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.
Not all plants use exactly the same epigenetic strategies. The best-characterised model system is an insignificant looking little flowering plant called Arabidopsis thaliana. It’s a member of the mustard family and looks like any nondescript weed you can find on any patch of wasteland. Most of the leaves grow close to the ground in a rosette shape. It produces small white flowers on a stem about 20–25 centimetres high. It’s been a useful model system for researchers because its genome is very compact, which makes it easy to sequence in order to identify the genes. There are also well-developed techniques for genetically modifying Arabidopsis thaliana. This makes it relatively straightforward for scientists to introduce mutations into genes to investigate their function.
Arabidopsis thaliana seeds typically germinate in early summer in the wild. The seedlings grow, creating the rosette of leaves. This is called the vegetative phase of plant growth. In order to produce offspring, Arabidopsis thaliana generates flowers. It is structures in the flowers that will generate the new eggs and sperm that will eventually lead to new zygotes, which will be dispersed in seeds.
But here’s the problem for the plant. If it flowers late in the year, the seeds it produces will be wasted. That’s because the weather conditions won’t be right for the new seeds to germinate. Even if the seeds do manage to germinate, the tender little seedlings are likely to be killed off by harsh weather like frost.
The adult Arabidopsis thaliana needs to keep its powder dry. It has a much greater chance of lots of its offspring surviving if it waits until the next spring until it flowers. The adult plant can survive winter weather that would kill off a seedling. This is exactly what Arabidopsis thaliana does. The plant ‘waits’ for spring and only then does it produce flowers.
The technical term for this is vernalisation. Vernalisation means that a plant has to undergo a prolonged cold period (winter, usually) before it can flower. This is very common in plants with an annual life-cycle, especially in the temperate regions of the earth where the seasons are well-defined. Vernalisation doesn’t just affect broad-leaved plants like Arabidopsis thaliana. Many cereals also show this effect, especially crops like winter barley and winter wheat. In many cases, the prolonged period of cold needs to be followed by an increase in day length if flowering is to take place. The combination of the two stimuli ensures that flowering occurs at the most appropriate time of year.
Vernalisation has some very interesting features. When the plant first begins to sense and respond to cold weather, this may be many weeks or months before it starts to flower. The plant may continue to grow vegetatively through cell division during the cold period. When new seeds are produced, after the vernalisation of the parent plant, the seeds are ‘reset’. The new plants they produce from the seeds will themselves have to go through their own cold season before flowering[274].
These features of vernalisation are all very reminiscent of epigenetic phenomena in animals. Specifically:
The plant displays some form of molecular memory, because the stimulus and the final event are separated by weeks or months. We can compare this with abnormal stress responses in adult rodents that were ‘neglected’ as infants.
The memory is maintained even after cells divide. We can compare this with animal cells that continue to perform in a certain way after a stimulus to the parent cell, such as in normal development or in cancer progression.
The memory is lost in the next generation (the seeds). This is comparable with the way that most changes to the somatic tissues are ‘wiped clean’ in animals so that Lamarckian inheritance is exceptional, rather than common.
So, at a phenomenon level, vernalisation looks very epigenetic. In recent years, a number of labs have confirmed that epigenetic processes underlie this, at the chromatin modification level.
The key gene involved in vernalisation is called FLOWERING LOCUS C or FLC for short. FLC encodes a protein called a transcriptional repressor. It binds to other genes and stops them getting switched on. There are three genes that are particularly important for flowering in Arabidopsis thaliana, called FT, SOC1 and FD. Figure 15.1 shows how FLC interacts with these genes, and the consequences this has for flowering. It also shows how the epigenetic status of FLC changes after a period of prolonged cold.
Figure 15.1 Epigenetic modifications regulate the expression of the FLC gene, which represses the genes which promote flowering. The epigenetic modifications on the FLC gene are controlled by temperature.
Before winter, the FLC gene promoter carries lots of histone modifications that switch on gene expression. Because of this, the FLC gene is highly expressed, and the protein it codes for binds to the target genes and represses them. This keeps the plant in its normal growing vegetative phase. After winter, the histone modifications at the FLC gene promoter change to repressive ones. These switch off the FLC gene. The FLC protein levels drop, which removes the repression on the target genes. The increased periods of sunlight during spring activate expression of the FT gene. It’s essential that FLC levels have gone down by this stage, because if FLC levels are high, the FT gene finds it difficult to react to the stimulus from sunlight[275].
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For a useful summary of the epigenetic control of vernalisation, see Ahmad et al. (2010),