It is synthesized in plastids from carotenoids and diffuses in all directions through vascular tissues and parenchyma. The transport of ABA can occur in both xylem and phloem tissues. It can also be translocated through parenchyma cells. The movement of abscisic acid in plants does not exhibit polarity like auxins. ABA is capable of moving both up and down in the stem. ABA promotes leaf senescence independent of ethylene and inhibits gibberellin induced enzyme production.
The ratio of ABA and gibberellins control seed dormancy.
Biosynthesis and metabolism
ABA is a naturally occurring compound in plants. It is a sesquiterpenoid (15-carbon), which is partially produced via the mevalonic pathway in chloroplasts and other plastids. It is synthesized partially in the chloroplasts and the biosynthesis primarily occurs in the leaves. The production of ABA is accentuated by stresses such as water loss and freezing temperatures. It is believed that biosynthesis occurs indirectly through the production of carotenoids. Carotenoids are pigments produced by the chloroplast, which have 40 carbons. Breakdown of these carotenoids occurs by the following mechanism. Violaxanthin is a carotenoid, which has forty carbons. It is isomerized and then split via an isomerase reaction followed by an oxidation reaction. One molecule of xanthonin is produced from one molecule of violaxanthonin and it is uncertain what happens to the remaining byproduct. The one molecule of xanthonin produced is unstable and spontaneously changed to ABA aldehyde. Further oxidation results in ABA. Activation of the molecule can occur by two methods:
i. In the first method an ABA-glucose ester can form by attachment of glucose to ABA.
ii. In the second method oxidation of ABA can occur to form phaseic acid and dihydrophaseic acid.
Role of abscisic acid (ABA)
- It plays role in inhibition of cell growth.
- ABA amount increases in developing seeds and promotes dormancy.
- If leaves experience water stress, ABA amounts increase immediately, causing the stomata to close.
- It induces seeds to synthesize storage proteins and inhibits the affect of gibberellins on stimulating de novo synthesis of α-amylase.
- It induces gene transcription especially for proteinase inhibitors in response to wounding which may explain an apparent role in pathogen defense.
Abscisic acid (ABA) is a sesquiterpene (15-carbon). It is synthesized almost in all cells that contain chloroplast and other plastids. The pathway of ABA begins with isopentenyl pyrophosphate (IPP) and leads to the synthesis of violaxanthin. Violaxanthin is converted to the neoxanthin, which is then cleaved to form ABA aldehyde. Transport of the ABA occurs through both xylem and phloem and also through parenchyma cells outside vascular bundles.
ABA brings about reduction in stomatal aperture and stomatal opening is also inhibited. Stomatal closure is driven by a reduction in guard cell turgor pressure caused by a large efflux of K+ and anions from the cell. By increasing cytosolic calcium concentration, ABA inhibits stomatal opening. Elevations in the concentration of cytosolic calcium stimulated by ABA by inducing both influx through plasma membrane channels and release of calcium into the cytosol from internal compartments, such as the central vacuole. Anion channels are activated by increased cytosolic calcium. Activation of anion channels causes an anion release from guard cells, which causes depolarization of the membrane. K+in channels deactivated by charges in membrane potential and activates K+out channels, resulting in K+ efflux from guard cells. K+ channels that open only at more negative potentials are specialized for inward diffusion of K+ and are known as inward-rectifying or simply inward, K+ Channels. Conversely, K+ channels that open only at more positive potentials are outward-rectifying or outward, K+ channels.
In addition, an increase in pH of the guard cell‟s cytosol caused by ABA which directly enhances K+out channel activity. The sustained efflux of both anions and K+ from guard cells via anion and K+out channels contributes to loss of guard cell turgor, which leads to stomatal closing. Proton-extruding H+-ATPases present on the plasma membrane drives stomatal opening. H+-ATPases can drive K+ uptake via K+in channels. Cytosolic calcium elevation in guard cells in the presence of ABA down-regulate both K+in channels and plasma membrane H+ -ATPases. This explains the mechanistic basis of stomatal opening inhibition in the presence of ABA.
The effects of ABA on stomatal apertures vary under red and blue light. Blue light-stimulated stomatal opening in a concentration-dependent fashion inhibited by increasing ABA concentrations, but there is no effect on red light-stimulated opening. These contrasting response to blue and red light can be explained by the effect of ABA on guard cell osmoregulation. ABA concentrations have been shown to inhibit proton pumping and potassium uptake, which are central to blue light-stimulated stomatal opening. On the other hand, guard cell photosynthesis and sucrose accumulation stimulated by red light and this osmoregulatory pathway appears to be insensitive to ABA.
Roots and Shoots growth On the roots and shoots growth, ABA has different effects and the effects are strongly dependent on the water status of the plant. When ABA levels are high under low water potential, a strong positive effect on root growth is exerted by the endogenous hormone by suppressing ethylene production and a negative effect on shoot growth. To reduce shoot growth only under water stress conditions, endogenous ABA acts as a signal.
Dormancy and germination For the development of desiccation tolerance in the developing embryo ABA is required, the synthesis of storage proteins and the acquisition of dormancy. Germination is inhibited by the high levels of ABA in maturing seeds. When ABA is removed or inactivated, many types of dormant seeds are germinated. Often, the ratio of ABA to gibberellins determines whether the seed remains dormant or germinates. The GA-dependent hydrolytic enzyme synthesis (such as α-amylase) is inhibited by ABA that is essential for the breakdown of storage reserves in seeds. Although less is known about the role of ABA in buds dormancy, ABA is one of the inhibitors that accumulate in dormant buds.
Vivipary Precocious germination and vivipary may exhibit by ABA-deficient embryos. Inactivated ABA or low levels of ABA can lead to precocious germination. For example, a maize mutant with grains that germinate while still on the cob lacks a functional transcription factor required for ABA to induce expression of certain genes. Precocious germination of red mangrove seeds, due to low ABA levels, is actually an adaptation that helps the young seedlings to plant themselves in the soft mud below the parent tree.
Hydraulic conductivity The hydraulic conductivity increases by ABA (decreasing the resistance to water movement across the membrane) and ion flux of root in response to water stress.