Understanding Plant Hormones


  • You’ve seen auxin in action. Well you haven’t seen the actual auxin molecule itself with the naked eye, but you’ve seen what it can do to a plant grown near a window. Have you ever wondered how a plant bends towards sunlight? Well, it has to do with auxin in the stem.
  • Auxin has lots of jobs but most importantly it stimulates growth, and if a plant doesn’t naturally produce auxin itself, it will die. So you can see auxin is pretty important.
  • The technical alias for auxin is indole-3-acetic acid or IAA (just incase you ever see it written is “IAA” – it means the same thing as “Auxin”).
  • Auxin is involved in cell growth and cell expansion, so it is produced primarily in parts of the plant that are actively growing like the stem (specifically, the very tiptop of the stem).
  • This is where it gets interesting. Auxin is transported (read: active process – requires energy) in one direction in a plant – downward from the top to the bottom, like a one-way road from the stem tip to the roots.
  • It is the only plant hormone known to do this.
  • Therefore the concentration of auxin is highest at the top of the plant and decreases as you get closer to the roots, this controls the overall shape of the plant and helps keep the primary stem of a plant the leader.


  • Have you ever noticed that if you put a really ripe, brown banana right next to a bunch of green bananas, the unripe bananas will ripen and turn yellow much faster? How does that happen? Well, the brown banana is communicating with the green bananas using a hormone called ethylene.
  • Ethylene is a plant hormone that affects ripening and rotting in plants. It is a particularly interesting plant hormone because it exists as a gas. No other plant hormone is gaseous!
  • Ethylene can be produced in almost any part of a plant, and can diffuse through the plant’s tissue, outside the plant, and travel through the air to affect a totally different plant. How cool is that!


  • When our bodies need water we feel thirsty.
  • The “thirst signal” signifies that we’re dehydrated and we need a drink of water. When a plant needs water, for example during a drought, it doesn’t have too many options. A rain dance is pretty much out of the question.
  • Plants produce a chemical messenger, called abscisic acid, to alert the rest of the plant that it is water stressed.
  • Abscisic acid is made in droughted leaves, droughted roots, and developing seeds and it can travel both up and down in a plant stem in the xylem or phloem sounding the alarm.
  • Think back to transport in plants, how does water typically move through a plant? (Reminder: soil -> roots -> stem -> leaves -> air) Water molecules exit a plant through tiny pores in the leaves called stomata. Each stoma (singular) has two kidney bean shaped bodyguards on either side of the pore, whose job it is to open and close the stoma.
  • When the guard cells are full of water, or turgid, the stoma is open. When water leaves the guard cells, they become flaccid, and the stoma is closed.


  • Who knew that fish could play a role in the discovery of a plant hormone? Aged herring sperm DNA can promote cell division.
  • The molecule that is responsible for this was named kinetin. Soon after, a substance that had the same biological effect as kinetin was found in plants, it stimulated plant cells to divide when in culture with auxin.
  • The substance was named cytokinin and it is involved in cell division and in the making of new plant organs, like a root or a shoot.
  • Cytokinins are produced in the root apical meristems (very tip of the roots) and travel upward hitching a ride with water and traveling up the stem through the xylem. The movement of cytokinins is passive – it does not require energy!
  • Cytokinins are like the fountain of youth in plants.
  • They delay senescence or the natural aging process that leads to death in plants.
  • In the cell cycle, cytokinins promote the movement from the G2 phase to the M phase. In other words, they encourage cells to divide!
  • Cytokinins are involved in repair, too.
  • If a plant becomes wounded, it can fix itself with the help of cytokinins and auxin. Remember how some hormones work together to affect plants? Well if the concentration of auxin and cytokinin are equal, then normal cell division will take place.
  • If the concentration of auxin is greater than cytokinin then roots will form. If the concentration of auxin is less than cytokinin then shoots will form.


  • Gibberellin causes some similar effects in plants as auxin, but it is a very different hormone.
  • Gibberellins were discovered originally in Japan.
  • A fungus called Gibberella fujikuroi infected rice plants and caused them to grow too tall and fall over. The infectious fungus produced a chemical that stimulated the growth in rice plants. The chemical was isolated and named Gibberellin after the fungus.
  • It was later found that plants naturally produce variations of these chemicals!
  • Gibberellins play an important role in several developmental stages in plants, but their claim to fame is making stems longer.
  • Gibberellins promote stem elongation between nodes on the stem.
  • A node is a place on a stem where a leaf attaches, so gibberellins elongate the internodes.
  • It is easiest to see the absence of gibberellin in dwarf plants and rosette plants – there is very little space between nodes on a stem and the leaves are clustered toward the base of the plant.

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