Have you ever wondered why tomato plants have so many little hairs on the stems and leaves? They are kind of neat to look at, but they actually serve a purpose too. The tiny hair-like structures are called trichomes, and they are there to help protect the plant from weather and herbivorous insect invasion. Tomatoes have several different types of trichomes that fall under two main categories- glandular and non-glandular.
Non-glandular trichomes are hair-like and come to a point. These types of trichomes are like feelers, they can sense the feel of a bug walking across the leaf surface and send a signal to the plant to boost it’s defenses and prepare for a herbivorous attack. Non-glandular trichomes can also protect the plant from rain and other elements by keeping the water elevated just above the leaf surface, allowing it to roll off more freely instead of touching the sensitive dermal cells below.
Glandular trichomes are also hair-like and come to a point, but beyond the point there is a rounded gland that looks much like a tiny dew drop resting on the tip of the trichome. This type of trichome can also sense the feel of an insect walking on the leaf, the movement causes the glands to rupture. The sticky substance that spills out from the ruptured trichome gland is a mixture of terpenes, acylsugars, alkaloids, and other defense related proteins. The purposes are to either trap the insect by causing their feet to stick to the surface, to act as a toxic poison to any insects that ingest it, or to cause aversion by the smell. The scent of this sticky mixture is what we know as the smell of “tomato vine”.
There are several genes that regulate the amount of trichomes a tomato plant may have. The first is the wooly gene (Wo), which makes a plant appear fuzzy or angora like. This gene increases the numbers of trichomes, but they are no longer than normal.
The Hirsute (Hr) gene promotes longer than average trichomes on the upper leaf surface and sometimes even the fruit itself. This is a dominant gene that is most prevalent in wild relatives. Another partial dominant gene is Hirtum (Hrt), which increases the number of these longer, larger trichomes and is also found more abundantly within wild relatives.
The final gene type is the hairless (hl) recessive, this gene suppresses the trichome growth into a curled or gnarled state, making it appear as though there are no hairs and as a result, the plant itself becomes brittle without proper protection.