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Auxin: How Are Plants Supposed to Live Without You?

Welcome to the first in a series of five articles dealing with the fab-five plant hormones: Auxins, Gibberellins, Cytokinins, and Abscisic Acid.

I will be expanding and updating each article through time so feel free to check back and see what’s new {KDH: 10/14/18}.

[Edited 10/15/18]

 Image 1: IAA, the gold standard for all auxins.

Image 1: IAA, the gold standard for all auxins.

Auxin, a term derived from the Greek word, auxein (meaning “to grow”), is the first class of plant hormones or growth factors to have ever been discovered [2]. That’s right, auxin is not a specific molecule, it is a class of molecules based on a solitary and shining example: Indole-3-acetic acid (from now on referred to as IAA).

IAA was not only the first plant hormone ever discovered and isolated, but it is widely considered to be the most ubiquitous and potent auxin in nature.

That is to say…

IAA is exactly like Micheal Bolton. Sure, there are other singers in the world, but when people say, “male vocalist,” they are really only referring to one man:

GIF 1: Micheal Bolton is widely considered to be the leading cause of romance worldwide.

Auxin Definition

Now that you have been adequately primed, let us define auxin as any plant-produced chemical with a similar structure and function to that of IAA: the Micheal Bolton of auxins.

However, as we shall see later, there may be auxins as-or-even-more interesting than IAA.

Auxin Function

Since the 1930s, plant scientists have understood auxin to have a hand in influencing cell elongation and, by proxy, localized tissue growth [I]. In addition to cell elongation, auxins influence a plant’s directional movement as influenced by light (phototropism) and gravity (gravitropism) as well as a plant’s general upward growth by ensuring the most vigorous cell division is happening at the top of the plant (apical dominance) [3]. This, in turn, inhibits the growth of lateral stems and influences plants to grow taller as opposed to wider/bushier. Conversely, auxins present in root tips influence root elongation; auxins present in stem tips influence stem elongation… so on and so on.

In plant tissue culture, auxins are added in varying ratios with cytokinins in gel media to promote the growth of cambium tissue (tissue associated with the xylem and phloem), as well as root initiation/elongation [4].

Essentially, wherever auxin is, cell elongation will commence… just like wherever Michael Bolton sings, romance will commence. However, just as The Boltanator has his fair share of impersonators on Planet, auxin has its own impersonators produced within the plant. We term these auxins as “endogenous auxins.”

EN-dodge-EN-us

Endogenous Auxins

Let us shout the name…

 Image 2: 4-chloroindole-3-acetic acid (4-Cl-IAA), indole-3-butyric acid (IBA) and phenylacetic acid (PAA): three plant-produced auxins with similar structure and function to that of IAA.

Image 2: 4-chloroindole-3-acetic acid (4-Cl-IAA), indole-3-butyric acid (IBA) and phenylacetic acid (PAA): three plant-produced auxins with similar structure and function to that of IAA.

In addition to IAA, there are at least three known auxins that are considered “endogenous,” meaning, produced inside [of the plant]; these auxins are 4-chloroindole-3-acetic acid (4-Cl-IAA), indole-3-butyric acid (IBA) and phenylacetic acid (PAA) (See Image 2 above). Let’s consider these one at a time. Shall we?

indole-3-butyric acid (IBA)

 Image 3: IBA makes plants hype about producing roots.

Image 3: IBA makes plants hype about producing roots.

A darling substance for, say, an aspiring scientist to explore, the true role and physiological effects of IBA are yet to be fully realized or understood, however, there is plenty of experimental and empirical evidence out there. Perhaps you need look no further than your local garden center.

IBA, being a chemical precursor to IAA, is understood to have a role in the formation of adventitious roots, or, roots that arise from any non-root section of a plant (e.g. from the cut end of a plant cutting).

Recent experiments conducted at Sapienza Università di Roma have shown IBA influences the creation of adventitious roots in plant cuttings (Arabidopsis thaliana cuttings to be exact) to a greater degree than even IAA [6]. In fact, it is more potent when used alone than when combined with kinetin (a member of another class of plant growth hormones altogether) [6].

Considering its role in stimulating the emergence of adventitious roots, it should come as no surprise that IBA is a commonly used agent in plant tissue culture micropropagation (a practice close to mine own heart) as well as general plant cutting activities (See Image 4 below).

 Image 4: This is the first search result for “plant rooting hormone” I saw on a search engine inquiry. BEHOLD! Indole-3-butyric acid is listed as the only active ingredient.

Image 4: This is the first search result for “plant rooting hormone” I saw on a search engine inquiry. BEHOLD! Indole-3-butyric acid is listed as the only active ingredient.

Further, experimental evidence suggests that IBA may have a role in stimulating growth and photosynthetic pigment production in a model studying the photosynthetic bacteria (cyanobacteria), Nostoc linckia [7].

Not too shabby for a second-fiddle.

Surely, the other auxins are much more boOOoooOoring!

4-chloroindole-3-acetic acid (4-Cl-IAA)

 Image 5: 4-Cl-IAA. This is an endogenous auxin with some pretty serious bragging rights.

Image 5: 4-Cl-IAA. This is an endogenous auxin with some pretty serious bragging rights.

Ahh, 4-Cl-IAA, your name pleases the ears like the tender words of a first true love.

Now, you might be thinking that 4-Cl-IAA is just IAA with a chlorine attached at the 4th carbon of the 1-H indole (the benzene-ring-looking-thingy) on IAA, but I’m here to tell you… uh… yes… that is pretty much how that goes.

But along with this chlorine atom—and totally graceful name— comes a considerably separate identity. That’s right, 4-Cl-IAA does some interesting things.

Unlike IAA, 4-Cl-IAA stimulates pericarp growth [8]. If that doesn’t impress you, SPIT OUT THAT APPLE. The pericarp is the delicious part of the fruit that surrounds the seeds (essentially plumped tissue arising from a plant’s ovary). Not only that, 4-Cl-IAA slows fruit ripening and spoilage by inhibiting ethylene response, and stimulates the biosynthesis of gibberellins (another class of plant growth hormones we will explore in the future) [8].

Additionally, 4-Cl-IAA has shown an ability to increase nitrogen, phosphorus, chlorophyll, sugar, and soluble protein levels in plants (all the good stuff) [8]. But, HEY, that’s not the whole story.

Remember when we found out that IBA has a greater ability to stimulate adventitious root growth than IAA? Well, a study in Japan has shown the root growth stimulating activity of 4-Cl-IAA to be 4 times higher than that of IBA [9].

WHAT?!

I said a study in Japan has shown the root growth stimulating activity of 4-Cl-IAA to be 4 times higher than that of IBA [9]… which, itself, is more potent than IAA in that regard.

Okay, I am pretty much sure this last auxin should just be a dull benzene-ringy-looking-thingy with nothing special going on and nothing incredible to wonder about. Don’t worry.

PREPARE TO GET BORED, PEOPLE!!!

Phenylacetic Acid (PAA)

 Image 6: PAA may be one of a plant’s untold resources in its fight against bullies.

Image 6: PAA may be one of a plant’s untold resources in its fight against bullies.

Phenylacetic Acid— can I call you PAA?— thank you for showing me how to grow and to whoop [donkey butt].

Plant

In addition to being recognized for its ability to promote generalized plant and root growth, recent studies have shown that PAA may also have a role influencing the production of enzymes associated with defense agains pathogenic microbes, namely, enzymes that combat the soft rot bacterium Pectobacterium carotovorum subsp. carotovorum [9]. Yes, like a devoted father, PAA is there to help a young plant learn when it needs to fight. I’m sure you have guessed this by now, but hold your plants on… there is even more.

Phenomenally, Amazingly, and Awesomely enough (clever, I know), a team of researchers from The Catholic University of Korea have used PAA to synthesize— get this— a nanoporous biodegradable mesh inspired by spiders (it is spun) that is capable of trapping drugs with an aim to use as an implant to slowly deliver chemotherapeutic medication [10].

PEOPLE, we are talking about a physical… biodegradable… mesh with pores small enough to trap dissolved molecules of medication. It is made with a hormone naturally produced in plants… our good-ole PAA.

Could this mesh be used to clean drinking water of pharmaceutical pollution?! I don’t know!

Dear Michael Bolton,

No offense, Michael, but writing this article has gotten me thinking. Even though many academic resources claim IAA is the most potent auxin and that other endogenous auxins only play minor roles, recent scientific evidence is starting to show otherwise.

This led me into a new thought process. Maybe there is a world outside of Michael Bolton.

Anyway, Michael, I think it is time that I start listening to other male vocalists.

I’d hate to picture you singing sad songs in your apartment over this, but I know that is exactly what you’re about to do after reading this (typical Bolton). I just want you to know that there are other music fans out there that are right for you… just… not me.

I hope you understand.

KDH

P.S.

Thank you for visiting my home on the web. There is still so much more to learn out there about auxins. There is a ton of recent news to sift through, chemical pathways to study, and perhaps a couple musical catalogs to explore outside of M. Bolten.

Anyhow, thank you!

Thanks for taking this brief-yet wild ride through the world of auxins. This article will continue to expand and be updated thanks to the continuing support of Pull Up Your Plants patrons. I encourage your feedback.

If you want to see this site grow and wish to become a patron, you can with a donation of as little as $5 a month by signing up at this LINK.

Thanks for your consideration.

Kevin Healey 10/14/18

References

[1] Ogden, L. E. (2017). Directional Signals. Natural History125(2), 6. Retrieved from http://search.ebscohost.com.ezproxy.pueblolibrary.org/login.aspx?direct=true&db=aph&AN=120709770&site=ehost-live

[2] Mahaja, D. P. (n.d.). Ph. D. Thesis, Mr. Dipesh P. Mahajan, School of Chemical Sciences, NMU, Jalgaon. (Unpublished master's thesis). North Maharashtra University.

[3] PLANT HORMONES, NUTRITION, AND TRANSPORT. (n.d.). Retrieved from https://www2.estrellamountain.edu/faculty/farabee/BIOBK/BioBookPLANTHORM.html

[4] The International Plant Growth Substances Association. (n.d.). Retrieved from https://pages.wustl.edu/ipgsa/auxins

[5] Simon, S., & Petrášek, J. (2011). Why plants need more than one type of auxin. Plant Science,180(3), 454-460. doi:10.1016/j.plantsci.2010.12.007

[6] Fattorini, L., Veloccia, A., Rovere, F. D., D’Angeli, S., Falasca, G., & Altamura, M. M. (2017). Indole-3-butyric acid promotes adventitious rooting in Arabidopsis thaliana thin cell layers by conversion into indole-3-acetic acid and stimulation of anthranilate synthase activity. BMC Plant Biology17, 1–14. https://doi.org/10.1186/s12870-017-1071-x

[7] Mansouri, H., & Talebizadeh, R. (2017). Effects of indole-3-butyric acid on growth, pigments and UV-screening compounds in Nostoc linckia. Phycological Research65(3), 212–216. https://doi.org/10.1111/pre.12177

[8] Jayasinghege, C. P. A., Ozga, J. A., Waduthanthri, K. D., & Reinecke, D. M. (2017). Regulation of ethylene-related gene expression by indole-3-acetic acid and 4-chloroindole-3-acetic acid in relation to pea fruit and seed development. Journal of Experimental Botany68(15), 4137–4151. https://doi.org/10.1093/jxb/erx217

[9] Sumayo, M. S., Son, J.-S., & Ghim, S.-Y. (2018). Exogenous application of phenylacetic acid promotes root hair growth and induces the systemic resistance of tobacco against bacterial soft-rot pathogen Pectobacterium carotovorum subsp. carotovorum. Functional Plant Biology45(11), 1119–1127. https://doi.org/10.1071/FP17332

[10] Yu, H. S., Lee, J. M., Youn, Y. S., Oh, K. T., Na, K., & Lee, E. S. (2018). γ-Cyclodextrin-phenylacetic acid mesh as a drug trap. Carbohydrate Polymers184, 390–400. https://doi.org/10.1016/j.carbpol.2017.12.078