A primary difference between plants and animals is that plants are in most cases static, unable to move and literally rooted to the spot. Plants that move do so passively, carried along by wind and water currents, unable to select their destination. A consequence of this lifestyle is that plants are unable to avoid changes in their environment. Plants cannot burrow into the ground to avoid high temperatures, move to water in times of drought, etc.
What are the types of environmental change that plants might face?
Here I am considering changes only in the physical environment, frequently referred to as abiotic factors. This is to separate them from biotic stress caused by the likes of herbivores and pathogens, which we discussed previously.
What can plants do when they encounter or are exposed to environmental conditions that are less than optimal?
Die
Escape - not physical escape by getting away from the hostile environment, but rather a strategy to avoid stress by rapidly completing the life cycle of the plant. As the environment changes, the plant responds by initiating flowering and seed production. By the time the stress is severe, the plant has reproduced and then dies or senesces.
Adapt - make changes that allow the plant to survive and continue to grow even under sub-optimal conditions
Adaptive responses of plants to environmental change is a complex field. Plants vary in the ways they adapt to the same environmental change. A common response is the activation of some protective mechanism(s). These protective mechanisms can take a variety of forms. Two examples of these responses are:
Manipulating and altering these responses has been the focus of several research and development programs, with the goal of improving the stress tolerance of plants. I will discuss a number of these to illustrate what is in being done in this area, focusing on the following examples:
There are, to my knowledge, no commercial products on the market that result from these manipulations. I believe it will be a number of years before these type of products will be available. In part this reflects the complexity of plant stress responses, the relative lack of knowledge of how these responses are controlled, and a focus on traits that are simpler to manipulate with more predictable results, such as herbicide tolerance and insect resistance.
First I will discuss how plants deal with stress imposed by a lack of water. Water deficit is frequently the result of drought conditions. In the US droughts occur every few years with dramatic effects on crop yields. The prevalence of drought in some areas determines what crops can be grown. For example, sorghum is the crop of choice in areas where it is too dry to grow maize reliably, such as in parts of Kansas and Texas. Another condition that also makes it difficult for plants to acquire water is high concentrations of salt in the soil, referred to as salinity stress. As an aside, this is a major problem with irrigation. In California, large acreages of land that have been irrigated in the past have now accumulated so much salt in the soil that crops can no longer be grown. Similar problems resulted in the decline of early civilizations in the Middle East.
While drought and salinity are quite different environmental conditions, they are frequently discussed together because they cause similar physiological problems for plants. In both cases, these stresses make it more difficult to keep water inside plant cells. Water will move out of the cytoplasm of the cell when the salt concentration is higher outside the cell than inside. How do plants respond to water deficit and/or salinity stress? One response is that cells synthesize compounds referred to as solutes which will allow water to be retained inside the cell. Not just any compound will function as a solute. These compounds must not be toxic and should allow normal metabolic processes to continue inside the cell. Examples of what are called compatible solutes include mannitol (a sugar alcohol), glycinebetaine (abbreviated as GB) and the amino acid proline. Yeast accumulates glycerol when it experiences similar environmental conditions. Plants differ in the solutes they produce in response to water deficit. Some accumulate mannitol, others accumulate glycinebetaine.
A strategy that has shown some promise is to provide transgenic plants with the capacity to produce an osmotic solute that they don't normally make, or to increase their capacity to accumulate a solute that they normally produce. As you might imagine, bacteria have been a source of genes for these experiments.
A bacterial gene called mtlD (from E. coli I believe) catalyzes the conversion of fructose-6-phosphate to mannitol-6-phosphate. Plants contain the substrate for this reaction, and they also contain an enzyme (a phosphatase) that converts mannitol-6-phosphate to mannitol. So, simply providing the mtlD gene should allow plants to make mannitol.
The first plants that were produced with this modification were initially thought to provide some protection against salt stress, but this was later shown to result from differences in growth rate rather than a real increase in tolerance. A modification of this approach was to target the protein encoded by this bacterial mtlD gene to the chloroplast, an organelle that is particularly sensitive to salt damage. Transgenic plants with this modification do have some increased salt tolerance.
I understand that Dekalb, the seed company that has been acquired by Monsanto in 1998, has been conducting field trials of transgenic corn engineered to produce mannitol as a protection against drought stress and the accompanying water deficit. Time will tell if this is able to provide an advantage under stress.
Glycinebetaine (GB) is a compatible osmotic solute produced by many organisms. In the plant kingdom, some species are GB accumulators while others use different solutes. The synthesis of GB in plants occurs in two steps catalyzed by different enzymes, as follows
Genes encoding these two enzymes have been cloned in the last few years. Efforts to use these genes to increase GB accumulation in transgenic plant have not been very successful to date. However, an alternative approach has been used with some success by a group in Japan. They identified a gene in a soil bacterium that encodes an enzyme called choline oxidase. This single enzyme is able to convert choline all the way to GB. This gene was then used to construct a chimeric gene that would express choline oxidase in plants, and specifically in chloroplasts. The chimeric gene had the following components:
This chimeric gene was then transferred into Arabidopsis, the little weed that is in itself of no agricultural importance but is a great tool for doing experiments and evaluating the function of genes. Transgenic Arabidopsis plants were shown to accumulate GB, as expected if the transgene was functioning properly. When the transgenic plants were tested under various stressful environmental conditions, the plants that accumulated GB were shown to be significantly more tolerant than normal plants. For example, transgenic plants were able to grow vigorously in medium containing sodium chloride, at levels that almost totally inhibited the growth of normal plants. In addition to tolerance to salt, these plants were also shown to be more tolerant to low temperatures. I remind you here that while these are significant results, it does not mean that we are about to see a large number of transgenic crops with improved drought tolerance. It takes a lot to go from an interesting observation in the laboratory to a successful product in the field.
It is plain for all to see that some plants are able to survive low temperatures whereas others are not. While it is not know precisely what is required for cold tolerance, it is known that plants respond to low temperatures in a number of ways. These responses include synthesizing a large number of proteins that are not expressed under normal conditions. The functions of these cold-induced proteins are also not well understood. However, I think it is fair to say that organisms typically do not waste their resources making proteins that do not serve an important and useful function. In this case, you might guess that the function of cold-induced proteins would be to help the plant survive low temperature conditions.
After genes for these cold-induced proteins were first cloned, experiments were carried out to see if constitutive expression of individual cold-induced proteins would increase the cold tolerance of plants, but the results were not promising. However, as with the case of pathogenesis-related proteins, it may be that the whole suite of cold-induced proteins is required to protect plants from cold temperatures rather than just one or two. How can this set of proteins be activated in transgenic plants?
The approach to this question has been to identify what activates all the genes that are expressed when it gets cold. The activator is a protein that switches on transcription of cold-induced genes, referred to as a transcription activator. The diagram below illustrates how this activator functions.
With this knowledge in hand, it has been possible to engineer plants that constitutively express this suite of cold-induced proteins. How has this been done?
The gene encoding the activator protein has been cloned. Normally this gene is itself activated by low temperatures. The promoter of this gene has been altered so that it is no longer induced by low temperatures but is constitutively (all the time) expressed. Transgenic plants with this modified activator now express constitutively the suite of genes that are normally induced by low temperatures.
When these transgenic plants were moved straight from normal conditions to -5û C they were able to survive, unlike normal plants. These results demonstrate that this approach of modifying the regulator or controller of a set of genes provides an excellent method to manipulate gene expression and perhaps to alter and improve the properties of plants. It is similar to one of the approaches I described to manipulate disease resistance in plants.
Why might it be useful to improve the cold tolerance of crop plants? In the spring, as temperatures begin to rise, many plants lose their ability to withstand low temperatures and become susceptible to late frosts. A good example is the production of peaches in Indiana. Growers cannot guarantee a good crop because large amounts of the crop are damaged from late frosts in perhaps 3 out of every 5 years. If these and other plants could be genetically protected from this cold injury, it might be able to expand the area of production for these crops and improve the reliability of crop production.
Aluminum (Al) is the most abundant metal ion in the rhizosphere. It is not required by plants but is very toxic to most plants if it is available in the soil. In Indiana and surrounding states, Al is not a problem because the soil is not acid. But where the soil has a low pH (acid conditions) Al is soluble in the soil solution and can severely inhibit plant growth. Al is highly toxic to the growing region of root tips but has little effect on other parts of the root. There are a number of species, and varieties within a species, that are resistant to Al and can grow in these acid soils. Over the last few years it has been shown that perhaps the most important mechanism of Al tolerance is one that prevents the Al from being taken up by the root tip. If this part of the root is protected, then the rest of the root system is able to continue functioning.
Tolerant plants are able to reduce Al uptake by secreting organic acids into the soil around the root tip. These organic acids (malate and citrate) are able to chelate (bind up, sequester) the Al that is in the soil solution right around the root tip. If the Al is bound to one of these organic acids, it cannot enter the plant root and cause damage.
How can this information be used to improve the tolerance of other plants to Al? The approach that has shown some promise is to express a bacterial gene encoding an enzyme that directs synthesis of citrate in roots of transgenic plants. The citrate produced in the roots is able to get out of the root and into the rhizosphere around the root where it chelates the Al. These transgenic plants are more tolerant of Al under acid conditions.
If this approach can be developed successfully in crop plants, it will provide a method to improve Al tolerance in many plants and perhaps to improve the agricultural productivity of crops grown under acid soil conditions. This may find widespread application in the tropics where acid soils are widespread.
An outcome of many different environmental stresses is the production of compounds generally referred to as reactive oxygen species (ROS). All organism are equipped with methods (enzyme pathways and metabolites) to deal with these compounds and the damage that ROS can cause inside cells. Among the enzymes that are involved in this protective mechanism are a family of proteins called glutathione S-transferases (GSTs). These enzymes play an important role in removing toxic compounds that accumulate in plant cells.
Transgenic plants have been engineered to overexpress GSTs. These plants have been shown to not only have increased GST activity but also to be able to grow better than normal plants, especially under low temperature conditions.
Overall, these examples highlight the potential to increase the tolerance of plants to environmental stresses. The two approaches used to date (increasing synthesis of protective metabolites and increased expression of protective proteins) have shown some promise. Time will tell if these or other more sophisticated approaches will be necessary to make changes in the stress tolerance of crop plants that are useful in agricultural practice.