1. Introduction to Plant Hormone Biochemistry
Plant hormones (phytohormones) are low-concentration signaling molecules
that regulate growth, development, and stress responses.
Their action depends on biosynthesis pathways, receptor recognition,
and downstream signal transduction cascades.
Major hormone classes include:
- Auxins
- Gibberellins (GA)
- Cytokinins
- Abscisic acid (ABA)
- Ethylene
- Brassinosteroids
- Jasmonates
- Salicylic acid
2. Hormone Biosynthesis Pathways
Auxin (IAA)
Synthesized mainly from tryptophan via the IPA (Indole-3-pyruvic acid) pathway.
Gibberellins
Derived from the terpenoid pathway via geranylgeranyl diphosphate (GGDP).
Abscisic Acid (ABA)
Synthesized from carotenoid cleavage in plastids.
Ethylene
Formed from methionine via S-adenosylmethionine (SAM) and ACC.
3. Signal Perception and Receptor Mechanisms
Hormones are perceived by specific receptors, triggering
molecular signaling cascades.
Examples
- Auxin receptor: TIR1 (F-box protein)
- Gibberellin receptor: GID1
- ABA receptor: PYR/PYL/RCAR family
- Ethylene receptor: ETR1
Hormone binding often results in:
- Protein degradation (ubiquitin-proteasome pathway)
- Activation of transcription factors
- Modulation of gene expression
4. Signal Transduction Cascades
Secondary Messengers
- Ca²⁺ signaling
- Reactive oxygen species (ROS)
- cGMP
- IP3
Protein Phosphorylation
Kinase cascades (MAPK pathways) transmit signals
from receptors to nucleus.
Transcriptional Regulation
Activation or repression of hormone-responsive genes
controls physiological outcomes.
5. Hormonal Crosstalk
Hormones rarely act independently. Crosstalk integrates signals:
- Auxin–cytokinin balance in root/shoot development
- ABA–ethylene interaction in stress response
- Jasmonate–salicylic acid antagonism in defense
Understanding crosstalk is essential for applied crop management.
6. Applied Relevance in Horticulture
- Rooting hormones (IAA, IBA)
- Fruit ripening control (ethylene management)
- Seed dormancy breaking (GA treatment)
- Drought tolerance via ABA regulation
- Flower induction control
Biochemical manipulation of hormone pathways is central
to modern horticultural production systems.
7. Laboratory Applications
- IAA quantification via HPLC
- Ethylene measurement using gas chromatography
- Gene expression analysis (RT-PCR) of hormone-responsive genes
- Calcium imaging in stress signaling studies
8. Open Learning Resources