Plant Selenium Metabolism: Genetic Manipulation, Phytotechnological Applications, and Ecological Implications - Environmental Contamination: Health Risks and Ecological Restoration - page 303

enhanced.sulfate.uptake.and.reduction;.this.may.serve. to.prevent.Se.from.
replacing.S.in.proteins.and.other.S.compounds.
The. genus. Stanleya ,. which. is. from. the. same. family. as. Arabidopsis .
(Brassicaceae),.contains.hyperaccumulators.and.nonhyperaccumulators..In.
Stanleya ,.similar.Se.tolerance.mechanisms.were.found.to.those.described.
earlier. for. Arabidopsis . (Freeman. et. al.. 2010).. The. plant. hormones. JA. and.
ethylene,. as. well. as. the. hormone. salicylic. acid,. appear. to. play. a. role. in.
regulating. Se. stress. response. in. Stanleya .. Probably. as. a. response. to. the.
elevated.levels.of.these.hormones,.hyperaccumulators.have.constitutively.
upregulated. expression. of. S. transporters. and. assimilatory. enzymes,. and.
hence.higher.levels.of.total.S,.reduced.S.compounds.(including.the.antioxi-
dant.glutathione),.and.higher.levels.of.total.Se..In.addition,.the.Se.hyperac-
cumulator. Stanleya . species. showed. interesting. Se. sequestration. patterns.
that. were. not. observed. in. nonhyperaccumulators.. Around. 90%. of. the.
accumulated.Se.was.present.as.methyl-SeCys.in.vacuoles.of.leaf.epidermal.
cells. (Freeman. et. al.. 2006b,. 2010).. This. may. indicate. Se-speciic. transport.
mechanisms. into. these. specialized. cell. types. in. hyperaccumulators.. In.
future.research,.it.will.be.very.interesting.to.identify.such.specialized.Se.
transporters,.as.well.as.the.key.genes.that.bring.about.the.enhanced.phyto-
hormone.levels.that.appear.to.trigger.the.cascade.of.reactions.that.together.
bring.about.the.hyperaccumulation.syndrome.
EcologicalAspectsofSePhytoremediation
Contribution of Microbes to Plant Se Uptake and Volatilization
Most. plant. species. have.one.or. more. types.of. symbiosis. with. bacteria.and.
fungi,.organisms.that. are.thought.to.play.an.important.role. in. the.biogeo-
chemistry. of. Se. by. mineralization,. immobilization,. or. volatilization.. The.
potential.effects.of.associated.microbes.on.the.fate.of.Se.in.their.host.plants.
offer.an.interesting.area.of.study.(Thompson-Eagle.et.al..1989,.de.Souza.and.
Terry.1997,.Pankiewicz.et.al..2006)..Different.types.of.plant-microbial.associa-
tions.may.have.different.effects.on.plant.Se.accumulation.and.volatilization..
Endophytic. or. epiphytic. bacteria. and. fungi. may. affect. Se. accumulation.
within. tissues. or. on. plant. surfaces,. and. root-associated. fungi. and. bacteria.
may. volatilize. or. change. the. oxidation. state. of. Se,. making. it. more. or. less.
available.for.plant.uptake.(Garbisu.et.al..1996,.Dowdle.and.Oremland.1998)..
Selenate-.and.selenite-reducing.bacteria.have,.for.instance,.been.isolated.from.
the.rhizosphere.of.the.Se.hyperaccumulator. A. bisulcatus .(Di.Gregorio.et.al..
2005,.Vallini.et.al..2005)..The.two.more.oxidized.forms,.Se(IV).and.Se(VI),.are.
relatively.bioavailable.to.plants.in.the.rhizosphere,.as.opposed.to.the.more.
reduced. elemental. form. of.Se.. Thus,. Se. oxidative. bacteria. may.enhance.Se.
availability.to.the.plant.while.Se.reducers.may.immobilize.Se.making.it.less.