Tough Plants

Plants come in all shapes and sizes. Some plants live in tropical rainforests, others live in parched deserts. Some are tiny and some are taller than a 30-story building and larger than 25 school buses. Some plants live for a few months, others live for hundreds of years—or even 1,000’s of years. During their lifetimes, plants face many challenges. Maybe it is too hot or too cold. Maybe the plant becomes lunch for a very hungry caterpillar or food for a plant-killing fungus. Unlike animals, plants cannot hide during a storm or move away from an insect attack. Instead, they must stand still and deal with every challenge that comes their way. This has made plants masters of self-defense and experts at adjusting themselves to changes in their environments.

How Do Plants Survive All the Challenges They Face?

One way plants can change to deal with challenges is through a process called natural selection. This is usually a very slow process in which those individuals that are best adapted to their environments produce the most offspring, or seeds. Being “best adapted” means that a plant has physical characteristics or behaviors that help it survive and make seeds. Adaptations could be sharp thorns that protect a plant from being eaten by a giraffe, or poisonous leaves that protect it from caterpillars. If these characteristics are “written” in the plant’s DNA (the instruction manual for how a living thing grows and functions) they are passed on to the seeds (Figure 1). Since successful individuals make more seeds than unsuccessful ones, useful characteristics become more common in the next generation of plants. Through natural selection, a species can gradually change over time and fine-tune itself to the environment where it lives.

As we mentioned, change through natural selection usually happens very slowly. In long-lived plants like trees, it can take many generations and hundreds of years for helpful changes in the DNA to spread and become common in the species. Luckily, there is another, much quicker way for plants to adapt to their environments. They can change when and how instructions in their DNA are carried out. This type of change is controlled by something called epigenetics—”epi” means that changes take place on top of, or in addition to, the genetic instructions written in the DNA. Epigenetic marks on the DNA control how compacted (“turned off”) or loose (“turned on”) the DNA is (Figure 1). Changes in the location of epigenetic marks allow for rapid changes in how a plant responds to its environment. These changes can also provide the plant with a “memory” of past stressful events and may be passed on to their seeds, making the next generation better prepared to face the challenges around them.

Studying epigenetic change is important to understand how plants adapt to their environments. This type of adaptation is especially important in long-lived trees, like conifers, as they may face a wide range of environmental conditions in their lifetimes. Conifers are part of an ancient group of plants called gymnosperms, which dominated the world around the time dinosaurs appeared and have adapted to live and thrive in extreme environments—from the dry deserts of Namibia (southwestern Africa) to the cold taiga of Siberia (Figure 2). Today, some of the most important timber species in the world (and almost all Christmas trees!) are conifers. Epigenetic changes allow conifer trees to remember the past and be better prepared for the future. Conifers can form epigenetic “memories” in response to many kinds of environmental cues, such as temperature conditions, insect attacks, and radioactivity.

Trees Use Epigenetic Changes to Remember the Past

One environmental signal that can trigger conifer memories is the temperature the baby trees experienced when the seeds were developing. Trees growing from seeds that developed at different temperatures show different timing in terms of when new needles appear in the spring (Figure 3). New needles appear earlier on trees from seeds that developed at lower temperatures, and trees from seeds that developed at warmer temperatures grow new needles later in the season. This temperature memory has been observed to last for at least 20 years, and researchers think it may last for the tree’s whole lifetime [1].

Another environmental cue that can create a memory in conifers is gamma radiation. The Chernobyl nuclear power plant disaster in April 1986 gave scientists a unique opportunity to understand how radiation can trigger conifer memories. Trees that grew up in an area that received high radiation from the accident look like bushes, while trees that received low to medium radiation look like normal trees. One reason for this difference could be that the varying radiation levels triggered unique epigenetic changes, by damaging the trees’ DNA [2]. So far, researchers do not know the nature of these epigenetic changes.

Conifers can also make memories when an insect lays her eggs on the needles. Researchers have noticed that needles with sawfly eggs on them are eaten less by the caterpillars that emerge from the eggs. Sawflies are a kind of wasp that feeds on plants and has larvae that are very similar to the caterpillars of butterflies. When the mother sawfly lays her eggs, she cuts the needles and releases special egg proteins into the needle. The wounding and contact with these proteins signal the needles to change the way they use their DNA instructions, helping them to defend themselves [3].

Treating conifers with plant-made defense chemicals can also trigger memories. When small spruce trees are treated with plant-made defense chemicals in the fall, they are better able to defend themselves against insect attack when they are planted out in the forest in the spring [4]. (To learn more about how plant-made chemicals help conifers better protect themselves, see our previous FYM article). Like with egg laying, defense chemicals tell a conifer to change how, and how rapidly, the DNA instructions for defense are carried out in response to insect attack.

Conclusions—Why is it Important to Study Epigenetics?

Trees remember the past—not with brains, like us, but through epigenetic changes that alter how the genetic instructions written in their DNA are carried out. There are still a lot of things we do not know about how and how long trees can remember the past. We know that various environmental cues cause different epigenetic changes and form unique types of memories. The kind of signal a tree receives, and whether it is received early or later in life, seems to affect how strong the memory is and how long a tree remembers. However, we do not yet understand how this works. For example, we still know very little about how memories created by one signal affect a tree’s ability to respond to a different signal. For example, will a memory of drought affect how a tree responds to an insect attack? Does root colonization by “good” microbes, such as fungi that supply the tree with water and minerals, make a tree better (or perhaps worse) at fighting an insect attack on the needles? Because trees experience many kinds of environmental cues during their long lives, it is important to understand how different memories work together. When we have learned more about tree epigenetics, we can perhaps grow new forests that are better at dealing with climate change or recovering from insect attacks or diseases.

Glossary

Natural Selection: ↑ The process by which individuals who are best able to survive in their surroundings produce the most offspring and pass on their instructions (DNA) for how to survive.

Adapted: ↑ Refers to an individual changing how it looks, acts, grows, or senses to better live in its surroundings.

DNA: ↑ The molecule that makes up the genetic instructions for how a living thing looks, acts, grows, and senses its surroundings. This genetic code is passed from parents to kids.

Epigenetics: ↑ Changing the way the genetic instructions written in the DNA are carried out, without changing how the instructions are written.

Conifers: ↑ Trees that make cones that hold their seeds and have needle-like leaves.

Gymnosperm: ↑ An ancient group of plants that produce seeds in cones or similar structures.

Taiga: ↑ A forest of the cold, subarctic region just south of the Arctic Circle.

Gamma Radiation: ↑ High-energy rays emitted from radioactive particles that causes burns and damage to DNA.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

Grants from the Research Council of Norway (Award 324129) and SNS Nordic Forest Research (Award SNS-131) were used to support the authorship.