Title : Effects of environmental contaminants on lignin biosynthesis in arabidopsis thaliana: implication for biofuel production
Marginal lands contaminated by anthropogenic pollutants can be turned into fields for biofuel production bringing the combined benefits of reducing dependency on fossil fuel, mitigation of carbon dioxide emissions, and land bioremediation (phytoremediation). However, prior research has shown that plant exposure to stress, including toxic chemicals (e.g., heavy metals), may lead to increased lignification of the plant biomass, therefore potentially impairing biomass digestibly and its conversion into bioethanol. The objective of the present study was to determine whether exposure to toxic organic contaminants may affect lignin biosynthesis in the model plant, Arabidopsis thaliana. Hydroponic plantlets were exposed to the antibiotic, chlortetracycline (CT), and the forever pollutants, polychlorinated biphenyls (PCBs). Toxicity testing indicated that exposure to CT and the PCB hydroxylated derivative, 4-hydroxy-2,5-dichlorobiphenyl (4'-OH-2,5-DCB) caused a dose-dependent reduction of the biomass, with an effect concentration 50% (EC50) of 8.1 ± 2.3 mg L-1 and 2.5 ± 1.9 mg L-1 for CT and 4-OH-2,5-DCB, respectively. The parent PCB, 2,5-DCB, did not show significant toxic effects at the highest concentration tested (25 mg L-1). The lignin content of the biomass was then compared in exposed plants and control, non-exposed plants using Fourier-transformed infrared spectroscopy (FTIR). Several spectral second-derivative absorbance peaks correlated with lignin were identified in the fingerprint region (~800 – 1800 cm-1), indicating a lignin increase of 16 to 40% in plants exposed to CT, and 22 to 102% in plants exposed to 2,5-DCB and 4'-OH-2,5-DCB. Whole genome expression analysis using RNA sequencing was conducted to determine the molecular bases of Arabidopsis exposure to the three pollutants. Each compound resulted in significant differential expression of more than 1,000 genes in exposed plants versus control plants (FDR adjusted p-value < 0.05, fold change > 2.0 or < 0.5). Exposure to 2,5-DCB and 4'-OH-2,5-DCB resulted in 5.1- and 4.9-fold enrichment of genes involved in the phenylpropanoid pathway (major lignin biosynthetic pathway), respectively. Exposure to CT also resulted in overexpression of genes involved in lignin biosynthesis, including cinnamyl alcohol dehydrogenases, beta-glucosidases, glycosyl hydrolases, and peroxidases. Exposure to the three compounds also induced genes involved in the metabolism of xenobiotic compounds and detoxification reactions, including glutathione S-transferases and cytochrome P-450s, which was consistent with their observed phytotoxic effects. Our results may have important implications for the production of bioethanol from lignocellulosic biomass generated through phytoremediation applications.