Roots on the Prowl

The Second Annual Oklahoma Evolution Road Trip is April 26-27. You can read about it here and the direct registration link is here.

And now for a story that you will probably think must be an April Fool’s joke. But it is real. We usually think of animals, especially charismatic predators, as foraging for food. They ramble through the underbrush looking for cute furry things to eat. In contrast, we usually think of plants as being passive. They are mere surfaces into which molecules and sunlight enter, and from which molecules exit: green leaf surfaces above ground, white leaf surfaces below ground. It is part of our bias, which goes all the way back to the first chapter of Genesis, of seeing animals as alive but plants as being merely a covering on the landscape.

But plants are active. They respond to their environments in many creative ways. Rather than to tell you about them, I will refer you to an interesting book by Daniel Chamovitz, What a Plant Knows. He stops short of endorsing the Trewavas idea that plants are intelligent, but Chamovitz certainly expands our view of plants as active respondents to their environments.

My own small contribution to this topic has been to develop a botany teaching activity in which students investigate foraging by roots. You can find this module online on the PlantEd page of the Botanical Society of America website; click on the document link at the top.

Roots do not just grow down into the soil. They have some kind of physiological feedback (which, not being a physiologist or molecular biologist, I cannot investigate) that allows them to proliferate when they encounter rich soil. They proliferate by growing a lot of branch roots. In contrast, they produce fewer branch roots and simply get on with the business of growing downward when they encounter poor soil.

Students can investigate root foraging by growing a sunflower seed in a clear glass cylinder that is filled with layers of rich soil alternating with nutrient-poor perlite. While soil and perlite are not identical in their physical properties, they are pretty similar (so long as the soil has enough peat in it to maintain air spaces); the main difference between them is the presence vs. absence of nutrients. Students can watch the roots grow downward through the perlite but proliferate in the soil.

Not only can they watch this process but they can measure it. They can measure the length of the roots using a map wheel. Of course, they can only measure the roots that are exposed to the glass, but this is likely to be the same in soil as in perlite. They can also, at the close of the experiment, harvest the plants and weigh the roots. And since, for each glass cylinder, they have a set of numbers, they can perform a statistical analysis.

This project also allows the students to consider experimental design. For example, it might make a difference whether the top layer is soil or is perlite. In other words, a cylinder with soil-perlite-soil-perlite might give different results from a cylinder that is perlite-soil-perlite-soil. So they try both arrangements. (It turns out to not matter.) Also, seeds placed on perlite might mold; soil microbes will prevent this. Therefore every cylinder has at least a thin layer of soil on top. Furthermore, it matters which species of plant you use. If you use a grass such as wheat or oats, the fibrous roots will not grow all the way down to the bottom of the cylinder. If you use beans, the roots will show no preference for soil over perlite, since the large seeds already have plenty of nutrients stored in them and the roots can form mutualistic associations with nitrogen-fixing bacteria. For both of these reasons, you would not expect bean roots to “care” whether the medium through which they are growing is rich in nutrients or not. It turns out that sunflower (which is a relatively small seed and does not form nitrogen-fixing nodules) is just about right.

This project also allows students to think of applications of this principle. Perhaps the most readily apparent application is that some plants are “hyperaccumulators” whose roots actually seek out toxic ions such as zinc or cadmium. Genetic engineers can make hyperaccumulator plants into superhyperaccumulator plants, if I may so call them. The roots of such plants seek out metal toxins in contaminated soil and remove them. This is fundamental to the process of bioremediation—the use of plants to clean up toxic waste sites.

What does evolution have to do with this? Natural selection has favored plants whose roots have ways of diverting their resources to areas of greatest benefit—e.g., that do not waste their resources growing roots in poor soil when there may be rich soil nearby. Presumably plants that grow in soil that is nearly always poor do not have this response (if anybody wants to investigate this, let me know at srice@se.edu. But why would plants seek out toxins? Some plants accumulate these toxins in their leaf vacuoles where they are not in contact with the metabolism of the cell cytoplasm but where they can spill out in the mouth of a herbivore that begins to eat the leaf. They constitute a chemical protection for the plant.

Just like my earlier report about the smoke-induced germination of wildflowers, this experiment required almost no budget. The most expensive part was the glass cylinders. But our department happened to have inherited a bunch of glass cylinders from the USDA. If you have some glass cylinders and a couple of map wheels lying around, consider trying this hands-on minds-on project.