Abscisic acid (ABA)

Unlike animals, plants cannot flee from potentially harmful conditions like

• drought
• the approach of winter

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. 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.

2. 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 plays a role in seed maturation, at least in some species, 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.

3. 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.

4. Closing of stomata

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.
• This initiates
  • a rise in pH in the cytosol
  • the formation of the "second messenger", cyclic ADP ribose (cADPR)
• Increased pH stimulates the loss of K⁺ and organic ions from the cell
• Rising levels of cADPR cause Ca²⁺ to move from the vacuole to the cytosol which blocks the uptake of K⁺ into the guard cell
• The combined effects result in a loss of solutes in the cytosol.
• This reduces the osmotic pressure of the cell and thus turgor.
• The stomata close.