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Archive for the ‘Plant Metabolism’ Category

SauromGutt.jpg“Hot Plants?”

In the previous post, the topic was how plants survive the cold. Although some perennial plants can withstand winter temperatures well below zero (F), plants certainly don’t generate body heat like mammals do in order to warm themselves.

Or do they?

There are a few plants in nature, like the remarkable Voodoo Lily (Sauromatum guttatum), that produce extraordinary heat when they flower. What actually warms up when the plant flowers is part of the inflorescence, called a spadix.

Typically, the plants do this to attract insect pollinators. But some, such as the Eastern skunk cabbage may actually use this mechanism against the cold.

In the case of the Voodoo Lily, flies are lured by chemical attractants, which are volatilized by the heat of the spadix. (The chemicals smell to us like putrid, rotting meat.)

The process of heat production by living organisms is called thermogenesis. And though it’s far from common in the plant kingdom, thermogenic plants occur in several plant families, especially the Araceae. Members of this plant family include the Eastern skunk cabbage and the giant carrion flower.


(from: Giant stinking flower reveals a hot secret)

Much fewer plants, however, are able to thermoregulate, that is, they actually regulate the temperature of thermogenesis within narrow limits. For an excellent slide-show about plant thermoregulation, see here (PDF).

How Do Plants Generate Heat?

meeuse.jpgMuch about what we know about how the Voodoo Lily spadix, for example, generates heat came from the research of Professor Bastiaan J. D. Meeuse.

Among his discoveries about heat production in plants, Dr. Meeuse and co-workers showed that a compound related to aspirin triggers pronounced heat production in the flowers and inflorescences of some thermogenic plants.

Briefly, heat generation in these plants is due to the massive activation of the alternative oxidase metabolic pathway in the mitochondria inside the plant cells.

Simply put, when this happens, instead of generating ATP as result of metabolizing sugars via oxidative phosphorylation, the mitochondria generate heat.

Bottom line: Though some plants can generate heat to promote flower pollination, it’s unlikely that they do so just to survive cold temperatures.

References

1. Meeuse B.J.D. (1966) The Voodoo Lily. Scientific American vol. 218, pp. 80-88.

2. Meeuse, B.J.D. (1975) Thermogenic Respiration in Aroids. Ann. Rev. Plant Physiology vol. 26, pp. 117-126. (Abstract)

HowPlantsWork © 2008-2011 All Rights Reserved.

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Roundup.jpgThe Last Roundup?

The herbicide that most Americans are likely familiar with is Roundup®.

Unlike the auxin-based herbicides I discussed in the previous post, Roundup® is not a selective herbicide. That is, it usually kills all green plants (except if the plant is Roundup Ready® or if the plant is a naturally Roundup®-resistant “superweed” – see below for more).

Roundup® is the Monsanto brand of the artificial chemical glyphosate, which was first synthesized in the 1970’s by Monsanto as a so-called “broad-spectrum” herbicide (i.e., kills all plants).

Glyphosate kills plants by specifically blocking the action of a key enzyme (5-enolpyruvylshikimate-3-phosphate synthase or EPSPS) that plants use to synthesize three amino acids (tyrosine, tryptophan and phenylalanine) that are essential components of all proteins. Without the ability to synthesize these amino acids, the plants will die.

Why doesn’t glyphosate similarly affect animals?

Humans and most other animals don’t have this enzyme, so glyphosate has no specific target as it does in plants*. (Since they do not have this enzyme, animals do not synthesize these three essential amino acids. They get them from their food.)

*Please note: This is NOT to say, however, that glyphosate is totally non-toxic to animals – more below.

Roundup may be losing its effectiveness, however, due to several factors.

Are Your Plants “Roundup Ready®”?

During the 1980’s there was a revolution going on in the plant sciences. Scientists discovered how to insert genes into the genome of some plants by using the bacterium Agrobacterium tumefaciens (At) as a genetic vector. Here’s how:

Scientists were able to use recombinant DNA technology to load genes into the Agrobacterium. And the bacteria were then able to “infect” susceptible plant tissue and deliver the genes straight into the plant cell’s genome. These foreign genes were actually “hard-wired” (stably inserted) into the plant’s genome. Moreover, these genes were able to be passed along to the plant’s offspring.

Simply put, Agrobacterium was like a taxicab, and the DNA was the passenger.

Using such technology, Monsanto scientists discovered a bacterial version of the enzyme EPSPS (see above) that was not affected by glyphosate, isolated the gene coding for it from the bacteria, and then inserted this bacterial gene into soybeans.

Roundup Ready® soybeans were born. Followed by Roundup Ready® cotton and canola. And when scientists learned how to genetically engineer grasses (Agrobacterium doesn’t work so well on grasses) using the so-called gene gun, along came Roundup Ready® corn. And maybe even Roundup Ready® turfgrass for lawns!

RR_1.jpg

Roundup® Ready crops have certainly contributed to the extensive use of glyphosate, making it the most widely used herbicide in the U.S. This has likely increased the evolutionary selective pressure on “weeds”, leading to the generation of naturally glyphosate-resistant plants, a.k.a., “superweeds”. (Figs. 1 & 2 from Ref. 1 below)

Here Come the Superweeds.

Recent articles, such as this one in, of all places, Business Week have discussed implications of the appearance of so-called “superweeds”.

RR_2.jpg

Examples of more recent reports of Roundup®-resistant weeds can be found here, here, and here. The increase in the number of such glyphosate resistant plant species has elicited warnings from environmental groups such as the Union of Concerned Scientists, as well as the international press.

In addition to naturally occurring gylphosate-tolerant weeds, there also is the risk of the spread of the genes conferring herbicide tolerance from GM (genetically modified) plants to native plants. Such events have recently been reported to have occurred in test plots of Roundup Ready® turfgrass in Oregon.

Briefly, the artificial gene conferring gylphosate-tolerance was discovered to be present in some native grass species growing adjacent to the test plots. (The genes presumably traveled via pollen from the GM plants to pollenate the native grasses.)

Thus, the possibility of the creation of superweeds via the escape of herbicide-resistance-conferring gene constructs is a very real possibility indeed.

May Increased Use of Glyphosate Also Be Toxic to Animals?

Though glyphosate has been considered one of the more benign pesticides, its environmental impacts are being reconsidered in light of some recent evidence to the contrary.

Reference
1. Boerboom, C. and M. Owen (2006) “Facts About Glyphosate Resistant Weeds”, Purdue University Extension, publ. GWC-1. (PDF)

Bottom Line: Though glyphosate kills plants by targeting an enzyme not present in animals, its overuse – due in part to GM Roundup Ready® crops – may be harmful to agriculture, to the environment, and, ultimately, to human health.

HowPlantsWork © 2008-2011 All Rights Reserved.

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