Like all omnivorous foods, peppers – both hot and sweet – have been created in unique, wise ways to make them appealing to eat and to help reproduce.

Plants and their fruits are sustenance for herbivores, including humans. They are often brightly colored – be it red, orange, yellow, purple, or green, inviting us to a delicious food. But not all plants are meant to be eaten. Indeed, some plants are designed to be unappetizing, either through look or smell; some even have thorns, or sticky, hairy surfaces to deter us from eating them. Another remarkable way that plants are protected against herbivores is the presence of unique chemicals that induce vomiting and pain, or may be toxic.

Many edible plants need to be eaten – it’s how they spread their seeds. Thus, they produce juicy, tasty skin and sweet-smelling aromas. Why, then, are hot peppers different? Though they are nicely colored and juicy, and sweet smelling, they are also hot, and not terribly pleasant for animals to eat.

A recent study at New Mexico State University’s Chile Pepper Institute determined the hottest chili to be the Trinidad Moruga Scorpion Chili. It was chosen from among 125 varieties. Researchers dried and ground it to powder, to isolate its active compound. This way, they were able to determine that the Trinidad Moruga Scorpion reaches about 1.2 million units on the Scoville heat scale. It is so potent that it could induce sweating and tears, and of course puts the mouth on fire.

Figure 1. Molecular mechanism of TRPV receptor activated by capsaicin. TRPVs are located on the surface of nerve cells where they normally respond to changes in temperature by releasing calcium ions. Those ions signal to intracellular machinery to fire nerve action to inform the brain about increased levels of heat. Capsaicin in hot peppers mimics this system, and thus fools brain to think mouth is hot.

What make hot peppers hot?

Animals are equipped with receptors, like TRPV1 in the mouth’s nerve endings, which sense heat. Hot peppers produce a chemical called capsaicin. Capsaicin binds to and activates TRPV1 receptors; thus we feel a heat similar to a burning sensation. In reality, capsaicin does not actually increase the temperature in the mouth, but instead mimics the same process (Figure 1). Since capsaicin mainly dissolves in oil instead of water, drinking water does not help much to get rid of the burning sensation. Cold water provides only temporary aid. However, the drinking of ayran (a Turkish yogurt drink) relieves hotness by removing capsaicin due to the presence of oil in ayran. Interestingly, although mammals have receptors for capsaicin, scientists have recently discovered that birds don’t.

Capsaicin deters mammalian consumption

It is an interesting phenomenon that peppers are hot but need to be eaten to propagate their seeds in different environments, which is done through the droppings of animals. If this is the case, why are hot peppers made unpleasant with capsaicin? To figure out the wisdom behind this contradiction, Joshua Tewksbury performed a study with a group of mice and birds, and found that birds do not distinguish between sweet and hot peppers in their diet. In this study, both mice and birds ate the same amount when fed with food mixed with sweet peppers. However, mice refused to eat foods mixed with hot peppers, while birds happily ate such food. Moreover, analysis of the droppings of birds and mice showed that the seeds passed through the bird’s digestion systems were intact and fully fertile and could germinate, while seeds eaten by mice were either crushed or semi digested so that they were not fertile. Thus, the role of capsaicin in hot peppers is to deter mammals that destroy their seeds while not disturbing birds. This is a great example of ingenious interdependence.

Capsaicin as antifungal agent of peppers

Figure 2. Capsaicin is not only protective against mammals but also fungus contaminations. A) Insects make peppers prone to fungus contamination by causing harm. B) Healthy pepper C) A pepper with fungal contamination. Modified Image from Tewksbury lab.

The infinite wisdom of capsaicin protects peppers against fungus as well. Another study by Tewksbury showed that hot peppers are relatively protected against fungal infections. Tewksbury demonstrated that increased doses of capsaicin inhibit the growth of fungus. This finding is in parallel with lower fungus growth in hot peppers compared to sweet peppers.

But what about insects? How could peppers be protected from insects?

Adaptations against insects

Interestingly, the skins of hot and sweet peppers have different levels of thickness. It has been suggested that this gives an advantage to sweet peppers. Furthermore, this protective layer is made of lignin, which is made of the same material as capsaicin and helps to protect from other threats. This allows peppers to adapt to many different environments. For instance, in the presence of fungal contamination, a pepper might be able to produce more capsaicin and decrease lignin production, or vice versa.

Why do we like to eat hot peppers then?

Humans differ from mammals in their love of hot peppers. There are different explanations why we like to eat hot peppers, despite them being painful. Some experts assert that hot peppers are good for our health by lowering blood pressure, having antimicrobial effects, and increasing salivation thus making a boring diet fun. On the other hand, some experts approach it from the perspective of human emotions and argue that we are actually after the pain produced by hot peppers. In addition, there are some studies suggesting that capsaicin could also suppress other pains.

Hot pepper or capsaicin as pain suppressor

Capsaicin in hot peppers could be used as a pain suppressor, as some studies suggested. A study using mice lacking TRPV1 heat receptors showed that the increased long term activation of TRPV1 by capsaicin could relieve pain following the accumulation of high doses of Ca2+ in the cells. This is accomplished by the suppression of both cellular activities and the transmission of pain through nerves.

Hot peppers are hot and we love them. It seems like we will continue consuming them. As every other art of creation, hot or sweet peppers are likely to have many more levels of wisdom awaiting us to discover.

References

• Tewksbury Lab. Retrieved from: http://faculty.washington.edu/tewksjj/res_pai.html on 18.2.2012.
• Yalgın. Ç. 2012. Acı biberler niye acı? (Why Are Peppers Hot?) Açık Bilim. Retrieved from: http://www.acikbilim.com on 18.2.2012.
• Zivkovic, B. 2006. Hot peppers – Why are they hot? A Blog Around The Clock.

Southern Pine Beetles (Dendroctonus frontalis, figure 1) are one of the most harmful pests in the world, causing hundreds of million dollars worth of damage to pine trees in the Southern United States. These beetles initiate their attack on a pine tree by having a small group of female beetles dig into the inner park (Figure 2) and phloem of the pine tree. Once this work is done these females will secrete a chemical that will draw other male and female beetles to the affected pine tree. This initial attack is followed by a massive attack by a larger number of beetles, enabling them to easily overcome the defenses of the pine trees due to the sheer number of beetles. Shortly after taking over the tree, mating begins between the male and female beetles. As part of the mating process the females fill the excavated galleries within the trees with eggs. After the eggs have been laid the adult beetles will then leave the tree and continue to attack other trees. Attacking trees and causing their death is of course a sad story and may not sound very interesting since most pests have similar attack strategies; however, one detail that we did not mention yet makes this process more intriguing. What do the larvae eat to complete their development within a gallery that is inside a dead tree? The answer is: Fungi.

Pine beetles establish a symbiotic life structure with a beneficial fungus (Entomocorticium sp. A), which is the main food source for their larvae. Adult pine beetles have a body compartment (mycangium), in which they can carry this fungus. When adult beetles dig galleries in tree barks, they inoculate these galleries with the fungi. This fungus will grow in these galleries that helps the beetles’ larvae complete their development by providing them with a source of nutrition. Fungi also benefit from this process by being transferred from one tree to another with the help of the beetles. This symbiotic life structure is threatened by the existence of an antagonistic fungus, (Ophiostoma minus, also known as blue stain fungus) and parasitic mites both of which the southern pine beetles also bring along. The blue stain fungus has no nutritional importance for larvae, this fungus can grow in the same galleries as the beneficial fungus and they can even outcompete them. In addition, the parasitic mites feed on the blue fungi and can prove harmful beetles as the amount of blue fungi increases. Therefore, beetles’ larvae cannot survive for long and thus the reproduction of the southern pine beetles can be disrupted without some form of defense.

A recent study by Scott et al. has shed some light on this complicated life structure [3]. These tiny beetles, which are only a few millimeters long, have a smart defense mechanism to prevent their larvae (Figure 3, pink arrow). As far as the history tells us, humankind started using pesticides about 5000 years ago, but these tiny beetles have been using them for preventing the growth of the blue-stain fungi long before humans started using pesticides. These studies showed that the symbiotic coexistence of southern pine beetles and the beneficial fungi (Figure 3, yellow circle) is maintained by a (actinomycetous) bacterium (Figure 3, inside the red square). This bacterium produces a previously unknown antibiotic compound (named mycangimycin), which selectively inhibits the growth of blue-stain fungus hence providing a significant advantage for the maintenance of the beneficial fungi (that is, the main food source of the larvae). How can this bacterium even be present in the freshly carved galleries in pine trees in the first place? The surprising answer to this question is that they are transferred to these galleries by the very same pine beetles. As mentioned earlier, adult pine beetles carry beneficial fungi in their body compartment (mycangium) and inoculate the galleries they carved with this fungus to provide food for their larvae. In addition to this beneficial fungi these beetles carry they also carry bacteria that can produce antibiotic compound to inhibit the growth of blue-stain fungi and consequently diminish the number of parasitic mites. This bacterium can grow inside these galleries and even in a body compartment of adult southern pine beetles. Interestingly, this antibiotic kills blue-stain fungi but does not significantly affect the growth of the fungi, which is the food source for larvae. Thus, the beneficial fungi can multiply in number and offer enough food for the development of beetle larvae. Some of these larvae manage to grow to adults and then leave to attack other pine trees carrying the same fungi and bacteria with them (Figure 4).

The interactions within the rest of the life kingdoms is not any less complicated than the symbiosis between pine beetles, fungi, and the bacteria. The interactions between animals, plants and microbes are very complex and also fragile. Removing or replacing any member of an ecosystem can often result in a serious failure in the ecosystem as was observed many times especially within the last century.

References

1. http://www.nsf.gov/news/news_summ.jsp?org=NSF&cntn_id=112319&preview=false.
2. http://entnem.ufl.edu/creatures/trees/southern_pine_beetle.htm.
3. Scott, J.J., et al., “Bacterial protection of beetle-fungus mutualism.” Science, 2008. 322 (5898): p. 63.