Abscisic acid (ABA)

Unlike animals, plants cannot flee from potentially harmful conditions like

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

Some 90% of the water taken up by a plant is lost in transpiration. Most of this leaves the plant through the pores — called stomata — in the leaf. Each stoma is flanked by a pair of guard cells. When the guard cells are turgid, the stoma is open. When turgor is lost, the stoma closes.
Discussion of gas exchange in the leaf.

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.

The mechanism:

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.

Most nondeciduous species in cold climates (e.g., pines) have "needles" for leaves. These are very narrow and have a heavy waterproof cuticle. The shape aids in shedding snow, and the cuticle cuts down on water loss.

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.

Other plant hormones
Auxin Brassinosteroids Cytokinins Ethylene Gibberellins Jasmonates Strigolactones
As you read about these various hormones, you will note that: (1) each hormone affects several, or even many, different processes and
(2) each process is, in turn, influenced by several different hormones.
How these overlapping signals are integrated to produce a particular response remains a topic of active research.

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4 January 2019