The Epigenetics Revolution

Yet animals and plants are surprisingly similar in many ways. This is especially the case when we consider the most advanced of our green relatives, the flowering plants. These include the grasses and cereals that we rely on for so much of our basic food intake, and the broad-leaved plants, from cabbages to oak trees and from rhododendrons to cress.

Animals and the flowering plants are each made up of lots of cells; they are multi-cellular organisms. Many of these cells are specialised for particular functions. In the flowering plants these include cells that transport water or sugars around the plant, the photosynthesising cells of the leaves and the food storing cells of the roots. Like animals, plants have specialised cells which are responsible for sexual reproduction. The sperm nuclei are carried in pollen and fertilise a large egg cell, which ultimately gives rise to a zygote and a new individual plant.

The similarities between plants and animals are more fundamental than these visible features. There are many genes in plants which have equivalents in animals. Crucially, for our topic, plants also have a highly developed epigenetic system. They can modify histone proteins and DNA, just like animal cells can, and in many cases use very similar epigenetic enzymes to those used by animals, including humans.

These genetic and epigenetic similarities all suggest that animals and plants have common ancestors. Because of our common ancestry, we’ve inherited similar genetic and epigenetic tool kits.

Of course, there are also really important differences between plants and animals. Plants can create their own food, but animals can’t do this. Plants take in basic chemicals in the environment, especially water and carbon dioxide. Using energy from sunlight, plants can convert these simple chemicals into complex sugars such as glucose. Nearly all life on planet earth is dependent directly or indirectly on this amazing process of photosynthesis.

There are two other ways in which plants and animals are very different. Most gardeners know that you can take a cutting from a growing plant – maybe just a small shoot – and create an entire new plant from this. There are very few animals where this is possible, and certainly no advanced ones. True, if certain species of lizard lose their tail, the animal can grow a new one. But they can’t do this the other way around. We can’t grow a new lizard from a discarded bit of tail.

This is because in most adult animals the only genuinely pluripotent stem cells are the tightly controlled cells of the germline which give rise to eggs or sperm. But active pluripotent stem cells are a completely normal part of a plant. In plants these pluripotent stem cells are found at the tips of stems and the tips of roots. Under the right conditions, these stem cells can keep dividing to allow the plant to grow. But under other conditions, the stem cells will differentiate into specific cell types, such as flowers. Once such a cell has become committed to becoming part of a petal, for example, it can’t change back into a stem cell. Even plant cells roll down Waddington’s epigenetic landscape eventually.

The other difference between plants and animals is really obvious. Plants can’t move. When environmental conditions change, the plant must adapt or die. They can’t out-run or out-fly unfavourable climates. Plants have to find a way of responding to the environmental triggers all around them. They need to make sure they survive long enough to reproduce at the right time of year, when their offspring will have the greatest chance of making it as new individuals.

Contrast this with a species such as the European swallow (Hirundo rustica) which winters in South Africa. As summer approaches and conditions become unbearable the swallow sets off on an epic migration. It flies up through Africa and Europe, to spend the summer in the UK where it raises its young. Six months later, back it goes to South Africa.

Many of a plant’s responses to the environment are linked to changes in cell fate. These include the change from being a pluripotent stem cell to becoming part of a terminally differentiated flower in order to allow sexual reproduction. Epigenetic processes play important roles in both these events, and interact with other pathways in plant cells to maximise the chance of reproductive success.