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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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