This experiment shows the important role of the skin as an organism’s first line of defense against pathogens. Like an “injured” tomato, a cut in your skin can become infected by pathogens, too. Bacteria—and sometimes fungi—can enter through the cut and take up residence in your tissues, causing an infection. Your intact skin is the primary structure protecting you from these kinds of infections.
The outer layer of your skin, or epidermis, forms a physical and chemical barrier that most pathogens can’t penetrate. (The mucous membranes lining your airways and gut have a similar role to play inside your body.) The cells of the skin are joined tightly together, and they’re full of a protein that makes them tough and resistant to bacteria. Your skin is also salty and slightly acidic, making it a less hospitable environment for microorganisms.
The skin of the fruit plays an identical role for the plant. Plant skin, or cuticle, contains wax and other molecules that help it retain water and prevent invasion by pathogens. When you injured the fruit by poking holes in it, microbes in the environment jumped at the opportunity to invade and colonize within the exposed tissue. You may have observed molds, bacteria, and various protists and nematodes growing in your injured fruit.
Even after injury and infection, plants still have ways to protect themselves. Plant cells produce a variety of antimicrobial enzymes and other molecules that can help inhibit the reproduction of microscopic pathogens, although they don’t target specific pathogens. The human body also has non-pathogen-specific defenses that respond to cell damage caused by infection. These systems are all part of the innate immune system. All plants and animals have versions of these systems.