Plant Selenium Metabolism: Genetic Manipulation, Phytotechnological Applications, and Ecological Implications - Environmental Contamination: Health Risks and Ecological Restoration - page 301

and. B. juncea . (Ellis. et. al.. 2004,. LeDuc. et. al.. 2004).. The. SMT. transgenics.
showed. enhanced. Se. accumulation,. in. the. form. of. methyl-SeCys,. as. well.
as.enhanced.Se.tolerance..The.expression.of.SMT.also.resulted.in.increased.
rates.of.Se.volatilization,.with.more.Se.volatilized.in.the.form.of.DMDSe.
Although.the.expression.of. A. bisulcatus .SMT.enhanced.Se.tolerance,.accu-
mulation,. and. volatilization,. the. effects. were. more. pronounced. when. the.
plants.were.supplied.with.selenite.as.opposed.to.selenate..This.suggests.that.
the.conversion.of.selenate.to.selenite.is.a.rate-limiting. step.for. the.produc-
tion. of. SeCys.. Therefore,. APS. and. SMT. B. juncea . transgenics. were. crossed.
to.create.double-transgenic.plants..The.APS.×.SMT.double.transgenics.accu-
mulated.up.to.nine.times.higher.Se.levels.than.wild.type.(LeDuc.et.al..2006)..
Most.of.the.Se.in.the.double.transgenics.was.in.the.form.of.methyl-SeCys:.
the.APS.×.SMT.plants.accumulated.up.to.eightfold.more.methyl-SeCys.than.
wild.type.and.nearly.twice.as.much.as.the.SMT.transgenics..Se.tolerance.was.
similar.in.the.single.and.double.transgenics.
These. genetic. engineering. studies. demonstrate. that. the. sulfate. assimila-
tion.and.volatilization.pathway.is.responsible.for.selenate.assimilation.and.
volatilization..APS.appears.to.be.a.rate-limiting.enzyme.for.the.assimilation.
of. selenate. to. organic. Se,. and. CgS. is. rate-limiting. for. DMSe. volatilization..
Enhanced.APS.expression.appears.to.trigger.selenate.uptake.and.Se.and.S.
accumulation,.perhaps.due.to.upregulation.of.sulfate.transporter.expression..
The. results. from. the. SL. and. CpNifS. transgenics. show. that. speciic. break-
down. of. SeCys. can. reduce. nonspeciic. incorporation. of. Se. into. proteins,.
enhancing.Se.tolerance..Overexpression.of.SL.or.CpNifS.led.to.enhanced.Se.
accumulation,.suggesting.that.introduction.of.this.new.sink.for.Se.upregu-
lates.Se.and.S.uptake..The.results.from.the.SMT.transgenics.show.that.SMT.
is.a.key.enzyme.for.Se.hyperaccumulation,.conferring.enhanced.Se.tolerance.
and.accumulation.when.expressed.in.nonhyperaccumulators..However,.for.
improved.Se.assimilation.and.detoxiication,.APS.needs.to.be.overexpressed.
together. with. SMT.. APS.×.SMT. double. transgenics. combine. the. ability. to.
reduce. selenate. to. selenite. and. SeCys. with. the. ability. to. methylate. SeCys.
and. thus. to. detoxify. the. increased. pool. of. internal. Se.. While. APS.×.SMT.
double.transgenics.show.signiicantly.enhanced.Se.tolerance.and.accumula-
tion,.they.do.not.approach.the.performance.of.Se.hyperaccumulator.species..
Further.research.is.needed.to.identify.additional.Se.tolerance.and.accumula-
tion.genes.in.these.specialized.plant.species.
Under.laboratory.conditions,.the.different.transgenics.showed.enhanced.
Se. tolerance,. up. to. ninefold. higher. Se. accumulation. and. up. to. threefold.
faster. Se. volatilization.. These. properties. are. all. useful. for. phytoremedia-
tion..To.test.the.transgenics'.potential.for.phytoremediation,.they.were.ana-
lyzed. for. their. capacity. to. accumulate. Se. from. naturally. seleniferous. soil.
and. from. Se-contaminated.sediment..When. grown.on. naturally. selenifer-
ous.soil.in.a.greenhouse.pot.experiment,.the.APS.transgenics.accumulated.
Se.to.threefold.higher.levels.than.wild-type. B. juncea ,.and.the.CgS.transgen-
ics.contained.40%.lower.Se.levels.than.wild.type.(Van.Huysen.et.al..2004),.all.