Lecture # 25 Development II: Hormones [Slides 1 , 2]

CONCEPTS:

1. Hormones are chemical signals that facilitate intercellular communication.
    Act at very low conc.
    Made in certain tissues and transported to target tissues.
2. Levels of hormones are regulated.
    By     Rate of biosynthesis
              Rate of degradation, and
             Transport
Life time of chemical messenger is limited.
3. Environmental signals cause changes in hormonal balance.

Growth Regulation by 6 Major Hormones
6 types of hormones:
1. Auxin:                     Stimulates cell elongation
2. Gibberellin (GA):      Broad functions , e.g.
                                    cell elongation, germination
3. Cytokinins:  induce cell division
4. Abscisic Acid [ABA]: suppresses growth, "stress" hormone
5. Ethylene:  causes fruit ripening, leaf abscission.
6. Brassinosteroid: stimulates cell elongation.

New Hormones:
Jasmonic Acid (JA)
Salicylic Acid
Systemin

Balance of hormones regulate growth & Development.
    i.e. ratio of hormones
    concentration of hormones

Environmental factors regulate hormone levels. e.g.

A. Phototropism- Growth of shoot towards light is due to changes in hormone levels.
B. Gravitropism- Root grows down and shoot grows up. 
C.  Drought causes increase in ABA.

MODE OF ACTION OF HORMONES

1. Signal perception by a receptor
2. Signal transduction and amplification
3. Final response
    a) Regulate activity of a protein
    b) Regulate gene expression (activation or repression) by  transcription factors
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BSCI 442