How Plants Use Cyanide to Protect Themselves

Since plants are fixed in place and cannot move to elude their attackers, they have developed an elaborate array of chemical defenses to protect themselves from herbivores and pathogens. One way in which they defend themselves is through the production of toxic secondary metabolites. One group of these compounds is known as preformed inhibitors or phytoanticipins. They are present in an inactive form. A large subgroup of these molecules contain hydrogen cyanide (HCN) bound to a sugar molecule. These compounds are activated upon attack and poison the attacking organism by interfering with cellular respiration.

Many Plants Produce Cyanogenic Glucosides as Secondary Metabolites

As opposed to secondary metabolites that are induced after infection or attack by herbivores, preformed inhibitors are always present in a plant. They remain inactive, however, until their activation is triggered by the attack of an herbivore, such as an insect, or a pathogen. There are several classes of these compounds. One important class is the cyanogenic molecules—those that contain cyanide. Over 2650 species of plants contain HCN, ranging from ferns to common crop plants such as sorghum and cassava.

The cyanogenic compound is kept inactive by being conjugated to a sugar group. It is then known as a cyanogenic glucoside. This conjugation also allows the compound to be stored at high concentrations in the plant’s vacuole or in protein bodies. Enzymes called beta-glucosidases release the cyanogenic molecule. There are a number of different beta-glucosidase enzymes, specialized for specific cyanogenic glucosides.

This enzyme cleaves off the sugar molecule, releasing the compound that contains the cyanide. The beta-glucosidase is often stored in a separate compartment. It only encounters the molecule that contains the cyanide when the tissue is disrupted, such as by being chewed by an insect or animal. Once the beta-glucosidase is released from its compartment, it is able to react with the cyanogenic glucoside. This releases an unstable molecule, which then produces free HCN.

Food Crops Often Contain Cyanide

A large number of commonly grown food crops contain cyanogenic glucosides. It is thought that these secondary metabolites have been inadvertently bred into crops as natural pesticides to confer resistance against insects and other pests. Maize, wheat, rice, and barley all contain some level of cyanide in their leaves. With most food crops, there is little cyanide found in the parts of the plant eaten by humans.

An exception is the common crop cassava, the major staple food in many developing countries. This plant has such large amounts of cyanide in its roots they have to be specially treated to remove the cyanide for them to be safely edible. Cyanide poisoning from cassava is common enough to cause a condition of chronic paralysis known as konzo. Cassava plants have been genetically engineered to reduce cyanide production to ameliorate this problem. Humans are more likely to have problems detoxifying cyanide when their diet is low in protein, such as when it is cassava-based.

Biochemistry of Cyanogenic Glucoside Metabolism in Sorghum

The pathway of cyanide metabolism in cyanogenic plants is most thoroughly understood for sorghum and cassava. In sorghum, dhurrin is the cyanogenic glycoside that contains the HCN. It is located in the epidermal cells of the plant, while the beta-glucosidases that are specific for dhurrin are located in the chloroplasts. This separation of enzyme and substrate is referred to as compartmentalization. They do not meet unless the tissue is degraded.

A number of enzymes are involved in the synthesis of dhurrin, including two cytochrome P450s and a UDPG-glucosyltransferase. The biosynthetic compounds are organized as a group, so that the product of one enzyme can travel quickly to the next enzyme. Also, this reduces the chances of the plant being poisoned by the cyanogenic product. After tissue damage, the dhurrin is cleaved by a beta-glucosidase, releasing an alpha-hydroxynitrile compound.This is then cleaved enzymatically, releasing HCN.

The whole biosynthetic pathway has been transferred to the model plant Arabidopsis thaliana. These genetically engineered plants now possess the capability to produce cyanide and have an increased ability to deter flea beetles. This experiment provided definitive evidence that HCN production is a plant defense mechanism against herbivores.

References:

Jones, A.1998. Why are so many food plants cyanogenic?  Phytochemistry 47:155-162

Morant, A.V., K. Jorgensen, C. Jorgensen, S.M. Paquette, R. Sanchez-Perez, B.L. Moller, S. Bak. 2008. B-glucosidases as detonators of plant chemical defense. Phytochemistry 69:1795-1813

Tattersall, D.B., S. Bak, P.R. Jones, C.E. Olsen, J.K.Nielsen, M.L. Hansen, P.B. Joh, R.L. Moller. 2001. Resistance to an herbivore through engineered cyanogenic glucoside synthesis. Science 293:1826-1828

The buzzing; the buzzing

As you well know, caterpillars are major pests of plants. What you may not know is that they feed constantly during their waking hours. Life is not all paradise for them, however since they can serve as food to other organisms. One of their afflictions is parasitic wasps, which lay their eggs on the caterpillars. The wasp larvae then eat them from the inside (isn’t Nature wonderful?). To avoid this fate, caterpillars have become very sensitive to the presence of wasps. They have sensory hairs that detect the airborne presence of the wasps. This causes them to “stop moving…or drop off the plant.”

Since caterpillars have such an extreme reaction in the presence of the parasitic wasps, Tautz and Rostas of the University of Wurzburg in Germany wondered if the sound of honeybees might have an adverse effect on the caterpillars. They studied beet armyworm caterpillars (Spodoptera exiguea) that feed on a large number of plants, efficiently sense the wasps’ airborne vibrations, then stop moving or drop off the plants in the presence of their parasites. They set up bell pepper or soybean plants in experimental tents. One tent was connected to a hive that enabled the bees to fly over the plants to two feeders filled with non-scented sugar solutions. They had another control tent that had the same number and species of plants and caterpillars, but did not have bees.

The caterpillars reacted to the bees just like the wasps and destroyed 60-70% less foliage than in the tent without the bees! The bees just flew over the caterpillars; they did not approach them. This indicates that the vibration alone of the bees buzzing by was enough to stop the caterpillars in their tracks! This suggests a novel way of protecting plants without using pesticides. One can only wonder if the caterpillars would evolve to ignore the vibrations of the bees and continue their munching. Despite our best efforts at plant control, the targets frequently evolve a resistance to our efforts.

Reference:
Tautz, J. and M. Rostas. 2008. Honeybee buzz attenuates plant damage by caterpillars. Current Biology 18:R1125-R1126.

 

Chili Peppers Fight Back

As you may know, spices have been precious commodities for millennia. Wars have been fought over spices, and when Alarich threatened Rome in 409 AD, he demanded precious furs and metals and 3,000 pounds of peppercorns as tribute to refrain from attacking. (He returned two years later and sacked Rome anyway.)

The reason for the importance of spices is thought to be their antimicrobial properties. Especially in the days before refrigeration, it was vital to make efforts to preserve food from spoiling. This was particularly true in the tropics and for meat dishes. One of these spices is chili peppers, which are thought to have been used by Mexicans as long as 7,000 years ago. Recent work by the Tewksbury lab at the University of Washington has shed light on the mechanism for the antimicrobial properties of chili peppers.

The main ingredient in chilis that gives them their bite, or “pungency” to be technical, is a secondary compound called capsaicin. Tewksbury et al. 2008 studied the effect of capsaicin on a fungus that infects an ancestral variety of chili called Capsicum chacoense. The fungus is introduced into the seed by the probing of insects that feed on the chilis (technically a fruit). They found that the more the insects probe, the greater the degree of infection in the chilis. They also looked at the effect of capsaicin on the growth of the fungus in artificial fruit varying in the amount of capsaicin and a related compound. The capsaicin inhibited the growth of the main fungal species that attacks the fruit, indicating that capsaicin is truly antimicrobial.

However, even more definitive evidence was obtained from populations of the ancestral chili plants that the researchers found growing throughout Bolivia, thought to be where the chili ancestors evolved. They found that the plants varied in their degree of pungency—some were spicy, while others had no capsaicin and were not spicy. This breakthrough of finding natural variability in pungency enabled them to test for the effect of capsaicin in wild populations of the chili plants. Using the degree of insect probing as a measure of infection, they found a strong correlation between the degree of pungency and the tendency to have been probed by an insect. This indicated that the fungal infection had exerted a selective pressure on the population of chili plants, driving them to be spicy to fight back against infection. Thus, the chilis fight back!

This is another example of the utility of plants’ arsenal of secondary compounds, many of which appear to have evolved to defend the plants against attack by microorganisms, insects, and vertebrates. This is a topic dear to my heart, and you will be sure to hear more about it in the future (if you read my blog).

Sources
Tewksbury, J.J. et al. 2008. Evolutionary ecology of pungency in wild chilies. Proceedings of the National Academy of Sciences USA 105:11808-11811

Sherman, P.W. and S.M. Flaxman. 2001. Protecting ourselves from food. American Scientist 89:142-151

By Helga George