How Do Plants Know When to Bloom?
Scientists like to use big words to impress the rest of us, so they have a term for how a plant decides when to bloom: vernalization. But making up a word for a phenomenon is not the same as explaining it.
Everybody observes that plants seem to just “know” that spring is here, when they put forth their glorious blossoming colors, but think about it: how can a plant, without eyes or a brain or a calendar, judge when it is safe to send out flowers? Through all the vagaries of weather they have an uncanny sense of timing. It’s especially puzzling how winter annuals do this, and biennials, which only bloom in the second year. How can a plant have a memory, and sense the seasons? What goes on in the genes, at the molecular level? How can the memory be preserved through multiple cell divisions?
This was the subject of two scientific papers in the the Jan. 8 issue of Nature,1,2 and an analysis by Christopher Surridge.3 The process is very complex and still mysterious in many respects. It involves quite a few genes and proteins, particularly histones which are part of the chromatin that wraps DNA, and additional signaling molecules like acetyl and methyl groups. Biochemists have found that, in many cellular processes, there are starters and stoppers: genes and proteins that initiate or suppress an action, and other genes and proteins that stop or re-enable them. For instance, a molecule might clamp onto a gene, making it impossible for the translation machinery to read it, and another molecule will remove the suppressor, allowing the gene to be read and transcribed into a protein. The complex dance of activators and repressors and signalling molecules can be triggered by the external environment and by other activities inside the cell.
If you can keep this all straight, vernalization goes something like this: a gene named FLC prevents flowering, and is normally expressed during the off-season. A cold snap induces the VIN3 protein to remove acetyl groups from the histones on the chromatin near this gene, signalling two other molecules (vernalization proteins VRN1 and VRN2) that this gene is silenced. Their job is to keep it that way, so that suppression of flowering is itself suppressed. The FLD gene, which promotes flowering, is then expressed. Somehow, FLD tells the molecules at the apical meristem (see 11/20/2003 entry), to send out the buds. Surridge explains, “Silencing is an effective means of controlling long-term gene expression, as it persists even after cells divide. In animals, switching silencing on or off is a well-known way to control development. It seems that plants share this system, using it to preserve the memory of winter’s passing.”
How does cold cause these reactions? What is known so far is just part of a more involved process. One of the papers2 admits, “How cold results in low FLC RNA and whether any post-transcriptional regulation occurs that feeds back to cause reduced transcription is unknown at present.” The other paper1 says, “The additional components that interact with VIN3, and VRN1 and VRN2, to repress FLC during and after vernalization are not known.” Undoubtedly there are other environmental cues that affect vernalization, such as length of daylight and nutrient availability.
A popular-level account from Reuters on these results can be found on MSNBC.com.
1Sibum Sung and Richard M. Amasino, “Vernalization in Arabidopsis thaliana is mediated by the PHD finger protein VIN3,” Nature 427, 159 – 164 (08 January 2004); doi:10.1038/nature02195.
2Ruth Bastow et al., “Vernalization requires epigenetic silencing of FLC by histone methylation,” Nature 427, 164 – 167 (08 January 2004); doi:10.1038/nature02269.
3Christopher Surridge, “Plant development: The flowers that bloom in the spring,” Nature 427, 112 (08 January 2004); doi:10.1038/427112a.
We hope this makes your gardening more thought-provoking. Inside a cell, there’s a flurry of activity. If one gene were expressed freely, it would never stop. Suppressors turn them off until they are needed, then other molecules remove the suppressors. Cues from the environment and signals from other genes can trigger switches in the genes in the right sequence. How could the cell, without a brain, “know” how to handle these signals, if it were not preprogrammed? The cell responds to feedback from the environment, monitors protein levels, and can branch to different pathways depending on conditions. Everything acts as if programmed with loops, switches, and conditional procedures designed in advance. The proof of the programming is in the results: a crocus emerges out of the last snow, showing forth its delicate beauty, unafraid of the clouds, knowing that sunny days filled with pollinators are just ahead. This is so amazing it should make us stand in awe of the Almighty Programmer.