Plants are under constant attack in the environment, with the attackers trying to breach the perimeter by chewing, ripping, dissolving, penetrating, or tearing open the organs, tissues, and cells making up the plant body. These attackers include everything from acellular viruses and single-celled bacteria to multicellular mammals. Plants have evolved defense systems to address this wide variety of potential attackers. Those systems include physical barriers and anatomical structures to prevent the penetration of an attacker, general chemical defenses that act as a deterrent against attack, and highly specific immune-type responses that recognize and protect against familiar pathogens.
Kinds of Pathogens
In addition to the multicellular attackers such as nematodes and insect larvae, single-celled microbes are a constant threat to a plant’s health. This class of attacker falls into one of three possible categories depending on the mode of action employed by the pathogen. On one end of the spectrum, some microbes seek to destroy the plant tissue and absorb as much of the remaining organic matter as quickly as possible. This category of pathogen is known as the necrotropic pathogens due to the fact that these microbes feed on dead tissue. On the other end of the spectrum, some microbes attempt to penetrate the host plant’s defenses and soak up as many organic resources as possible without the host mounting a defense response. These are biotrophic pathogens, attempting to evade host detection and keep the host tissue alive because it benefits them the most to do so. These are the microbes that often induce tumor formation in host organs by transferring genes into the host cell that encode hormone biosynthesis enzymes, resulting in increased cell division. These pathogens are the equivalent of a squatter taking up residence in a home and building and addition onto the kitchen so they can get more to eat. In between these extremes are the hemibiotropic pathogens, which keep the host tissue alive for a time after invasion, but eventually kill it and take its resources.
The first line of defense plants present to an invading pathogen is the thick cuticle found on the surface of the epidermis throughout the aerial tissues of the plant. The cuticle is made up mostly of waxes and fatty acid esters known as cutins that are secreted by the epidermal cells. In addition to making the surface more water-tight, these waxes and cross-linked esters deter some smaller animal herbivores and many microbes.
While the cuticle forms an effective barrier on leaves and stems during primary growth, stems undergoing secondary growth secrete a different kind of coating for protection. These organs produce a layer of cells that contribute to an increase in girth of the stem by dividing in a ring around the stem parallel to the surface, forming cork. These cork cells, which make up part of the bark of stems, synthesize a waterproof substance called suberin, which is secreted into the apoplast and permeates the cell wall of cork cells. Once suberin is manufactured and secreted, these cells can no longer take up water and die. Suberin is a polyphenolic substance and is not only hydrophobic but also has some antimicrobial properties as well as providing a physical barrier.
Trichomes, or hair cells, provide a physical impediment on the surface of many stems and leaves that deters the feeding of many insects. Not only do trichomes offer a physical challenge, they are often filled with secondary compounds that are irritants or toxins to feeding insects or other animals. Cooking herbs are an abundant example, often having leaves covered in terpene-bearing glandular trichomes that burst when contacted. Another example is poison ivy (Toxicodendron radicans), which secretes an irritating oil from trichomes called urushiol.
If an attacker manages to make it through cuticle covering the surface of the plant, perhaps through a wound opening, it will still find one last line of defense: the tough, rigid, lignified cell wall. Made up of cellulose and other structural polysaccharides impregnated with the polyphenol lignin, the cell wall presents a formidable barrier for microbial attackers like bacteria and fungi. Some species of fungi form elaborate multicellular structures with their hyphae (called an appresorium) that can puncture the highly-pressurized cell wall, while other fungi and bacteria secrete cellulose-degrading enzymes that help to digest the wall away.
Once the structural defenses have been compromised, the plant begins to mount one or several simultaneous defense responses leading to only a few outcomes. In many cases, the results of an attack include the increased biosynthesis of secondary compounds through the induced expression of genes encoding enzymes in terpene or phenolic compound synthesis. These molecules are always being synthesized by the cell at a baseline rate, but that rate increases when an attack is detected.
Another outcome of infection can be the initiation of programmed cell death at or near the site of infection. When the plant dectects certain features of a pathogen or herbivore, these can trigger those cells to cut themselves off from the rest of the plant to try to contain the infection. If you inspect a leaf, it is not uncommon to find brown spots scattered throughout the leaf. These spots are a common indicator of such cell death activity.
Pathogen and herbivore attack can also induce the plant to produce signal molecules that communicate an infection throughout the rest of the plant body. Salicylic acid and jasmonic acid are two such hormones that serve to prime the immune response of distant organs in advance that a pathogen or herbivore has attacked.