The Sixth Sense: Where am I?

Updated: Feb 22, 2021

If you’ve ever closed your eyes and swayed, you’re closer to understanding how a plant perceives their environment. An organism managing to navigate itself and understand its environment is an impressive feat—most notably, the sense of proprioception. Another name for it is "the sixth sense," and it provides us with the ability to understand where our body exists in relation to itself and its environment.

Swaying with your eyes closed, feeding yourself, using the pedals in a car, and other activities that rarely require consistent visual attentiveness are examples of this phenomena. Performing these actions successfully is—in part—due to our body consistently receiving information about gravity. Through otoliths in our inner ear, that fall in response to gravity, our sense of balance is realized. In plants, the story is similar. Gravity sensing otoliths in our inner ear are similar in function as the statoliths in a plant, which are located in the root cap and endodermal tissue. If you turn a plant on its side, the root cap will turn down towards gravity, and the shoot will respond in the opposite direction—positive and negative gravitropism, respectively. Additionally, if a plant’s tip position is recorded through the day, you will see a pattern that varies in size, shape, and speed; a process called circumnutation. Through these functions, plants survey their environment, find balance, and decide how to grow.

While the sway-dancing is innate to plants, their patterns will be less dynamic in the absence of gravity; in order to fully appreciate their environment, plants need gravity to help understand where they are. Once their gravitational information is received, the plant hormone auxin is distributed to areas that need elongation. In the shoot, elongation occurs on the side receiving light and opposes gravity. In the root cap, auxin will instruct the root to travel downwards. As you can imagine, receiving all these inputs could result in a plant falling over, which is why sensing gravity and understanding where to go is imperative to a plant’s survival. Similarly, if we dance with our eyes closed, we can thank our gravity perception to stay upright on the dance floor.