1. Growth Hormones
Syed Abdullah Gilani

2. The Growth Hormone
In higher plants, regulation and coordination of metabolism, growth, and morphogenesis depend on chemical signals (hormones) from one part of the plant to another.
Chemical messengers, “hormones”, are responsible for the formation and growth of different plant organs.
Hormones are chemical messengers that are produced in one cell or tissue, and modulate cellular processes in another cell by interacting with specific protein receptors.

3. Most plant hormones are synthesized in one tissue and act on specific target sites in another tissue at vanishingly low concentrations.
Endocrine hormones: transported to sites of action in tissues distant from their site of synthesis.
Paracrine hormones: act on cells “sites” adjacent to the source “site” of synthesis.

4. Plant development is regulated by six major types of hormones:
auxins, gibberellins, cytokinins, ethylene, abscisic acid, and brassinosteroids.
Other signaling molecules that play roles in resistance to pathogens and defense against herbivores have also been identified in plants
jasmonic acid, salicylic acid, polypeptide systemin).
Auxin was the first growth hormone discovered in plants.

5. Auxins

6. Auxin and cytokinin differ from the other plant hormones in one important aspect: they are required for viability.
Whereas other plant hormones act as on/off switches that regulate specific developmental processes, auxin and cytokinin appear to be required at some level more or less continuously.

7. Darwins’ concept:
A signal is produced in the tip, travels to the growth zone, and causes the shaded side to grow faster than the illuminated side.
Major breakthrough by Went (1926):
The substance promoted the elongation of the coleoptile sections in the absence of the unilateral light source, it was named “auxin” from the Greek “auxein”, meaning “to increase” or “to grow”.

9. Biosynthesis and Metabolism of Auxin
The principal auxin in higher plants is Indole-3-Acetic Acid (IAA):
Auxins affect many developmental processes besides cell elongation.
Auxins can be defined as compounds with developmental biological activities similar to those of, or associated with, IAA.

10. Sites of auxin synthesis:
Biosynthesis of IAA is associated with rapidly dividing and rapidly growing tissues, especially in shoots.
Sites of auxin synthesis:
shoot apical meristems
young leaves
root apical meristems
young fruits and seeds

11. Auxin Transport
The apex-base polarity of shoots and roots is dependent on the polarity of auxin transport.
Basipetal transport:
In excised oat coleoptile sections, IAA moves mainly from the apical to the basal end, (basipetally), i.e. unidirectional transport called “polar transport”.
Auxin is the only plant growth hormone known to be transported polarly.
Acropetal transport
In roots, acropetal transport of auxin is towards the tip, occurs in the phloem.

12. General Properties of Polar IAA Transport
Tissues differ in the degree of polarity of IAA transport.
Polar transport proceeds in a cell-to-cell fashion
The velocity of polar auxin transport ranges from 2 to 20 cm/h.
The major sites of polar auxin transport in stems, leaves, and roots is the vascular parenchyma tissue (xylem).

13. Gibberellins

14. Gibberellins: Regulators of Plant Height
Plant growth and development are regulated by several hormones acting both individually and in concert.
Gibberellins (GAs) were the second group of plant hormones to be characterized.
At least 136 naturally occurring GAs have been identified.
The GAs all share a similar chemical structure but few of them have biological activity.

15. In any plant, there are only a few bioactive GAs, and their levels are correlated with stem length.
Gibberellins are best known for their promotion of stem elongation.
GA-deficient mutants have dwarf phenotypes.

16. 1- Discovery and Chemical Structure
GAs were discovered in the 1930s in Japan.
A fungal disease (termed “foolish seedling”) caused rice plants to grow too tall and eliminated seed production
induced by a chemical secreted by a pathogenic fungus, Gibberella fujikuroi, that infected the plants.
Chemical isolated from the fungus called gibberellin
gibberellin A and B were identified.

17. At about the same time, Japanese scientists isolated and identified
In the 1950s, Britain & US researchers purified a compound from Gibberella and named it gibberellic acid.
At about the same time, Japanese scientists isolated and identified
gibberellin A1 (GA1)
gibberellin A2 (GA2), and
gibberellin A3 (GA3) – GA3 was identical to gibberellic acid.

18. GA3 showed spectacular responses in the elongation growth of dwarf and rosette plants.
Do plants contain their own endogenous GA?
In 1958, GA1 was the first GA to be identified from a plant extract, from the bean, Phaseolus coccineus.

19. 2- Effects of GAs on Growth and Development
Endogenous GAs influence a wide variety of developmental processes:
stem elongation, seed germination including the loss of dormancy and the mobilization of endosperm reserves.
In reproductive development, GA can affect the
transition from the juvenile to the mature stage,
floral initiation,
sex determination, and
fruit set (initiation of fruit growth following pollination).

20. Commercial applications of Gibberellins
The major uses of gibberellins (GA3), applied as a spray or dip, are:
to manage fruit crops,
to malt barley, and
to increase sugar yield in sugarcane.
In some crops a reduction in height is desirable, and this can be accomplished by the use of gibberellin synthesis inhibitors.