Post on 10-Jan-2016
description
Abscisic Acid And Water Stress
Edita Deli
Discovery
Role in Plants
Structure
Biosynthesis
Role in water stress
Recent advances
Applications of Abscisic acid
Bioassays
Introduction
One of the plant hormones Called stress hormone Transported through xylem and phloem, up and down the stem ABA produced in leaves transported through phloem and ABA
produced in roots is transported through xylem
Discovery
In 1963 the substance that promotes the abscission of cotton fruits was purified and crystallized and named abscisin II by Ohkuma (C15H20O4).
At about the same time a substance that promotes bud dormancy was purified from the sycamore leaves and called dormin.
Dormin = abscisin , named Abscisic acid (ABA).
Structure
(S)-cis-
Aliphatic ring3 methyl groups
End Carboxyl group orientation determines cis and trans isomers
Nearly all naturally occurring ABA is in cis form
There are S and R enantiomers, S is natural form and it is active in fast response to ABA such as stomata closure
Both are active in long-term responses (changes in protein synthesis)
Initiation and maintenance of dormancy of seed and bud The ABA content is low early in embryogenesis, reaches a maximum
at halfway and then gradually decreases as the seed reaches maturity Zygotic genotype controls the level of ABA in the embryo and
presence of ABA as well as absence of GA result in embryo dormancy.
Maternal genotype determines the amount of ABA in the seed coat and seed-coat imposed dormancy
ABA promotes synthesis of late-embryogenesis-abundant (LEA) proteins involved in high desiccation tolerance of the embryo
Role in Plants
Inhibits Precocious germination and Vivipary
Role in Plants
Inhibits shoot growth and promotes root growth at low water potential when its levels are high.
Result is increase in root:shoot ratio at low water potentials
independently of and not through stimulation of ethylene, ABA seems to be initiating agent and ethylene acts at later stage
Regulates gene expression under certain stress conditions (heat shock, low temperatures, salt tolerance)
Few DNA elements are involved in transcriptional repression by ABA such as Gibberellin Response Elements (GARE-s) which mediate the gibberellin inducible ABA-repressible expression of the barley alpha-amylase gene
Promotes leaf senescence (Muns, 1993 and Saab1991)
Inhibits opening of stomata (as a response to water stress)
Role in Plants
ABA coming from the plastids promotes the metabolism of fruit ripening
Bioassays
Imunoassays
Biological
Physiochemical -- more reliable
Coleoptile growth inhibition – 10-7M
Inhibition of germination
Stomatal closure high sensitivity,10-9, also little affected by other plant growth regulators
gas chromatography or High Performance Liquid Chromatography (HPLC), detects 10-13g of ABA
Recognition of antibodies from mice and rabbits injected with the growth regulator, can detect 10-13 g of ABA, easier to do than HPLC
Biosynthesis
ABA is synthesized via the terpenoid pathway IPP Isopentenyl pyrophosphate is a precursor for the
synthesis of C40 xanthophyl zeaxanthin. Zeaxanthis is then converted to 9’-cis-neoxanthin
through several steps. 9’-cis-neoxanthin is oxidatively cleaved to form the C15
xantoxin which is then converted to ABA aldehide. ABA aldehide is oxidized to form ABA
Biosinthesis
ABA Synthesis - New discoveries
Xanthoxin is formed exclusively from neoxanthin (rather than from either violaxanthin). Recent data show that most neoxanthin in spinach and broccoli green tissue appears to be in the cis-form
(Strand et al, 2000, Biochem Systematics Ecology 28: 443-455)
Recent papers from W. Hartung and colleagues suggest that, "Glycosylation of ABA may be a mechanism to allow for the export of ABA from the cells independent of the prevailing cytoplasmic proton concentration and transmembrane proton gradients.
(Dietz et al, 2000, J. Experimental Botany 51: 937-944; Sauter and Hartung, 2000, J. Experimental Botany 51: 929-935)
ABA and Water Stress
90% of the water taken up by a plant is lost in transpiration.
Most of this is lost through the stomata in the leaf.
Stomata
S = guard cellN = subsidiary cell
E = Epidermal cell
substomatal chamber
Each stoma is flanked by a pair of guard cells. When the guard cells are turgid, the stoma is open. When turgor is lost, the stoma closes.
Stoma
ABA is the hormone that triggers closing of the stomata when soil water is insufficient to keep up with transpiration.
Redistribution of ABA in the leaf - Under water stress pH of xylem sap increases, this favors formation of the dissociated form of ABA which is not readily taken up by mesophyll cells so more ABA reaches guard cells via the transpiration stream and thus stimulates closure of the stomata
The mechanism: ABA binds to receptors at the surface of the plasma membrane of the
guard cells, this initiates a rise in pH in the cytosol and the formation of the "second messenger", cyclic ADP ribose (cADPR)
Increased pH stimulates the loss of K+ and anions from the cell while ABA induced depolarization of the membrane induces the long term loss of K+
Rising levels of cADPR cause Ca2+ to move from the vacuole to the cytosol, which then blocks the uptake of K+ into the guard cell
ABA and Water Stress - Closing of stomata
The combined effects result in a loss of solutes in the cytosol. This reduces the osmotic pressure of the cell and thus turgor so the
stomata close. Receptor has not been identifed and it is not known whether the
hormone must enter the cell to be effective or whether it binds to outer cell membrane
Increase in cytosolic Ca might be responsible for stomata closure In addition to stomatal closure ABA inhibits light induced stomatal
opening by inhibiting inward K+ channels
Closing of stomata
Felle et al, 2000,, “ Dinamics of ionic activities in the apoplast of the sub-stomatal cavity of Viva faba leaves during stomatal closure evoked by ABA and darkness
Tracing the ion content in sub stomatal cavities In order to continuously record the ion exchange between guard cells
and surrounding apoplast during stimulus induced stomatal movements they inserted ion-selective microelectrodes with heat-polished tips into substomatal chamber through the stomatal opening.
When the electrical contact with the apoplastic fluid was achieved the electrode was retracted a little bit.
In the neighboring stoma they placed a voltage reference electrode. Whith electrodes positioned this way they were able to record ion
concentrations for several hours. K+ 1.4 – 4.7 mM, Cl- 0.67 – 2.5mM, Ca2+ 35 – 89 mM lightadapted leaves, half open stomata
When they fed 10-5 ABA into xylem through the cut petiole - stomata closed within 15 – 30 minutes.
Ion activities changed, all peaked 9-10 min after ABA addition Ca2+ apoplastic activity and H+ activity decreased while Cl-
activity increased K+ activity leveled of at 10 mM Similar effects were observed when the stomata closed in the
response to darkness
Felle et al, 2000
Commercial applications
ABA analogs have extensive uses in horticulture, agriculture and forestry and there are many commercial applications for this product that include:
Growing seedlings in tissue culture (e.g. conifers) Reduction of seedling transplantation shock in fruit trees, vegetables,
ornamentals and flowering annuals Treatment of potatoes and barley to delay sprouting during storage Control of non seasonal sprouting of canola and soft white wheat Control growth rates of plants to grow compact, sturdy, plants Increase survival rates of transplanted seedlings exposed to stress conditions
(cold, drought, salt) Seed coating (e.g. canola seeds) for dormancy regulation and improved
seedling performance