Winter Awakens Spring Flowering

To avoid the dangers of frost, many plants have evolved a knack for waiting for winter to pass before flowering. But the season’s cold weather is key to making it happen. Rick Amasino investigates this process — and the genes behind it — to reveal invaluable insights for farmers and plant breeders. Interview by Kaine Korzekwa MS’16

Rick Amasino
Photo by Frederic Bouche

It’s springtime in Wisconsin again. Home gardeners and farmers are busy tending to their beds and fields, relishing the fresh sprouts of flowers, vegetables, and crops. It begs the question: What happens in the inner workings of plants as they prepare for spring? What’s the science that governs the growing season for different flora?

Rick Amasino, a plant biochemist and professor in the Department of Biochemistry, may have the answers — or at least some of them. He studies plant development and, specifically, how and when plants produce flowers. In 2016, his expertise earned him a place on a National Academies of Sciences, Engineering, and Medicine committee tasked with investigating the impacts of genetically engineered crops.

Many plants have effectively evolved a way to avoid flowering prior to winter. Instead, they use the cold season to help activate flowering when the weather warms. Amasino’s research sheds light on what conditions a plant must experience in order to flower. In particular, he focuses on unraveling the genetic basis of the effects these conditions have on plants as they stimulate or repress flowering. His findings may allow other scientists and plant breeders to develop crops that are more efficient and have higher yields of food or energy.

How do plants respond to spring?

There are a wide range of responses. For example, some plants need to be exposed to winter cold to flower in the spring, whereas others form spring flowers as a result of being exposed to the decreasing hours of sunlight during the fall season. Apple and cherry trees are in this latter category — their flowers are actually formed in the previous fall in buds that become dormant. Then, when it gets warm the following spring, everything that was crammed into those buds in the fall just unfolds. Other plants like lilies, for example, require exposure to cold in order to flower. When they are growing in the fall, flowering is blocked. But over winter, the block is removed and they flower in the spring. The underlying processes for this involve a lot of biochemistry, and that’s what we’ve studied in my lab. Specifically, we study how flowering is blocked in the fall and how exposure to cold results in the removal of this block. The block removal process is known as vernalization; this word is derived from vernal, which means “relating to spring.”

Are there any more examples of plants that need winter to flower?

Some common examples include many of the vegetables we plant in the spring, such as cabbage, carrots, and beets. We don’t usually see these particular vegetables flowering because they will not flower until they experience winter, and we harvest them before they have a chance to flower. Many grasses go through this process as well.

Why should we be interested in this process?

This requirement to go through winter in order to flower is important agriculturally; food plants keep growing without flowering all summer long and, therefore, the part which we consume can get very large. However, if you left a carrot in the ground after the summer, it would flower the next spring, and the underground part of the carrot we eat would become shriveled as it provides the nutrients for flowers to form.

If it gets warmer earlier, is that a problem?

An early warming trend in itself isn’t problematic if it continues into spring, but our climate is likely to be more variable than that. So, if we have unusually high temperatures late in the winter and cherry blossoms in Door County open, but then we get a blast of cold afterward, the flowers will be destroyed and fruit cannot form.

What’s going on on the inside of the plant that determines whether or not it flowers?

In the plants we study that require winter, there is a gene encoding a repressor protein that is expressed in the fall that prevents the plant from flowering. Then, over the winter, control of the repressor gene is altered in a way that the repressor is no longer expressed. Consequently, plants can flower when it gets warm, and they resume growth in the spring in the absence of the repressor protein. We’ve recently published research specifically on the small Mediterranean grass called Brachypodium. Previous work has shown that a gene called VRN1 is responsible for activating flowering in these grasses after the winter. But what’s the repressor gene keeping VRN1 in check in the fall? That was previously unclear. We did genetic screens and found several of the genes that repress the VRN1 gene prior to winter. We just published a scientific paper on one of these, calling it RVR1, for its role in repressing VRN1.

Why are gene discoveries like this important for this area of research?

Scientists that breed cereal grains may find this newly identified gene interesting. However, we think it could also impact biofuels research. I am part of the U.S. Department of Energy’s Great Lakes Bioenergy Research Center (GLBRC) here on campus. Although switchgrass, which can be used to make biofuels, doesn’t go through the vernalization process, there’s a good chance that taking the RVR1 gene from Brachypodium and putting it in switchgrass will delay switchgrass flowering. Delaying switchgrass flowering to various extents may improve yield.

Why is understanding this process important?

In basic research like ours, we often don’t know where exactly it’s going, but it often ends up having practical relevance. Our goal is to understand the biochemical pathways that plants have evolved to flower at certain times of the year. But in crops, in which the timing of flowering is important, this research can be applicable. For example, we share our unpublished work with wheat breeders who can translate some of the knowledge into increased efficiency in a breeding program. Also, our work has revealed basic principles of how genes are regulated, which has implications for many areas. Another example of applicability, although not directly from our research, was useful for sugar beet farmers, who plant in the spring. A spring cold spell will trigger some of the sugar beets to flower, and flowering plants do not produce the part of the beet the farmers harvest. Scientists in Europe modified genes involved in the flowering response to cold and came up with a sugar beet variety that doesn’t flower if it is exposed to cold. Now farmers can plant their beets in the fall rather than the spring to allow them to have a much longer growing season and to grow bigger — and they don’t have to worry about the beets flowering. This has significantly increased the yield per acre of sugar beets.

What’s your next step in this research?

We are going to continue to work with other GLBRC researchers to study Brachypodium and how different varieties of the plant live and persist in winters that have varying temperatures and lengths. How did one variety evolve a system tweaked to require 16 weeks of cold? Why does another one require just two weeks of cold? In other words, what’s the genetic and biochemical difference between the requirement for a short winter versus a long winter? Grasses are really important crops, and this model for studying flowering can tell us a lot about how they work