Hexahydro-1,3,5-trinitro-1,3,5-triazine
(RDX) is an explosive part of munitions extensively used by the military in
training ranges having a global market size of around $10 billion.
This
explosive does not degrade in the environment, it is absorbed into groundwater,
and very toxic to soils organisms and humans. Therefore, an accumulation of the
toxic material has happened, and cases of human contamination have been
reported.
Not only
active firing ranges are contaminated, but also manufacturing sites due to
exhaustion plumes dump locations, minefields, and decommissioned sites. It is
estimated that 10 million hectares of military land are contaminated in The United States. Since the armed forces continually need live-fire training, it
is extremely important to have a permanent strategy to contain and remediate
RDX.
There is a soil
bacterium, Rhodococcus rhodochrous 11Y, that can degrade RDX. The genes
responsible (xplA and xplB ) were identified and the breakdown of the
explosive was understood. It was then proposed that a transgenic grass carrying
these genes could be the solution for soil decontamination.
In the transgenic
switchgrass, RDX is transformed into naturally occurring plant metabolites with
no harvesting necessary. After promising results in the lab, a field trial was
authorized in Fort Drum, New York with the condition that all flower heads were
removed from the transgenic plants in the trial plots so no cross-breeding with
native plants could occur.
There were three
soil treatments (no RDX, 1mg kg–1 RDX and 100mg kg–1 RDX)
and three vegetation treatments (no plant control (NPC), wild-type
untransformed switchgrass (WT) and XplA/XplB-expressing switchgrass (TG))
replicated three times for a total of 27 plots.
RDX was not
detected in any of the tissues sampled from the transgenic lines over the third
growing season and across all three plots treated with the highest
concentration. In 3 years of observation, the concentration of RDX in water from
the transgenic plots was always less than either control and significantly
less than the no plant control. The XplA/XplB-expressing plants took up and
metabolized equivalent to an RDX removal rate of 27 kg ha–1.
It was then confirmed
that the transgenic switchgrass is capable of remediation and a transformation method
for another grass (Pascopyrum smithii) native to many states in the US
was developed as well. This looks to be a cost-effective and sustainable strategy to prevent
groundwater contamination around military ranges.
By Rosana Segatto
Reference
Cary, T.J., Rylott, E.L., Zhang, L. et al. Field trial demonstrating phytoremediation of the military explosive RDX by XplA/XplB-expressing switchgrass. Nat Biotechnol (2021). https://doi.org/10.1038/s41587-021-00909-4
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