Abscisic acid (ABA)

Unlike animals, plants cannot flee from potentially harmful conditions like

• drought
• freezing
• exposure to salt water or salinated soil.

They must adapt or die.

The plant hormone abscisic acid (ABA) is the major player in mediating the adaptation of the plant to stress.

Here are a few examples.

1. Closing of stomata

In angiosperms and gymnosperms (but not in ferns and lycopsids), ABA is the hormone that triggers closing of the stomata when soil water is insufficient to keep up with transpiration.

• ABA binds to receptors at the surface of the plasma membrane of the guard cells.
• The receptors activate several interconnecting pathways which converge to produce
  • a rise in pH in the cytosol;
  • transfer of Ca²⁺ from the vacuole to the cytosol.
• These changes
  • stimulate the loss of negatively-charged ions (anions), especially NO₃⁻ and Cl⁻, from the cell and also
  • the loss of K⁺ from the cell.
• The loss of these solutes in the cytosol reduces the osmotic pressure of the cell and thus turgor.
• The stomata close.

2. Protecting cells from dehydration

ABA signaling turns on the expression of genes encoding proteins that protect cells — in seeds as well as in vegetative tissues — from damage when they become dehydrated.

3. Root growth

ABA stimulates root growth in the direction of soil moisture, a phenomenon called hydrotropism.

4. Bud dormancy

ABA mediates the conversion of the apical meristem into a dormant bud. The newly developing leaves growing above the meristem become converted into stiff bud scales that wrap the meristem closely and will protect it from mechanical damage and drying out during the winter.

ABA in the bud also acts to enforce dormancy so if an unseasonably warm spell occurs before winter is over, the buds will not sprout prematurely. Only after a prolonged period of cold or the lengthening days of spring (photoperiodism) will bud dormancy be lifted.

5. Seed maturation and dormancy

Seeds are not only important agents of reproduction and dispersal, but they are also essential to the survival of annual and biennial plants. These angiosperms die after flowering and seed formation is complete. ABA is essential for seed maturation and also enforces a period of seed dormancy. As we saw for buds, it is important the seeds not germinate prematurely during unseasonably mild conditions prior to the onset of winter or a dry season. ABA in the seed enforces this dormancy. Not until the seed has been exposed to a prolonged cold spell and/or sufficient water to support germination is dormancy lifted.

6. Abscission

ABA also promotes abscission of leaves and fruits (in contrast to auxin, which inhibits abscission). It is, in fact, this action that gave rise to the name abscisic acid.

The dropping of leaves in the autumn is a vital response to the onset of winter when ground water is frozen — and thus cannot support transpiration — and snow load would threaten to break any branches still in leaf.

7. Seedling growth

ABA inhibits stem elongation probably by its inhibitory effect on gibberellic acid.

8. Apical dominance

ABA — moving up from the roots to the stem — synergizes with auxin — moving down from the apical meristem to the stem — in suppressing the development of lateral buds. The result is inhibition of branching or apical dominance.