How plants remember winter, in order to flower in spring

Scientists at the John Innes Centre (JIC) Norwich, have discovered the molecular change that allows plants to remember winter.

Many plants need a cold period (3-8 weeks at 4° – 8°C) early in their growth to stimulate them to flower, this is called vernalisation, and without a suitable cold treatment flowering is delayed. JIC scientists have identified many of the genes involved in this process but their latest discovery is a chemical modification that occurs on one of these genes, which locks it in an ‘off’ state after exposure to cold. Thus the plant ‘remembers’ throughout its life that it has been exposed to a period of cold temperature early in its growth. This helps the plant to avoid flowering in winter and to flower in the favourable conditions of spring. Understanding the biology of this chemical key may allow scientists to alter vernalisation requirements in crop plants. It will also add to studies on the evolution of molecular ‘memories’ as these are common to diverse organisms, including fruit flies and humans. This discovery is described for the first time tomorrow (8th January), in the international scientific journal Nature.

“This is a very exciting discovery because it adds to our understanding of how plants, and other organisms, use chemical modifications to lock genes on or off and so control their growth and development”, says Professor Caroline Dean (Associate Research Director and leader of the research team at JIC). “In the depths of a British winter we are all looking forward to the coming of spring when we can forget the cold. But many plants will be ‘remembering’ the winter cold to ensure that, come spring, they flower at the right time. Understanding this process is of tremendous scientific importance but is also of practical interest as flowering time can have a big effect on crop yields”.

Histone H3 is one of a class of proteins that coat the DNA molecule. Modification of histone H3 can affect the activity of adjacent genes. When the JIC scientists studied histone H3 molecules bound close to a gene, (FLC)(1), known to be important in vernalisation they found that the chemical structure (actually the methylation) of the histones was different in plants that had experienced a period of cold. This particular methylation causes other proteins to coat the DNA so hiding the FLC gene from the machinery which reads it.

The JIC scientists used a common weed, Arabidopsis thaliana (Mouse-ear or Thale Cress) in their studies. Arabidopsis would flower 3-4 weeks after seed germination if it were not for the activity of the FLC gene, which inhibits flowering and delays the production of flowers for up to 3 months. A period of low temperatures (3-8 weeks at 4°- 8°C), reduces the activity of the FLC gene. In this ‘low activity’ state, its ability to inhibit flowering is reduced and so plants flower more quickly, hence the acceleration of flowering after a cold treatment.

Methylation of histone H3 is the lock used by plants to keep the FLC gene in the ‘off’ position. How it is unlocked in the next generation is so far unknown.

Identification of histone H3’s role in vernalisation follows a series of discoveries that have gradually unravelled the process. A previous step was the discovery of two genes (called VRN1 and VRN2) (2) which are required for the plant to ‘remember’ it had had a cold treatment. In plants where the VRN genes were not working (because of mutations) the activity of FLC was reduced in the cold, as normal, but on return to warm conditions its activity increased again. It was as if these plants had not been vernalised and they did not flower for several months. VRN1 and 2 were somehow ‘locking’ the FLC gene in the ‘low activity’ position that is induced by cold so that, even when returned to warm conditions, FLC activity remained low and the plants were able to flower in a few weeks.

By taking advantage of the relative simplicity of the genetics of Arabidopsis the scientists were able to isolate the normal and mutant forms of the VRN genes and compare their structures with those of other plant and animal genes. VRN1 was found to be similar to known genes that produce DNA binding proteins. VRN2 proved to be closely related to a gene in fruit flies, which has a role in providing a molecular memory that ‘locks’ specific genes into a particular level of activity over prolonged periods of time and/or cycles of cell division and growth.

This comparison led the researchers to look at the proteins that are closely associated with the DNA that contains the FLC gene. Changes in histone H3 methylation were found to account for the maintenance of the reduced activity of FLC (caused by prolonged cold) and that the VRN1 and 2 proteins were the factors causing the changes in methylation.