Some plants can endure the cold,
Even freezing temps surviving
Gene transcription levels change ten-fold
But the important thing’s the timing.
CBFs start the response pathway,
Activating others to join the fray,
This works when there’s not a sudden freeze one day
It’s bad enough to lose a limb,
But plants must protect their meristems!
For those readers who are not compliant with our rodent overlord and have not seen the movie Frozen, here is a link to the Youtube clip with corresponding song.
Ice in biological systems is just as bad for plants as it was for poor Anna in Frozen. The sharp edges of ice crystals tend to rip biological membranes to shreds in ways they can no longer function or be repaired. In freezing temperatures, crucial biomolecules slowly lose mobility and grind to a halt. Both of these consequences violate central tenets of living systems. Cells need intact membranes to maintain the integrity of the ion gradients responsible for fueling bioenergetics, and all biochemical reactions depend on small motions in all molecules. Thus, all living things want to keep their cytoplasm from turning into snowflakes. So, how do plants deal with freezing temperatures? It’s not like they can just go inside their castles next to a warm fire. And for the record, acts of true love aren’t very helpful either. Today’s post explores how plants adapt to falling temperatures.
Temperature is an important factor governing the molecular details of plants’ lives. The activities of the enzymes that carry out routine metabolism are affected by the ambient temperature. The fluidity of plants’ numerous important membrane systems is also influenced by temperature. Consequently, plants must make significant internal adjustments to adapt to colder temperatures. Even though the leaves of your cold-hardy plants may not look any different in summer compared to winter, there are many molecular-level changes with respect to the types of proteins and membrane lipids that the cells contain. These changes require an energy commitment that makes it wasteful or impractical for plants to always have cold-tolerance turned on, so they have an elaborate system for inducing cold-tolerance.
Very few plant species would be able to survive Elsa’s magical wrath, which turned a normal summer day into deep winter. For the most optimal cold-survival, plants need exposure to lengths of cold-but-not-freezing temperatures to trigger internal pathways to get ready for freezing temperatures. This generally works well for plants on Earth* since the cooling temperatures of autumn precede the first snowfalls of winter. During this time, plants are acclimating at the molecular level for the onslaught of months of freezing temperatures. It shouldn’t be that surprising that cold-acclimation is linked to plants’ circadian clock and timekeeping mechanisms for day-length since these are also tightly connected with seasonal temperature changes.
Plant scientists have been working out the details of just what plant cells are doing during this acclimation time. One of the first things triggered by colder temperatures is the gene expression of the CBFs (C-repeat Binding Factors). These CBF regulatory proteins then activate other genes responsible for the grunt work of protecting cells from freezing temperatures. This layered response gives the plants more ways to regulate the biochemical changes as well as simplifies the initial activation of the pathway. The exact functions of the structural proteins which confer freezing-tolerance are still under investigation, but they have roles like stabilizing membranes under freezing temperatures or producing cryoprotectant molecules like sugars. Plants also change expression of genes controlling development so that growth is controlled during dangerously cold seasons. Teasing out the underlying mechanisms plants use to protect themselves from freezing temperatures is an active area of research in plant science.
Researchers are also interested in how cold-acclimation systems differ between plant species in order to gain insights as to why plant species have such differences in cold-tolerance (say, citrus vs. pine). Is it because the less cold-tolerant species don’t have some of the structural proteins necessary to protect cells during freezing temperatures? Or is it because the cold-sensitive species lack or have less responsive regulatory proteins for acclimation? Can the genes responsible for cold-acclimation be transferred to cold-sensitive plants to confer better freezing tolerance? Plant scientists are diligently working to find answers for these questions.
Hey, I’m sure there is some agribusiness that would love to open the market for citrus crops up to farmers in Minnesota,**but I think more modest adjustments in hardiness for certain agriculturally important crops are what they are aiming for. Even so, you may be wondering at this point, “Haven’t scientists been wailing for years about how the climate is getting warmer not colder? Why bother with cold-tolerance now? Can’t we just wait it out?” I’d say, “Sure, when there is beachfront property in Arkansas, go ahead and grow grapefruit there.” However, the story is a little more complicated than that, and it’s also important to mention that freezing-tolerance isn’t all about the thermometer. A big part of dealing with freezing temperatures means coping with less available liquid water. In studying freezing-tolerance, scientists have also uncovered connections with drought tolerance. That’s right, the same systems that kick in to handle frost intersect and merge with those conferring drought-tolerance. Thus, the more we know about freezing tolerance, the more we learn about drought tolerance. I think we can all agree that finding more ways to increase drought tolerance in plants is useful in places other than fairytales.
* I can only hypothesize that the flora of fairytale worlds has a much quicker cold-tolerance induction response or that there is other magic to mitigate sudden frost damage.
**Hey, maybe there’s an idea for finally getting rid of that awful citrus greening that’s decimating Florida’s citrus groves. I have a feeling the insect vector that carries the disease will have a difficult time surviving winters in the northern Midwest.
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