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Cytokinins: Do You Have What it Takes?

THIS ARTICLE IS A WORK IN PROGRESS.

 Image 1: Cytokinin. One does not simply “say” they are a cytokinin [3].

Image 1: Cytokinin. One does not simply “say” they are a cytokinin [3].

“Cytokinesis,” from the Greek kyto, meaning, “hollow vessel” (referring to “the cell”) and -kinesis, meaning, “activation” (used in reference to the activation of cell division) is simply interpreted to mean “cell division” [9]. Now, it wouldn’t be too far of a leap in logic to think that cytokinin should mean “a plant hormone which promotes cell division” [I].

Well, you cannot simply put on shiny pants and start punching people and later convince the police that you are a “boxer.” You need advance permission, training, somebody to pay you to do it, drunken spectators… etc etc.

Similarly, as a prospective cytokinin, you cannot simply burst into the cytokinin dojo, start promoting cell division, and tell everyone that you are a cytokinin. No way!!!! Do you want to die?! You’ve got to look at yourself in the mirror with a critical eye. You must be honest with yourself; who am I? What am I made of?

You must ask yourself…

Do I have a similar structure to adenine (See Image 2 below) [I]?

 Image 2: Yes, this is the same adenine from the fab-four DNA nucleobases (G-C- A -T).

Image 2: Yes, this is the same adenine from the fab-four DNA nucleobases (G-C-A-T).

Do I affect similar physiological changes in plants as kinetin (e.g. can I influence cell division, bud/cell/leaf enlargement, and chlorophyll synthesis; see image 3 below) [I]?

 IMAGE 3: Kinetin. Kinetin was the first cytokinin ever discovered [1].

IMAGE 3: Kinetin. Kinetin was the first cytokinin ever discovered [1].

You must ask yourself, can I influence the growth of lateral buds and break apical dominance in tissue culture [1]?

If you honestly answered yes to all of those questions, maybe…MAYBE… you’ve got the goods.

So, hot shot, let’s take a crack at the title. What is a cytokinin?

Cytokinin Definition

*This definition relates only to adenine-type cytokinins created and found within plants.

Definition: a cytokinin is any one of a class of chemicals which resemble adenine in structure, and kinetin in function (e.g. promoting cell division, bud or shoot formation, breaking apical dominance, cell enlargement, and/or chlorophyll synthesis etc) [1].

Endogenous Adenine-Type Cytokinins

 Image 4: A Woefully Incomplete Group of Endogenous Cytokinins:  meta -topolin,  cis -zeatin,  trans -zeatin,  dihydro -zeatin, and isopentenyladenine. These are but a few cytokinins which are produced inside of the plant.

Image 4: A Woefully Incomplete Group of Endogenous Cytokinins: meta-topolin, cis-zeatin, trans-zeatin, dihydro-zeatin, and isopentenyladenine. These are but a few cytokinins which are produced inside of the plant.

Before we move on, let us consider this: plant hormones differ from (y)our hormones in a very striking way. While a specific mammalian hormone may influence a narrow set of factors, a plant hormone,— say, a specific cytokinin— may have a relatively massive range of physiological effects (termed pleiotropic effects) [2].

That is to say, plant hormones get in there and start busting heads all over the place.

Each cytokinin may have characteristic properties and influence. Today we talk about cytokinins that are considered “endogenous,” meaning, produced inside [of the plant].

Let’s consider some individually.

Meta-Topolin

“Leaves, HOLD!!!!!!”

 Image 5:  meta -Topolin (mT).

Image 5: meta-Topolin (mT).

An aromatic cytokinin in the chemical sense (see: aromaticity), 3-[(7H-purin-6-ylamino)methyl]phenol, or, meta-troponin (mT), was discovered in poplar leaves 1997 [4] [5]. It has shown greater bioactivity than benzyladenine and zeatin in its influence in shoot regeneration in vitro [5], and has exhibited an influence in delaying leaf senescence (the process of leaves withering and dis-attaching from the plant) [5] [6].

Can you do that stuff? I didn’t think so.

Moving along.

Cis-Zeatin

“AHHHHHHhhhHHHHhhHhh!!!!!!”

 Image 6: cis-zeatin.

Image 6: cis-zeatin.

There is a lot we do not know about the cytokinin (Z)-2-methyl-4-(7H-purin-6-ylamino)but-2-en-1-ol, which we all know and love by the stage name, cis-zeatin, or “cZ.”

What studies are starting to suggest is that—in addition to the expected kinetin-like activities— cZ is also starting to look like a stress-signal used by plants in response to pressure due to the occurrence of a pathogenic infection or generalized herbivory [7]. This has pretty cool implications depending on how you look at it.

What, WHAT?! Moment…

If we interpreted plant metabolites manufactured in direct response to stimuli (like herbivory or disease) as communication, we could interpret cis-zeatin as a scream. If we interpreted the release of cis-zeatin as a scream, we could ask ourselves: what “message” does the scream convey?

Grasshopper!? Powdery mildew!? Aphids!? Drought!? Is it a plant analogue to pain? Does it signal, “pay physiological attention here. I am sustaining damage which could become life-threatening.”

What if we could consider any given balance of physiologically active plant metabolites at any given time, how specific of a message could we get?

“I’m generally happy but I’m getting a little thirsty.”

“I’ve detected an unknown fungus on my root in the past hour.”

Now, due to the general pleiotropic nature of plant hormones, I doubt we will be getting very specific “messages” anytime soon, but the REALIZATION that we could conceivably understand a plant’s general existential “concerns” through the nuances of its own real-time chemical signature… that kind-of inspired me to wax science-fiction in your face. Sorry.

Exit Cytokinin and Tangent. Transfer to main article.

Thank you for riding, have a great day!

Now, this is not an organic chemistry website, so before we move on to trans-zeatin, we can get away with quickly outlining the differences between cis-zeatin and trans-zeatin through interpretive dance. That’s right! No rules, compadre. Hint: notice the footwork (see “Video 1” below)!

Video 1: Cis/Trans-Zeatin Interpretive Dance Moves Are Aesthetically Pleasing, or In Other Words, “Fly.”

trans-Zeatin

“The nitrogen is over there, pal!”

TRANS ZEATIN.png

Have you ever heard the adage, “the grass is always greener on the other side?” Well, essentially, this adage cautions people against moving around too much in search of something better. The adage implies that if one does not take care to appreciate what they have, they will waste their life wandering in search of some forever-illusive happiness “on the other side.”

But what if there really is no grass on the hill? What then?

Cows need grass, my wife needs coffee, and plants need nitrogen; if any one of these amazing beings cannot move to the substance of their desire, somebody will die.

In the big-bad world outside, plants really do find themselves situated in places with patchy sources of nitrogen. Recent studies suggest that trans-zeatin has a hand in influencing the movement of roots toward these pockets of nitrogen and even modifying any given root’s ability to transport said nitrogen when needed [8]. This, of course, is done through hormone mediated cell division.

This sounds a little like a plant analogue to cognizant decision-making doesn’t it? Hey, it doesn’t hurt to wonder.

Conclusion

Biochemists and botanists are discovering new compounds and properties under the cytokinin umbrella all of the time. This article, as well as other articles dealing with other classes of plant hormones, will be experiencing an ongoing expansion. Keep an eye out.

If you have heard any recent news on cytokinin research, please let us know in the comments below.

References

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

[2] George, E. F. (1993). Plant propagation by tissue culture (Vol. 1). Edington: Exegetics.

[3] Currier & Ives. (ca. 1883) The champion slugger--"Knocking 'em out" / Edw. W. Kemble, del. , ca. 1883. [New York: Currier & Ives] [Photograph] Retrieved from the Library of Congress, https://www.loc.gov/item/90707243/.

[4] National Center for Biotechnology Information. PubChem Compound Database; CID=11557770, https://pubchem.ncbi.nlm.nih.gov/compound/11557770 (accessed Oct. 21, 2018).

[5] Strnad, M., Hanuš, J., Vaněk, T., Kamínek, M., Ballantine, J. A., Fussell, B., & Hanke, D. E. (1997). Meta-topolin, a highly active aromatic cytokinin from poplar leaves (Populus × canadensis Moench., cv. Robusta). Phytochemistry, 45, 213–218. https://doi.org/10.1016/S0031-9422(96)00816-3

[6] Palavan-Unsal, N., Cag, S., Cetin, E., & Buyuktunser, D. (2002http://search.ebscohost.com.ezproxy.pueblolibrary.org/login.aspx?direct=true&db=aph&an=11166480&site=ehost-live). Retardation of senescence by meta-troponin in wheat leaves. Journal of Cell and Molecular Biology. Retrieved October 21, 2018, from http://search.ebscohost.com.ezproxy.pueblolibrary.org/login.aspx?direct=true&db=aph&AN=11166480&site=ehost-live

[7] Schäfer, M., Brütting, C., David Meza-Canales, I., Groβkinsky, D. K., Vankova, R., Baldwin, I. T., & Meldau, S. (2015). The role of cis-zeatin-type cytokinins in plant growth regulation and mediating responses to environmental interactions. Journal of Experimental Botany66(16), 4873–4884. https://doi.org/10.1093/jxb/erv214

[8] Poitout, A., Crabos, A., Petřík, I., Novák, O., Krouk, G., Lacombe, B., & Ruffel, S. (2018). Root Responses to Heterogeneous Nitrate Availability are Mediated by trans-Zeatin in Arabidopsis Shoots. doi:10.1101/242420

[9] Memije, A. (n.d.). A Dictionary Of Prefixes, Suffixes, And Combining Forms. Retrieved from https://www.academia.edu/26371550/A_Dictionary_Of_Prefixes_Suffixes_And_Combining_Forms











Kevin HealeyComment