Physalis Blog Post
Guest Post: What Makes Tomatillos Sticky?
The following is a guest post from Dr. Craig Schenck, a postdoctoral research associate in the Department of Biochemistry and Molecular Biology at Michigan State University, who studies plant metabolites and their role in plant defense. Connect with him on twitter at @caschenck_bio to talk more about plant defense and chemistry.
If you’ve ever encountered a tomatillo, you know that hidden beneath the papery husk is an extremely sticky fruit. Flies may even be stuck to the surface because it’s so sticky. Have you ever wondered why this fruit is so sticky? The short answer is defense.
Unlike humans and other mammals that can escape when harsh conditions arise, plants are rooted in place. Plants have developed many ways to deal with environmental stressors. Some employ physical defenses like spines or thorns, while others use the production of chemicals. These varied physical and chemical defenses are part of what makes each plant so unique.
Tomatillo, also known as a husk tomato, is not a green tomato, but its own distinct species (Physalis ixocarpa). The tomatillo and tomato are in the nightshade plant family, which includes other crop species like potato, pepper, and eggplant. Nightshades are well-known for their ability to make abundant and diverse chemical defenses. One class of chemical defense compounds are acylsugars (Figure 1). They consist of a sugar core, typically sucrose, with carbon chains attached. In most nightshade species acylsugars are made in the trichomes, which are tiny hair-like structures on the leaf and stem surfaces. However, in tomatillo acylsugars are found on the fruit surface, explaining why the fruit is sticky. The acylsugar location is one of many reasons why tomatillo is such a cool plant.
Figure 1 An example of an acylsugar structure. Tomatillo fruit are sticky due to copious amounts of acylsugars on the surface.
Another layer of tomatillo defense is the papery husk, or calyx, that surrounds the fruit (Figure 2). This is common to species that are closely related to tomatillo, including goldenberry (Physalis peruviana) and groundcherry (Physalis pruinosa). The calyx provides a physical barrier protecting the developing fruit.
Figure 2 A ripe tomatillo with its calyx still attached (left) and removed (right).
Due to both chemical and physical defenses, tomatillo is a hardy crop that is widely cultivated in Central America (with most of the production and consumption in Mexico). Although some domestication has occurred, there is untapped potential for breeding efforts aimed at improvement, but this has been hampered by a lack of genetic resources for tomatillo. However, the genomes of tomatillo and closely-related species are currently being sequenced, which will provide tools for scientists to begin to understand how chemical defenses are made. Together with breeding, we can use this knowledge to develop tomatillo varieties with enhanced yield, quality, and defense. Through enhancement of defense traits, like acylsugars, we may be able to produce varieties that require less herbicide and pesticide inputs.
We also need to increase the awareness of these amazing plant species, not only because of their interest to scientists like me, but also to chefs, cooks, and others who are looking to expand their ingredient repertoire. Tomatillo and its relatives are versatile ingredients, but also healthy alternatives to more mainstream fruits.
Bountiful varieties of tomatillo size, shape, and color can be found. The fruit is often a vibrant green but can also have hints of deep-purple. Fruit size can vary but is typically about the size of a tennis ball. It is common to find tomatillos at the market with their husks still attached. Fresh tomatillos are tangy and less sweet than a tomato, but after roasting the tanginess fades. Tomatillos are the critical component of salsa verde. Check out this blog for an excellent recipe. They can also be used in place of tomato for a unique take on a Bloody Mary. Play around and see how fun cooking with new ingredients can be. I recommend rinsing the fruit prior to use. This will remove the acylsugars and anything else that may be stuck to the fruit!
Craig Schenck
Michigan State University
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