Plant hormones- Auxins & Gibberellins

1. PLANT HORMONES
Auxins & Gibberellins

2. PLANT HORMONES
In all plants, there occur in minute quantities of certain
substances (plant growth regulators or phytohormones) which
REGULATE GROWTH AND DIFFERENTIATION.
Five major types of growth substances are recognized:
• Auxins
• Gibberellins (both concerned with cell enlargement and differentiation)
• Cytokinins (concerned with cell division);
• Abscisic acid (with resting states like lateral buds);
• Ethylene (with senescence-ageing),

3. 1. Auxins
The auxins are weak organic acids with the
acidic group positioned at the end of the side chain
attached to an unsaturated ring(s) system. Presently,
Indole acetic acid (IAA) and other natural or synthetic
growth-regulating substances having structures and
functions similar to IAA, are termed auxins.

4. SITES OF BIOSYNTHESIS
IAA is synthesized from tryptophan or indole
primarily in leaf primordia and young leaves, and
in developing seeds.
TRANSPORT
IAA transport is cell to cell, mainly in the
vascular cambium and the procambial strands,
but probably also in epidermal cells.
Transport to the root probably also involves
the phloem.

5. EFFECTS
CELL ENLARGEMENT – auxin stimulates cell
enlargement and stem growth.
CELL DIVISION - auxin stimulates cell division in the
cambium and, in combination with cytokinin, in tissue
culture.
VASCULAR TISSUE DIFFERENTIATION - auxin
stimulates differentiation of phloem and xylem.
ROOT INITIATION - auxin stimulates root initiation on
stem cuttings, and also the development of branch roots
and the differentiation of roots in tissue culture .

6. TROPISTIC RESPONSES - auxin mediates
the tropistic (bending) response of shoots and
roots to gravity and light .
APICAL DOMINANCE - the auxin supply
from the apical bud represses the growth of
lateral buds.
LEAF SENESCENCE - auxin delays leaf
senescence. ! Leaf and fruit abscission - auxin
may inhibit or promote (via ethylene) leaf and
fruit abscission depending on the timing and
position of the source

7. FRUIT SETTING AND GROWTH - auxin induces
these processes in some fruit
ASSIMILATE PARTITIONING - assimilate
movement is enhanced towards an auxin source possibly
by an effect on phloem transport .
FRUIT RIPENING - auxin delays ripening.
FLOWERING - auxin promotes flowering in
Bromeliads.
GROWTH OF FLOWER PARTS - stimulated by
auxin. Promotes femaleness in dioecious flowers (via
ethylene).

8. 2. Gibberellins
Gibberellins are synthesized in the apices of
young leaves and roots and are transported
through xylem.
Anti-giberellins are synthetic compounds
which interfere with the synthesis of gibberellins
in the plant body, hence called growth retardants
e.g. Maleic hydrazide, Phosphon D,
Chlorocholine chloride (CCC).

10. SITES OF BIOSYNTHESIS
GAs are synthesized from glyceraldehyde-3-
phosphate, via isopentenyl diphosphate, in young
tissues of the shoot and developing seed.
Their biosynthesis starts in the chloroplast
and subsequently involves membrane and
cytoplasmic steps.

11. TRANSPORT
Some GAs are probably transported in the
phloem and xylem. However the transport of the
main bioactive polar GA1 seems restricted.

12. EFFECTS
STEM GROWTH –
GA1 causes hyperelongation of stems by
stimulating both cell division and cell elongation.
This produces tall, as opposed to dwarf,
plants.
BOLTING IN LONG DAY PLANTS –
GAs cause stem elongation in response to
long days

13. INDUCTION OF SEED GERMINATION –
GAs can cause seed germination in some seeds that
normally require cold (stratification) or light to induce
germination.
ENZYME PRODUCTION DURING GERMINATION –
GA stimulates the production of numerous enzymes,
notably α-amylase, in germinating cereal grains.

14. FRUIT SETTING AND GROWTH –
This can be induced by exogenous
applications in some fruit (e.g., grapes). The
endogenous role is uncertain.
Induction of maleness in dioecious flowers .

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