Abscisic Acid: A Seed Maturation and Antistress Signal
Contents • Occurrence, Chemical Structure, and Measurement of ABA • Biosynthesis, Metabolism, and Transport of ABA • Developmental and Physiological Effect of ABA
Abscisic Acid (ABA) • Physiologists suspected that the phenomena of seed & bud dormancy were caused by inhibitory compounds. • The experiments led to the identification of a group of growth-inhibiting compounds, including dormin
Upon discovery that dormin was chemically identical to a substance that promotes the abscission of cotton fruits, abscissin II • Thecompound was renamed abscisic acid (ABA)
The chemical structures of the S (counterclockwise array) and R (clockwise array) forms of cis-ABA, the (S)-2-trans form of ABA, and lunularic acid. The numbers in the diagram of (S)-cis-ABA identify the carbon atoms.
Occurrence, Chemical Structure, and Measurement of ABA • ABA is a ubiquitous plant hormone in vascular plants.
ABA has been detected in living tissues from the root cap to the apical bud. • It is synthesized in almost all cells that contain chloroplasts or amyloplasts.
Chloroplasts in Leaf Cells Potato Amyloplasts
The chemical structure of ABA determines its physiological activity • ABA is a 15-C compound that resembles the terminal portion of some carotenoid molecules. • Nearly all the naturally occurring ABA is in the cis form
ABA also has an asymmetric C atom at position 1´ in the ring, resulting in the S and R(or+ and -) enantiomers. The S enantiomer is the natural form
ABA is assayed by biological, physical, and chemical methods • Avariety of bioassays have been used for ABA • inhibition of coleoptile growth • germination • GA-induced α-amylase synthesis
Coleoptile growth, the classic bioassay devised for auxins is also used for ABA detection in plant extracts by measurement of coleoptile growth inhibition.
This bioassay has adequate sensitivity (minimum detectable level is 10–7 M) and shows a linear response in the range of 10–7 to 10–5 M, but it has some disadvantages.
Physical methods of detection are much more reliable than bioassay. • The most widely used techniques are those based on gas chromatography or HPLC.
Biosynthesis, Metabolism, and Transport of ABA • ABA is synthesized from a carotenoid intermediate • Biosynthesis takes place in chloroplasts and other plastids.
The pathway begins with isopentenyl diphosphate (IPP)and leads to the synthesis of the C40 xanthophyll violaxanthin.
Maize mutants (viviparous; vp) that are blocked at other steps in the carotenoid pathway also have reduced levels of ABA and exhibit vivipary.
ABA concentrations in tissues are highly variable • ABA biosynthesis and concentrations can fluctuate dramatically in specific tissues during development or in response to changing environmental conditions. of ABA and exhibit vivipary.
Part of this increase is due to increased expression of biosynthetic enzymes, but the specific enzymes depend on the tissue and the signal.
Upon rewatering, the ABA level declines to normal in the same amount of time.
ABA can be inactivated by oxidation or conjugation • A major cause of the inactivation of free ABA is oxidation
Free ABA is also inactivated by covalent conjugation to another molecule; monosaccharide
As water stress begins, some of the ABA carried by the xylem stream may be synthesized in roots that are indirect contract with the drying soil.
The lack of early ABA accumulation in roots may reflect either rapid transport of ABA or transport of a distinct long-distrance signal, possible even an ABA precursor.
Although a concentration of 0.3 μM ABA in the apoplast is sufficient to close stomata, not all of the ABA in the xylem stream reaches the guardcells.
The major control of ABA distribution among plant cell compartments follows the “anion trap” concept.
The dissociated (anion) form this weak acid accumulates in alkaline compartments and may be redistributed according to the steepness of the pH gradients across membranes.
Stress-induced alkalinization of the apoplast favors formation of the dissociated form of abscisic acid, ABA-,which does not readily cross membranes.
That ABA is redistributed in the leaf in this way without any increase in the total ABA level. • Therefore, the increas in xylem sap pH may function as a root signal that promotes early closure of the stomata.
Developmental and Physiological Effects of ABA • ABAplays primary regulatory roles in the initiation and maintenance of seed and bud dormancy and in plant’s response to stress.
ABA regulates seed maturation • Seed can be divided into three phases of approximately equal duration:
1. During the first phase, which is characterized by cell division and tissue differentiation
2. During the second phase, cell divisions cease and storage compounds accumulate.
3. During the final phase, the embryo becomes tolerant to dessication, and the seed dehydrates, losing up to 90% of it water.
As a consequence of dehydration, metabolism comes to a halt and enters a quiescent (resting) state.
The latter two phases result in the production of viable seeds with adequate resources to support germination and the capacity to wait weeks to years before resuming growth.
ABA inhibits precocious germination and vivipary • ABA added to the culture medium inhibits precocious germination.