Cancer is a prevalent disease among humans and animals, affecting millions of lives across the globe. It can be caused in a variety of ways and can affect virtually every part of our bodies, ranging from skin cancer caused by prolonged exposure to the sun’s harmful UV rays to lung cancer that results from carcinogenic substances smokers inhale. However, plants do not die of cancer despite sometimes being exposed to the sun for over a thousand years – and they do not use any sunscreens!
Humans and animals who live to a certain age are very likely to get cancer one day. We see this situation mostly in our pets which have been specially bred and protected from predators and diseases. Cancer has become part of our lives and remains a top world health priority. The probability of prostate cancer is roughly 80% in 80-year-olds, 90% in 90-year-olds, and 100% in 100-year-olds. However, these statistics yet again do not apply to trees.
WHAT IS CANCER?
Cancer is a disease caused by the uncontrolled growth of cells that have become abnormal in a part of the body. These abnormal cells are not foreign invaders that have entered our bodies from outside but are instead our own cells. However, in time, various factors such as radiation, viruses, and chemical substances that they are exposed to cause the accumulation of errors, or mutations, in the genetic codes of cells. Some of them then acquire very different characteristics and become alien to their own body.
With old age, errors arise in the genetic code in an increased rate when our cells divide by copying their own DNA. External factors, such as “free radicals” and various radiations that affect our DNA, play a role in these errors. For young people, when there are too many errors in a cell’s genome, a process called “apoptosis” takes place after which faulty cells die before they can multiply in a potentially cancerous manner and forming a tumor. However, sometimes these accumulated errors cause the cell’s growth process to become stuck in the “on” position, and the cell begins to grow and divide continuously. Cells that emerge with out-of-control divisions ignore the commands coming from the healthy cells of the body, continue to grow and reproduce according to the erroneous commands from the cell’s disrupted genome. This situation lasts until death.
WHAT IS A TUMOR?
Cell growths that result from defective genome proliferation will eventually form a mass called a tumor. Some cells grow very slowly and stop at a certain size after a while and do not spread anymore and are called benign tumors. Masses formed by fast-growing, defective (cancerous) cells are known as malignant tumors. Cells that detach from malignant tumors and grow rapidly can attach to another organ where they will begin to grow again when they enter the bloodstream. The process by which cancerous cells start from a tumor and spread all over the body to different organs is known as metastasis.
WHY DOES THIS PROCESS NOT HAPPEN IN PLANTS?
One of the most destructive features of cancer when it enters metastasis is the mobility of malignant cells to varying degrees according to their type. Blood vessels, i.e. the transportation pathways of the circulatory system, act like a highway for cancer cells. As the blood vessels surround the entire body, a single cancerous cell can travel to and settle almost anywhere in the body, from the toes to the head.
Plant cells have a vital feature that is different from human and animal cells. In plants, cells do not change their locations because their cells are surrounded by a very rigid, strong, and impenetrable wall outside of normal plasma membranes. Cell walls are made of cellulose, which constitutes the main substance of plants, and form the wooden structures that ensure the plants stand upright and harden while at the same time locking each cell in place and preventing it from migrating within the organism.
Another important difference that is unique to plants is that they do not have blood circulation in which cells are carried; they have a circulatory system in which only water and food are carried. This system is often used to pump water from the roots to the leaves and to transport organic products such as sugar, which is a product of photosynthesis, from the leaves down. Therefore, there are no blood cells or immune system cells in these carrier channels, which are known as wood and roe tubes (xylem and phloem), in plants.
In addition, animal cells are specifically employed in tissues and organs such as muscle, bone, liver, and skin during embryonic development. Thus, when they divide only new cells of the same type are created. Tumors that occur in animal tissues can metastasize into different tissues and disrupt different organs. We can think of animal and human biology as a very complex system in which each cell, tissue, and organ has a task and purpose. In such a system, all elements work in cooperation for the continuation of life. This system is of a kind of irreducible complexity. A human being cannot live without organs like brain, heart, or lungs, while plants, on the other hand, have fewer simpler internal structures which are not as vital. When plant cells divide, they retain their ability to form new cells of any type. This is called totipotency.
In plants, every necessary structure can be recreated from the few tissues they have. For this reason, a gardener can grow new plants from the roots, branches, or leaf parts of a plant.
Plants are equipped with very powerful antioxidants to protect them from the sun’s harmful rays and mutations that may be caused by radiation. Therefore, tumors can develop only due to bacteria, viruses, fungi, parasites, and insects. For example, in a situation that we can call “information confusion” that occurs when Agrobacterium Tumefaciens bacteria insert some of its DNA into the plant’s DNA, an anomaly occurs in the plant’s genome. Cells that go through a rapid growth process and form tumors are not normally classified as cancer, since they simply remain in that area and cannot be transported elsewhere. Since the tumors cannot spread to the whole plant, they may cause only minor distress at most in a specific area rather than a fatal disease such as cancer. Just as the plant continues to grow around a rock that it encounters, it grows around the tumor as well. The tumor can continue to grow for years, but does not spread to the rest of the plant, meaning there is no metastasis.
In summary, plants can also be cancerous, but a cancerous tumor is not a deadly threat to a plant, as its cells are immobile and do not have vital and complex organs like humans and animals. Thanks to the cellulose walls gifted to the them, plants continue their role in the ecosystem by continuing to grow with healthy cells around the tumor as if nothing had happened.