Can some plants access past experiences so that this information can be incorporated into new responses, such as flowering?
The answer is: Yes! (as illustrated in the photo on the the left).
This picture shows a cabbage plant that was grown for five years in the laboratory of Dr. Rick Amasino. (For size comparison the little girl – who was the same age as the cabbage – is shown.)
Cabbage is a biennial plant and requires exposure to the environmental cue of prolonged winter cold in order to flower the second spring after planting. This promotion of flowering by cold is called vernalization.
The large cabbage shown in this picture has never been vernalized and cannot flower. The little girl is holding a cabbage plant (presumably between 1 to 2 years old) that has been vernalized.
In biennial plants, such as cabbage, competence to flower in the spring requires a previous “cold” treatment, sometimes months earlier.
A requirement for vernalization permits biennials to become established during the fall without the risk of flowering as winter begins. During the winter, these plants experience and “remember” a cold treatment, which enables them to flower during the favorable conditions of spring.
But how do such plants retain a “memory” of winter?
Much of what we currently know about how this works comes from the work of Dr. Amasino and his colleagues at the University of Wisconsin using the experimental plant Arabidopsis thaliana (see figure below right).
Briefly, the cold treatment elicits epigenetic changes in the cells of the shoot apical meristem (SAM) of the plant. (The SAM will ultimately give rise to the flower.)
Simply put, cold somehow causes parts of the SAM cells’ DNA to be physically blocked so that no gene transcription can take place in selected regions of the DNA. (Histones likely serve as the “padlocks” on the DNA.)
Remarkably, these blocks remain on the DNA even through the process of cell division (mitosis). Thus, even though vernalized cells divide, they retain this “memory” of a cold treatment.
These blocks do NOT persist through meiosis, however, so that the next generation of plants can use this vernalization strategy.
How do these epigenetic changes promote spring flowering?
The key appears to be the removal of a factor that actually blocks flowering. This factor, a transcription factor called FLC, blocks flowering by inhibiting genes required to switch the SAM from vegetative to floral development in Arabidopsis. (To learn much more about how this works, please see here.)
The epigenetic changes in response to the cold treatment result in the blockage of the gene coding for FLC. Thus, by the time warm spring temperatures role around, the absence of FLC renders the plant competent to flower.
If, however, vernalization does not occur, then the presence of FLC either delays or completely blocks flowering (e.g., see the 5-year old cabbage above).
A similar story appears to take place in winter wheat and winter barley.
Recent findings re. vernalization:
Research reported from the lab of Dr. Caroline Dean indicates that vernalization involves polycomb proteins, which suggests that gene silencing in plants – at least in connection with vernalization – may resemble that in insects and mammals.
Briefly, polycomb proteins actually alter the structure of small parts of the chromatin itself, thus, effectively silencing the genes in this section of the DNA by adding a “bend” or “kink” in the chromatin.
This would be analogous to having a bend or kink in a zipper, so that when the slider hits the kink, it can’t unzip the chain. It’s physically blocked.
Simply put, these investigators showed that a small strand of RNA that they named “COLDAIR” (ha ha) actually serves to help direct the polycomb proteins to the gene coding for the FLC, thus helping to shut it off. And what’s even more remarkable is that COLDAIR is a transcript of a non-coding region (intron) of the FLC gene itself. (Pretty cool, huh?)
Bottom line: Biennial plants have evolved a cellular mechanism that allows them to “remember” that they have experienced winter, so that they don’t flower prematurely.
Müller, R. and J. Goodrich (2011) “The Footprints of Winter: Epigenetic marks laid down during the cold months of the year allow flowering in spring and summer.” The Scientist, Vol. 25, p.57. (Full Text)
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