These authors contributed equally to this work Summary• Certain plant species hyperaccumulate selenium (Se) up to 0.6% of their dry weight. It is not known whether Se hyperaccumulation offers the plants any advantage. In this study the hypothesis was tested that Se can protect plants from invertebrate herbivory or fungal infection.• Indian mustard ( Brassica juncea ) plants grown with or without Se were subjected to herbivory by caterpillars ( Pieris rapae ) and snails ( Mesodon ferrissi ), or to fungal infection by a root /stem pathogen ( Fusarium sp.) and a leaf pathogen ( Alternaria brassicicola ).• When given a choice between leaves with or without Se (0.1% Se of leaf d. wt), the caterpillars strongly preferred leaves without Se ( P < 0.01), while the snails preferred leaves containing Se ( P < 0.015). When consumed, the Se leaves were lethal to the caterpillars. The snails showed no toxicity symptoms, even though their tissue Se concentrations were comparable with the caterpillars. Se-containing plants were less susceptible to infection by both fungi.• In conclusion, Se was shown to protect Indian mustard plants from fungal infection and from herbivory by caterpillars, but not by snails.

Selenium (Se) is an essential element for many organisms but is toxic at higher levels. CpNifS is a chloroplastic NifS-like protein in Arabidopsis (Arabidopsis thaliana) that can catalyze the conversion of cysteine into alanine and elemental sulfur (S 0 ) and of selenocysteine into alanine and elemental Se (Se 0 ). We overexpressed CpNifS to investigate the effects on Se metabolism in plants. CpNifS overexpression significantly enhanced selenate tolerance (1.9-fold) and Se accumulation (2.2-fold). CpNifS overexpressors showed significantly reduced Se incorporation into protein, which may explain their higher Se tolerance. Also, sulfur accumulation was enhanced by approximately 30% in CpNifS overexpressors, both on media with and without selenate. Root transcriptome changes in response to selenate mimicked the effects observed under sulfur starvation. There were only a few transcriptome differences between CpNifS-overexpressing plants and wild type, besides the 25-to 40-fold increase in CpNifS levels. Judged from x-ray analysis of near edge spectrum, both CpNifS overexpressors and wild type accumulated mostly selenate (Se VI ). In conclusion, overexpression of this plant NifS-like protein had a pronounced effect on plant Se metabolism. The observed enhanced Se accumulation and tolerance of CpNifS overexpressors show promise for use in phytoremediation.

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To investigate selenium tolerance mechanisms in Arabidopsis thaliana, genetic and physiologic studies were performed in the three Arabidopsis accessions Landsberg erecta (Ler), Columbia (Col) and Wassilewskija (Ws). Accession Ler was significantly less tolerant to selenate than Ws and Col, whereas Ws was less tolerant to selenite than the others. Analysis of selenium tolerance in F 1 and F 2 plants obtained from crosses between these accessions suggest that multiple genes are involved in selenate tolerance and a single major gene controls selenite tolerance in these populations. Bulked segregant analysis in two F 2 populations indicated that molecular marker ciw7 on chromosome 4 is linked to selenite tolerance, and three molecular markers on chromosomes 1, 3 and 5 (nga111, ciw4 and ciw8, respectively) are linked to selenate tolerance. The ecotypic variation in selenite tolerance appeared to be correlated with root levels of non-protein thiols. Also, the shoot tissue levels of selenocysteine (SeCys) and selenocystine were correlated with tolerance to both selenate and selenite. Judging from RT-PCR results, several sulfate transporters and S assimilatory enzymes appear to be upregulated by selenate and selenite at the transcriptional level. A potential SeCys methyltransferase was expressed at lower levels in selenite-sensitive Ws than in the other two, when grown on selenite. Together, these studies show that there is substantial intraspecific variation in tolerance to selenate and selenite in Arabidopsis, and provide insights into the genetic and biochemical mechanisms underlying the observed ecotypic differences.