, the right combination of hormones is vital to achieve the desired
behavioural characteristics of cells and the productive development of plants as
a whole.
• The plant growth regulators are classified into synthetic and native. The
synthetic regulators are also known as exogenous regulators and the native are
called the endogenous,
• Five major classes of plant hormones are mentioned: auxins, cytokinins,
gibbereilins, abscisic acid and ethylene.
• However as research progresses, more active molecules are being found and
new families of regulators are emerging; one example being polyamines
(putrescine or spermidine). Plant growth regulators have made the way for
plant tissue culture techniques, which were a real boon for mankind in
obtaining therapeutically valuable secondary metabolites.

6. 1. Auxins
• The term auxin is derived from the Greek word auxein which means to grow.
Generally compounds are considered as auxins if they are able to induce cell
elongation in stems and otherwise resemble indoleacetic acid (the first auxin
isolated) in physiological activity. Auxins usually affect other processes in
addition to cell elongation of stem cells but this characteristic is considered
critical of all auxins and thus ‘helps’ define the hormone.

7. • Auxins were the first plant hormones discovered. Charles Darwin was among the first scientists to pool in plant
hormone research. He described the effects of light on movement of canary grass coleoptiles in his book ‘The
Power of Movement in Plants’ presented in 1880.
• The coleoptile is a specialized leaf originating from the first node which sheaths the epicotyl in the plants
seedling stage protecting it until it emerges from the ground. When unidirectional light shines on the coleoptile, it
bends in the direction of the light.
• If the tip of the coleoptile was covered with aluminium foil, bending would not occur towards the unidirectional
light. However if the tip of the coleoptile was left uncovered but the portion just below the tip was covered,
exposure to unidirectional light resulted in curvature toward the light.
• Darwin’s experiment suggested that the tip of the coleoptile was the tissue responsible for perceiving the light
and producing some signal which was transported to the lower part of the coleoptile where the physiological
response of bending occurred. When he cut off the tip of the coleoptile and exposed the rest of the coleoptile to
unidirectional light curvature did not occur confirming the results of his experiment.

8. • Indole acetic acid (IAA) is the principle natural auxin and other natural
auxins are indole-3-acetonitrile (IAN), phenyl acetic acid and 4-
chloroindole-3-acetic acid. The exogenous or synthetic auxins are indole-
3-butyric acid (IBA), α-napthyl acetic acid (NAA), 2-napthyloxyacetic
acid (NOA), 1-napthyl acetamide (NAD), 5-carboxymeth-yl-N, N-
dimethyl dithiocarbamate, 2,4-dichlorophenoxy acetic acid (2,4-D), etc.

9. • Functions of auxin
• Stimulates cell elongation.
• The auxin supply from the apical bud suppresses growth of lateral buds. Apical dominance is the inhibiting influ-ence of the
shoot apex on the growth of axillary buds. Removal of the apical bud results in growth of the axillary buds. Replacing the
apical bud with a lanolin paste containing IAA restores the apical dominance. The mechanism involves another hormone -
ethylene. Auxin (IAA) causes lateral buds to make ethylene, which inhibits growth of the lateral buds.
• Differentiation of vascular tissue (xylem and phloem) is stimulated by IAA.
• Auxin stimulates root initiation on stem cuttings and lateral root development in tissue culture (adventitious rooting).
• Auxin mediates the tropistic response of bcircumstancesending in response to gravity and light (this is how auxin was first
discovered).
• Auxin has various effects on leaf and fruit abscission, fruit set, development, and ripening, and flowering, depending on
the circumstances.

10. 2. Cytokinins
• Cytokinins are compounds with a structure resembling adenine which promote cell
division and have other similar functions to kinetin. They also regulate the pattern
and frequency of organ production as well as position and shape. They have an
inhibitory effect on senescence. Kinetin was the first cytokinin identified and so
named because of the compounds ability to promote cytokinesis (cell division).
Though it is a natural compound, it is not made in plants, and is therefore usually
considered a ‘synthetic’ cytokinin. The common naturally occurring cytokinin in
plants today is called zeatin which was isolated from corn.

11. • Cytokinin have been found in almost all higher plants as well as mosses, fungi, bacteria, and also in many
prokaryotes and eukaryotes. There are more than 200 natural and synthetic cytokinins identified. Cytokinin
concentrations are more in meristematic regions and areas of continuous growth potential such as roots, young
leaves, developing fruits, and seeds.
• Haberlandt (1913) and Jablonski and Skoog (1954) identified that a compound found in vascular tissues had
the ability to stimulate cell division. In 1941, Johannes van Overbeek discovered that the milky endosperm
from coconut and other various species of plants also had this ability. The first cytokinin was isolated from
herring sperm in 1955 by Miller and his associates. This compound was named kinetin because of its ability to
promote cytokine-sis (cell division). The first naturally occurring cytokinin was isolated from corn in 1961 by
Miller and it was later called zeatin. Since that time, many more naturally occur-ring cytokinins have been
isolated and the compound was common to all plant species in one form or another.
• The naturally occurring cytokinins are zeatin, N6 dim-ethyl amino purine, isopentanyl aminopurine. The syn-
thetic cytokinins are kineatin, adenine, 6-benzyl adenine benzimidazole and N, N’-diphenyl urea.

12. • The naturally occurring cytokinins are zeatin, N6 dim-ethyl amino purine,
isopentanyl aminopurine. The syn-thetic cytokinins are kineatin, adenine, 6-
benzyl adenine benzimidazole and N, N’-diphenyl urea.

13. Functions of cytokinin
 Stimulate cell division (cytokinesis).
 Stimulate morphogenesis (shoot initiation/bud forma-tion) in tissue culture.
 Stimulate the growth of lateral (or adventitious) buds-release of apical dominance.
 Stimulate leaf expansion resulting from cell enlarge-ment.
 May enhance stomatal opening in some species (Figure 6.2).
 Promotes the conversion of etioplasts into chloroplasts via stimulation of chlorophyll synthesis.
 Stimulate the dark-germination of light-dependent seeds.
 Delays senescence.
 Promotes some stages of root development.

14. 3. Ethylene
• Ethylene has been used in practice since the ancient times, where people
would use gas figs in order to stimulate ripen-ing, burn incense in closed
rooms to enhance the ripening of pears. It was in 1864, that leaks of gas
from street lights showed stunting of growth, twisting of plants, and
abnormal thickening of stems.
• In 1901, a Russian scientist named Dimitry Neljubow showed that the
active component was ethylene.

15. • Doubt 1917, discovered that ethylene stimulated abscission. In 1932 it was
demonstrated that the ethylene evolved from stored apple inhibited the
growth of potato shoots enclosed with them.
• In 1934 Gane reported that plants synthesize ethylene. In 1935, Crocker
proposed that ethylene was the plant hormone responsible for fruit
ripening as well as inhibition of vegetative tissues. Ethylene is now known
to have many other functions as well.

16. • Functions of ethylene
• Production stimulated during ripening, flooding, stress, senescence, mechanical damage, infection.
• Regulator of cell death programs in plants (apoptosis). Stimulates the release of dormancy.
• Stimulates shoot and root growth and differentiation (triple response).
• Regulates ripening of climacteric fruits.
• May have a role in adventitious root formation. Stimulates leaf and fruit abscission.
• Flowering in most plants is inhibited by ethylene. Mangos, pineapples and some ornamentals are
stimu-lated by ethylene.
• Induction of femaleness in dioecious flowers. Stimulates flower opening.
• Stimulates flower and leaf senescence.

17. 4. Gibberellins
• Unlike the classification of auxins which are classified on the basis of function,
gibberellins are classified on the basis of structure as well as function. All
gibberellins are derived from the ent-gibberellane skeleton. The gibberellins
are named GA1. GAn in order of discovery. Gibberellic acid was the first
gibberellin to be structurally characterized as GA3. There are currently 136
GAs identified from plants, fungi and bacteria.

18. • They are a group of diterpenoid acids that functions as plant growth regulators influencing a range
of developmental processes in higher plants including stem elongation, germination, dormancy,
flowering, sex expression, enzyme induction and leaf and fruit senescence.
• The origin of research into gibberellins can be traced to Japanese plant pathologists who were
investigating the causes of the ‘bakanae’ (foolish seedling) disease which seriously lowered the
yield of rice crops in Japan, Taiwan and throughout the Asian countries.
• Symptoms of the disease are pale yellow, elongated seedlings with slender leaves and stunted roots.
Severely diseased plants die whereas plants with slight symptoms survive but produce poorly
developed grain, or none at all.
• Bakanae is now easily prevented by treatment of seeds with fungicides prior to sowing. In 1898
Shotaro Hori demonstrated that the symptoms were induced by infection with a fungus belonging to
the genus Fusarium, probably Fusarium heterosporium Necs.

19. • In 1912, Sawada suggested that the elongation in rice-seedlings infected with bakanae
fungus might be due to a stimulus derived from fungal hyphae.
• Subsequently, Eiichi Kurosawa (1926) found that culture filtrates from dried rice seedlings
caused marked elongation in rice and other sub-tropical grasses. He concluded that
bakanae fungus secretes a chemical that stimulates shoot elongation, inhibits chlorophyll
formation and suppresses root growth.
• Although there has been controversy among plant pathologists over the nomenclature of
bakanae fungus, in the 1930s, the imperfect stage of the fungus was named Fusarium
moniliforme (Sheldon) and the perfect stage, was named as Gibberella fujikuroi (Saw.) Wr.
by H.W. Wol-lenweber. The terms ‘Fujikuroi’ and ‘Saw’ in Gibberella fujikuroi (Saw.) Wr.
were derived from the names of two distinguished Japanese plant pathologists, Yosaburo
Fujikuro and Kenkichi Sawada.

20. • In 1934, Yabuta isolated a crystalline compound from the fungal culture filtrate that inhibited
growth of rice seedlings at all concentrations tested.
• The structure of the inhibitor was found to be 5-n-butylpicolinic acid or fusaric acid. The
formation of fusaric acid in culture filtrates was suppressed by changing the composition of the
culture medium. As a result, a noncrystalline solid was obtained from the culture filtrate that
stimulated the growth of rice seedlings. This compound was named gibberellin by Yabuta.
• In 1938, Yabuta and his associate Yusuke Sumiki finally succeeded in crystallizing a pale yellow
solid to yield gib-berellin A and gibberellin B (The names were subsequently interchanged in
1941 and the original gibberellin A was found to be inactive.)
• Determination of the structure of the active gibberellin was hampered by a shortage of pure
crystalline sample. In the United States, the first research on gibberellins began after the Second
World War.

21. • In 1950, John E. Mitchell reported optimal fermentation procedures for the
fungus, as well as the effects of fungal extracts on the growth of bean (Vicia
faba) seedlings.
• In Northern USDA Regional Research Laboratories in Peoria, large scale
fermentations were carried out with the purpose of producing pure gibberellin A
for agricultural uses but initial fermentations were found to be inactive. Further
researches were carried out by Sumiki in 1951, Stodola et al., 1955, Curtis and
Cross, 1954 regarding gibberellins and finally the gibberllic acid was
determined by its chemical and physical properties.

22. • In 1955, members of Sumuki group, succeeded in sepa-rating the methyl ester of gibberellin A into three compo-nents,
from which corresponding free acids were obtained and named gibberellins Al, A2, and A3. Gibberellin A3 was found to
be identical to gibberellic acid. In 1957, Takahashi et al. isolated a new gibberellin named gibberellin A4 as a minor
component from the culture filtrate.
• In the mid 1950s, evidence that gibberellins were naturally occurring substances in higher plants began to appear in the
literature. Margaret Radley in the UK demonstrated the presence of gibberellin-like substances in higher plants.
• In the United States, Bernard Phinney et al were the first to report gibberellin-like substance in maize. This was followed
by the isolation of crystalline gibberellin Al, A5, A6 and A8 from runner bean (Phaseotus multiflorus).
• After 10 years the number of gibberellins reported in the literature isolated from fungal and plant origins rapidly
increased. In 1968, J. MacMillan and N. Takahashi concluded that all gibberellins should be assigned numbers as
gibberellin A1-x, irrespec-tive of their origin.
• Over the past 20 years using modern analytical techniques many more gibberellins have been identified. At the present
time the number of gibberellins identified is 126

23. Functions of gibberellins
• Stimulates stem elongation by stimulating cell division and elongation. GA controls internode
elongation in the mature regions of plants. Dwarf plants do not make enough active forms of
GA.
• Flowering in biennial plants is controlled by GA. Bien-nials grow one year as a rosette and
after the winter, they bolt (rapid expansion of internodes and formation of flowers).
• Breaks seed dormancy in some plants that require strati-fication or light to induce
germination.
• Stimulates α-amylase production in germinating cereal grains for mobilization of seed
reserves.

24. • Juvenility refers to the different stages that plants may exist in. GA may help
determine whether a particular plant part is juvenile or adult.
• Stimulates germination of pollen and growth of pollen tubes.
• Induces maleness in dioecious flowers (sex expres-sion).
• Can cause parthenocarpic (seedless) fruit development or increase the size of
seedless fruit (grapes).
• Can delay senescence in leaves and citrus fruits. May be involved in
phytochrome responses.

25. 5. Abscisic Acid
• Natural growth inhibiting substances are present in plants and affect the normal
physiological process of them. One such compound is abscisic acid, a single
compound unlike the auxins, gibberellins, and cytokinins.
• It was called ‘absci-sin II’ originally because it was thought to play a major role
in abscission of fruits. At about the same time another group was calling it
‘dormin’ because they thought it had a major role in bud dormancy. Though
abscisic acid generally is thought to play mostly inhibitory roles, it has many
promoting functions as well.

26. • In 1963, when Frederick Addicott and his associates were the one to identify
abscisic acid. Two compounds were isolated and named as abscisin I and
abscisin II. Abscisin II is presently called abscisic acid (ABA). At the same
time Philip Wareing, who was studying bud dormancy in woody plants and
Van Steveninck, who was studying abscission of flowers and fruits
discovered the same compound.

27.  Functions of abscisic acid
• The abscisic acid stimulates the closure of stomata (water stress brings about an increase in ABA synthesis) (Figure 6.3).
• Involved in abscission of buds, leaves, petals, flowers, and fruits in many, if not all, instances, as well as in dehiscence of
fruits.
• Production is accentuated by stresses such as water loss and freezing temperatures.
• Involved in bud dormancy.
• Prolongs seed dormancy and delays germination (vivipary).
• Inhibits elongation.
• ABA is implicated in the control of elongation, lateral root development, and geotropism, as well as in water uptake and
ion transport by roots.
• ABA coming from the plastids promotes the metabolism of ripening.
• Promotes senescence.
• Can reverse the effects of growth stimulating hormones.