Intro to Phytohormones

Good afternoon, everyone! It’s a lovely, cold, sunny day here in the middle of Alabama, and I’m coming to you live with the last bit of studying that I’m going to get done before I go keyboard-mash my way to a SciFi novella. (What can I say? Old habits die hard.) As Junhi and co. studiously pore over their physics homework on the library table across the divider from me, I sit here, fangirling over plant hormones.

Yes, you read that right, fangirling. (Have I mentioned that I want to be a phytochemist? Well, I do. I want to be a phytochemist.) Plant hormones are great.

“Agree to disagree, June!” you may be tempted to say. If you’re more like my roommate, Sapphire, you may be saying something more along the lines of, “That’s it, this is the final wedge between us.” Oh, come on! Before you make such hasty, sweeping statements, allow me to make my case.


I said before that plants are like aliens among us, and I continue to stand by that statement. They remain in a single spot for their entire lives. They can produce food from sunlight. They can regrow severed organs (or regrow from severed organs). The can–shudder–synthesize aromatic amino acids.

As different as plants are from humans, some things in biology, apparently, just work. Hormones are one of those things. Mammals have many hormones, as I’m sure you’re aware (my pet hormone is T3), that control many, many processes in our bodies. Plants, as it turns out, have them too.

Because we’re elitists, we’ve given plant hormones their own names–phytohormones. There are a great number of them, but for our purposes, we’re going to stick with seven: auxin, gibberellins, cytokinins, ethylene, abscisic acid, salicylic acid, and jasmonic acid.

You may recognize some of those (salicylic acid looks suspiciously like something you have in your bathroom, doesn’t it?), or you may not. Doesn’t matter. In the coming days, I’ll be publishing a blog post going over each of their structures and functions. Right now, however, it’s important to understand phytohormones in a general sense.

Phytohormones play a role in pretty much every part of a plant’s life. Embryogenesis? Check. Germination? Check. Cell division and elongation? Check. Cell differentiation? Check. Organogenesis? Yup. Sex determination? Yup. Reproduction? Yep. Stress response (abiotic and biotic)? Yepperino.

In fact, most hormones, when you really look at them, do most things, which is incredibly frustrating if you’re an aspiring biochemist. (“Ah, salicylic acid is an immune hormone! Wait, what are you doing messing around with seed germination? Uuugh…”) Thankfully, there are other commonalities, too, besides the processes that they like to muck around with.

For example, all hormones have to start somewhere–in other words, they have to be synthesized. Hormone biosynthesis is really tightly controlled, and many hormones can be conjugated with (attached to) other substituents (amino acids, carbohydrates, etc.) to change their activity. Sometimes, conjugation activates them, but other times, it marks them for storage or breakdown. The effect of the marker depends on the hormone.

Hormones also have to be transported, if they’re going to be of much use. (How helpful is it that the root can scream, “ASDFAKSDJ WE HAVE NO WATER” if the leaves can’t hear it?) Hormones can be transported through the vascular tissue either toward the shoot (through the xylem) or toward the root (through the phloem), or they can move across cell membranes and through special transporters. (Transmission through the vascular tissue is especially fascinating, because there are some hormones that only move in one direction–we call this “polar transport.”)

After a hormone reaches its destination, it must be detected. This is the job of particular proteins called receptors, which bind the hormone and end up fiddling with something else in the cell in response. In some cases, hormone binding results in phosphorylation of other proteins, which alters their activity and results in changes in gene transcription. Other times, hormone binding results in the breakdown of an inhibitor protein, which allows for the activation of the process the inhibitor was, well, inhibiting. (In my notes, I’ve written, “Destroy something to send a message–hecka,” and that’s pretty much how I feel about it.)

The resulting responses are diverse. Hormones can alter cell proliferation, elongation, and differentiation. They can promote or suppress branching in the shoot or root. They can open or close ion channels (which is important for, say, closing stomata in response to drought). They can trigger germination, or an immune response, or death. The possibilities are endless.

That was pretty convincing, right? You see that this isn’t hard, and it has the potential to be cool? (Because it does. It really really does.) You’re not gonna bail on me?

Awesome, because we’re just getting started.

You do you, fam.



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