Abscisic acid

• IUPAC: (S) -5 - (1- hydroxy-2 ,6,6 -trimethyl -4-oxo -cyclohex -2 -en- 1-yl) -3-methyl - cis, trans- penta-2 ,4- dienoic acid
• Abscisic
• Dormin
• ABA

Yellowish solid

186 ° C

120 ° C.

Poorly soluble in water soluble in methanol, ethanol, diethyl ether, chloroform, 1M sodium hydroxide

Template: Infobox chemical / molecular formula search available

Abscisic acid or abscisic acid (deprecated Dormin ) is a phytohormone (plant hormones ) with generally inhibitory effect. Chemically it is one of the monocyclic sesquiterpenes. International common is the abbreviation of ABA Engl. abscisic acid.

ABA comes in higher plants, mosses, algae, fungi and cyanobacteria before, but not in other bacteria, archaea and liverworts.

Structure

In contrast to most other plant hormones abscisic acid is at a single substance and not a group of substances. Structurally, ABA is a monocyclic sesquiterpene. However, the biosynthesis is carried out from the cleavage product of a Tetraterpens ( see below).

Properties

Abscisic acid forms colorless crystals that are well and poorly soluble in water in organic solvents. Abscisic acid is very sensitive to light. Light exposure occurs a photochemical rearrangement to the physiologically ineffective trans isomer.

Biosynthesis

The herbal biosynthesis takes place mainly in leaves as a general stress response (especially in drought stress ). As a signal for synthesis usually serves a waste of cellular turgor. The synthesis generally follows the other tetraterpenes from isopentenyl pyrophosphate (IPP ) which over the Methylerythritolphosphatweg ( MEP pathway ) is formed in the chloroplasts of the leaves. Intermediate are formed during the synthesis of terpene xanthophylls zeaxanthin and violaxanthin. From them ABA is formed in the cytoplasm by cleavage and oxidation.

Inactivation of abscisic acid is carried out by oxidative degradation to Phaseinsäure and dihydro- Phaseinsäure. In addition to free abscisic acid found in plant tissue and their glucose esters and O- glucoside. These conjugates of abscisic acid are regarded as inactive transport and storage forms. Is transported in the xylem ABA largely through the phloem from the leaves to the root, small amounts reversed in the xylem from the root to the shoot.

Discovery history

The name abscisic acid based on the fact that the acid was found in high concentrations in discarded cotton leaves ( first described as a factor Abscisin II). For application on leaf and fruit stalks they actually causes the shedding ( abscission ). This effect is limited to a few plants and is not directly caused by the abscisic acid, but. Caused by one of its ethylene release Regardless, she was described as the dormancy -inducing factor Dormin until it turned out by chemical analysis that it was the same substance at Abscisin II and Dormin. However, the name Abscisin prevailed, though Dormin ( sleep of Latin dormire =) would have been functionally appropriate.

Effects

Abscisic acid generally acts antagonistically to other growth-promoting phytohormones and is therefore a natural growth inhibitor. Along with the growth-promoting auxins, gibberellins and cytokinins, abscisic acid regulates aging processes, leaf fall, flowering, fruit ripening, seed and bud dormancy, stomatal transpiration and other developmental processes of the plant.

General effects

The two primary effects of abscisic acid can be divided into two groups:

Induce and maintain dormancy of plant organs

• Seed dormancy: ABA is formed during late embryogenesis in the embryo. Preventing the immediate germination on the parent plant ( viviparous ) and induces the production of storage proteins and dehydrins (especially LEA proteins) for protection from osmotic stress. As an antagonist effect gibberellins ( GAs ) and lead to stratification for germination, where exclusively on the ratio of ABA to GA (similar differentiation processes the auxin / cytokinin ratio).
• Dormancy ABA also induces dormancy of new shoots of deciduous trees in winter. As an antagonist here gibberellins act which after vernalization in spring bolting trigger.

Responses to biotic and abiotic stress

• Osmotic stress ( drought stress, heat stress, salt stress ): restriction of transpiration (ABA induced by ion fluxes to stomatal closure, so that less water evaporates ) and limitation of photosynthesis. The phytohormone ABA induces this at the molecular level the opening of anion channels, which anions escape from the closed cell. Thereby, the positive membrane potential ( depolarization ), which in turn open potassium channels and output K ions flow out of the cell into the apoplast. The water flows out of the cell into the apoplast after, the turgor in the guard cells decreases and the stomata close. When drought stress (water stress), the stress-induced ABA concentration, water use efficiency correlates linearly with the water conductivity of the leaves while increasing.
• Cold Stress: causes dormancy ( buds slowed growth, forms protective cuticle ) and setting the primary and secondary growth (inhibition of cell division in the cambium )
• Plant pathogens

Signal transduction

Short-term physiological adaptations

• Stomataschluss:

When water stress, the pH rises in the apoplast of the mesophyll of about 6.3 to about 7.2. This dissociated abscisic acid with elimination of a proton to its anion. Due to its negative charge, the ABA - ion can diffuse through cell membranes much worse and less is thus received by the mesophyll cells. Instead, it binds to an increased G protein- coupled receptor in the cell membrane of the guard cells of stomata. This leads on the one hand for short-term opening of cellular calcium channels and depolarization by influx of Ca2 into the cell. On the other hand activates the G- protein, phospholipase C, and hence the liberation of inositol triphosphate (IP3 ), which in turn leads to the release of intracellular in the vacuole and the endoplasmic reticulum stored calcium into the cytoplasm. Both effects of the cytoplasmic Ca 2 levels, and thus the positive charge of the cell increases. Ca2 functions in plant cells as a second messenger. It inhibits proton pumps in the cell membrane (H - ATPases that normally active H out of the cell pump ) and further depolarization by the opening of other ion channels. Cl - and K and possibly Malat2 flows from. Osmotically coupled flows of water also. Therefore the decrease in the closed cell turgor, so that they close.

Longer-term differentiation

• Altered gene expression:

The intracellular signal transduction of ABA and gene activation is still largely unknown. Relatively well studied is the EM gene of wheat that contains, among other regulatory sequences an ABA - responsive element ( ABRE ). Among the long -term effects of ABA in the plant is one of an increase in the hydraulic conductivity of the roots, and increased root growth, while the other growth ( in the shoots, buds, leaves) is inhibited. In seedling ABA induced as mentioned above, the formation or deposition of storage materials and dehydrins.

In the protonema of mosses, ABA induces very specific the conversion of photosynthetically active cells to vegetative spores, called Brachycyten. Abscisic acid has a broad spectrum of activity, its mechanism of action is still unclear. The content of abscisic acid is the plant organ and its development state -dependent, but is on average about 100 micrograms per kg fresh weight.