Seemingly innocuous plants can harbor dangerous toxins. Many have developed a bitter taste, which our sophisticated sense of taste can pick up.
In biology, it's all about survival. So much so that the sense of taste, or gustation, allows us to differentiate between nutritious and potentially harmful foods.
According to Charles S. Zuker, Ph.D. - a professor of biochemistry and molecular biophysics at the University of Columbia in New York City, NY - "An organism's survival can depend on its ability to distinguish attractive tastes like sweet from aversive ones like sour and bitter."
"We are born to be averse to sour or bitter tastes and attracted to sweet things," explains Hojoon Lee, Ph.D., a member of one of Prof. Zucker's research teams.
So how does our body pick up these potentially harmful tastes among the myriad complex flavor profiles?
Five basic tastes
The chemical constituents in our food are detected by taste receptors on the tongue. Sweet taste, for example, is mostly conveyed by sugars, but also by more complex proteins and alcohols, which contain sugar constituents.
Salty taste comes from salt crystals in food, and umami, or savory taste, comes from glutamate, which is found foods such as seaweed, soy sauce, Parmesan cheese, and tomatoes.
Sour taste comes from acids, such as lemon juice or vinegar, but it can also indicate fermented or rotting food. Bitter taste, however, is more complex, and there are several different molecules that can activate its receptors. This diversity is important, as bitter is the number one danger signal.
The complex flavors we experience when we eat food are a combination of the five tastes, so millions of such combinations are possible.
Receptors for each taste
Two cell types work hand-in-hand to allow us to pick up taste signals. The first one is the taste receptor cell (TRC), and the second is the ganglion neuron that connects the TRC to the brain.
There are five different types of TRCs: one for each taste. Specialized taste receptors on these cells pick up taste signals in our food. A new contender on the block is fat, which some scientists believe represents the sixth taste.
TRCs are found clustered together in taste buds concentrated along the sides and the back of the tongue. Each taste bud contains between 10 and 50 TRCs.
Interestingly, TRCs have a very short lifespan: the entire population is renewed every 1 to 3 weeks.
'Handshake' connects cells and neurons
Every time a TRC is renewed, the connection to a ganglion neuron that encodes the matching taste must be re-established. Until now, this sophisticated biology has baffled scientists.
Prof. Zucker and his colleagues recently identified signaling molecules produced by TRCs that allow matching ganglion neurons to home in and make a connection.
In their study, which is published in the journal Nature, they show that a group of molecules called semaphorins are involved in guiding the "handshake" between matching TRCs and neurons.
To show how this works, they turned on the sweet signaling molecule in bitter TRCs in mice. This led to a re-wiring of the taste system with sweet neurons now connected to bitter TRCs.
As a result, the mice in their experiment "did not avoid the bitter water," explains Prof. Lee, as their brains received sweet signals due to the switch.
Understanding how this process works not only provides clues into fundamental biology but might also open up new avenues of treatment for those living with taste loss.
Although food has transcended from mere nourishment to a pleasurable pastime for many, our sense of taste remains our first line of defense against poisonous foods.