We often think of plants as hunks of life that just sit there, not really doing much, letting themselves get pushed around by the other moving organisms. Well Wednesday Science is here to debunk that myth for you all, starting with some interesting information about plants’ defenses, including immune systems. Say what, Wednesday Science? Yes! Plants have immune systems too!
A plant’s first line of defense against any pathogen is, of course, it’s “skin,” or epidermis. However, when a baby caterpillar comes by to eat some juicy leaf, the plant's insides are now exposed to microbes who can come inside! In addition, plants have many natural pores to let air molecules inside, though that just means another entrance for pathogens as well. And of course, the baby caterpillar, cute as it is, is also harmful for the plant! Not because it causes disease, but because, well, it destroys plant leaves!
First, we’ll talk about what plants do against the tiny microbes with their immune systems. All pathogens have certain components that make them recognizable by plants, called pathogen-associated molecular patterns (PAMPs). For example, bacteria have “tails,” or flagella, that help them move. The protein in flagella, or flagellin, can be recognized by plants. After recognition, the plant can then secrete proteins called phytoalexins, which have fungicidal or bactericidal (killing fungi or bacteria) properties. This first method of internal defense is known as the plant’s innate immune system.
You also happen to have an innate immune system! Humans’ innate immune systems are actually quite similar to plants’, except that we secrete what are called complement proteins instead of phytoalexins to combat pathogens. However, we differ from plants in what comes next: protections for if the innate immune system fails. Pathogens can secrete effectors, or molecules that inhibit the proper functioning of the innate immune system, so having the aptly-named effector-triggered immunity is helpful.
Effector-triggered immunity comes in two types: the hypersensitive response and systemic acquired resistance. The hypersensitive response restricts the spread of pathogens by breaking down pathogens’ cell walls, as well as increasing the strength of the plant’s cell walls. This cell wall strengthening appears as “lesions” on a leaf. It may look bad, but it's a lot like a fever; it is unappealing in the moment but it eventually helps you survive. This response is usually localized, or in a specific area of a plant, but can also be triggered by the more general systemic acquired resistance pathway in plants. This process is triggered by the production of substance methylsalicylic acid, which can travel around the plant as a “warning signal.” Once it reaches other parts of the plant, it can be converted to salicylic acid, which activates plant defenses in a more specific region of the plant.
These defenses are usually very effective against microbes like bacteria, fungi and viruses, but what about the baby caterpillar? And just toughening up your cell walls isn’t going to do anything against the mighty bison of the midwest! You’ve probably seen spikes and spines on some plants, such as roses and cacti. Tiny hairs on plant epidermis called trichomes can also give plants an unpleasant texture. For more molecular-based defenses against herbivory, plants secrete the protein systemin. A lot like the systemic adaptive response, this affects cells all around the entire plant, or the system. It does this by stimulating the production of jasmonic acid, which signals to cells all around the plant to produce molecules blocking herbivores’ digestive enzymes. In addition, several plants also secrete distasteful or toxic compounds that deter most herbivores. Sorry, caterpillar, but the plant deserves to live too!
And that’s how plants defend themselves: against both the tiny invaders as well as the large invaders. Plants may stay relatively still, but they need their personal space too!
Post by: Roshni P. M.
Photo credits: https://unsplash.com/photos/hDyO6rr3kqk