With cherry trees blooming from Jamaica Pond to the Public Gardens, Boston and the rest of New England experienced a reprieve from winter weather earlier this season. In fact, December 2006 was the warmest December on record in Boston, with the average monthly temperature reaching 6 degrees higher than normal. But what does this dramatic and sustained increase in temperatures mean for gardens and gardeners in the spring?

Who needs the cold, anyway?

Many plants in northern climates tightly regulate their flowering and reproduction, saving their energies to produce seed only after the winter is over. Most of these plants sense and integrate a variety of cues from their environment, including the length of daylight hours, temperature, water availability, and length of the cold season. Some plants, including the early spring-blooming forsythia, require a long period of cold temperatures in order to prepare for blooming in the spring. This requirement of a certain period of cold for a plant to be capable of flowering is called vernalization. In most plants, scientists are unsure what causes the vernalization response. However, recent research in the weedy mustard-related plant, Arabidopsis thaliana, has begun to uncover a complex system of environmental sensing that is linked to flowering.
Arabidopsis is a small flowering plant indigenous to Europe that is frequently used by scientists as a model organism for the study of plant biology and genetics. Arabidopsis thaliana was the first plant genome fully sequenced, allowing scientists to better correlate changes, or mutations, in the DNA sequence with visible alterations in the growth, shape, and flowering capacity of the plant. Like many northern plants, winter varieties of Arabidopsis require a cold spell of at least 4-6 weeks to enable the plant to respond to the lengthened daylight hours of spring and initiate flowering. These studies suggest that vernalization acts like a circuit breaker—it must be turned on, but doesn’t directly control the final activation. Once vernalization is achieved, additional cues (including long daylight hours) act as the “on” switch that ultimately trigger flowering. This mechanism ensures that these plants can’t be fooled into blooming by a brief stint of warm weather while Mother Nature waits to freeze those buds with a cold snap the following week.
Other varieties of Arabidopsis do not show the same reliance on vernalization as the winter varieties described above. Scientists have traced the cause of this variation to mutations in several genes, one of which—FLC—constitutes the main vernalization circuit breaker. In Arabidopsis winter varieties, the FLC gene (short for Flowering Locus C) is turned on and therefore FLC protein is produced. FLC protein in turn binds to and shuts down production of several genes required for flowering. In the spring varieties, mutations in the FLC gene prevent production of functional FLC protein. Thus, there is no repression of the flowering genes, and the requirement for vernalization is abolished.
In Arabidopsis winter varieties, cold weather induces gradual production of several vernalization-related proteins that work to turn off the FLC gene, releasing the flowering genes to be activated by other pathways (such as length-of-day sensing mechanisms). Interestingly, once FLC has been turned off, it is maintained in an “OFF” state, even when the vernalization-related proteins are no longer expressed. In fact, an Arabidopsis plant can “remember” that it has passed through winter for up to 300 days, biding its time until it senses the long daylight hours indicative of spring.

How does Arabidopsis remember winter?

It was recently shown that the vernalization-related proteins (vrn1, vrn2, vin3, vrn5 and lhp1) do not simply bind to the FLC gene to block its activation. In all organisms more complex than bacteria (including plants and humans), the DNA of the genome is found in a region of the cell called the nucleus, and is packaged into complex structures around proteins called histones. If a gene is in a region of tightly packaged DNA, it is frequently inaccessible to the machinery required for it to be turned on. Thus, these genes are typically “OFF” unless the DNA is first unwrapped from the histone packaging. This method of gene regulation is common across all higher organisms and represents a mechanism to maintain genes in “ON” or “OFF” states for longer periods of time, often even when a cell divides.< br /> Scientists studying Arabidopsis found that the vernalization-related proteins are similar to human proteins involved in packaging the DNA. In particular, several vernalization-related proteins appear to modify the histones (and even the DNA itself) around the FLC gene so that the FLC gene DNA is more tightly packaged. Thus as cold weather continues, these proteins slowly turn off the FLC gene by making it inaccessible. This circuit breaker mechanism then ensures that the plant “remembers” that it has passed through winter, allowing the flowering genes to respond to the long daylight hours of spring.

So what does this mean for Boston’s green thumb?

For Bostonians, the first blooms of spring may rely heavily on the weather over the rest of the winter. Woody vernalization-dependent shrubs—including forsythia (the first harbinger of spring), lilacs, and pussywillows—may show delayed or decreased blooming if the current cold spell doesn’t last long enough to flip on the molecular circuit breaker. Bulb plants—from crocuses and tulips to garlic and shallots—also require vernalization and may show decreased or delayed blooming. The required temperatures and length of vernalization may vary significantly amongst these plants, so predictions of the degree of damage are likely only as good as weather predictions.
In contrast, other common New England plants do not require vernalization—including cherry trees, azaleas, and rhododendrons. However, these plants may also feel the consequences of the warm December. Many plants without a vernalization requirement began blooming in December and may show delayed or decreased abundance of flowering, having expended resources once already for reproduction. While December 2006 was a time for picnicking under cherry trees, the flowers of April 2007 may respond reciprocally, leaving gardeners wondering how to adapt if warm weather trends continue in the coming years.

— Leta Steffen, Harvard University

For More Information:

For a good review of the history of the field and the genes involved, check out “Vernalization, Competence, and the Epigenetic Memory of Winter”. [Amasino, R. (2004) Plant Cell 16:2553-2559.]

Vrn5 is one of the most recently identified vernalization-related genes and its activity can be read about in “The PHD Finger Protein VRN5 Functions in the Epigenetic Silencing of Arabidopsis FLC”. [Greb T, Mylne J, Crevillen P, Geraldo N, An H, Gendall A, and Dean C. (2007) Current Biology 17:73-78]

Wikipedia also has a brief synopsis of vernalization. <http://en.wikipedia.org/wiki/Vernalization>

And to learn about how you should deal with specific plants in your own garden, start with <http://www.sciencedaily.com/releases/2007/01/070129161909.htm>

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