The Epigenetics Revolution полностью

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 rites of spring

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.