The froth is alive
Cell biology is a bit off my usual interests, but the title of this paper was just irresistible: Turning a plant tissue into a living cell froth through isotropic growth. Leaving aside the isotropic growth for now, the idea of a “living cell froth” intrigued me, even when I found it the day before one of my exams a few weeks ago.
First, it’s worth explaining that cells aren’t all that different from bubbles. A soap bubble is a thin layer of water and soap molecules, separating one bit of air from another. A cell membrane is, roughly speaking, a thin layer of fat separating one watery bit from another. A froth is where, rather than free floating bubbles, lots of them are joined together, forming many compartments of air or water.
Living things obviously don’t grow as froth, though (froghoppers produce froth, but the insect itself is hiding inside). So what’s the difference? All cells have a degree of internal structure and organisation from the ‘cytoskeleton’—networks of tiny fibres running along inside the cell membrane, and anchored to it. Imagine gluing thin threads to the inside of a balloon: you’d be able to pull it into particular shapes, so long as the pressure wasn’t enough to unglue them.
Perhaps more importantly, most of life also has cell walls. Animals, and a few microbes like amoebae, are the exception here—plants, algae, fungi, bacteria, and others all have cell walls, made from a variety of materials. Plant cell walls make up the ‘fibre’ in our diet. Returning to our imaginary balloon, imagine blowing it up inside a sock: its size and shape will be limited by the sock, and, crucially, you can’t blow it up until it bursts (cells can also burst; it’s called ‘lysis’). There’s a twist in the tale, though—the cell wall is produced by the cell itself. It’s as if the balloon, while it’s being blown up, can decide on its own size and shape.
Enough about balloons; what did they do?
‘They’ are a group of French scientists, in Paris and Lyon. They grew the favourite lab plant, thale cress (known as Arabidopsis), and treated it with a particular chemical, oryzalin, which damages plants by affecting the cytoskeleton. Normally, there are a number of threads (microtubules) lying parallel inside the cell. These, in turn, act as a template for the production of the cell wall, determining which direction the cell will be allowed to expand, and where it will be held in. When oryzalin is added, the plant still makes cell walls, but they’re just produced wherever the physics of cell membranes leaves them, leading to a froth.
It’s not just looking something that looks like a froth, either: alongside microscope images of misshapen plant shoots, they’ve analysed the geometry of the froth to quantify the differences, and even made computer models of froth for comparison. A bit of maths suggests that, if you slice through an unstructured froth, and measure all the angles between membranes, they’ll be near 120° (you need three meeting to get an angle, and 360° ÷ 3 = 120°). Sure enough, the oryzalin-treated ‘frothy’ plants had their membranes at angles close to 120°, whereas the angles in normal plants were spread much wider. The modelling also suggested a link to cell division: when the model cells couldn’t split themselves, the structure became ‘frothier’.
Besides some nice pictures (which I’ll put up if I get permission), I can’t see any immediate practical implications. Froth is not a particularly good way to make plants grow. Further out, though, understanding how plants shape themselves could let us make stronger plants, or thinner ones, or more or less straight ones, or whatever seems like a good idea at the time. There’s also interest in the structure of the cell wall, because it represents a lot of stored energy, which we can’t yet use in biofuels (running cars on wood would be tricky). Maybe it’s even relevant to the origins of life; some theories do suggest that the first cells were formed from a sort of foam, before later evolving the ability to control their shape.
Reference: Corson, F., Hamant, O., Bohn, S., Traas, J., Boudaoud, A., & Couder, Y. (2009). From the Cover: Turning a plant tissue into a living cell froth through isotropic growth Proceedings of the National Academy of Sciences, 106 (21), 8453-8458 DOI: 10.1073/pnas.0812493106