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