Molecular and physiological mechanisms associated with root exposure to mercury in barley.

Mercury (Hg) is a toxic metal that affects plant growth. Here the effect of Hg exposure on plant growth and leaf gas-exchange together with gene expression in roots is reported for barley. Hg was mainly accumulated in roots and only very small amounts were found in the shoots. Chlorophyll fluorescence and net photosynthesis were not affected by Hg. Nevertheless exposure to Hg reduced shoot and root growth, the shoot to root ratio, stomatal conductance, carbon isotope discrimination and expression of an aquaporin transcript, whereas abscisic acid related transcripts were over-expressed. These results suggested some degree of limitation to water uptake causing a moderate water stress when plants are exposed to Hg. Microarray (MapMan) analysis revealed changes in the transcription of genes involved in nitrogen metabolism, which were accompanied by decreased nitrogen concentrations in the shoots, together with an increase in transcripts associated with secondary metabolism, stress, inhibition of DNA synthesis/chromatin structure and cell organization elements. Moreover, Hg induced the expression of many transcripts known to be involved in the uptake, accumulation, transport and general responses to other heavy metals. It is concluded that barley is able to accumulate high amounts of Hg in roots through several transcriptional, metabolic and physiological adjustments.

Detection and quantification of unbound phytochelatin 2 in plant extracts of Brassica napus grown with different levels of mercury.

The mercury (Hg) accumulation mechanism was studied in rape (Brassica napus) plants grown under a Hg concentration gradient (0 microm-1,000 microm). Hg mainly accumulated in roots. Therefore, the presence of phytochelatins (PCs) was studied in the roots of the plants. The high stability of the PC-Hg multicomplexes (mPC-nHg) seems to be the main reason for the lack of previous Hg-PC characterization studies. We propose a modification of the method to detect and quantify unbound PC of Hg in plant extracts via high-performance liquid chromatography coupled to electrospray tandem mass spectrometry and inductively coupled plasma mass spectrometry in parallel. We separated the PC from the Hg by adding the chelating agent sodium 2,3-dimercaptopropanesulfonate monohydrate. We only detected the presence of PC after the addition of the chelating agent. Some multicomplexes mPC-nHg could be formed but, due to their large sizes, could not be detected. In this study, only PC(2) was observed in plant samples. Hg accumulation was correlated with PC(2) concentration (r(2) = 0.98).