Expression of Genes Involved in Heavy Metal Trafficking in Plants Exposed to Salinity Stress and Elevated Cd Concentrations

Many areas intended for crop production suffer from the concomitant occurrence of heavy metal pollution and elevated salinity; therefore, halophytes seem to represent a promising perspective for the bioremediation of contaminated soils. In this study, the influence of Cd treatment (0.01–10.0 mM) and salinity stress (0.4 M NaCl) on the expression of genes involved in heavy metal uptake (irt2–iron-regulated protein 2, zip4–zinc-induced protein 4), vacuolar sequestration (abcc2–ATP-binding cassette 2, cax4–cation exchanger 2 pcs1–phytochelatin synthase 1) and translocation into aerial organs (hma4–heavy metal ATPase 4) were analyzed in a soil-grown semi-halophyte Mesembryanthemum crystallinum. The upregulation of irt2 expression induced by salinity was additionally enhanced by Cd treatment. Such changes were not observed for zip4. Stressor-induced alterations in abcc2, cax4, hma4 and pcs1 expression were most pronounced in the root tissue, and the expression of cax4, hma4 and pcs1 was upregulated in response to salinity and Cd. However, the cumulative effect of both stressors, similar to the one described for irt2, was observed only in the case of pcs1. The importance of salt stress in the irt2 expression regulation mechanism is proposed. To the best of our knowledge, this study is the first to report the combined effect of salinity and heavy metal stress on genes involved in heavy metal trafficking.

Designing yeast as plant-like hyperaccumulators for heavy metals

Hyperaccumulators typically refer to plants that absorb and tolerate elevated amounts of heavy metals. Due to their unique metal trafficking abilities, hyperaccumulators are promising candidates for bioremediation applications. However, compared to bacteria-based bioremediation systems, plant life cycle is long and growing conditions are difficult to maintain hindering their adoption. Herein, we combine the robust growth and engineerability of bacteria with the unique waste management mechanisms of plants by using a more tractable platform-the common baker’s yeast-to create plant-like hyperaccumulators. Through overexpression of metal transporters and engineering metal trafficking pathways, engineered yeast strains are able to sequester metals at concentrations 10–100 times more than established hyperaccumulator thresholds for chromium, arsenic, and cadmium. Strains are further engineered to be selective for either cadmium or strontium removal, specifically for radioactive Sr90. Overall, this work presents a systematic approach for transforming yeast into metal hyperaccumulators that are as effective as their plant counterparts.