Analysis of plant Pb tolerance at realistic submicromolar concentrations demonstrates the role of phytochelatin synthesis for Pb detoxification.

Lead (Pb) ranks first among metals with respect to tonnage produced and released into the environment. It is highly toxic and therefore an important pollutant of worldwide concern. Plant Pb uptake, accumulation, and detoxification mobilize Pb into food webs. Still, knowledge about the underlying mechanisms is very limited. This is largely due to serious experimental challenges with respect to Pb availability. In most studies, Pb(II) concentrations in the millimolar range have been used even though the toxicity threshold is in the nanomolar range. We therefore developed a low-phosphate, low-pH assay system that is more realistic with respect to soil solution conditions. In this system the growth of Arabidopsis thaliana seedlings was significantly affected by the addition of only 0.1 μM Pb(NO3)2. Involvement of phytochelatins in the detoxification of Pb(II) could be demonstrated by investigating phytochelatin synthase mutants. They showed a stronger inhibition of root growth and a lack of Pb-activated phytochelatin synthesis. In contrast, other putative Pb hypersensitive mutants were unaffected under these conditions, further supporting the essential role of phytochelatins for Pb detoxification. Our findings demonstrate the need to monitor plant Pb responses at realistic concentrations under controlled conditions and provide a strategy to achieve this.