Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Abscisic Acid (ABA)

• Bennet-Clark and Kefford (1953) described the presence of an inhibitor of coleoptile elongation in oats
• about 10 years later, Addicott et al. found a substance that stimulated abscission of fruits in cotton and they named it abscisin II
• about the same time, Wareing found a substance in sycamore leaves that promoted dormancy in buds and called it dormin
• it was soon clear that these were the same substance
• a conference in 1967 straightened out the name and it was decided to call the hormone abscisic acid (ABA).

• a single structure, not a family of related structures like the gibberellins
• sesquiterpene (i.e., terpenoid) - C15 - made from 3 isoprene units
• occurs as cis form; S-enantiomer is natural and active form
• found in all green plants, also in some mosses, algae, and fungi
• related to lunularic acid which is found in liverworts.

III. Biosynthesis

• plastids
• most tissues, especially leaves and seeds
• terpene pathway
• IPP is the first intermediate; not derived from mevalonate (mevalonate-independent); comes from intermediates of glycolysis
• ABA derived from the breakdown of carotenoids.  Evidence:  maize mutants blocked in carotene synthesis → low levels of ABA → vivipary (see below)
• Pathway:  IPP → farnesyl pyrophosphate → carotenoids (C40; violaxanthin) → xanthoxin → ABA

IV. Bioassays/Analysis

A. Bioassays � there are several including:

• inhibition of seed germination
• inhibition of GA induced alpha-amylase production

B. Analysis � Gas chromatography, HPLC, and immunoassay

V. Disposal/Regulation 

• rapid changes in endogenous levels (up to 100x within a few days)
• ABA levels can be regulated by: (a) degradation; (b) compartmentalization; (c) transport; (d) conjugation to a sugar or other molecule; and (e) conversion (oxidation) into phaseic acid and dihydrophaseic acid.
  

VI. Transport - xylem and phloem (greater amounts)

VII. Actions

A. Growth Inhibitor
    Widespread growth inhibitor; often antagonistic of GA actions

B. Maintains or "seals in" bud and seed dormancy (i.e., prevents germination)
    In fact, ABA is made during the terminal stages of embryo development. Among it's roles in seed dormancy is to: (1) provide desiccation tolerance of the embryo by promoting synthesis of proteins involved in the process; and (b) promote accumulation of seed storage proteins.

C. Prevents vivipary
    Development of the embryo without a dormant period. Some evidence: viviparous mutants have reduced [ABA]; and fluridone stimulates treatment stimulates vivipary (fluridone is an inhibitor of carotenoid biosynthesis that blocks ABA production)

D. Inhibits auxin induced growth (seems to block the H⁺ pump)

E. Stomatal closure under water stress (remember our unit on gas exchange?)

F. Abscission & senescence
    Involved, though perhaps only a minor role


VIII. Mechanism of action

A. Effects on plasma membrane

B. Inhibits protein synthesis

C. Regulation of genes (transcription)