The Selfish Plant 4 – Plant Proprioception?

Proprioception: sensing of the posture, shape and relative orientation of body parts

Are Plants Self-Aware?

No, I’m NOT referring to plant self-awareness as it relates to plant “consciousness”. (If you’re interested in such metaphysical aspects of plant physiology, please see here, for example.)

What I’m referring to is the notion that plants are physically self-aware. In other words, can a plant perceive the configuration of its own body?

In zoology this concept is sometimes referred to as “proprioception”.

The term “proprioception” was first used over 100 years ago by neurophysiologist Charles Scott Sherrington to define an animal’s sense of the relative position of the parts of its body.

Close your eyes. Now touch your nose with the tip of a finger. That’s an example of an ability made possible by proprioception.

In animals, mechanisms of proprioception are complex, the result of multiple inputs, both internal and external, all integrated and interpreted by the brain.

In the past few years, some plant scientists have suggested that proprioception also occurs in plants and, moreover, significantly affects plant development.

Shape-Sensing in Plants?

As far as I know, a paper published in 2013 (see Ref. 1 below) may have been the first to propose that proprioception is a significant factor in plant development. Briefly, the authors analyzed shoot growth in response to gravity and concluded that the observed curvature was a result not only of shoot gravity-sensing but also localized self-sensing of the shoot shape.

“Localized” may be the key word with regard to invoking proprioception in plants, and also how this concept differs in plants compared to proprioception animals.

On using the term “proprioception” with regard to plant development: “The overall picture is still rudimentary, but it bears some similarity to animals, with two types of proprioception. The first is a basic proprioception that allows for shape and growth activity control in the meristems and growth zones. Proprioceptive capacities are shared by all cells. In growing organs at later stages (e.g. stem, roots), a second type of proprioception may involve more specialized cells and cross-talk with other sensory specialized cells (e.g. the statocytes involved in gravitropic sensing) to achieve proper posture control and shape resilience to the mechanical hazards of the outer world…” (from Ref. 2 below)

Seems to me that what they’re saying here isn’t really new; they’re just trying to appropriate an animal physiological concept that relates to the whole organism and to apply it to plants at the cellular level.

That plant growth and form are the result of both internal (hormones, mechanical) and external (gravity, light, wind) factors have been known for decades (centuries?). Heck, even I had a post on the internal, mechanical affects on plant development in 2012: How Stress Shapes Plants.

Thus, borrowing the term “proprioception” from zoology and trying to apply it to plants may be a bit too much of a stretch. (Please see here for my rant regarding why I think we shouldn’t, in general, try to zoomorphize plants.) Would using the concept of automorphogenesis be more appropriate?

Anyway, what’s new and exciting now, however, is the computational and molecular research being conducted on how internal mechanical forces may affect differential gene expression, which, ultimately, determines plant development.

But back to the question: Are Plants Self-Aware?….

Do Plants “Know” How Big They Are?

One way plants may be able to determine their relative size is by “node counting”. That is, the more nodes (stem buds/leaves) the plant has, the bigger (more productive) it is. (For all you scholars out there, an exhaustive review of “node counting” can be found here.)

A plant may also gauge its size by how far the shoot apical meristem (SAM) is from the roots. Or a plant may determine its overall size by how big a root system it has. There is scientific evidence for all of these possibilities. However, the key to all of them is that the nodes, the roots, or both produce chemical signals (likely one or more of the common plant hormones) that travel via the phloem to the SAM.

Bottom Line: Are plants self-aware, physically speaking? At the whole plant level, I think not. Certainly, not like animals are. Active areas of plant growth and development may be affected by local, physical factors and also by plant hormones originating from distal parts of the plant. But lacking a central processing unit, i.e., brain, plants don’t really have a way to integrate multiple inputs to create an awareness of a single, unified whole shape.

References

1. Bastien, R., T. Bohr, B. Moulia and S. Douady (2013) “Unifying model of shoot gravitropism reveals proprioception as a central feature of posture control in plants.” PNAS, Vol. 110, pp. 755–760. doi: 10.1073/pnas.1214301109 (Full Text)

2. Hamant, O. and B. Moulia (2016) “How do plants read their own shapes?” New Phytologist, Vol. 212, pp. 333–337. (Full Text)

3. Chelakkot, R. and L. Mahadevan (2017) “On the growth and form of shoots.” Journal of the Royal Society Interface, 2017 14 20170001; DOI: 10.1098/rsif.2017.0001. Published 22 March 2017. (Full Text – PDF)

“The Shape I’m In” by The Band

The final stop on our “Selfish Plant” tour = Selfish Genes in Plants

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