Hormonal Signalling in Plants (ABA, Auxin, Ethylene)

What plant hormones are

Plant hormones are signalling molecules that coordinate growth, development, and stress responses.

They operate at very low concentrations and act as information carriers, not nutrients.

---

Abscisic Acid (ABA)

Role

ABA is the primary stress hormone in plants.

It is involved in: - Stomatal closure - Drought response - Root-to-shoot stress signalling - Growth suppression under adverse conditions

Agronomic significance

• ABA rises rapidly under water stress, salinity, high VPD, or root oxygen limitation
• Elevated ABA reduces transpiration and photosynthesis
• Effects can persist after stress conditions improve

ABA explains why plants may remain “closed down” even after irrigation or cooling.

---

Auxins

Role

Auxins regulate: - Cell elongation - Apical dominance - Root initiation - Directional growth responses

Auxins help define where growth happens.

Agronomic significance

• Auxin distribution controls root architecture
• Stress disrupts auxin transport
• Imbalanced auxin signalling leads to uneven growth, poor rooting, and abnormal morphology

---

Ethylene

Role

Ethylene is a gaseous hormone involved in: - Senescence - Fruit ripening - Stress responses - Growth inhibition under oxygen stress

Agronomic significance

• Ethylene accumulates under poor aeration and high stress
• Promotes leaf ageing and abscission
• Can suppress root and shoot growth

Ethylene often increases when roots experience hypoxia or compaction.

---

Hormonal cross-talk

Plant hormones do not act independently: - ABA interacts with auxin to suppress growth under stress - Ethylene modifies auxin sensitivity - Combined signals determine whether growth continues or pauses

Key concept

Hormones explain why plants prioritise survival over growth when stressed.