Early transcriptomic response of Arabidopsis thaliana to polymetallic contamination: implications for the identification of potential biomarkers of metal exposure.

Heavy metal contaminated sites are frequently characterized by the simultaneous presence of several heavy metals. However, many studies report metal-induced plant responses after long-term exposure to just one metal. By contrast, whole genome expression microarrays were employed here to investigate the early (3 h) transcriptional responses of Arabidopsis thaliana plants exposed to polymetallic treatment (Pb, Hg, Cu, Cd, Co, Ni, Zn, and Mn) at low (L) and high (H) concentrations. After 3 h of exposure to polymetallic treatment, a total of 1315 noticeably (≥2-fold) and significantly (P < 0.05) differentially expressed genes were identified: 656 and 351 upregulated and 314 and 200 downregulated genes in L and H treatments, respectively. Functional analysis revealed that many genes involved in oxidative stress and perception/signalling/regulation systems were activated. Genes encoding proteins involved in hormone regulation (jasmonic acid, abscisic acid, ethylene, and auxins), glucosinolate metabolism and sulphur and nitrogen transport were also modulated. RT-qPCR analysis of four downregulated (AOP2, SAUR16, BBX31, and MTPC3) and upregulated genes (ASN1, DIN2, BT2, and EXL5), markedly responsive to both L and H treatments, validated our microarray data and suggested the potential of some of these genes (AOP2, SAUR16, ASN1, and DIN2) as early biomarkers of metal exposure. Relevant changes in gene expression occur as early as 3 h after exposure to polymetallic treatment. Four genes deserve further studies as novel putative biomarkers of early metal exposure and also owing to their potential implications in stress-related mechanisms: sulphur balance (AOP2), phytohormone regulation of plant growth and development (SAUR16), ammonium detoxification (ASN1) and senescence (DIN2).