Imaging technologies have come a long way since the invention of the microscope 400 or so years ago. Now we can look at the circulatory system in a developing chicken embryo or the hair cells on a terrapin’s inner ear, but there’s one very familiar place that remains a mystery: the inside of a plant pot.
There’s a good reason why it would be great to have a peek into the peat – to learn what’s going on between the plant roots and soil-dwelling microbes in situ (something we’ve written about before). These interactions are environmentally and economically important, and many have been intensely studied. The symbiosis between rhizobia and plant roots allows nitrogen fixing, and many pathogens, such as fungi of the genus Phytophthora, use roots as a route into a plant.
So what’s stopping us looking at the roots? The answer’s fairly obvious: you can’t see through soil. You can overcome this hurdle by using non-optical imaging techniques such as MRI to look at root structures, much as you can use them to see inside your head – but they can’t pick up light emitted by fluorescent proteins, for example.
I read a short paper this week that is making early inroads into another option: make soil see-through. Well, not soil precisely, but lumps of Nafion – a polymer invented in the 1960s that’s used in fuel cells. By grinding the Nafion down, researchers in Scotland have made it into particles similar to those found in soil.
Nafion isn’t normally see-through, but it has a refractive index very close to that of water. What that means in practice is that when you add water, the polymer disappears (this video shows something similar).
The Nafion particles that have been produced aren’t totally identical to soil, but they’ve been altered to have similar water and nutrient availability. It seems to work: plant growth in the transparent soil is comparable to growth in the real thing.
The researchers have already visualised GFP-labelled E. coli O157:H7 colonising the roots of lettuce seedlings. This bacterium, which is an important human pathogen, can grow quite happily on salad vegetables. The above picture is from the paper, showing the fluorescent E. coli on the lettuce roots.
This demonstrates how useful this technology could become. Although the authors concede that it’s not perfect for every type of plant–microbe interaction, there’s so much we don’t know about life in the loam that any insights we can get should advance both plant science and microbiology significantly.
Downie H, Holden N, Otten W, Spiers AJ, Valentine TA, & Dupuy LX (2012). Transparent soil for imaging the rhizosphere. PloS one, 7 (9) PMID: 22984484