Phaseic acid is a terpenoid catabolite of

abscisic acid Abscisic acid (ABA) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to environmental s ...

. Like abscisic acid, it is a plant hormone associated with photosynthesis arrestSharkey, T.D. "Effects of Phaseic Acid and Dihydrophaseic Acid on Stomata and the Photosynthetic Apparatus". Plant Physiol. 65(2): 291--297. and abscission.

Function

Abscisic acid (ABA) is a multifunctional plant hormone, playing roles in

germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fer ...

, seasonal growth patterns, and stress response. ABA levels are believed to be regulated in part by control of ABA catabolism, specifically by oxidation to form phaseic acid. Phaseic acid can therefore be thought of as a degradation product of ABA, although it may have other functions. The introduction of high phaseic acid concentrations have been found to impede stomatal closure and reduce photosynthesis in ''Arabidopsis'' but this may be a result of

product inhibition Product inhibition is a type of enzyme inhibition where the product of an enzyme reaction inhibits its production. Cells utilize product inhibition to regulate of metabolism as a form of negative feedback controlling metabolic pathways. Product i ...

rather than recognition of phaseic acid by a

receptor Receptor may refer to: * Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a ...

. Phaseic acid inhibits glutamate receptors in mouse brain.

Biosynthesis

Early precursors

Phaseic acid is an

isoprenoid The terpenoids, also known as isoprenoids, are a class of naturally occurring organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeably with "terpenes", ...

, which means that it is derived from

isoprene Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. Isoprene is an unsaturated hydrocarbon. It is produced by many plants and animals ...

units. The activated terpene geranylgeranyl pyrophosphate is combined with itself to produce the common

carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, and Fungus, fungi. Carotenoids give the characteristic color to pumpki ...

precursor, lycopene.

Carotenoid precursors

Phaseic acid is a product of abscisic acid, which is itself the product of the C₄₀

zeaxanthin via at least four enzymatic steps. Zeaxanthin is epoxidized to form violaxanthin or neoxanthin. The C₁₅ end of the molecule is then cleaved by an epoxycarotenoid epoxygenase to form

xanthoxin Xanthoxin is an intermediate in the biosynthesis of the plant hormone abscisic acid Abscisic acid (ABA) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stom ...

, an aldehyde.

Modification of xanthoxin

Xanthoxin is reduced at the epoxy group and then hydroxylated at the aldehyde group, producing abscisic acid. The 8' hydroxylation of abscisate, abscisic acid's conjugate base, produces 8'-hydroxyabscisate. 8'-hydroxyabscisate cyclizes via nucleophilic attack of the existing ring by the 8' hydroxy group to interconvert with phaseate. The former process is known to be mediated by 8' abscisic acid hydroxylases, a family of

NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require NAD ...

-dependent enzymes. Saito et al. have demonstrated that, in the case of arabidopsis, these hydroxylases are independent of any regulatory mechanism downstream of translation itself.Saito S. (2004). "Arabidopsis CYP707As encode -abscisic acid 8'-hydroxylase, a key enzyme in the catabolism of abscisic acid". Plant Physiol. 134(4):1439-49. The latter process is reported to occur without enzymatic intervention,Milborrow B.V. "The cyclization of 8'-hydroxy abscisic acid to phaseic acid in vivo". Phytochemistry 27:757-759. as it has been found to occur spontaneously in vitro. Image:Xanthtoaba.svg, Xanthoxin is converted enzymatically to abscisic acid Image:Abscisictophaseic.svg, Abscisic acid is oxidized to form phaseic acid

References

{{Reflist Carboxylic acids Sesquiterpenes Tertiary alcohols Ketones Oxygen heterocycles