An Overview of Vernalization in Plants

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In the Journal of American Society of Plant Biologists, Plant Cell. 2004 October; 16(10): 2553–2559.  Richard Amasino explores the history of research into the process of vernalisation.

Vernalisation is the process by which seeds or plants are prepared for spring, by a long period of winter cold.

Vernalisation is explained in terms of a gene which regulates germination and flowering.  Active, it prevents germination or flowering, but it can be switched off by freezing temperatures.  This effectively gives the plant a “memory” of winter which prepares it for spring.

The process is now used by horticulturists to promote flowering at any desired time by artificially subjecting the seed to below freezing temperatures.  This can be done in the traditional way, by sowing the seed in beds or pots outdoors throughout winter, or by putting seed in a freezer or refrigerator in tropical climates which do not experience long, freezing winters, or, in cooler countries, at any time of the year.

Originally, stratification, or exposure to prolonged frost, was recognized only as the physilogical process by which prolonged exposure to cold temperatures promoted seed germination. Seed dormancy was thought to have two causes, the physical rigidity of the seed coat, which could be broken by scarification, and an inner dormancy which required environmental factors of moisture and cold to modify.

It was known that some chemical which prevented germination was broken down by frost, in the same way that saponins, and other chemicals on a seed coat, needed to be washed off before germination could occur.  In dry countries, this ensured that seed of cereal crops such as Quinoa, would not germinate until the soil was wet enough to allow the young plant to survive.

However, since the various genome-mapping projects, this process has been investigated at molecular level, particularly in Arabidopsis  and Henbane.

In the early 1900s, the Russian geneticist, Lysenko, worked with two strains of wheat: one could be sown in autumn and one in spring.  The winter wheat responded to cold by flowering early. Although Lysenko attributed this to a genetic cause, he lost credibility by falsifying his experiments to support Russian propaganda.  He insisted that the change was not merely applicable to one season, but could be transmitted genetically. This was a nonsense, since the property was protective and therefore it would have been counterproductive if it could be easily reversed. But the Russian propaganda machine used it to argue that induced toughness in the Russian people could be transmitted  from one generation to another.

The process of vernalisation enables plants to germinate in autumn and take advantage of the favourable pre-winter growing period, without flowering prematurely. After winter, they are enabled to flower quickly to enable them so set seed early, giving the seed a longer period to ripen, fall and germinate before winter returns.

It was therefore concluded that vernalisation affected an epigenetic switch, a “mitotically stable acquisition of the competence to flower” which can be transmitted to new cells as the plant grows, until the right trigger conditions of temperature, moisture, light or day length are satisfied.   Some researchers argue that the term epigenetic is not strictly accurate, since the induced trait is only transferable by mitosis, to growing cells in the parent plant, and is not heritable in its modified form.

In order for vernalisation to work, plants must be able to tell if spring has really arrived, and not be fooled by short, early periods of mild weather. They therefore needed long periods of cold to prepare them to flower.  But cold alone was not always enough.  Plants such as henbane also needed a specific day-length to promote flowering.  So vernalisation was a preparation for flowering, but not the trigger which caused it. It is now known that there are three genes in wheat,  VRN1, VRN2, and FT (VRN3), which work together to enable  recognition of optimal  temperature, moisture and day-length for flowering.

Vernalisation research has been used to make seed storage more efficient, since it is now known that seeds can be stored cold, but vernalisation can be reversed by heat treatment if required.  Vernalisation is also used to prepare spring bulbs for Christmas flowering, whilst reversing heat treatment is used to prepare onion sets, and other vegetables, by preventing them from flowering, in order to divert their energy to the production of larger bulbs, tubers, roots or leaf-heads.;jsessionid=57858745B50F76978CC07DEF00E52F09.tomcat1?fromPage=online&aid=4815856

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