One of my siblings got addicted to eating bitter kola sometime ago. At a point, she started having palpitations of the heart. Each time her blood pressure was checked, however, it came out fine. At a point, the palpitation became worse and was accompanied by serious fatigue. She decided to see a doctor. While at the hospital, she was munching bitter kola. She told us that just before it was her turn to be attended to by the doctor, she did a soul search on what could be causing the condition. She then discovered that the only strange thing she indulged in was her addiction to bitter kola. She picked up her phone and checked the side effects of bitter kola. She was shocked by what she saw on Google: bitter kola had been the problem all along! She was checked by the doctor that day; her blood pressure was normal. She had to confess to the doctor that she just discovered that it was bitter kola that had been causing her palpitations. That was the last day she ever ate bitter kola and the heart palpitations also stopped! Why was bitter kola giving her palpitations? You will understand shortly.
This week, I will be discussing how plants defend themselves. Although the emphasis will be on the chemical form of defence known as anti-nutrients, it is not a bad idea if we intimate ourselves with other forms of plant defence. Animals employ various strategies/defence mechanisms to avoid their predators. Behavioural responses, like running away, are very important. In contrast, plants are sedentary, meaning that they cannot move. Roots anchor them to the soil, so, plants cannot run. Therefore, they have developed physical and chemical defences to protect themselves against herbivores, which are animals that live by eating plant tissues.
Plant defence against herbivores or host-plant resistance is a range of adaptations evolved by plants which improve their survival and reproduction by reducing the impact of herbivores. Plants can sense being touched and they can use several strategies to defend against damage caused by herbivores. Many plants produce secondary metabolites, known as allelochemicals. These chemical defences can act as repellents or toxins to herbivores or reduce plant digestibility.
Another defensive strategy of plants is changing their attractiveness. To prevent over-consumption by large herbivores, plants alter their appearance by changing their sizes or qualities, reducing the rate at which they are consumed. Other defensive strategies used by plants include escaping or avoiding herbivores at any time in any place. For example, by growing in a location where plants are not easily found or accessed by herbivores or by changing seasonal growth patterns. Another approach diverts herbivores toward eating non-essential parts or enhances the ability of a plant to recover from the damage caused by herbivory.
Plants can also communicate through air. Pheromone release and other scents can be detected by leaves and regulate plant immune response. In other words, plants produce volatile organic compounds to warn (the most common response is for the other plants to shed their leaves) other plants of danger and change their behavioural state to better respond to threats and survival. These warning signals produced by infected neighbouring trees allow the undamaged trees to provocatively activate the necessary defence mechanisms. Some plants can also produce plant defensive proteins which upon ingestion end up poisoning the herbivore. A form of defence can be physical. For example, a rose’s thorns, technically called prickles, are extremely important for its survival. They do everything from helping rose bushes grow to keeping them safe from predators and casual flower pickers.
The evolution of chemical defence in plants is linked to the emergence of chemical substances that are not involved in essential photosynthetic and metabolic activities. These substances, secondary metabolites, are organic compounds that are not directly involved in the normal growth, development or reproduction of organisms and often produced as by-products during the synthesis of primary metabolic products. Some of these include Phenolics, Flavonoids and Tannins.
What is nature’s purpose for anti-nutrients?
The name comes from how they function in your body once you eat them. They block or interfere with how your body absorbs other nutrients. Thus, anti-nutrients may decrease the amount of nutrients you actually get from your food. They most commonly interfere with the absorption of calcium, iron, potassium, magnesium and zinc.
Plants evolve these compounds as a defensive mechanism against insects, parasites, bacteria and fungi infestations. For example, some anti-nutrients can cause food to taste bitter; animals won’t want to eat it, leaving the seed, for instance, to provide nourishment for future seedlings. However, if birds and animals were to eat the fruits containing the seeds and digest them to obtain additional nutrients and calories, then, there will be no seeds left for the continuation of the plant species. Some anti-nutrients block the digestion of seeds that are eaten. The seeds disperse when they come out of the other end of the animal’s faecal matter and can go on to grow new plants.
Some of these compounds may also prevent the activity of the digestive enzymes so that even if the seeds are consumed, they are excreted without altering their physiology. Both of these survival tactics help the plant species grow and spread. In terms of foods that people eat, you will most commonly find anti-nutrients naturally occurring in whole grains and legumes. Thus, anti-nutrients are part of the plant’s chemical defence system against invaders in order to ensure its survival.
Anti-nutrients are currently undergoing a change in image very similar to the one dietary fibre experienced. At one point, scientists thought dietary fibre was bad for people. Since fibre could bind to nutrients and pull them out of the digestive tract in poop, it seemed like something to avoid. But now, scientists know that dietary fibre is incredibly important and they encourage its consumption. In the same way, rather than something to avoid, many anti-nutrients are now considered health-promoting nutraceuticals and functional foods due to their numerous benefits. Saponins, common in legumes, can boost the immune system, reduce the risk of cancer, lower cholesterol, lower blood sugar response to foods, result in fewer cavities, reduce the risk of kidney stones and combat blood clotting seen in heart attacks and strokes. Lectins, found in cereal grains and legumes, are associated with reduced risk of cardiovascular disease, diabetes, cancers and becoming overweight or obese.
Tannins, commonly found in teas, coffees and processed meats and cheeses, are antioxidants that can inhibit the growth of bacteria, viruses, fungi and yeast and may decrease cholesterol levels and blood pressure. Phytate, found in wheat, barley, rice and corn, is associated with increased immune function and cancer cell death as well as reduced cancer cell growth and spread. They also have antioxidant properties and can reduce inflammation. Glucosinates, found in brassica vegetables like cauliflower, inhibit tumour cell growth.
The most widely studied anti-nutrients include:
- Phytate (phytic acid): Mainly found in seeds, grains and legumes. Phytate reduces the absorption of minerals from a meal. These include iron, zinc, magnesium and calcium.
- Tannins: A class of antioxidant polyphenols that may impair the digestion of various nutrients.
