What happens to the TNT once it enters the plant?
Plants detoxify TNT by transforming it and then conjugating the breakdown products to plant metabolites such as glycose, and then sequestering these macromolecules into vacuoles or incorporating them into the cell wall through lignification. These processes primarily occur in the root of these plants as seen my poplar trees in which 75% of radioactivity from radiolabeled TNT remained in the root tissues. Detoxification of xenobiotics within plant tissue is expected to follow the "green liver" model and consists of 3 phases. Phase 1 is activation of the compound for example hydroxylation. This transformation step of TNT generally begins with sequential reduction of the nitro side groups by the plants endogenous nitroreductases to hydroxylamine intermediates and then into amino derivatives. Phase 2, where the chemical is conjugated with sugars and polypeptides. One important step in this process is its conjugation to tripeptide glutathione which is mediated by glutathione S-transferases GST's.
Arabidopsis
Microarray data on Arabidopsis plants exposed TNT at levels which cause
phytotoxic effects (10?M TNT) reviles a number of genes being up-regulated
which could be involved in TNT transformation. Of particular interest
was the up-regulation of glucosyltransferases and anthranilate benzoyltransferases
which is involved in detoxification of many xenobiotics. The rate limited
step in TNT transformation occurs in the reaction converting TNT to hydroxylamine
as reported by Subramanian et al 2006. The enzyme involved in this reaction
should have further testing done on its ability to increase a plants ability
to transform TNT.
Tobacco
Wlid Type tobacco plants are resistant to TNT up to 5 mg/Kg.
Poplar
Hybrid Poplar trees are resistant to TNT up to 5mg/Kg. Studies show hybrid poplar exposed to 5mg/L of TNT breakdown TNT into insignificant levels after 48 hours. The presence of ADNT and DANT in this experiment peaked at 24 hours which a concentration of 8% and 6% respectively and then decreased to <2%.
Microarray analysis of poplar trees exposed to TNT have shown an increase in 2 tau class GST genes which showed significant homology to GST's found in Arabidopsis which are essential in detoxification of herbicides. This study also showed significant induction of 12-oxophytodienoate reductase and indole 3-acetate B-glucosyl transferase which could possibly play a role in TNT detoxification and tolerance.
Rice
Germination rates in rice have been shown to decrease as concentrations
of TNT increase beyond 500mg/Kg. Plants grown in soils with TNT concentrations
below 150mg/kg exhibited an increase in shoot biomass. Plants grown in
soils with TNT concentrations at 200mg/kg exhibited an increase in root
growth by 280%. However concentrations that increased plant growth also
had higher germination inhibition suggestion that it would be best to
grow plants in a nursery and then transfer them to the location of contamination.
Aspen
Wild type aspen trees are resistant to TNT up to 10mg/Kg
Datura innoxia
Wild type Datura innoxia plants are resistant to TNT up to 750mg/Kg.
This plants has been shown to be capable of transforming TNT and stoaring
the matabolites at concentrations up 30 times that of the TNT concentration
in the soil. After 2 weeks only 4-9% of the applied TNT remained in the
soil.
Indian Mustard (Abutilon avicennae)
Studies using Indian mustard on soils contaminated with TNT has been
shown to enhance the bacterial densities of bacteria capable of breaking
down TNT in addition to directly taking up and transforming TNT.
