Inoculation of selenium hyperaccumulator Stanleya pinnata and related non-accumulator Stanleya elata with hyperaccumulator rhizosphere fungi--investigation of effects on Se accumulation and speciation.

Little is known about how fungi affect elemental accumulation in hyperaccumulators (HAs). Here, two rhizosphere fungi from selenium (Se) HA Stanleya pinnata, Alternaria seleniiphila (A1) and Aspergillus leporis (AS117), were used to inoculate S. pinnata and related non-HA Stanleya elata. Growth and Se and sulfur (S) accumulation were analyzed. Furthermore, X-ray microprobe analysis was used to investigate elemental distribution and speciation. Growth of S. pinnata was not affected by inoculation or by Se. Stanleya elata growth was negatively affected by AS117 and by Se, but combination of both did not reduce growth. Selenium translocation was reduced in inoculated S. pinnata, and inoculation reduced S translocation in both species. Root Se distribution and speciation were not affected by inoculation in either species; both species accumulated mainly (90%) organic Se. Sulfur, in contrast, was present equally in organic and inorganic forms in S. pinnata roots. Thus, these rhizosphere fungi can affect growth and Se and/or S accumulation, depending on host species. They generally enhanced root accumulation and reduced translocation. These effects cannot be attributed to altered plant Se speciation but may involve altered rhizosphere speciation, as these fungi are known to produce elemental Se. Reduced Se translocation may be useful in applications where toxicity to herbivores and movement of Se into the food chain is a concern. The finding that fungal inoculation can enhance root Se accumulation may be useful in Se biofortification or phytoremediation using root crop species.

Inoculation of Astragalus racemosus and Astragalus convallarius with selenium-hyperaccumulator rhizosphere fungi affects growth and selenium accumulation.

Little is known about how fungi affect plant selenium (Se) accumulation. Here we investigate the effects of two fungi on Se accumulation, translocation, and chemical speciation in the hyperaccumulator Astragalus racemosus and the non-accumulator Astragalus convallarius. The fungi, Alternaria astragali (A3) and Fusarium acuminatum (F30), were previously isolated from Astragalus hyperaccumulator rhizosphere. A3-inoculation enhanced growth of A. racemosus yet inhibited growth of A. convallarius. Selenium treatment negated these effects. F30 reduced shoot-to-root Se translocation in A. racemosus. X-ray microprobe analysis showed no differences in Se speciation between inoculation groups. The Astragalus species differed in Se localization and speciation. A. racemosus root-Se was distributed throughout the taproot and lateral root and was 90 % organic in the lateral root. The related element sulfur (S) was present as a mixture of organic and inorganic forms in the hyperaccumulator. Astragalus convallarius root-Se was concentrated in the extreme periphery of the taproot. In the lateral root, Se was exclusively in the vascular core and was only 49 % organic. These findings indicate differences in Se assimilation between the two species and differences between Se and S speciation in the hyperaccumulator. The finding that fungi can affect translocation may have applications in phytoremediation and biofortification.

Characterization of rhizosphere fungi from selenium hyperaccumulator and nonhyperaccumulator plants along the eastern Rocky Mountain Front Range.

PREMISE OF STUDY: Selenium-hyperaccumulator plants can store over 1% (dry mass) Se in their tissues, despite the toxicity of this element at high concentrations across eukaryotes. These levels of Se can have widespread effects on the plant's ecological partners, including herbivores and pathogens. Still other partners seem to have coevolved Se tolerance. This is the first known study addressing the rhizosphere mycoflora of Se hyperaccumulators and aims to evaluate the rhizospheric fungal diversity and Se tolerance to further the knowledge of how these organisms interact with their host plants and survive in these extreme habitats. METHODS: Rhizosphere fungi were isolated from Se-hyperaccumulator and nonaccumulator plant species collected from five sites in Colorado and Wyoming; four seleniferous sites and one nonseleniferous site. 259 isolates were identified to genus or species and evaluated for Se tolerance. KEY RESULTS: Among the 24 represented genera, 11 comprised 86% of the isolates. The majority of isolates from the seleniferous sites were unaffected by 10 mg·L(-1) Se, irrespective of host plant (hyperaccumulator vs. nonaccumulator), while rhizosphere fungi from a control, nonseleniferous site were highly sensitive to Se at 10 mg·L(-1) and as a group were significantly less (α = 0.05) tolerant than the isolates from the seleniferous sites. CONCLUSIONS: Even though Se is a commonly used antifungal agent, these results suggest that rhizosphere fungi from seleniferous habitats have widespread Se tolerance, likely an adaptive advantage in their Se-rich habitat.

Constitutive expression of a high-affinity sulfate transporter in Indian mustard affects metal tolerance and accumulation.

The Stylosanthes hamata SHST1 gene encodes a high-affinity sulfate transporter located in the plasma membrane. In this study the S. hamata SHST1 gene was constitutively expressed in Indian mustard [Brassica juncea (L.) Czern.] to investigate its importance for tolerance and accumulation of various oxyanions that may be transported by SHST1 and for cadmium, which is detoxified by sulfur-rich compounds. The transgenic SHST1 lines SHST1-12C and SHST1-4C were compared with wild-type Indian mustard for tolerance and accumulation of arsenate, chromate, tungstate, vanadate, and cadmium. As seedlings the SHST1 plants accumulated significantly more Cd and W, and somewhat more Cr and V. The SHST1 seedlings were less tolerant to Cd, Mo, and V compared to wild-type plants. Mature SHST1 plants were less tolerant than wild-type plants to Cd and Cr. SHST1 plants accumulated significantly more Cd, Cr, and W in their roots than wild-type plants. In their shoots they accumulated significantly more Cr and somewhat more V and W. Shoot Cd accumulation was significantly lower than in wild-type, and As levels were somewhat reduced. Compared to wild-type plants, sulfur accumulation was enhanced in roots of SHST1 plants but not in shoots. Together these results suggest that SHST1 can facilitate uptake of other oxyanions in addition to sulfate and that SHST1 mediates uptake in roots rather than root-to-shoot translocation. Since SHST1 overexpression led to enhanced accumulation of Cr, Cd, V, and W, this approach shows some potential for phytoremediation, especially if it could be combined with the expression of a gene that confers enhanced metal translocation or tolerance.

Selenium protects plants from phloem-feeding aphids due to both deterrence and toxicity.

• Certain plant species hyperaccumulate selenium (Se) to 1000 mg kg-1 d. wt, even from low-Se soils. It is not known whether Se hyperaccumulation offers these plants any advantage. In this study the hypothesis was tested that Se can protect plants from phloem-feeding herbivores. • Indian mustard (Brassica juncea) grown with or without Se was subjected to colonization by green peach aphids (Myzus persicae). • In choice feeding experiments the aphids clearly avoided Se-containing plant material, and were able to detect and avoid Se-containing leaves with levels as low as 10 mg Se kg-1 d. wt. In nonchoice feeding experiments aphid population growth was inversely correlated with leaf Se concentration. The leaf Se concentration leading to a 50% reduction in aphid population growth was 1.5 mg kg-1 d. wt, and ≥ 10 mg Se kg-1 d. wt was lethal. • In summary, Se can protect plants from feeding by aphids at leaf levels two orders of magnitude lower than those found in hyperaccumulators in the field. These results shed light on the possible functional significance of Se hyperaccumulation.

Selenium accumulation protects Brassica juncea from invertebrate herbivory and fungal infection.

• 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 protects the hyperaccumulator Stanleya pinnata against black-tailed prairie dog herbivory in native seleniferous habitats.

Elemental hyperaccumulation in plants is hypothesized to represent a plant defense mechanism. The objective of this study was to determine whether selenium (Se) hyperaccumulation offers plants long-term protection from the black-tailed prairie dog (Cynomys ludovicianus). Prairie dogs are a keystone species. The hyperaccumulator Stanleya pinnata (prince's plume) co-occurs with prairie dogs in seleniferous areas in the western United States. Stanleya pinnata plants pretreated with high or low Se concentrations were planted on two prairie dog towns with different levels of herbivory pressure, and herbivory of these plants was monitored over 2 years. Throughout this study, plants with elevated Se levels suffered less herbivory and survived better than plants with low leaf Se concentrations. This study indicates that the Se in hyperaccumulator S. pinnata protects the plant in its natural habitat from herbivory by the black-tailed prairie dog. The results from this study support the hypothesis that herbivory by prairie dogs or similar small mammals has been a contributing selection pressure for the evolution of plant Se hyperaccumulation in North America. This study is the first to test the ecological significance of hyperaccumulation over a long period in a hyperaccumulator's natural habitat.

Selenium accumulation protects plants from herbivory by Orthoptera via toxicity and deterrence.

To investigate whether selenium (Se) accumulation in plants provides a chemical defense against generalist insect herbivores, the feeding preference and performance of a mix of orthopteran species were investigated. The selenium hyperaccumulator Stanleya pinnata and accumulator Brassica juncea were used in herbivory studies in the laboratory, and S. pinnata was also used in a manipulative field experiment. In laboratory studies, both crickets and grasshoppers avoided plants pretreated with selenate, while those given no choice died after eating leaves with elevated Se (447 +/- 68 and 230 +/- 68 microg Se g(-1) DW, respectively). B. juncea has previously been shown to accumulate selenate, while S. pinnata hyperaccumulates methyl-selenocysteine. Thus, these findings demonstrate that both inorganic and organic forms of selenium protect plants from herbivory. Grasshoppers fed S. pinnata contained methylselenocysteine in their midgut and absorbed this form into surrounding tissues. In a manipulative field experiment, methylselenocysteine protected S. pinnata from invertebrate herbivory and increased its long-term survival rate over an entire growth season. * In native habitats of selenium hyperaccumulators, orthopterans represent a major group of insect herbivores. Protection offered by organic selenium accumulation against these herbivores may have promoted the evolution of selenium hyperaccumulation in plants.