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Archive for the ‘Plant Stress’ Category

Stress Causes Genes To Jump

When plants experience environmental stress, such as very hot temperatures, interesting things may happen inside plant cells at the genetic level.

For instance, heat stress (typically, leaf temperatures above 95o F for several hours) may increase the activity of “jumping genes” within the plant genomes.

The scientific name for such mobile genetic elements is transposon, from the fact that these pieces of DNA can “transpose” or “jump” from one place within the plant cell’s genome to another location.

Since transposons insert themselves randomly within the genome, they may land inside a functional gene. This is somewhat like throwing a genetic “monkey wrench” into the functional gene, effectively rendering it non-functional, that is, causing a gene mutation.

(Interestingly, if the transposon “jumps” back out of the effected gene, then its normal function may be restored.)

The American geneticist Dr. Barbara McClintock first discovered and described the nature of transposons. For this she was awarded a Nobel Prize in 1983. (A brief summary of her discovery of transposons can be found here.)

3288895386 836fff930fErasing “Bad” Memories?

Some have suggested that the fact that some transposons are activated by stress contributes to evolution (adaptation to stressful environments, for example) by helping to “stir the genetic pot”, so to speak (see Ref. 1 below, e.g.)

Another way of thinking about this that, if these genetic changes are passed onto the plant’s offspring, then this serves as sort of a trans-generational “memory” of environmental stress.

A recent paper (Ref. 2 below), however, provides evidence that plants may actually have mechanisms that suppress these “memories” by effectively “erasing” the new, stress-induced transposons (called retrotranspons) from the genome prior to sexual reproduction (i.e., flowering).

The “erasers” in this case turn out to be small pieces of RNA called small interfering RNAs (siRNAs). (Such siRNAs may provide epigenetic means to regulate gene expression via RNA interference.)

The gist of the paper is perhaps best expressed via the Editor’s Summary:
The transcription of repetitive elements such as retrotransposons — mobile genetic elements constituting more than 40% and 60% of the human and maize (corn) genomes, respectively — is normally repressed, to prevent their unchecked dissemination throughout the genome. Ito et al. show that heat stress in Arabidopsis plants induces transcription of the ONSEN retroelement. Accumulation of ONSEN is suppressed by small interfering RNAs (siRNAs). In the absence of siRNAs, new ONSEN insertions appear in the progeny, having transposed during differentiation. These results imply a memory of stress that is counteracted by siRNAs, providing a way of preventing transgenerational retrotransposition in plants facing environmental stress.”

Bottom Line: Plants may possess genetic mechanisms to accelerate evolution in response to changing environments, but they may also have “brakes” on such systems as well.

References:

1. Pierre Capy, Giuliano Gasperi, Christian Biémont and Claude Bazin (2000) “Stress and transposable elements: co-evolution or useful parasites?”Heredity 85:101–106

2. Hidetaka Ito, Hervé Gaubert, Etienne Bucher, Marie Mirouze, Isabelle Vaillant and Jerzy Paszkowski (07 April 2011) “An siRNA pathway prevents transgenerational retrotransposition in plants subjected to stress.” Nature 472:115–119.

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Can Chemically Inducing Dormancy Help Plants Cope With Stress?

sleepyhead.jpgThis was a very bad year for wheat in Russia because of an extraordinarily hot and dry summer. And many climatologists believe this may be only a glimpse of the not-too-distant future in many parts of the world. If so, then it is of critical importance to find ways to make crop plants more drought and heat tolerant.

One way to do this, of course, is to develop new varieties of crops that can better withstand hot, dry summers. Unfortunately, using conventional plant breeding, this typically takes a very long time, sometimes decades.

But what if it was possible to spray plants with a chemical that induces them to become more tolerant of heat and drought?

Some scientists believe that research reported in the last couple of years have brought us steps closer to such a scenario.

Abscisic Acid = The Plant’s “Sleeping Pill”?

green_pills.jpgAs mentioned in a previous post, the plant hormone Abscisic Acid (ABA) triggers the closing of stomata in plant leaves in response to water stress. It also is well known in promoting dormancy in seeds of many plant species. (ABA may also play a role in maintaining a state of dormancy in other parts of plants, but scientific evidence for this is equivocal.)

In a way, plant dormancy is somewhat analogous to sleep in that it may induce a sort of near inactive metabolic state in plants.

In such a state, plant cells and tissues are usually better able to withstand extremes of cold, heat, and drought. Briefly, this is due to the expression of protective proteins, such as dehydrins, and to changes in the cellular structures, such as the composition of cell membranes.

What ABA does is trigger the cellular processes involved in such changes. The question that has confounded scientists for many years is exactly how ABA does this.

Targets and messengers.

It turns out that 2009 and 2010 have been very good years for research on the mode of action of the plant hormone ABA.

In 2009, several laboratories, including Sean Cutler’s lab at UC Riverside, identified ABA receptors, that is, cellular proteins that specifically bind to ABA.

And this year, much more was learned about the docking site for ABA on these proteins and about what these proteins do. (See here and here for more about this.)

Briefly, these proteins function as phosphatases, which often function to regulate the activity of other enzymes. In this way, they act as an army of messengers, greatly amplifying and elaborating the ABA signal.

Artificially Inducing Dormancy?

Information about the ABA docking site may allow scientists to develop chemicals, such as pyrabactin, that can mimic the action of ABA, but that are way more stable and cheaper than the natural plant hormone ABA itself.
Some scientists envision the use of such chemicals as a way to artificially induce a transient dormancy-like state in growing crop plants, such as wheat for example, in order for them to survive episodes of extreme heat and drought.

For example, Dr. Mike Sussman, from a recent article regarding his work on ABA puts it this way: “”Since they cannot walk or run, plants have developed an interesting and complicated system for sensing and responding very quickly to dehydration and other stresses,” says Sussman, noting that, on average, a plant is composed of 95 percent water. “Most plants have what’s called a permanent wilting point, where if water content goes below 90 percent or so, they don’t just dehydrate and go dormant, they dehydrate and die.”

Figuring out how to trigger a dormant state, such as exists naturally in seeds, which are 10 percent water and can in some cases remain viable for hundreds of years, could be key to creating plants that survive drought in the field, Sussman explains.

It’s possible that someday farmers will spray their wheat fields with such chemicals, which will induce the plants to “sleep through” an unusually hot, dry period.

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Selaginella_lepidophylla.jpg“Near-Death Experience”

One of the main problems for plants when they colonized terrestrial environments on Earth nearly a half billion years ago was how to survive the dryness.

Today, much of a typical vascular plant’s anatomy, morphology and physiology is dedicated to obtaining and retaining water.

Although most vascular plants can tolerate brief periods (hours to days) of water stress, most plants are killed by long periods (weeks to months) of drought.

Some plants, however, display the remarkable ability to survive near total desiccation (less than 5% relative water content), which causes them to appear dead. But when rehydrated, these plants can be revived. Hence, they are often referred to as “resurrection plants”.

Probably the most well-known “resurrection plant” is the species Selaginella lepidophylla (see photo above). This resurrection plant (a.k.a., Rose of Jericho and Siempre Viva) belong to a group of plants called lycopods.

Myrothamnus_flabellifolius_5_Myrothamnaceae.jpgOther plants sometimes called the “resurrection bushes”, are members of the the genus Myrothamnus, and, in paticular, the resurrection bush Myrothamnus flabellifolia has recently been the object of study by scientists interested in why these plants are so desiccation tolerant.

Bringing Dehydrated Plants “Back To Life”

The nature the desiccation tolerance of resurrection plants has interested plant scientist for many years.

If the cellular mechanisms for such remarkable drought tolerance was understood, and the genes involved identified, then it may be possible to use this information to improve drought tolerance in crop plants. (As previously mentioned here, this may become of increasing concern due to “global weirding”.)

But let’s not get side-tracked with issues regarding global climate change….

…how do resurrection plants work?

When “dead”, these plants exist in a quiescent, desiccated state. That is, their metabolism is at or near zero, along with a significant reduction in cell and tissue volume. (see refs 1 & 2 below, for example).

What has happened at the cellular level to allow these plants to survive such an extreme state, often for a long time?

Desiccation tolerance in resurrection plants involves a combination of molecular genetic mechanisms, metabolic and antioxidant systems as well as macromolecular and structural stabilizing processes.” (from Ref 3 below)

Briefly, the onset of water loss apparently sets into motion a series of cellular events that can be summarized as follows:

Dehydration –> Activation of “desiccation-related” genes –> (1) Alterations in metabolism and (2) Production of “protective” proteins

(1) Alterations in metabolism: (a) accumulation of protective solutes such as sucrose, trehalose, and proline that stabilize proteins and cellular membranes, (b) production of antioxidant compounds (such as galloylquinic acids), and (c) biochemical alterations in membrane and cell wall composition.

(2) Production of “protective” proteins such as “dehydrins” and “expansins” that help preserve the structural integrity of intracellular organelles and the cell walls.

In some ways, responses similar to how plants tolerate sub-freezing cold temperatures (see previous post)

Are All Seed Plants “Resurrection Plants”?

In a way, seeds are vessels for “resurrection plants”.

As we’ve seen above, a “resurrection plant” is a plant that can withstand extreme desiccation without dying and that can be revived with the addition of water.

In each viable, dry seed is an embryonic plant in an extremely desiccated state, but still alive (at least as long as the seed’s food reserves last). This desiccated state allows the seed embryo to withstand periods of cold, drought, heat, etc. Unless the seed is dormant, adding water revives the embryo, and the seed germinates.
Many, if not most, of the cellular mechanisms for desiccation tolerance mentioned above for resurrection plants have also been shown to be relevant to seeds.

So, an interesting question comes to mind: Did the cellular mechanisms for desiccation tolerance that evolved in primitive, spore-producing plants such as the lycopods (think Selaginella) help “pave the way” for the appearance of seed-producing plants?

Bottom line: Some unusual so-called “resurrection plants” have the ability to withstand extreme desiccation using cellular mechanisms not unlike those found in dry seeds.

References
1. Moore, J.P., et al. (2006) “Response of the Leaf Cell Wall to Desiccation in the Resurrection Plant Myrothamnus flabellifolius.” Plant Physiology Vol. 141, pp. 651–662. (Abstract)

2. Layton, B.E., et al. (2010) “Dehydration-induced expression of a 31-kDa dehydrin in Polypodium polypodioides (Polypodiaceae) may enable large, reversible deformation of cell walls.” American Journal of Botany Vol. 97, pp. 535-544. (Abstract)

3. Moore, J.P., et al. (2009) “Towards a systems-based understanding of plant desiccation tolerance.” Trends in Plant Science Vol. 14, pp. 110-117. (Abstract)

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witchazel.jpgThe End of Winter?

We experienced a relatively warm January here in western Washington (record-setting, in fact). Some daffodils are already coming up and the witch hazels are blooming.

But most of the trees haven’t started to leaf out because they haven’t yet emerged from winter dormancy.

Most woody perennial plants (mainly trees and shrubs) that must survive winter in the temperate latitudes do so by becoming dormant in the fall.

The deepest state of this dormancy called “endodormancy” – often referred to in horticulture as “rest” – allows them to survive cold temperatures, sometimes well below -30o F. (This adds new meaning to the phrase “chilling out”.)

Cold Acclimation (a.k.a., Cold Hardening)

Some plants, but not all, can become more cold tolerant or cold hardy simply by being exposed to near freezing temperatures. This is sometimes called cold hardening or cold acclimation. Interestingly, plants that are more cold hardy are generally also more tolerant to cellular desiccation. Indeed, improved cold hardiness may also be induced by mild drought. So, it appears that some of the cellular things the plant does to become cold hardy are the same as it does to become more drought tolerant. Icy_Tree.jpg

What happens at the cellular level during cold acclimation to make the plants more cold tolerant was described in a previous post. Briefly, the plants may synthesize new proteins that inhibit ice crystal formation or stabilize cellular structures against the cold.

For some plants, it may take only a few days to become cold-hardened. For other plants, especially those entering endodormancy, it may take weeks to become fully cold-hardened.

Of course, the degree of cold hardiness varies greatly among plant species. (USDA Plant Hardiness Zone Map) “So-called chilling-sensitive plants, such as the tropical banana and the semitropical avocado, can be severely injured or even killed by long-term exposure to temperatures (50 degrees Fahrenheit, for example) that are well above freezing. By contrast, chilling-resistant plants, such as garden peas and potatoes, survive brief periods of frost but are killed when freezing conditions continue for more than about four hours. Cold-hardy plants, on the other hand, tolerate extended periods of freezing, and laboratory tests indicate that cold hardness in some of these plants permits them to survive at temperatures as low as minus 75 degrees Fahrenheit.” (from Ref. 1 below)

pussy_willow.jpgThe Big Chill

Plants that exhibit the greatest cold hardiness are those that become dormant. When these plants make the transition to dormancy, one of the first stages in this process is similar to cold acclimation. This actually may be triggered in many plants by longer nights and reinforced by cooler temperatures.

But these plants go beyond chilling tolerance into a so-called “resting state” (endo-dormancy or mid-winter hardiness) that renders them extremely cold hardy. This resting state is characterized by a temporary suspension of growth and a lower rate of metabolism.

Dormancy is broken after exposure to long periods (weeks to months) of cool temperatures (typically below 40o F) and shorter nights (among other factors), though cold deacclimation and reacclimation may take place. (see Ref. 3 below)

How global warming may affect the complex process of tree winter dormancy has certainly been on the minds of some plant scientists (e.g., see Ref. 4 below).

The plant hormone ABA is generally thought to be a chemical signal that induces cold acclimation and dormancy in many plants. (Lots of new information about how ABA works has been published in the past few months, so maybe that would be a good subject for next time.)

References

1. John W. Einset “What Determines a Plant’s Cold Hardiness?” Arnold Arboretum, Harvard University (PDF)

2. Mark Longstroth Dormancy and Cold Hardiness in Fruit Crops

3. Kalberer, S.R., M. Wisniewski and R. Arora. (2006) “Deacclimation and reacclimation of cold-hardy plants: Current understanding and emerging concepts.” Plant Science vol. 171, pp.3-16. (PDF)

4. Legave, J.M., I. Farrera, T. Almeras and M. Calleja. (2008) “Selecting models of apple flowering time and understanding how global warming has had an impact on this trait.” Journal of Horticultural Science & Biotechnology vol. 83 pp. 76–84. (PDF)

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frosty_leaves.jpgPlants Can’t Come In From The Cold

Imagine for a moment that you had to stand outside your house or apartment, without moving, all winter long…and that you were naked.

How long do you think you could last?

Not long, especially if the temperature went below freezing. And certainly not if the temperature went below 0o F (-18o C).

In temperate zones on Earth that’s what perennial plants must be able to do to survive. And even annual plants may have to withstand an early or late frost in order to complete their life cycles.

But, you may reply, plants are not warm-blooded organisms like mammals. What difference is it to them whether it’s cold or not?

How Cold Kills Plants

What happens if water freezes inside at plant?

At least a couple of things can happen…both bad for the plant.

The first, and likely lethal for the plant, is ice crystal formation inside cells. This has just about the same effect on a plant cell as the little alien chestburster did on the actor John Hurt in the movie Alien. (Sorry, but it’s an effective analogy.) It’s lethal.

If, however, the water freezes outside the cells, in the intercellular spaces, this may lead to the extreme desiccation of the plant. That is, it’s sort of the same as if the plant was drying out.

Other cold-temperature effects on plants include (1) decrease in enzyme activity and (2) changes in the fluidity of cellular membranes, both of which could severely harm plant cells, and, thus, the plant as a whole.

How Do Plants Cope With the Cold?frosty_tree.jpg

To answer this question we have to consider plants at the cellular level.

How do the cells of cold-tolerant plants survive sub-freezing temperatures, i.e., withstand dehydration and, more importantly at very low temperatures (below 0o F or -18o C) , avoid the formation of ice crystals in the cell?

1. Accumulation of solutes (sucrose, mainly, but also other organic compounds such as proline) by the cells to depress the freezing point of water (think salting ice on the sidewalk) and to stabilize membranes. (But this can only be effective at temperatures from 32o F to 20o F.)

2. So-called “antifreeze” proteins help prevent ice crystals from forming in the extracellular spaces (outside cell); plant cells that make these proteins typically secrete them into cell wall region (intercellular spaces).

3. The plant cells may synthesize proteins called “dehydrins”, which are inside the cell (cytoplasm), may bind water molecules and alter the collective structure of water in the cell to stabilize membranes.

4. Plant cells can alter lipid composition of cellular membranes in order to adjust the fluidity (functionality) to colder temperatures.

red_leaf.jpgCan Plants Generate Their Own Heat?

A silly question? I think not.

Some unusual plants, by partially uncoupling their cellular mitochondria, can generate small amounts of heat. (Please see ref #1 below for more information) But this is likely not very significant with regard to cold tolerance, however. (More on this interesting topic here.)

Bottom Line: Plant cells survive sub-freezing temperatures by adjusting their solutes, proteins, and membrane lipids in order to withstand desiccation and to avoid ice crystal formation.

Reference

1. Seymour, Roger S. (1997) “Plants That Warm Themselves.” Scientific American, March 1997, pp. 104-109. (Summary)

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192742837_e99d0c1a40.jpgAs previously mentioned….if most climatologists are correct, then parts of the Earth’s surface may experience increasing episodes of heat and drought as a result of global “weirding”. (see here and here for recent examples)

Some of the effects of heat on photosynthesis was considered in part 1 of this post.

But what about drought (a.k.a., long term water stress)?

How does the lack of water affect plant photosynthesis?

When plants lose more water than they can take up from the soil, they become water stressed.

Short-term or diurnal water stress can often be manifested in plants on hot, dry afternoons by drooping or flaccid leaves.

Long-term water stress may occur over days or weeks or longer. Such drought conditions certainly inhibit the growth of plants. But such conditions may even inhibit the most critical process in plants, namely, photosynthesis.

The Stomata Are The Keystoma1.jpg

What also may be happening to water-stressed leaves can not be observed without a microscope. That is, the small pores on the leaves called stomata that allow for leaf/air gas exchange may be closed.

This stomatal closure in response to water stress is often triggered by the plant hormone abscisic acid (ABA). In many plants ABA is produced in the leaves in response to water stress.

When plants close their stomata to conserve water, then they effectively cut off the main supply of CO2 for photosynthesis.

Interestingly, as atmospheric CO2 increases from the continued burning of fossil fuels, this may partially compensate for the inhibitory effects of water deficits on plant photosynthesis. (see here for more on this)

Drought + Sunlight May Also Damage A Key Photosynthetic Enzyme

Research on the effects of water stress on photosynthesis has revealed that decreased CO2 availability in bright light leads to formation of reactive oxygen species. These damage the chloroplast ATP synthase, decreasing ATP content and disrupting the photosynthetic Calvin cycle.

Under these circumstances, photosynthesis becomes insensitive to elevated CO2.

Bottom line: Water deficits inhibit photosynthesis by causing stomatal closure and metabolic damage.

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20070621_drought.jpgIf most climatologists are correct, then parts of the Earth’s surface may experience increasing episodes of heat and drought as a result of global “weirding”. (see here for a current example)

In a previous post in this blog, I briefly introduced the complex subject of how increasing atmospheric CO2 may affect plant photosynthesis.

There is some evidence that suggests that, on a global scale, plant photosynthesis may increase due to the elevated levels of the carbon source for this biochemical process, namely, CO2.

But will any increases in global photosynthesis provided by higher levels of CO2 be lost due to heat and drought resulting from global weirding?

Some recent research suggests that the answer probably is yes. These investigators showed that a 4 degree C (about 8.5o F) increase in temperature above background led to decreased carbon absorption by a simulated grassland.

chloroplastsfigure1.jpgHow Heat Affects Photosynthesis

Among the many biochemical processes in plants, photosynthesis is one of the most sensitive to inhibition by elevated temperatures.

Is it the proteins (enzymes) that catalyze the chemical reactions that comprise photosynthesis that are so heat sensitive or the chloroplasts themselves? Since the chloroplasts consist of intricate lipid bilayer structures, is it likely that even moderately high temperatures “melt”, and thus disrupt, the whole process? It appears that the chief suspect is an enzyme.

The world’s most abundant and most important enzyme is RuBisCo, since it catalyzes the first step in carbon fixation (a.k.a., the Calvin cycle), that is, the conversion of CO2 into sugars in the stroma of chloroplasts. (see Figure 1 above)

2322699374_ee8f2b7711.jpgBut it’s not RuBisCo that is the heat-sensitive culprit, but apparently an associate enzyme called RuBisCo activase. Rubisco activase’s chief role is to serve as an activator and regulator of RuBisCo. Specifically, RuBisCo activase helps convert RuBisCo from its inactive to active state.

Much scientific evidence (see here and here, for example) supports the hypothesis that RuBisCo activase may be the key to the heat sensitivity of plant photosynthesis. Despite this, there is still controversy over the limiting processes controlling photosynthesis at elevated temperature.

Bottom line: Photosynthesis in land plants may both benefit (higher CO2) and suffer (higher temps) as a result of global weirding.

Next time: Part 2, how drought (long-term water stress) effects photosynthesis.

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