Sunday, June 6, 2021

Frankenstein's Plants: Natural Genetic Engineering in Grasses


Frankenstein foreground from Wikipedia (Universal Studios, NBCUniversal). Background is Phragmites australis

The grasses (Family Poaceae) are the most successful and economically important plant family today. They are supremely adaptable and hardy, and exist in numberless hordes from the rainforests of the equator, to the driest deserts of the American West, and even to the cold and almost lifeless vastness of Antarctica. 

The ability of species in this family to adapt to different environmental conditions has been one of the factors which has made it so successful, and researchers have recently elucidated one of the reasons for this adaptability.

A new study has discovered that many grasses can readily absorb novel genetic information from  their environment, and incorporate them into their own genome (Hibdige et al, 2021)! They do this via a process called Lateral Gene Transfer (LGT), where a species can acquire new adaptive genes and traits from completely different species without any sexual reproduction.

Echinochloa crus-galli had evidence of at least 10 LGT

The groundbreaking study showed that many grass species have been involved in lateral gene transfer of multiple genes, some of the genes coming from other grass species that are far removed in terms of  evolutionary relationship.

The researchers looked at 17 species for protein coding LGT, examining from 15,000+ to  well over 30,000 genes for each species tested. They found that most of the genes had an evolutionary history similar to the species that harbored them, indicating that they had been passed down through the generations from parent to offspring as expected. But more than a hundred genes were found where the evolutionary history of the gene did not match the species.

Zea mays (maize/corn) had at least 11 LGT

This evidence of LGT was found in 13 of the 17 species tested, with Alloteropsis semialata showing 20 instance of LGT! The species that showed evidence of LGT included not only wild species, but several domesticated ones as well, including maize, millet, and wheat. In this case, maize (Zea mays) had 11 LGTs received from Chloridoideae and Paniceae, while wheat (Triticum aestivum) had 10 LGTs received from Andropogoneae, Chloridoideae and Paniceae. The researchers noted that the LGTs may be beneficial for the crops, as the transferred genes included functions related to abiotic stress tolerance and disease resistance. 

Such instances of LGT are an amazing way for a grass to "leap frog" the slower evolutionary pathways and suddenly acquire traits very quickly, some of which are not only useful but staggering in their significance. For example, in another earlier study (Christin et al, 2012), researchers looked at the grass genus Alloteropsis, which contain species that  use C3 or C4 photosynthesis. They discovered  that fundamental elements of the C4 pathway (which is very adaptive in hotter and drier environments) were acquired via LGT from C4 grass taxa that diverged from the Alloteropsis group more than 20 million years ago!  

Distribution of LGT in tested grass species

The mechanism of this lateral transfer of genes is still unresolved, although the researchers noted that it seems to be more prevalent in species that have rhizomes. Such grasses have the capability of producing full individuals from broken pieces of their rhizomes, so one could imagine a situation where an LGT between these below ground structures and a concomitant separation of the rhizome later could lead to new separate individuals with sexual apparatus containing the gene. 

I am reminded again of the rhizomes of cogon grass (Imperata cylindrica), which have very pointed tips and have been shown to puncture other species of grass as the cogon expands. Perhaps such instances of hostility was a gateway to the prevalence of LGT in grasses, or perhaps it is only one of many multiple mechanisms for such an absolutely amazing phenomenon.

The sharp pointed rhizome of Imperata cylindrica

Literature Cited

Christin PA, Edwards EJ, Besnard G, Boxall SF, Gregory R, Kellogg EA, Hartwell J, Osborne CP. Adaptive evolution of C(4) photosynthesis through recurrent lateral gene transfer. Curr Biol. 2012 Mar 6;22(5):445-9. doi: 10.1016/j.cub.2012.01.054. Epub 2012 Feb 16. PMID: 22342748.

Hibdige, S.G.S., Raimondeau, P., Christin, P.-A. and Dnning, L.T. (2021), Widespread lateral gene transfer among grasses. New Phytol.