Abscisic acid

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Structural formula
Structural formula of (+) - abscisic acid
Structure of (+) - abscisic acid
General
Surname Abscisic acid
other names
  • ( S ) -5- (1-Hydroxy-2,6,6-trimethyl-4-oxo-cyclohex-2-en-1-yl) -3-methyl- cis , trans -penta-2,4-dienoic acid ( IUPAC )
  • Abscisic acid
  • Dormin
  • ABA
Molecular formula C 15 H 20 O 4
Brief description

yellowish solid

External identifiers / databases
CAS number
  • 21293-29-8 (+) - abscisic acid
  • 14375-45-2 (±) -abscisic acid
EC number 244-319-5
ECHA InfoCard 100.040.275
PubChem 5280896
ChemSpider 4444418
Wikidata Q332211
properties
Molar mass 264.32 g mol −1
Physical state

firmly

Melting point

186 ° C

Sublimation point

120 ° C

solubility
safety instructions
GHS labeling of hazardous substances
no GHS pictograms
H and P phrases H: no H-phrases
P: no P-phrases
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Abscisic acid or abscisic acid (outdated dormin ) is a phytohormone (plant hormone) with a general inhibitory effect. Chemically, it is one of the monocyclic sesquiterpenes . The abbreviation ABA from English is in international use. from scisic a cid .

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

structure

In contrast to most other phytohormones, abscisic acid is a single substance and not a group of substances. In terms of structure, ABA is a monocyclic sesquiterpene . However, biosynthesis takes place from the cleavage product of a tetraterpene (see below).

properties

Abscisic acid forms colorless crystals that are readily soluble in organic solvents . The solubility in water depends on the pH value : abscisic acid dissolves poorly in acidic and neutral solutions, and good in alkaline solutions. Abscisic acid is very sensitive to light. Light exposure occurs a photochemical rearrangement to physiologically ineffective trans - isomer .

biosynthesis

Plant biosynthesis mainly takes place in leaves as a general stress response (especially in the case of drought stress). A decrease in the cellular turgor usually serves as the signal for synthesis . The synthesis generally follows that of other tetraterpenes from isopentenyl pyrophosphate (IPP), which is formed in the chloroplasts of the leaves via the methylerythritol phosphate (MEP) route . Intermediate products are the xanthophylls zeaxanthin and violaxanthin formed during terpene synthesis . ABA is formed from them in the cytoplasm by cleavage and oxidation.

Abscisic acid is inactivated by oxidative degradation into phaseic acid and dihydro-phaseic acid. In addition to free abscisic acid, the plant tissue also contains its glucose ester and O - glucoside . These conjugates of abscisic acid are considered to be inactive forms of transport and storage. ABA is largely transported in the conductive tissue by the phloem from the leaves to the root, while small amounts are also transported in the xylem from the root to the shoot.

Discovery story

The name abscisic acid is based on the fact that the acid (first described as factor abscisin II ) was found in high concentrations in discarded cotton leaves. When applied to leaf and fruit stalks, it actually causes them to be shed ( abscission ). However, this effect is limited to a few plants and is not caused directly by abscisic acid, but by a release of ethylene that it causes . Independently of this, it was also described as the dormin factor causing dormin until it was found by chemical analysis that abscisin II and dormin were the same substance. The name Abscisin caught on, although Dormin (from Latin dormire = to sleep) would have been functionally more appropriate.

Effects

Abscisic acid generally has an antagonistic effect on growth-promoting phytohormones and is therefore a natural growth inhibitor. Together with the growth-promoting auxins , gibberellins and cytokinins , abscisic acid regulates aging , leaf fall , flower formation, fruit ripeness , seed and bud dormancy, stomatal transpiration and other development processes of the plant.

General effects

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

Triggering and maintaining the dormancy of plant organs

  • Seed dormancy : ABA is formed in the embryo during late embryogenesis. It prevents immediate germination on the mother plant ( viviparia ) and induces the formation of storage proteins and dehydrins (especially LEA proteins ) to protect against osmotic stress. Gibberellins (GAs) act as antagonists and lead to germination after stratification , whereby the decisive factor is the ratio of ABA to GA (similar to the auxin / cytokinin ratio in differentiation processes).
  • Bud dormancy : ABA also induces bud dormancy of the new shoots of deciduous trees in winter. Gibberellins also act as antagonists here, which trigger shooting in spring after vernalization .

Responses to biotic and abiotic stress

  • osmotic stress ( drought stress , heat stress, salt stress): restriction of transpiration (ABA induces the closure of the stomata via ion flows , so that less water evaporates) and restriction of photosynthesis . The phytohormone ABA induces the opening of anion channels at the molecular level, whereby anions escape from the guard cell. This makes the membrane potential more positive (depolarization), which in turn opens up potassium delivery channels and K + ions flow from the cell into the apoplast. The water from the cell flows into the apoplast, the turgor in the guard cells decreases and the stomata close. In the case of drought stress ( water stress ), the stress-induced ABA concentration correlates linearly with the water conductivity of the leaves, while the water use efficiency increases.
  • Cold stress : causes bud rest (slows down bud growth, forms protective scales) and cessation of primary and secondary growth in thickness (inhibition of cell division in the cambium )
  • Plant pathogens

Signal transduction

Short-term physiological adjustments

  • Stomatal closure:

In the event of water stress, the pH value in the apoplast of the mesophyll increases from approx. 6.3 to approx. 7.2. As a result , abscisic acid dissociates to form its anion , splitting off a proton . Due to its negative charge, the ABA ion can diffuse through cell membranes much more poorly and is therefore less absorbed by the mesophyll cells. Instead, it increasingly binds to a G-protein-coupled receptor in the cell membrane of the guard cells of the stomata. On the one hand, this leads to the brief opening of cellular calcium channels and depolarization due to the influx of Ca 2+ into the cell. On the other hand, the activated G protein , the phospholipase C , and hence the release of inositol trisphosphate (IP 3 ), which in turn to the release of intracellular in the vacuole and the endoplasmic reticulum stored calcium in the cytoplasm leads. Both effects increase the cytoplasmic Ca 2+ level and thus the positive charge of the cell. Ca 2+ acts as a second messenger in plant cells . It inhibits proton pumps in the cell membrane (H + -ATPases, which normally actively pump H + out of the cell) and further depolarization by opening other ion channels . Cl - and K + and possibly malate 2− flow out. Osmotically coupled, water also flows out. This sinks the turgor in the guard cells so that they close.

Longer-term differentiation

  • Altered gene expression:

The intracellular signal transduction of ABA and the gene activation is still largely unknown. The EM gene in wheat , which contains an ABA-Responsive-Element ( ABRE ) in addition to other regulatory sequences , has been relatively well investigated . The long-term effects of ABA in the plant include an increase in the hydraulic conductivity of the roots and increased root growth, while other growth (in the shoot, buds, leaves) is inhibited. As mentioned above, ABA induces the formation or storage of storage substances and dehydrins in the seedling .

In the protonema of the moss , ABA induces very specifically the conversion of photosynthetically active cells into permanent vegetative spores , the so-called brachycytes . Abscisic acid has a broad spectrum of activity, the mechanism of which is still unclear. The abscisic acid content depends on the plant organ and its state of development, but is on average around 100  µg per kg of fresh mass.

literature

Web links

Individual evidence

  1. ↑ a b c d data sheet abscisic acid from Sigma-Aldrich , accessed on January 12, 2013 ( PDF ).Template: Sigma-Aldrich / name not given
  2. ^ A b c Richard J. Lewis, Sr .: Hawley's Condensed Chemical Dictionary . 15th edition. Wiley-Interscience, 2007, ISBN 978-0-471-76865-4 (English).
  3. ↑ Growth Regulators. In: Plant Tissue Culture Protocols. Sigma-Aldrich Co. LLC., Accessed January 7, 2012 .
  4. ↑ Barbara Steuer, Thomas Stuhlfauth, Heinrich P. Fock: The efficiency of water use in water stressed plants is increased due to ABA induced stomatal closure . In: Photosynthesis Research . 18, No. 3, 1988, pp. 327-336. doi : 10.1007 / BF00034837 .
  5. ↑ Decker EL, Frank W, Sarnighausen E, Reski R: Moss systems biology en route: phytohormones in Physcomitrella development Archived from the original on February 4, 2012. (PDF) In: Plant Biol (Stuttg) . 8, No. 3, May 2006, pp. 397-405. doi : 10.1055 / s-2006-923952 . PMID 16807833 .