“Unzipped”: How Plants Grow – Part 3

Turgor Pressure + Plant Cell Wall “Loosening” = Growth.

In the previous post, we explored the question of whether or not plants grew at night.

The main point of this story was that plant cell turgor pressure is the force that drives plant cell enlargement, and thus plant growth.

But turgor pressure alone is not enough to result in the enlargement of plant cells.

In order to yield to this turgor pressure, the rigid plant cell wall needs to become more extensible or “loosened”.

How to envision plant cell wall “loosening”?

Here’s a visual aid I sometimes used in plant science classes when I wanted students to think about the nature of plant cell enlargement:

Hold your hands in front of you, and interlace your fingers together tightly.

With your fingers tightly gipped together – really tight – try pulling your hands apart. You’ll likely have to use a fair amount of force to get them to slide apart.

Now relax the tight grip among your fingers. Of course, now it takes a lot less force to slide your fingers apart.

In this case, your fingers are analogous to the bundles of cellulose that constitute the main structural component of primary plant cell walls.

The force you are applying to try to pull your fingers apart is analogous to the plant cell turgor pressure trying to force the cellulose strands apart.

And, finally, the tight grip you applied to keep your fingers from sliding apart when you tried to pull them apart is analogous to the biochemical/biomechanical (bc/bm) interactions that bind together the strands of cellulose, to keep them from sliding apart.

And by loosening your grip on your fingers, you mimicked what happens when some of these bc/bm* interactions are “unzipped” from the cellulose. As a consequence, the normally rigid plant cell wall becomes much more extensible, “loosened”, and it yields to the force of turgor pressure….and the plant cell enlarges.

But what is the nature of the bc/bm* interactions among the cellulose microfibrils, and how are such interactions “unzipped” to allow for cell wall “loosening” and, thus, growth?

The Mechanisms of Plant Cell Wall “Loosening”: An Enigma

Stretching (pun intended) well over 50 years, the story of the scientific research regarding the mechanism of plant cell growth, and, in particular, how the plant hormone auxin stimulates plant cell elongation, is long, complex, and not without controversy. (For example, see Refs. 1 and 5 below.)

Here’s a brief synopsis of this story (in somewhat chronological order):

Using auxin-induced plant cell elongation in young seedlings as a model system, investigators concluded that auxin stimulates cell enlargement by promoting cell wall extensibility (cell wall “loosening”).

Auxin does not directly interact with the plant cell walls to do this, and it requires living plant cells to elicit cell wall “loosening”.

Auxin triggers receptive plant cells to export some “wall-loosening factor(s)” from the cells into the cell wall region.

The “wall-loosening factor” turns out to be hydrogen ions. That is, auxin acts, at least in part, by causing receptive cells to acidify the cell wall region, which quickly (within minutes) induces cell wall “loosening”. This is the crux of the so-called “Acid Growth Theory”.

Auxin brings about cell wall acidification in receptive cells by stimulating the activity of plasma membrane-bound ATPase proton pumps that actively transport H⁺ ions from the cytoplasm across the cell membrane into the cell wall region.

Cell wall acidification leads to the activation of pH-sensitive proteins (enzymes?), already present in the cell wall region, that cleave (unzip?) load-bearing chemical bonds between cellulose and matrix polymers (e.g., pectins and hemicelluloses), leading to cell wall “loosening”.

Some of these pH-sensitive, wall-loosening proteins have been isolated and identified and are called “expansins”. “The movement of expansin along the cellulose surface may disrupt loosely-bound matrix polymers, with the result that the wall polymers move, or creep, resulting in turgor-driven wall extension.“

Recent experimental results (e.g., see especially Ref. 4 below), however, have challenged conventional models of the primary plant cell wall and, consequently, cellular and molecular mechanisms involved in cell wall “loosening”.

The complex and dynamic structural nature of plant cell walls, as well as experimental challenges involved in studying actively-growing plant organs and tissues, are just two reasons why the precise mechanisms involved in cell wall “loosening” leading to plant cell enlargement have evaded so many researchers for so long.

Bottom Line: Though the “unzipping” of bc/bm* interactions among cellulose microfibrils is likely critical for cell wall “loosening”, at the present time it’s unknown exactly what’s being “unzipped” and how.

*bc/bm = biochemical/biomechanical

References

1. Rayle, D. L. and R. E. Cleland (1992) “The acid growth theory of auxin-induced plant cell elongation is alive and well.” Plant Physiology, Vol. 99, pp. 1271-1274. (Full Text)

2. Cosgrove, D. J. (2016) “Catalysts of plant cell wall loosening.” F1000Research, Vol. 5, F1000 Faculty Rev–119. doi: 10.12688/f1000research.7180.1 (Full Text)

3. Arsuffi, G. And S. A. Braybrook (2018) “Acid growth: an ongoing trip.” Journal of Experimental Botany, Vol. 69, pp. 137–146. doi: 10.1093/jxb/erx390 (Abstract)

4. Cosgrove, D. J. (2018) “Diffuse growth of plant cell walls.” Plant Physiology, Vol. 176, pp. 16-27. doi: 10.1104/pp.17.01541 (Full Text)

5. Majda, M. and S. Robert (2018) “The role of auxin in cell wall expansion.” International Journal of Molecular Sciences, Vol. 19, 951. doi:10.3390/ijms19040951. (Full Text)

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