August 28, 2014

Review: Genomic-scale exchange of mRNA between a parasitic plant and its hosts

ScienceVol. 345 no. 6198 pp. 808-811 

 (click here to access the original article)

 Gunjune Kim1,  Megan L. LeBlanc1,  Eric K. Wafula2, Claude W. dePamphilis2,  James H. Westwood1,*
1Department of Plant Pathology, Physiology and Weed Science, Virginia Tech, Blacksburg, VA 24061, USA
2Department of Biology and the Huck Institutes of the Life Sciences, Pennsylvania State University, University Park, PA 16802, USA

* Author for correspondence:

We ordinarily think of parasitic organisms living off the nutrients of their host without returning anything of benefit. However, it is another matter to consider that some parasites may not only benefits from host nutrients but acquire their mRNA transcripts as well. Nonetheless, the process of gene acquisition through horizontal gene transfer provides one of several mechanisms by which an organism can acquire a significant amount of DNA from another, so it should come as no surprise to learn that RNA may play a role as an intermediate in some cases. A report last week by groups at Penn State and Virginia Tech, led by James Westwood, describes a parasitic plant, Cuscuta pentagona,  which share significant amounts of mRNA transcripts with two model host organisms, Arabidopsis and tomato.

Kim et al. published the result of extensive RNAseq efforts and host-parasite interactions to characterize the extent of the bidirectional exchange of mRNA molecules through haustoria, specialized organs that facilitate the uptake of nutrients and water by the parasite. Based on their findings, nearly half of the mRNA transcriptome of Arabidopsis could be located in the interface zone between the two species, and many were found in the Cuscuta stem far from their point of origin in the host. While mRNA travel in phloem is well documented, the researchers found not only mRNAs representing species known to travel in phloem but also mRNA species that are typically found only in the cytoplasmic environment.

This indicates that acquisition of novel genes by horizontal gene transfer may in some cases proceed via the intermediacy of RNA, a prospect which so far has been rarely documented. While additional research will provide more insights into how such an exchange of mRNAs may affect the metabolism of the parasite and host, each of which receives a mRNA complement from the other during this exchange, it is tempting to speculate on whether such mRNAs are translated into functional proteins after crossing into the other species. Could a parasite use such mRNAs as metabolic clues to gauge its engagement with its host? Can a parasitic plant send in its own mRNAs to affect the expression of host genes for its benefit? 

Finally, this fascinating reports helps us to redefine what constitutes the boundaries of an organism. It may even have implications for the genetic manipulation of domesticated plants. If the phenomenon described here by Kim et al. is widespread in nature, the concept of different organisms of distinct species exchanging genetic information should not necessarily alarm us if it turns out that mother nature, once again, has done it first, has always done so, and has done so on a scale we can scarcely imagine.

h/t Briardo

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