Is an inhibitor of growth with retardant action. It is present in all plant tissues. Abscisic acid is an antagonist of auxin, gibberellins and cytokinins.
It accumulates in large quantities during autumn, in the period of transition to a state of latency, particularly in reaction to stress.
Has the ability to form glycoside by interacting with glucose, thus representing an inactive non-toxic form. ABA is also inactivated by hydroxylation in the endoplasmic reticulum.
Biosynthesis
Is synthesized in mature leaves and fruits. ABA biosynthesis occurs via two pathways.
• from mevalonic acid →isopentenyl pyrophosphate →heranylpirophosphate;
• by carotenoid and violaxanthine decomposition →xanthoxin →abscisic acid.
It was found that induction of ABA synthesis occurs during genome reprogramming and synthesis of increased amounts of ABA-inducing polypeptides, of which lectins are more significant (especially agglutinins in wheat).
Transport
ABA is transported from its synthesis location through the phloem and xylem to the parenchyma, the descendent flow being three times greater than the ascendant.
Mechanism of action
The primary action is carried in the membranes of target cells which contain protein receptors specific for ABA, influencing the lipid phase of the membranes, causing degradation of hydrocarbon bonds and of membrane hydrophilic groups.
ABA influences the level of calcium in the cytosol by opening calcium channels (calcium is a secondary messenger in phytohormone signaling).
ABA mechanisms of action determine plant response reactions:
• immediate or late.
The most studied immediate response is stomatal closure induced by ABA, by altering the concentration of CO2 in intercellular spaces and the turgor pressure. ABA slows potassium ion accumulation by inhib-iting the corresponding proton pumps in the plasmalemma (Fig. ). It has an important influence on water transport, increasing membrane permeability.
The late response is represented by the influence of endogenous ABA on transcription of genes into mRNA and cytoplasmic protein synthesis. Among the reactions of late response to ABA content increase under stress conditions are phenomena like RNA and protein synthesis inhibition.
In a concentration of 1–10 μM ABA reduces RNA polymerase activity by 22–38 % in active chromatin, blocks the synthesis of α-amylase and protease, alters the activity of nuclear and cytoplasmic protein kinase, chloroplast differentiation, synthesis of chlorophyll and enzymes photosynthetic apparatus .
In isolated chloroplasts ABA and cytokinins are inactive therefore we can say that their action is determined by the nuclear genome.
Biological significance
ABA has the following properties:
- contributes to the distribution of metabolites in plants, in case of repeating stress
- In wheat and soybean, stimulates the movement of photosynthesis products from the leaves into the ear
- Accumulates sucrose in seeds, sweet fruits, reserve tissues of the roots
- Promotes leaf detachment by forming the separation layer between the petiole and the peduncle
- Induces the synthesis of shock proteins and of certain osmotins and dehydrins
- Inhibits seed germination and stem growth;
- Contributes to oriented movements of the root in the soil
- It has anti-gibberellin, anti-auxin, anti-quinine action
- Maintains plant dormancy (latency)
- Increases plant resistance to stress factors
- Prevents seed germination in the fruit.
Practical applications
It is used to control the number of flowers and fruits per unit of surface to achieve the optimum yield in terms of quality and quantity.