How plants defend themselves
Plant cells are not able to move, so every single cell must be able to defend itself against attack from invading pathogens like fungi and bacteria.
Plants have developed two strategies to deal with these invadors.
REMEMBER: All of the things that are helping plant cells to protect themselves are the PROTEINS inside! Proteins are the tools that cells have to work with and perform cell functions. Different proteins can help in different ways. Click here to learn more about proteins in general!
Proteins are sometimes hard to visualize because they are very small and look different depending on what their job is.
As an example, to the right is the structure a protein that you may be familiar with. Hemoglobin is found in our red blood cells and its job is to carry oxygen from the lungs to the rest of the body. It may just look like a jumble of lines - but the way that each individual protein is formed (out of its building blocks - amino acids!) will make it be able to perform different jobs!
R-state haemoglobin with low oxygen affinity: crystal structures of deoxy human and carbonmonoxy horse haemoglobin bound to the effector molecule L35. Yokoyama, T., Neya, S., Tsuneshige, A., Yonetani, T., Park, S.Y., Tame, J.R. Journal: (2006) J.Mol.Biol. 356: 790-801
The first line of defense occurs when plant cells detect the presence of some sort of foreign invader (by detecting general characteristics such as the flagella of bacteria or the chitin found in fungi cell walls) and alert surrounding cells by releasing certain molecules that tell other cells to amp up their defenses. For example, alerted plant cells may secrete molecules that are harmful to invadors or build up their cell walls for extra protection.
The second line of defense exists to protect plants from sneakier invadors. Some pathogens have figured out how to disable the plant's first line of defense. When plant cells recognize that they are being attacked, they fight back. Just as pathogens have evolved to disable plant defenses, plants have evolved to be able to recognize these disabling attacks and counter them.
There are different signs that help indicate which kind of defense is activated in a plant and in response to what kind of pathogen. By studying these signs and what is going on in the genome of plants, researchers can try to figure out which parts of the plant genome and which proteins are involved in which kinds of defense.
For example, one of the signs of a plant activating the second line of defense is something called the hypersensitive response (HR). Basically, plant cells right around an infection site will kill themselves to stop a pathogen from spreading throughout the plant. This prevents pathogens from getting access to things that they need (like water and nutrients) and usually stops the infection! You have probably noticed this phenomenon on leaves in your backyard! Plants are faced by attackers constantly and it is only through strategies like this one that they are able to thrive through challenges.
image source: Freeman, B.C. and G.A. Beattie (2008) An Overview of Plant Defenses against Pathogens and Herbivores. The Plant Health Instructor. DOI: 10.1094/PHI-I-2008-0226-01.The following cartoon depicts what might happen in a plant cell being attacked by a pathogen! (Click "Play Movie" to watch):
Remember that this cartoon is simplified! The roles of the various proteins (A, B, and C) are performed by many proteins in a real plant cell. A HUGE number of proteins are involved and there are many ways that both plant and pathogen can respond to fight back against each other.
As you may have noticed, the plant immune system is very complicated! Only a small amount of information is currently known. There are so many different proteins and pathways involved that there are endless research possibilities on the subject. Research is so exciting because there is so much in the world to study and understand! Before starting on a specific research story, continue on to learn more about the wide world of scientific research!