lecture 5: PLANT HORMONES

Plant hormones

are small molecules that regulate plant growth

and development at extremely low concentrations

Julius von Sachs

higher plants, regulation and coordination of metabolism, growth, and morphogenesis often depend on chemical signals from one part of the plant to

another.

- This idea originated in the nineteenth century with the German botanist

Sachs

proposed that chemical messengers are responsible for the formation and

growth of different plant organs.

He also suggested that external factors such as gravity could affect the

distribution of these substances within a plant

hormones

Plants also produce signaling molecules known as. , which significantly

influence development even at extremely low concentration

auxins, gibberellins, cytokinins, ethylene, and abscisic acid

Plant development was thought to be regulated by only five types of hormones:

sunlight, water, oxygen, minerals

What do plants need for their growth and development?

Phytohormones

intrinsic factors that regulate the growth and development of plants. These are called plant hormones or

Phytohormones

are chemical compounds present in very low concentrations in plants

- regulate plant development, growth, longevity and reproductive

processes

Plant Hormones

are crucial in processes such as vernalization, phototropism, seed

germination, dormancy, and more, interacting with external factors

Synthetic plant hormones

are also used externally to regulate and optimize crop

production

Plant hormones control all the growth and development activities like

- cell division

- enlargement

- flowering

- seed formation

- dormancy and abscission

Main functions of plant hormones:

1. Plant Growth Promoters

2. Plant Growth Inhibitors

Based on their action, plant hormones are categorized into two categories:

"to grow"

Auxin means

Auxin

They are widely used in agricultural and horticultural practices. They are found in growing apices of roots

and stems and then migrate to other parts to act

Auxin

First growth hormone to be discovered in plants

phototropism

Charles Darwin and his son Francis studied plant growth phenomena involving tropisms. One of their interests was the bending of plants toward light. This phenomenon, which is caused by differential growth, is called

canary grass (Phalaris

canariensis)

Darwins used seedlings of

coleoptile

the youngest leaves are sheathed in

a protective organ called the

coleoptile

are very sensitive to light, especially to blue light. If illuminated on one side

with a short pulse of dim blue light, they will bend (grow) toward the source of the light

pulse within an hour

growth zone

The region of the coleoptile that is

responsible for the bending toward the light, called the

Frits Went

This research culminated in the demonstration in 1926 by of the presence of

a growth-promoting chemical in the tip of oat (Avena sativa) coleoptiles

"to increase" or "to grow

auxin from the Greek auxein

indole-3- acetic acid (IAA)

mid-1930s it was determined that auxin is

IAA biosynthesis

associated with rapidly dividing and rapidly growing tissues,

especially in shoots

shoot

apical meristems,

young leaves,

and developing fruits and seeds

are the primary sites of IAA synthesis

stem elongation,

apical dominance,

wound healing,

and leaf senescence

longitudinal gradient of auxin from the shoot to the root affects various

developmental processes, including

Phloem

a significant amount of auxin transport also occurs

in the

Phloem

probably the principal route by which auxin is

transported acropetally (i.e., toward the tip) in the root. Thus, more than one pathway is

responsible for the distribution of auxin in the plant

1) Phototropism, 2) Gravitropism, 3) Thigmotropism

3 main guidance systems that controls the orientation of plant growth

Phototropism

This is growth with respect to light, which is expressed in all shoots and some roots

This guidance system ensures that leaves will receive optimal sunlight for

photosynthesis

Gravitropism

This is growth in response to gravity

This guidance system enables roots to grow downward into the soil and shoots to grow

upward away from the soil, which is especially critical during the early stages of

germination

Thigmotropism

This is growth with respect to touch

This guidance system enables roots to grow around rocks and is responsible for the ability of the shoots to climbing plants to wrap around other structures for support

gibberellins

They are acidic in nature. These are found in higher plants and fungi.

There are more than 100

1950s

In the the second group of hormones, the gibberellins (GAs), was characterized

Gibberellins

large group of related compounds (more than 125 are known)

that, unlike the auxins, are defined by their chemical structure rather than by their

biological activity

are most often associated with the promotion of stem growth, and the

application of gibberellin to intact plants can induce large increases in plant height

foolish seedling,"

or bakanae, disease

Rice farmers in Asia had long known of a disease that makes the rice plants grow tall but eliminates seed production. In Japan this disease was called the

Gibberella fujikuroi

▪ Plant pathologists investigating the disease found that the tallness of these plants was

induced by a chemical secreted by a fungus that had infected the tall plants. This

chemical was isolated from filtrates of the cultured fungus and called gibberellin after

, the name of the fungus

the mature stage, as well as floral initiation, sex determination, and fruit set.

In reproductive development, gibberellin can affect the transition from the juvenile to

genes

In plants with unisexual flowers instead of hermaphroditic ones, the determination of

flower sex is controlled by.

1. the activation of vegetative growth of the embryo

2. the weakening of growth-constraining endosperm layer surrounding the embryo

3. the mobilization of stored food reserves in the endosperm

Seed germination may require gibberellins for one of several possible steps

increase the stalk length of seedless grapes.

Because of the shortness of the individual fruit stalks, bunches of seedless

grapes are too compact and the growth of the berries is restricted

A major use of gibberellins is to

Gibberellin

is sometimes used to speed up the malting process

malt

The germinated seeds are then dried and pulverized to produce "," consisting mainly of a mixture of amylolytic (starch-degrading) enzymes and partly digested

starch

Sugarcane (Saccharum officinarum)

one of relatively few plants that store their

carbohydrate as sugar (sucrose) instead of starch (the other important sugar-

storing crop is sugar beet)

Sucrose

is stored in the central vacuoles of the internode parenchyma cells.

Cytokinins

play an important role in cytokinesis process. are naturally

synthesized in the plants where rapid cell division occurs e.g. root apices, shoot buds,

young fruits, etc. Movement of cytokinins is basipetal and polar.

cytokinins

discovered in the search for factors that stimulate plant cells to

divide (i.e., undergo cytokinesis)

Cytokinins

appear to mediate many aspects of light-regulated development,

including chloroplast differentiation, the development of autotrophic metabolism, and

leaf and cotyledon expansion

1. Cytokinins modify apical dominance and promote lateral bud growth

2. Cytokinins delay leaf senescence

3. Cytokinins induce bud formation in a moss

4. Cytokinins promote movement of nutrients

5. Cytokinins promote chloroplast development

6. Cytokinins promote cell expansion in leaves and cotyledons

7. Cytokinins regulate growth of stems and roots

FUNCTIONS OF CYTOKININS

exhibit several characteristics that point to roles

played by cytokinin in plant physiology and development:

1. The shoot apical meristems of cytokinin-overproducing plants produce more

leaves

2. The leaves have higher chlorophyll levels and are much greener.

3. Adventitious shoots may form from unwounded leaf veins and petioles.

4. Leaf senescence is retarded.

5. Apical dominance is greatly reduced.

6. The more extreme cytokinin-overproducing plants are stunted, with greatly

shortened internodes.

7. Rooting of stem cuttings is reduced, as is the root growth rate.

These cytokinin-overproducing plants

Ethylene

- It acts as a growth promoter as well as an inhibitor.

- Occurs in gaseous form.

- It is synthesized in the ripening fruits and tissues undergoing senescence.

- It regulates many physiological processes and one of the most widely used hormones in agriculture.

H.H. Cousins in 1910.

The first indication that ethylene is a natural product of plant tissues was published by

Ethylene

can be produced by almost all parts of higher plants, although the rate of

production depends on the type of tissue and the stage of development. In general,

meristematic regions and nodal regions are the most active in ethylene biosynthesis.

flooding, chilling, disease, and

temperature or drought stress.

during leaf abscission and flower

senescence, as well as during fruit ripening. Any type of wounding can induce ethylene

biosynthesis, as can physiological stresses such as

amino acid methionine

is the precursor of ethylene, and ACC (1-aminocyclopropane-1-carboxylic acid) serves as an intermediate in the conversion of methionine to ethylene

Gymnosperms and lower plants, including ferns, mosses, liverworts, and certain

cyanobacteria

all have shown the ability to produce ethylene

Ethylene biosynthesis

stimulated by several factors, including developmental state,

environmental conditions, other plant hormones, and physical and chemical injury

climacteric

All fruits that ripen in response to ethylene exhibit a characteristic respiratory rise before

the ripening phase called a.

non-climacteric

In contrast, fruits such as citrus fruits and grapes do not exhibit the respiration and

ethylene production rise and are called

C l i m a c t e r i c

A p p l e

A v o c a d o

B a n a n a

C a n t a l o u p e

C h e r i m o y a

Fig

M a n g o

O l i v e

P e a c h

P e a r

P e r s i m m o n

P l u m

T o m a t o

N o n c l i m a c t e r i c

Bell pepper

C h e r r y

C i t r u s

G r a p e

P i n e a p p l e

S n a p b e a n

S t r a w b e r r

abscisic acid

It is a growth-inhibiting hormone. ABAs act as an antagonist to GAs. It inhibits

plant metabolism and regulates abscission and dormancy.

"stress hormone" as it increases the tolerance of plants.

Abscisic acid is also called

dormin

These early experiments led to the identification of a group of growth-inhibiting

compounds, including a substance known as purified from sycamore leaves

collected in early autumn, when the trees were entering dormancy.

abscisic acid (ABA)

that dormin was chemically identical to a substance that promotes the

abscission of cotton fruits, abscisin II, the compound was renamed

Ethylene

the hormone that triggers abscission and that ABA-induced

abscission of cotton fruits is due to ABA's ability to stimulate ethylene production.

Abscisic acids can

plays primary regulatory roles in the initiation and maintenance of seed

and bud dormancy and in the plant's response to stress, particularly water stress.

ABA

influences many other aspects of plant development by interacting,

usually as an antagonist, with auxin, cytokinin, gibberellin, ethylene, and

brassinosteroids

Embryogenesis

ABA levels in seeds peak during

ABA

is synthesized in almost all cells

that contain plastids and is transported via both the xylem and the phloem

ABA

promotes desiccation tolerance in the embryo

promotes the accumulation of seed storage protein during embryogenesis