Abscisic acid

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Abscisic acid

Abscisic acid (ABA), also known as abscisin II and dormin, is a plant hormone

Plant hormone

Plant hormones are chemicals that regulate plant growth. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations....
. It functions in many plant developmental processes, including bud dormancy

Dormancy

Dormancy is a period in an Organism Biological life cycle when growth, development, and physical activity is temporarily suspended. This minimizes metabolism and therefore helps an organism to conserve energy....
.

was originally believed to be involved in abscission

Abscission

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Abscisic acid (ABA), also known as abscisin II and dormin, is a plant hormone

Plant hormone

Dormancy

Function

ABA was originally believed to be involved in abscission

Abscission

Abscission is the shedding of a body part. It most commonly refers to the process by which a plant intentionally drops one or more of its parts, such as a leaf, fruit, flower or seed, though the term is also used to describe the shedding of a claw by an animal, and is also the word used to describe the separation of daughter cells at the end...
but researchers no longer believe this to be the case. ABA-mediated signalling also plays an important part in plant responses to environmental stress and plant pathogens. The plant genes for ABA biosynthesis and sequence of the pathway have been elucidated. ABA is also produced by some plant pathogenic fungi via a biosynthetic route different from ABA biosynthesis in plants.

Abscisic acid owes its names to its role in the abscission of plant leaves. In preparation for winter, ABA is produced in terminal buds

Bud

In botany, a bud is an undeveloped or Plant embryogenesis shoot and normally occurs in the axil of a leaf or at the tip of the Plant stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately....
. This slows plant growth and directs leaf primordia to develop scales to protect the dormant buds during the cold season. ABA also inhibits the division of cells in the vascular cambium

Cambium

In botany the cambium is a layer or layers of tissue, also known as meristems, that are the source of cells for secondary growth. There are two types of cambium...
, adjusting to cold conditions in the winter by suspending primary and secondary growth.

Abscisic acid is also produced in the root

Root

In vascular plants, the root is the organ of a plant body that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial root or aerating ....
s in response to decreased soil water potential

Water potential

Water potential is the potential energy of water relative to pure free water in reference conditions. It quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects including surface tension....
and other situations in which the plant may be under stress. ABA then translocates to the leaves, where it rapidly alters the osmotic potential of stomatal guard cells, causing them to shrink and stomata to close. The ABA-induced stomatal closure reduces transpiration

Transpiration

Transpiration is the evaporation of water from the aerial parts of plants, especially leaf but also Plant stems, flowers and roots. Leaf surfaces are dotted with openings called stoma that are bordered by guard cells....
thus preventing further water loss from the leaves in times of low water availability.

Several ABA mutant

Mutant

A mutant is an individual, organism, or new genetic character arising or resulting from an instance of mutation, which is a base-pair sequence change within the DNA of a gene or chromosome of an organism resulting in the creation of a new character or Trait not found in the wild type....
Arabidopsis thaliana
Arabidopsis thaliana

Arabidopsis thaliana , is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, Arabidopsis is popular as a model organism in plant biology and genetics....
plants have been identified – both those deficient in ABA production and those insensitive to its action. ABA-deficient plants show defects in seed

Seed

A seed is a small Plant embryogenesis plant enclosed in a covering called the seed coat, usually with some Food storage. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant....
dormancy, germination

Germination

Germination is the process whereby growth emerges from a period of dormancy. The most common example of germination is the sprouting of a seedling from a seed of an flowering plant or gymnosperm....
, stomatal regulation and some mutants show stunted growth and brown/yellow leaves. These mutants reflect the importance of ABA in seed germination and early embryo development.

Biosynthesis

Abscisic acid (ABA) is an isoprenoid plant hormone, which is synthesized in the plastid

Plastid

Plastids are major organelles found in plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell....
al 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway; unlike the structurally related sesquiterpene

Sesquiterpene

Sesquiterpenes are a class of terpenes that consist of three isoprene units and have the molecular formula C15H24. Like monoterpenes, sesquiterpenes may be acyclic or contain rings, including many unique combinations....
s, which are formed from the mevalonic acid

Mevalonic acid

Mevalonic acid is a key organic compound in biochemistry. The anion of mevalonic acid, the predominant form in biological media, is known as mevalonate....
-derived precursor farnesyl diphosphate (FDP), the C₁₅ backbone of ABA is formed after cleavage of C₄₀ carotenoid

Carotenoid

Zeaxanthin

Zeaxanthin is one of the two carotenoids contained within the retina of the eye. Within the central macula, zeaxanthin is the dominant component, whereas in the peripheral retina, lutein predominates....
is the first committed ABA precursor; a series of enzyme-catalyzed epoxidations and isomerizations, and final cleavage of the C₄₀ carotenoid

Carotenoid

Carotenoids are organic compound pigments that are naturally occurring in chromoplasts of plants and some other photosynthesis organisms like algae, some types of fungus and some bacterium....
by a dioxygenation reaction yields the proximal ABA precursor, xanthoxin, which is then further oxidized to ABA. Abamine has been patented by the Japanese researchers Shigeo Yoshida and Tadao Asami, which are very reluctant to make this substance available in general, neither commercially nor for research purposes.

Location and timing of ABA biosynthesis

Released during desiccation
Desiccation

Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately-well sealed container....
of the vegetative tissues and when roots encounter soil compaction
Compaction

Compaction may refer to:* Soil compaction, for mechanically induced compaction near the ground surface* Compaction , part of the process of lithification involving mechanical dewatering of a sediment by progressive loading under several km of geomaterial...
.
Synthesized in green fruit
Fruit

The term fruit has different meanings dependent on context, and the term is not synonymous in food preparation and biology. In botany, which is the scientific study of plants, fruits are the ripened Ovary of flowering plants....
and seed
Seed

A seed is a small Plant embryogenesis plant enclosed in a covering called the seed coat, usually with some Food storage. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant....
s at the beginning of the wintering period
Mobile within the leaf
Leaf

In botany, a leaf is an above-ground plant Organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin, to expose the cells containing chloroplast to light over a broad area, and to allow light to penetrate fully into the tissues....
and can be rapidly translocated from the roots to the leaves by the transpiration stream in the xylem
Xylem

In vascular plants, xylem is one of the two types of transport tissue, phloem being the other. The word "xylem" is derived from classical Greek language ????? , "wood", and indeed the best known xylem tissue is wood, though it is found throughout the plant....
.
Produced in response to environmental stress, such as heat stress, water stress, salt stress.
Synthesized in all plant parts, e.g. roots, flowers, leaves and stems
Plant stem

A stem is one of two main structural axes of a vascular plant. The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaf, inflorescence , conifer cones or other stems etc....

Effects

Induces stomatal closure, reducing transpiration to prevent water loss.
Inhibits fruit ripening
Responsible for seed dormancy by inhibiting cell growth – inhibits seed germination
Germination

Germination is the process whereby growth emerges from a period of dormancy. The most common example of germination is the sprouting of a seedling from a seed of an flowering plant or gymnosperm....
Inhibits the synthesis of Kinetin
Kinetin

Kinetin is a kind of cytokinin, a class of plant hormone that promotes cell division. Kinetin was originally isolated by Carlos Miller and Folke K....
nucleotide
Downregulates enzyme
Enzyme

Enzymes are biomolecules that catalysis chemical reactions. Almost all enzymes are proteins. In enzymatic reactions, the molecules at the beginning of the process are called Substrate , and the enzyme converts them into different molecules, the products....
s needed for photosynthesis
Photosynthesis

File:Seawifs global biosphere.jpgPhotosynthesis is a metabolic pathway that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight....
.