Plants Defend Themselves Chemically

Udo Blum, professor of botany, Department of Botany, North Carolina State University, Raleigh.

Plants contain and may subsequently release into the environment molecules that provide them with defenses against disease, nematodes, herbivores, and other plants. Because of genetic differences and continuous selection pressures from the natural environment, the distribution of such molecules is irregular. In addition, the concentrations of such plant molecules are determined by the environment in which they grow. Thus potential levels of chemical defenses are determined by the genetic makeup as well as the growth environment of the plant. In cases where molecules must first enter the soil before acting on an associated species, the role of soil factors such as microorganisms, amount of organic matter, and soil pH also are important in determining the active concentrations.

Little information is available on the mode of action of most of these molecules. Such information can be useful in manipulating interactions to the benefit of agriculture. For example, toxicity of simple phenolic acids in soil solutions can be minimized or enhanced by manipulating soil pH and soil nutrition.

The plant kingdom is a vast storehouse of chemical molecules waiting to be identified, isolated, manipulated, and used. With the advent of molecular biology and biotechnology, new opportunities for effectively using such molecules in agriculture are now at hand.

Before discussing specific plant chemical defenses, let us look at the kinds of molecules found in plants.

Plant Molecules

Apart from the short period during seed germination when plants are heterotrophic (dependent on their seed stores for energy), all higher green plants are autotrophic (capable of obtaining energy from the sun). This fact provides a clue to the enormous biosynthetic abilities of plants. Plants can make universally required molecules such as DNA, RNA, carbohydrates, lipids, and proteins from inorganic materials that include CO, H2O and mineral nutrients (N, P, K, etc.). They also can synthesize all the complex molecules which contribute to the makeup of their tissues. Carbohydrates, lipids, proteins, and nucleic acids are important for growth and development in both plants and animals.

However, among the compounds synthesized by plants area series of complex materials that appear to have no immediately obvious metabolic function in plants (called secondary metabolites). These include alkaloids (complex and bitter-tasting substances such as quinine, morphine, and nicotine), phenolics (such as tannins), and terpenes (compounds found in odor-giving oils such as peppermint).

Over the last 100 years 5 percent of the world's plant species have been examined for alkaloids resulting in the isolation and identification of 5,600 alkaloids. All alkaloids contain nitrogen, frequently as part of a heterocyclic carbon ring system. They are classified either according to their ring system or according to the amino acid from which they are derived.

Plants also produce thousands of compounds that contain one or more phenolic residues. These compounds can be divided into major groups according to the number of carbon atoms in their skeleton. Most phenolics arise from a common biosynthetic intermediate, phenylalanine or its close precursor, shikimic acid.

Finally, there are probably more terpenes than any other group of plant products. The term "terpene" is used to denote branching compounds containing five carbon units. Terpenes are essential to plant growth since chlorophyll and carotenoids, which are essential for photosynthesis, also are derivatives of terpenes. The roles of alkaloids and phenolics in plant metabolism have not been clearly identified. (For more information, you may wish to read J. B. Harborne's book, Introduction to Ecological Biochemistry, Academic Press, 1983.)

Plants provide substances found in about a fourth of all prescription drugs.

Plant Chemical Defenses

Have you ever wondered why some plants are resistant to fungi, bacteria, nematodes, or herbivores while others are clearly susceptible to such organisms? Or wondered why some plants are more effective in reducing the growth of associated plants than others? There is ample evidence to show that at least some of these differences in behavior are associated with the presence and absence of chemical defenses produced by plants.

Defense Against Disease.

Plants are resistant to the majority of micro-organisms, including pathogens, that come in contact with them. Much of this resistance is associated with plant-produced molecules. Each plant species or even variety has its own unique set of molecules. Tomatoes make a number of cyclic and acyclic mono- and sesqui-terpenes, for example, a-pinene and a-humulene respectively, which fight fungi. This was discovered when it was observed that plants with many glandular hairs, sources of these compounds, were infected less by fungi than were plants with fewer glandular hairs.

Plants also produce a variety of substances called phytoalexins. These substances are produced by a plant only after it comes in contact with the pathogen. Each species produces only one or a few phytoalexins that usually are unique to closely related species. Most phytoalexins within a plant tribe or family are similar in structure. For example, cotton sesqui-terpenes isolated from Xanthomonas-inoculated leaves (bacterial pathogen) have been shown to exhibit antibacterial activity. Much smaller amounts of these compounds were isolated from both inoculated susceptible as well as uninoculated resistant leaves. Chemically characterized phytoalexins have been isolated from at least 15 plant families.

(For the source of the examples cited here and additional information, you may wish to read A. Stoessl's Secondary plant metabolites in preinfectional and postinfectional resistance," The Dynamics of Host Defence. J. A. Baily and B. J. Deverall (eds.), Academic Press, 1983.)

Defenses Against Nematodes. In a number of instances plant root exudates and decomposing plant debris have been observed to be antagonistic toward nematodes. For example, the presence of asparagus roots in soil leads to a decline in the number of stubby-root nematodes. In addition, the number of nematodes dc not increase even in the presence of a suitable host if its roots are intermingled with those of asparagus.

(For additional information you may wish to read N. A. Minton's "Plant-nematode relationships of an allelopathic nature," Report of the Research Planning Conference on the Role of Secondary Compounds in Plant Interactions. USDA, 1977.)