Abscisic Acid

Abscisic acid (ABA) is a plant hormone with roles in growth, development, and stress responses, earning it the label of a stress hormone. As a sesquiterpene (C15), ABA is crucial for seed development, protein synthesis, and the production of osmolytes that help plants withstand environmental and biotic stress. It acts as a general growth inhibitor and regulates metabolic activities. Synthesized in roots in response to stress, ABA is transported to leaves, although leaves can also produce it. Overall, ABA plays a vital role in coordinating plant responses to various challenges.

1.0Discovery of Abscisic Acid

As plant hormone research advanced, scientists isolated auxins from ether extracts and found these substances disrupting the Avena coleoptile curvature test. Initially aiming to eliminate these interferences, researchers later considered these inhibitors as potential growth regulators. 

Paper chromatography, introduced in 1953, revealed the inhibitory substance, inhibitor β, alongside auxin (IAA). Kefford suggested a link between inhibitor β, found in axillary buds and dormant potato tuber layers, and apical dominance. Simultaneously, researchers noted inhibitors in buds and leaves associated with dormancy in woody plants. In 1964, P. F. Waring coined the term "dormin" for these dormancy-inducing substances.

In a related study, substances hastening abscission, labeled "abscission II," were isolated from senescing leaves of bean, cotton, and lupin fruits. In the mid-1960s, three labs reported the purification of abscisin II, inhibitor β, and dormin, all proving chemically identical.

Nomenclature disputes ensued, but a panel recommended the name abscisic acid (ABA) to the 1967 International Conference on Plant Growth Substances. Universally accepted, today, ABA is a common term in plant science.

2.0Biosynthesis of Abscisic Acid 

The biosynthesis of abscisic acid (ABA) in plants is a complex process intricately tied to the carotenoid biosynthetic pathway. ABA, a crucial plant hormone, originates from carotenoids synthesized in plastids. Zeaxanthin, a carotenoid, serves as a pivotal precursor for ABA.

3.0Physiological effects 

Stomatal closure: In drought conditions, leaves generate abundant abscisic acid (ABA), leading to the rapid closure of stomata within 1 to 2 minutes. ABA acts as a messenger, facilitating water conservation independently of protein synthesis. The mechanism involves ABA binding to proteins on the plasmalemma's outer surface in guard cells, enhancing a quick response to water stress. This binding renders the plasmalemma more positively charged, stimulating the transport of ions (especially K+) from guard cells to epidermal cells. The loss of ions causes water to exit guard cells via osmosis, leading to their collapse and resulting in the closure of stomatal apertures.

Delays seed dormancy: ABA often postpones seed germination in various species, and the ABA levels in seeds commonly decrease during germination in many plants. This suggests that ABA plays a role in regulating seed dormancy in certain instances. However, it's important to note that this conclusion doesn't apply universally, as many seeds can germinate without any significant changes in ABA levels.

Controls bud dormancy: While it was initially believed the abscisic acid hormone exclusively regulated bud dormancy, it is now recognized that cytokinins and the synthesis of ethylene induced by IAA likely also play significant roles in influencing bud dormancy.

Counteracts the effects of other hormones: ABA hormone acts in opposition to the effects of other hormones, exemplified by:

(a) ABA inhibiting the cell growth stimulated by IAA.

(b) ABA suppressing amylase production in seeds treated with gibberellin.

(c) ABA promoting chlorosis, a process counteracted by cytokinins.

This antagonistic action may arise from ABA's role as a Ca2+ antagonist, potentially interfering with Ca2+ metabolism and inhibiting the stimulatory effects of IAA and cytokinins. While ABA typically reduces gene activity, there are instances where it stimulates gene expression, such as in the synthesis of mRNAs for storage proteins in developing wheat grains.