Remediation of asbestos with poplar and willow species and gene regulation network behind asbestos toxicity tolerance in trees

Asbestos is a naturally occurring fibrous silicate mineral commonly used in many industries. Due to the definite link between asbestos exposure and lung carcinoma, production and utilization of asbestos have been banned in many countries. Although this ban reduces the possibility of occupational exposure of humans to asbestos, it is reported that asbestos exposure from natural sources is at a very high level in some parts of the world, including Turkey. Therefore, the afforestation of asbestos mines and natural sources is highly important for human health. In the current study, poplar and willow species collected in an asbestos-contaminated Turkish village were grown under asbestos-contaminated soils and tested for their tolerance to asbestos toxicity. The SEM, XRD and elemental analysis revealed that asbestos-contaminated soil rich in metals (Mg, Fe, Al, Cr, Ni, Co) and accumulation of these metals in the leaves and roots of the plants is the main reason for the asbestos toxicity in the trees. Deficiencies of plant nutritional minerals (K, B, Na) in asbestos also caused less tree growth in these contaminated soils. Elemental analysis on the plant tissues indicated that willow species grown in asbestos-contaminated soils accumulated threefold fewer asbestos metals and performed more biomass growth than poplars. Microarray-based transcriptome analysis on the asbestos-treated leaves and roots of trees revealed upregulation of genes functional in metal detoxification in poplar and Salicycilc acid-regulated toxicity tolerance in willow. Transcripts functional in ROS scavenging, enzymes/protein protection, and metal chelating are the most strongly upregulated genes in response to asbestos toxicity in poplar and willow tissues. ABC transporters, Heavy metal-associated isoprenylated plant protein, Glutathione S-transferases, and Glucosyltransferases represented much higher gene expression in asbestos-treated poplars than willow. Expressions of these genes were associated with the uptake of excess asbestos metals, their translocation to the leave, and cellular detoxification in poplar. On the other hand, asbestos-treated willow leaves and roots differentially induced the expression of the genes functional in salicylic acid (SA) biosynthesis and SA acid-regulated genes (Kunitz type protease inhibitors, pathogenesis-related genes and chitinases, etc). This SA-mediated gene regulation in willow tissues was linked to lower accumulation of asbestos-related metals and higher tolerance to asbestos toxicity in trees for the first time.