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A chemical found in chillies protects the plant from a fungus as well as our guts from bacterial infections.
Biologist Joshua Tewksbury has peeked into an aeons-old war between a plant and a fungus in rural Bolivia, and completed a long-standing puzzle about chillies. In a study of wild capsicum plants native to eastern Bolivia, Tewksbury has shown that the plants are loaded with a chemical that appears uniquely designed to protect them from a fungus called Fusarium.
Human taste buds have long been familiar with the chemical — capsaicin. It is the ingredient from chillies that goes into hundreds of dishes from cuisines worldwide — from Andhra chicken curry to Gaeng Phed, a spicy red curry from Thailand, to spicy Mexical lentils. Capsaicin in chillies provides the spicy taste to dishes. For plants, Tewksbury — of the University of Washington in Seattle — has found, capsaicin is self-defence against a microbe.
Microbes, it turns out, may help explain two elements of a puzzle about chillies. Why are chillies spicy? Why did humans begin to eat chillies — a spicy, even painful, fruit — in the first place?
One question was solved 10 years ago. Jennifer Billing, an undergraduate at Cornell University, scanned dozens of cookbooks and compiled a list of more than 4,500 recipes representing meat-based cuisines from 36 countries.
Then Billing and Paul Sherman, a professor of neurobiology and behaviour at Cornell, analysed temperature and rainfall patterns and cultivation ranges of 43 spice plants in each of those countries, and the anti bacterial properties of each plant.
The exercise threw up a distinct pattern on the map. The world appeared to have a hot zone — a band on either side of the equator where temperatures are high and the food tends to be spicy hot.
India, Thailand and Malaysia were at the top of the hot climate and hot food list. Sweden, Finland and Norway were the coldest countries with the least spicy food. The scientists also found that spices were microbe killers. Garlic, onion and oregano were the most efficient, wiping out virtually all bacteria, followed by cinnamon, cumin and thyme that kill 80 per cent of bacteria. Capsicum and chillies eliminate about 75 per cent of bacteria.
The Cornell biologists proposed that humans began to add spices to their food centuries ago — without realising it — to lower the risk of food-borne microbial infections. Some bacteria that might enter human stomachs through food have the potential to kill. The taste for spicy food, Sherman and Billing postulated, was a trait that would be beneficial — culturally and genetically.
“People who enjoyed food with anti bacterial spices probably were healthier, especially in hot climates,” said Sherman. “They lived longer, they left more offspring — and they taught their offspring how to cook food (with spices).”
The new study has solved the second botanical puzzle about chillies. A spicy fruit on first analysis is perplexing to evolutionary biologists. A plant makes fruit to lure insects and animals to eat and disperse its seeds to facilitate reproduction. “So it doesn’t make sense for a fruit to be painfully hot,” said Douglas Levey of the University of Florida, Gainesville, who was part of the six-member team that studied Bolivian chillies.
The researchers found that capsaicin significantly slows microbe growth and protect the fruit from Fusarium. Their findings were published recently in the Proceedings of the National Academy of Sciences.
Eastern Bolivia was the ideal locale for this study. A variety of wild capsicum grows there along a stretch of some 1,600 kilometres. Some are spicy and pungent, while others lack capsaicin and are less or not at all spicy.
Scars left by insects feeding on capsicum are used by the fungus as gateways into the fruit. The researchers counted the insect scars, analysed rates of fungal attack and levels of capsaicin in the plants. They found that hot plants were hotter with higher levels of capsaicin in areas where fungal attacks were common. In areas with few insects — and thus less danger of fungal attack — the plants were less spicy.
The studies consistently showed that a high level of capsaicin was associated with lower seed mortality from fungal attack. The findings appear to be general and could also be applicable to chillies grown elsewhere, including India, said Tewksbury. “There are reasons to suspect that fungi and microbes are general targets of these capsaicins,” Tewksbury told KnowHow.
But while capsaicin slows microbial growth and protects the fruit from Fusarium, it doesn’t interfere with seed dispersal. “Birds don’t have the physiological machinery to detect the spicy chemical and continue to eat peppers and disperse the seeds,” Levey said.
The study shows that the use of chillies by humans appears to mirror the evolutionary function of capsaicin. “The capsaicin in chillies may have protected early humans from microbial infections,” said Tewksbury.
Researchers argue that before the advent of refrigeration, it was probably beneficial to eat chillies, particularly in the hot tropics. Studies suggest that all chillies originated in South America, and explorers carried the plants to Europe and elsewhere. Today, scientists estimate, one in four humans worldwide consumes chillies daily. “The use of chilli peppers as a spice has spread to nearly every culture within 20 degrees of the equator,” said Levey, “and it tends to decline as you move toward the poles.”
Sources: The Telegraph (Kolkata, India)