Saturday, February 27, 2016

Best Biochemist: ABA

Growth hormones are critical to plant development, much in the same way they are for animals. Previously, I wrote about ethylene, the ripening hormone. Today, we'll look at abscisic acid (ABA), an important growth and stress hormone.

Biochemically, ABA is a 15 carbon derivative of zeaxanthin, a carotenoid, with the following chemical structure.

public domain
ABA was discovered over 50 years ago in abscising leaves. Abscission is the process by which plants detach leaves or ripened fruit. Thus the name abscisic acid. Though we now know that ABA does not play a role in abscission itself but rather cessation of growth that precedes the actual detachment process. In the 50 years since discovery, ABA has been shown to play a role in a multitude of pathways. These include: seed dormancy, embryo development, germination, cell division, flower induction, stress responses (drought, salt, cold, UV, pathogens), and stomatal closure. Let's start at the beginning of the plant's life cycle and take a look at the roles ABA plays in these processes.

Seeds in the ground remain dormant, in a state of suspended growth. The production of ABA is high during late embryo maturation which drives dormancy. Dormancy must be broken for the seed to germinate. When it is time for the seed to germinate, gibberellic acid and ethylene production results in a cascade that mutes the detection of ABA breaking dormancy. Germination then occurs and the plant grows.

As plants grow they can encounter many abiotic stressors. Personally, I research abiotic stress biology so I find the stress responsive role ABA plays to be one of the most interesting. Plant perception of environmental conditions such as, high salt, low water, cold, and wounding, all involve ABA. These stressors increase ABA levels which in turn activates stress responsive genes. The main function of ABA in all of these stresses seems to be controlling the water level within the plant in a number of ways, including gene activation, increasing water uptake in the roots, and closing the stomata.
Stomata Personal Image
Stomata are pores on the leaf that allow gas exchange, including water vapor. To close the stomata, ABA binds to receptors on the guard cells that control opening and closing of the stomata. These receptors allow the flow of ions such that the guard cells lose water and close the pores. This prevents water from leaving the leaf!

Flowering is an important stage in plant development, resulting in the production of seeds and the next generation of the species. As with all developmental phases, several hormones play important roles. The ratio of each and amount of cross-talk between the various hormone pathways results in the change of developmental stage.  Floral development requires an increase in ABA and decrease in ethylene to the proper ratio.

As the plants age, they will inevitably undergo senescence and die. The biosynthesis of ABA, along with a suite of other hormones, is increased while cytokinin is decreased. ABA works in a positive feedback manner. As ABA levels increase, water and minerals are pushed out of the leaf and into the stem. This results in dehydration of the leaf and further production of ABA. This sets up the leaf for abscission which is controlled by ethylene.

As we can see, ABA is an incredibly important hormone. It plays a role in almost every developmental stage a plant undergoes from germination to senescence. How exactly the plant knows when ABA means flowering, dehydration stress, or senescence is still a mystery. It is probably in part to genetic control, age of the plant, and coordination with other hormone levels. Plant growth and development is just as complicated as it is in animals.

References
Beaudoin, N., et al., 2000. Interactions between Abscisic Acid and Ethylene Signaling Cascades. The Plant Cell 12:7 1103-1115 http://www.plantcell.org/content/12/7/1103.full.html

Daszkowska-Golec, A., 2013. Open or Close the Gate - Stomata Action Under the Control of Phytohormones in Drought Stress Condition. Frontiers in Plant Science 4 138 doi 10.3389/fpls.2013.00138 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3652521/

Finkelstein, R. 2013. Abscisic Acid Synthesis and Response. Arabidopsis Book. 2013; 11: e0166. Published online 2013 Nov 1. doi:  10.1199/tab.0166 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3833200/

Lee, I., et al, 2011. Age-dependent action of an ABA-inducible receptor kinase, RPK1, as a positive regulator of senescence in Arabidopsis leaves. Plant Cell Physiology 52(4) 651-662 http://www.ncbi.nlm.nih.gov/pubmed/21382977

Tuteja, N. Abscisic Acid and Abiotic Stress Signaling. Plant Signaling & Behavior 2(3) 135-138 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2634038/ 

Wilmowicz, E., 2008. Ethylene and ABA interactions in the regulation of flower induction in Pharbitis nil. Journal of Plant Physiology 165 (18) 1917-1928  http://www.ncbi.nlm.nih.gov/pubmed/18565620

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