Transcriptomic changes induced by arsenic stress in hydroponically grown Salix purpurea

Arsenic (As) is a toxic element for plants that is one of the most common anthropogenic pollutants found at contaminated sites. Despite severe effects on plant metabolism, several species can accumulate substantial amounts of arsenic and endure the associated stress. However, the genetic mechanisms involved in arsenic tolerance remains obscure in many model plant species used for land decontamination (phytoremediation), including willows. The present study aims to assess the potential of Salix purpurea ‘Fish Creek’ for arsenic phytoextraction and reveal the genetic responses behind arsenic tolerance, phytoextraction and metabolism. Four weeks of hydroponic exposure to 0, 5, 30 and 100 mg/L revealed that plants were able to tolerate up to 5 mg/L arsenic. Concentrations of 0 and 5 mg/L of arsenic treatment was then used to compare alterations in gene expression of roots, stems and leaves using RNA sequencing (Illumina HiSeq 2000). Differential gene expression revealed transcripts encoding proteins putatively involved in entry of arsenic into the roots, storage in vacuoles and potential transport through the plant as well as primary and secondary (indirect) toxicity tolerance mechanisms. A major role for tannin as a compound used to relieve cellular toxicity is implicated as well as unexpected expression of the cadmium transporter CAX2, providing a potential means for internal arsenic mobility.