We examined the ability of yellow poplar (Liriodendron tulipifera) tissue cultures and plantlets to express modified mercuric reductase (merA) gene constructs. Mercury-resistant bacteria express merA to convert highly toxic, ionic mercury, Hg(II), to much less toxic, elemental mercury, Hg(O). Expression of merA in transgenic plants might provide an ecologically compatible approach for the remediation of mercury pollution. Because the alteration of the bacterial merA gene sequence is necessary for high-level expression in Arabidopsis thaliana, yellow poplar proembryogenic masses (PEMs) were transformed with three modified merA constructs via microprojectile bombardment. Each construct was synthesized to have altered flanking regions with increasing amounts of modified coding sequence. All merA constructs conferred resistance to toxic, ionic mercury in independently transformed PEM colonies. Stability of merA transgene expression increased in parallel with the extent of gene coding sequence modification. Regenerated plantlets containing the most modified merA gene (merA18) germinated and grew vigorously in media containing normally toxic levels of ionic mercury. The merA18 plantlets released elemental mercury at approximately 10 times the rate of untransformed plantlets. These results indicate that plants expressing modified merA constructs may provide a means for the phytoremediation of mercury pollution.

Although the economic, environmental, and aesthetic benefits of green roofs have been recognized for decades, research quantifying these benefits has been limited—particularly in the U.S. Green roof usage and research is most prevalent in Germany, but can also be seen in several other European countries and Canada. If green roof installations are to be successful in Michigan and the rest of the U.S., then a better understanding of what specific taxa will survive and thrive under harsh rooftop conditions in this geographic area is required. Nine simulated rooftop platforms containing three commercially available drainage systems were installed at Michigan State University. Eighteen Michigan native plants planted as plugs and nine Sedum spp. planted as either seed or plugs were evaluated over three years for growth, survival during both establishment and overwintering, and visual appearance. All Sedum spp. tested were found to be suitable for use on Midwestern green roofs. Of the eighteen native plant taxa tested, Allium cernuum L., Coreopsis lanceolata L., Opuntia humifosa Raf., and Tradescantia ohiensis L. are suitable for use on unirrigated extensive green roofs in Michigan. If irrigation is available, then other native species are potential selections.

No abstract

Because of greater interest in green roofs in the United States, it is critical to increase the number and geographic range of proven plant resources for long-term survival on rooftops. Successful plant taxa for extensive green roofs must establish themselves quickly, provide high groundcover density, and tolerate extreme environmental conditions. Furthermore, dead load weight restrictions on many buildings may limit the substrate depth that can be applied. The objective of this study was to evaluate the effect of substrate depth on initial establishment and survival of 25 succulent plant taxa for green roof applications in the midwestern United States. Survival, initial growth, and rate of coverage were compared for plants grown in three substrate depths (2.5, 5.0, and 7.5 cm) on 24 roof platforms. Plant coverage was determined from image analysis of weekly digital photographs. Results indicate deeper substrates promote greater survival and growth; however, in the shallowest depth of 2.5 cm, several species continued to persist. Of the 25 species initially planted, only 47% survived in the deepest substrate of 7.5 cm. Recommended species at the depths tested for climates similar to southern Michigan include Phedimus spurious Raf. ‘Leningrad White’, Sedum acre L., S. album L. ‘Bella d'Inverno’, S. middendorffianum L., S. reflexum L., S. sediforme J., and S. spurium Bieb. ‘Summer Glory’. Subsidiary species that are present at specific substrate depths but may not exhibit an ability to cover large areas include S. dasyphyllum L. ‘Burnatii’, S. dasyphyllum L. ‘Lilac Mound’, S. diffusum W., S. hispanicum L., and S. kamtschaticum Fisch. The primary deterrent for these subsidiary species was little to no survival at 2.5 cm. Deeper substrates promoted greater survival and growth for nearly all species tested.

Methylmercury is a highly toxic, organic derivative found in mercury-polluted wetlands and coastal sediments worldwide. Though commonly present at low concentrations in the substrate, methylmercury can biomagnify to concentrations that poison predatory animals and humans. In the interest of developing an in situ detoxification strategy, a model plant system was transformed with bacterial genes (merA for mercuric reductase and merB for organomercurial lyase) for an organic mercury detoxification pathway. Arabidopsis thaliana plants expressing both genes grow on 50-fold higher methylmercury concentrations than wild-type plants and up to 10-fold higher concentrations than plants that express merB alone. An in vivo assay demonstrated that both transgenes are required for plants to detoxify organic mercury by converting it to volatile and much less toxic elemental mercury.