Listen to part of a lecture in a botany class.

P: So, we've learned that plants like animals have complex immune systems. Plants' immune systems, of course, evolved to protect them from pathogens, which are disease causing microbes, such as bacteria and fungi that may infect plants. For example, we discussed how individual plant cells fight off some of the harmful microbes. However, when fighting off pathogens, a plant's immune system is not on its own. Plants get help from a wide variety of microbes in the soil. Soils are teeming with these tiny, beneficial microorganisms. Any idea how they might reduce the risk of infection? Think in terms of nutrient availability, Jessica?

S: Well, if the soil is loaded with these good microbes, the good microbes might use up all the essential elements in the soil that the bad ones also need to survive.

P: Right! The good and bad microbes compete for the same nutrients. The good ones which are much more plentiful, usually crowd the harmful ones out. Even when some pathogens managed to survive, there's usually not enough of them to kill any nearby plants. This is the general defense that is provided to plants by beneficial microbes in the soil. But certain microbes also target specific pathogens. Take the pseudomonas bacteria, pseudomonas bacteria protect wheat plants from disease caused by a particular fungus that attacks the roots of the plant. Pseudomonas bacteria produce a substance that's toxic, poisonous to the disease causing fungus.

S: So from the plant's point of view, the toxic substance is medicine?

P: That's a good way of thinking about it. When wheat fruits get damaged by the fungus, they leak out nutrients, which the pseudomonas bacteria feed on. Thus, the sick plants attract the bacterial species that poisons the dangerous fungus.

S: Wow, so wheat plants are able to fight off the disease because they produce food for the bacteria that cures it. P: Fascinating, isn't it?

P: Of course, plant defenses aren't perfect. As any farmer will tell you, crops are always vulnerable to disease. It's a constant battle. To kill pathogens, many farmers and gardeners use pesticides. But pesticides aren't always successful either. And as you know, they often have undesirable side effects for both people and the environment. So, based on today's lecture, what do you think might be another way for people to protect plants?

S: I'm guessing, could we put additional beneficial microbes into the soil?

P: Garden stores do sell reproductive cells that come from beneficial microbes. The idea is that once you add these reproductive cells to the soil, the microbes will grow and multiply. The thing is microbes rarely flourish when transported to a new environment. You see, microbes naturally live in complex ecosystems where they depend on local conditions and a variety of local species. Transplanting microbes into your garden or field isn't going to do much if they can't survive there. A better approach may be to support the protective microbes that are already in the soil. In one experiment, researchers added a product known as seed meal to the soil in an apple orchard. The seed meal used in the experiment, contained the husks, the outer shells of mustard and canola seeds. Once in the soil, seed meal increased the growth of a type of microbe that helps defend apple trees from diseases.