Temperature is something we worry about in our lives all the time. What temperature is it outside so you know what to wear and what are the conditions going to be in coming days or weeks. Farmers have to worry about this as well, as excessive periods of heat or cold snaps can prove damaging to the crops they are trying to grow and either event can lead to other stresses like droughts or flooding, respectively.
But farmers and scientists have a way to fight back against these growth stresses, by imbuing crops with resistance genes to protect them.
What Is Thermotolerance And How To Deal With It?
From a science-based perspective, plant resistance to temperature scenarios are grouped into three different components, all jointly under the label of thermotolerance. This relates to genetic components that confer resistance to temperature changes. These groupings are genes related to high temperatures, genes related to cold temperatures, and genes related to freezing temperatures. The latter two are kept separate because, from a genetic perspective, plants deal with them differently.
In this following article though, much like the studies that will be referenced, the latter two will be discussed together at the same time, but do keep a note that there are often different tactics needed for each.
Effects and Responses
When dealing with these temperature stresses, a lack of resistance in a particular plant can lead to symptoms such as low rates of germination or seed growth, a slowing of general plant development, reduced capabilities of photosynthesis for converting light into energy, and, of course, death.
Some of the responses to these conditions on the part of plants include manipulation of how the plant cell membranes are composed to better protect them, up-regulation of genes for transcription factors involved in stress responses, production of certain macromolecules for cell signaling and starting physical responses, and, if necessary, removing oxidative toxins that have built up due to temperature effects.
The Less Understood Components
Another factor that is still being worked on in order to uncover is how epigenetics plays a role in responses to temperature-related stress. We know that various types of RNA are involved and that the process of transcription includes memory toward past gene activation that helps prepare for faster reactions when encountering such conditions again. Farmers and scientists can visually see the effects from this, but the entirety of how it is happening is still not well understood.
This part of the genome is under continued research, however, with transcriptional analysis and overall mapping of the transcriptome, the entire bulk of RNA messaging, being conducted. As epigenetics, transcriptomics, and proteomics are fields still in their infancy, we will have to wait and see what new discoveries they will find in the future. Thus, this article will mostly be avoiding including these components when discussing temperature effects. Though genes related to transcription factors themselves will come up frequently as points of interest.
The Efforts of Farmers
Not all of the activity around protecting plants comes from the science end of things. Farmers do their part as well in order to help keep their crops healthy. One of the most common methods is to purposefully time the most stress-sensitive growing period of any particular crop to coincide with the best weather conditions of the year, in order to ensure the least amount of possible stress for the plants.
This can include altering the amount of fertilizer and irrigation provided in order to minimize the stress on the plants, such as providing more water during a hot period or allowing the plants more nutrients from fertilizer if a recent flooding stripped some of those nutrients from the soil due to runoff.
There is one other more recent method known as pre-conditioning. In order to help ensure that the crops survive an upcoming hot or cold spell, farmers will apply chemical agents that induce and create the plant temperature stress response beforehand. This means the plants will begin producing the metabolites, transcription factors, and all the rest already mentioned before the temperature shift occurs, giving them immediate protection against it. UV light can also be used to induce such a phenomenon.
Determining Temperature Stress Genes
A major difficulty in researching thermotolerance is finding and isolating the genes responsible. Not all plants have the genes, for one, or at least not ones that are worth anything in severe conditions. Thus, screening for the cultivars that do have strong stress responses and then figuring out which genes cause this response is one of the main segments of research in the field.
This process is not helped by the fact that even within a single type of temperature response, such as to heat, there are a number of different mechanisms that plants can end up employing, each separate from the rest. Thus, to isolate them, general characteristics and guidelines had to be set out for the determination of proper response genes.
A simple method is to measure the growth rate of seedlings under different temperature conditions. A Heat Tolerance Index (HTI) was even set up for testing high temperatures and seeing how quickly the plants bounce back from being exposed to high heats.
Another option is to determine the so-called thermostability of the plant’s cell membrane by measuring the amount of electrolytes lost during temperature exposure. Obviously, rate of photosynthesis is an additional method, as is stability of chlorophyll content within individual cells.
The Importance of Reproduction
The final area to mention before delving into the topic of temperature itself and some of the specific plants being researched is to note the effects that temperature has on the reproductive system of plants, as this often interferes with other measurements. Or needs to be taken into account when performing experiments.
When referring to the most temperature-sensitive time for plants, it is often during their reproductive flowering period. Any sudden temperature change during this period can result in the plant being unable to properly complete its reproductive cycle. The genes that help control and increase resilience during these periods are some of the most useful and continued transcriptome and genomic scans have been taken to find these specific genes.
Types of Crop Thermotolerance
The three types of temperature tolerance (two as to be grouped together and for the purposes discussed in this article) have different negative results on plants, their growth, their reproduction, and other facets of their development. Those should be mentioned very briefly.
The effects of heat shock and the development of heat tolerance has been a long-studied field, originating in the 1960’s, when proper molecular investigations began. For research into temperature, the model organism Arabidopsis has continued to prove invaluable. They have shown that heat shock proteins appear to be highly conserved, gene wise, even within completely different plant species. Abbreviated as Hsp’s, they are capable of insulating cells against high heat and even protect against other stresses as well.
It has been observed that plants that have been exposed to lethal temperatures, but managed to survive it, are able to respond quicker and more favorably to high temperatures in the future. This has been termed acquired thermotolerance and is separate from the passive “basal” thermotolerance.
Cold temperatures have a similar phenomenon. Crops normally grown in hilly areas are more likely to have to deal with the effects of a sudden cold snap. The plants exposed to such and survive are often capable of developing cold acclimation, as it is termed. Those plants used to such areas have genes directly related to this process, while crops normally suited for tropical areas do not. Tobacco plants are often used as a model organism for cold conditions in addition to Arabidopsis, especially to how well they take to transgenically testing genes from other plants.
Acquired tolerance and acclimation is where the similarities end though. Heat stress usually results in the production of oxidative agents that then cause damage to crop development and reduce the size of yields by affecting seed growth. In severe conditions, reproduction can be halted altogether due to this effect on the seeds.
Cold, meanwhile, directly reduces crop growth itself from the germination all the way to the reproductive stage. It can cause forced sterility by not allowing enough energy for the plant to enter the reproductive cycle itself, whether through interfering with nutrient uptake or photosynthesis. This is how it affects yields, though as you can see in a manner much different than heat.
That seems to be enough of a talk on how plants respond to temperature in general. The basics, at least. Now, let’s go over what scientists around the world have been actively researching and what fruits that research has born thus far for specific plants.
With those basics down, here is our first specific crop topic of discussion.
Rice Temperature Tolerance
The crops most often affected by temperature are those that are usually grown in more temperate climates, not used to extreme heat or cold conditions. Rice is included in this category, preferring temperatures around 30 degrees Celsius (mid 80’s in Fahrenheit). Though its cultivation throughout history has resulted in modern cultivars preferring tropical environments to grow in, further reducing its cold capabilities.
Heat and Rice
Excessively high temperatures can be devastating to the growth of rice and they can be more susceptible than other plants generally, due to heat transmitting through the water they are grown in. This is also true for the opposite when facing freezing temperatures.
Past research has identified two genes of interest capable of helping. A particular transcription factor found in some cultivars of rice and a chaperone gene from Arabidopsis have been used and overexpressed to reduce the amount of leaf necrosis suffered from rice plants after exposure to heat shock. It is not known, however, whether this expression during non-heat scenarios may interfere with proper growth.
An alternative was found from another Arabidopsis gene, this time a transcriptional regulator that is able to positively regulate the heat stress response in rice without any reduction in growth capability, even during non-stress periods. An additional co-activator gene was able to ensure this, making it so that the genes in general are only turned on during the stress response.
Some other examples of recent findings include continued research into a transcription factor family known as NAC that is useful for general and all-purpose stress response. A particular gene from the family known as SNAC3 was found to give rice higher tolerance to temperature, drought, and other stresses. The transgenic plants also produced less cell-damaging oxidative molecules like hydrogen peroxide.
Another study was able to take a gene from an already thermo-tolerant cultivar of wheat and transgenically use it in rice. The gene codes for several heat shock protection elements and is activated not just during the necessary flowering reproductive stage, but also during the seedling stage if necessary. The researchers used Green Fluorescent Protein (GFP) genes in order to measure its times of activation to confirm when it is turned on.
A final study was able to identify and test an existing rice gene that helps with heat tolerance by altering the stomata, pores for gas exchange on the underside of leaves, to restrict the intake of hot air. At the same time, the gene is able to help deal with the hydrogen peroxide buildup that occurs in plants during heat stress. Manipulating the genome to overexpress this gene would likely help increase heat tolerance for rice in general.
Cold and Rice
Due to being a tropically grown plant, rice at least has a large number of genes to deal with high heat, even if they are not always effective enough. Cold, on the other hand, is another beast entirely. It is especially damaging during germination and the seedling stage, where it can easily kill off a rice plant altogether if the temperature drops too low at the wrong time.
To that end, knowing what rice has to work with is an important step in improving its cold resistance. In March of this year, researchers ran an entire genomic map on all the genes in rice, in order to identify the clusters that have to do with cold tolerance. They were able to identify 42 loci across several different primary cultivars of rice, with the JAPONICA cultivar providing more resistance genes than the INDICA cultivar.
Now, more specialized research into those genes and a determination on which ones can be amplified to boost rice resistance can move forward. For the moment, the research is ongoing into the rice genome. Hopefully there will be more to report on this in the future.
Outside of the rice genome, researchers also in March were able to transfer a drought and cold protein encoding gene called CcCDR from pigeonpeas into the Indica cultivar. This was able to impart several types of stress tolerance and at a high level of efficiency. So, even if work on the rice genome itself doesn’t go as well as desired, there are other transgenic options that can be used to improve rice crops.
Soybean Temperature Tolerance
As a crop, soybeans are even more encumbered by changes in temperatures than other alternatives. Particular the impact of heat and drought. Its ongoing preferred temperate growth results in it losing its nitrogen fixation capabilities under high temperatures. But, let’s get right into a discussion on heat..
Heat and Soybeans
Any temperatures above 26 degrees Celsius (78 degrees Fahrenheit) begins to negatively affect the growth of soybeans and results in seeds with significant weight loss. Any higher of a growing temperature and reproductive defects can emerge, including a complete shutdown of the reproductive flowering cycle.
When under heat stress, they begin to lose a quarter to a full half of their chlorophyll content and their photosynthesis capabilities reduce by 12%. It is due to all of these factors that heat resistance is a top priority for soybean research, especially because of the crop’s prominence in daily food intake around the world.
At the same time, soybeans have to deal with drought as another combined concern under such conditions. Their growth already requires a significant amount of water and heat stress only compounds this issue. Luckily for soybeans, they already contain several genes of importance for temperature and drought resistance combined and researchers are already investigating how to amplify these effects.
This combined gene resistance can be shown into research on DRE transcription factors, which stands for dehydration-responsive elements. These genes, and specifically the DREB1 gene in soybeans is not just a primary resistance factor for drought conditions, but also is activated when facing any number of other problems, including heat stress. The DREB1 transcription factors in turn then activate a number of other genes downstream on the genome to begin the plant response to whatever stress it is facing.
Overexpression of this transcription factor and gene is a prime target in biotechnology research currently in order to create soybeans with a faster response time and thus more stable resistance to any number of issues.
Another area of focus has been on the heat shock transcription factors found in soybeans. As you can see, the particular topics of interest begin to be fairly similar even with different crops. This is because many plants share general characteristics of genes. Unfortunately, they are still separate genes themselves, so the research often doesn’t translate directly to being useful in other plants. Though sometimes it does and, even if not, the use of transgenic crops can fix the issue regardless.
As with many other resistance genes in soybeans, even the heat shock ones respond to more than just temperature stress. When Hsf-34 was overexpressed, the researchers found that it resulted in heat and drought tolerance being increased. Multi-focal gene responses are a boon to research on soybeans and any other plants that work the same, as it allows benefits across the board with far less overall tinkering required.
Cold and Soybeans
Surprisingly (or maybe not so surprisingly for some of you), modern strains of soybeans are fairly hardy against cold temperatures, with the ability to recover rapidly even from frost damage to their main stem, so long as the plant isn’t killed outright. It is more susceptible than corn, due to having most of its seedling growth cycle occur aboveground rather than under, but all in all, it does quite well.
This is likely due to breeding in Japan over centuries to deal with the heavy snowfall in the region. Soybeans make up the foundation of cuisine in the country, forming the basis for flavors including soy sauce, miso, tofu, and all the variety of uses of soybeans in cooking recipes, in addition to fermentation.
So, while there is not as much to say on this particular topic, one recent piece of research has identified an ethylene response factor (ERF) gene named GmERF9. These factors are involved in a variety of biological processes, including pathogen response. But this particular one helps in responding to drought and cold conditions. This identification means that it can be both amplified and act as a candidate for transferring to other crops to help with these stresses.
Wheat Temperature Tolerance
As another main food crop for the world, wheat is on par with rice for its necessity in human cuisine and the diets of many cultures. It is even pickier than rice, however, when it comes to temperature requirements, preferring warm temperatures between 21 and 24 degrees Celsius (70 to 75 degrees Fahrenheit) at all times.
Heat and Wheat
Unlike rice, wheat is more impacted negatively by hot temperatures than by cold. This is understandable due to it being a Mediterranean crop in recent history, rather than a subtropical one like rice. And these impacts directly go toward reducing wheat yield and the production of oxidative stress molecules harms plant growth even further.
Heat-tolerant cultivars of wheat have been found to develop genes involved with ferritin, an iron containing protein. Higher production of ferritin appears to reduce the harm caused by oxidative agents and also make the leaves of the plant iron-rich. This, in turn, allows it to be more capable of growing in a high heat environment. Thus, overexpression of iron-producing genes seems to be an option for increasing general heat tolerance, along with transgenically giving those genes to cultivars and even other crops that lack it.
Another important factor in wheat is, like with most plants, the production of starches for use in the nutritious coating around seeds. Sugars made via photosynthesis are converted to starches for this purpose. Heat stress reduces this process’ functionality and thus causes wheat to produce smaller seed kernels and lower yield. There are, however, temperature stable version of starch production genes available.
Transgenic wheat produced with such variant genes have been found to have 34% larger kernels as compared to control groups. Further work on these genes could yield even larger results, ones that have the possibility of being transferred to other crops as well.
Cold and Wheat
As noted just a moment ago, wheat does fairly well in the cold. Several cultivars of wheat are specifically called winter wheat for the expectation of them being grown over the winter season and wait for spring to flower.
That doesn’t mean that wheat can’t be damaged from the cold, but that it usually requires other factors to intervene. Dry soil is one such factor, as dry soil is more susceptible to temperature changes and will rapidly transfer it into the plants growing within it.
Similarly, any incident of sudden warm weather could be catastrophic, as the wheat will discard their cold resistance in expectation of spring. A return to cold after that often leads to the wheat dying off wholesale by no longer having their molecular and genetic resistance capabilities turned on.
For the wheat itself, research has shown that one of the genes that contributes to their cold tolerance are those related to aquaporin proteins. These are cell membrane proteins that assist in transferring water in and out of the cell. A particular gene for this, named TaAQP7, has been found to cause protein accumulation in the leaves during a frost.
When transgenically tested in tobacco, it was found to cause more root growth and overall plant development. It is possible that this allows the growing wheat to still obtain nutrients from deeper in the ground even if the topsoil freezes during a cold snap, helping to keep it alive. Meanwhile, its accumulation in the leaves implies that it helps leaf growth to maximize photosynthetic intake.
Wrapping Up, But So More To Say
I can immediately proclaim that this article is not comprehensive in the slightest. It is a brief overview of some avenues for three major food crops that are being investigated for biotechnology uses. There are more studies than I can count that I wasn’t able to include here. But we’re already running over 3000 words as it is.
My only hope is that this article helped inform you about how temperature tolerance works in plants and how rice, soybeans, and wheat deal with those topics in general. If you, dear reader, learned something from this text, then that is at least an accomplishment.
I’m sure at some point in the future there will be more to come. Corn wasn’t even able to be discussed and that’s a significant crop as well to consider.
But, until next time.
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Photo CCs: Soybean fields at Applethorpe Farm from Wikimedia Commons
