Prof. Annette Nassuth

When it gets cold outside, plants can’t bundle up to stay warm. Instead, some plants have developed survival strategies that help them tolerate freezing temperatures. Prof. Annette Nassuth in the Department of Molecular and Cellular Biology is trying to find out why Ontario wine grapevines can survive sub-zero temperatures while their sun-loving European counterparts get frost-bitten.

“We are looking at the different genes that change the programming of the plants and help them cope with freezing,” says Nassuth.

When ice crystals form within a plant cell, the cell membrane becomes brittle and breaks, causing the cell to die. What happens is that ice crystals form outside of the cell, so water leaves the cell to balance the amount of water on both sides of the membrane. When the cell loses water, its membrane shrinks like a balloon. During the thawing process, water tries to enter the cell, but the shrunken membrane can’t expand to hold more water and breaks.

“Most of the damage that’s being done is not necessarily caused by the freezing process, but by the thawing process,” says Nassuth.

Plants that grow in northern climates are always prepared for freezing temperatures.

“They have more sugars, osmolytes, special proteins and membrane lipids in their cells,” says Nassuth. The sugars act like antifreeze while osmolytes help the cell retain water. When a plant cell doesn’t have enough water, the proteins in its membrane unfold and become inactive.  Water helps keep the proteins folded in the right position, while other proteins help prevent the membrane from breaking. Lipids also keep the membrane flexible to accommodate changing water levels.

Plants that live in temperate climates aren’t prepared for cold weather because this requires energy, so they have to acclimate or else they will die when the temperature drops.

In the fall, cooler temperatures and shorter days trigger the synthesis of compounds that help plants acclimate. The same process happens when gardeners “harden off” seedlings by placing them outside for several hours a days before planting them. Exposure to cooler temperatures gets the plants accustomed to life outdoors.

“Whether a plant makes it through the winter or not depends a bit on how fast they do that cold acclimation,” says Nassuth. “There’s a big difference among plants. Some do it faster than others.”

Since wild grapes experience winters year after year, “Our hypothesis is that wild grapes have a superior mechanism,” says Nassuth. “Either they react faster to cold, or they can change to a larger extent so that they can withstand much lower temperatures than the cultivated grape.”

Genetics also determine the maximum temperature extremes that a plant can endure. “The challenge with freezing tolerance is that it’s not just one gene that does it; it’s a whole bunch of them,” says Nassuth. The genes work together, causing a chain reaction in the plant, which makes it difficult to isolate the genes responsible for freezing tolerance. Another complication is that wine grapes are cross-bred from two parents, so it’s hard to know if one or both parents are the source of genes that make their offspring intolerant to cold.

Nassuth is comparing the genes of wild grapes and cultivated grapes to identify the differences between them. To find out how those differences affect grapevines, she over-expressed the genes in Arabidopsis, a common lab plant.

She found that the genes play a role in freezing and drought tolerance, but they also affect plant growth, influencing the number of leaf pores, called stomata, which regulate water loss. Genes are also responsible when the plant produces smaller leaves or flowers later in the growing season. This research was recently accepted for publication in the journal Plant, Cell & Environment.

“We tried to tease out from these genes what kinds of things they activate,” says Nassuth.  Positive attributes could be used in plant breeding while negative attributes, like later flowering, would be avoided. Isolating these genes could help breeders develop wine grapevines that can better withstand weather extremes.