title: Roles of rhizobial symbionts in selenium hyperaccumulation in Astragalus (Fabaceae).
abstract: OBJECTIVE
  Are there dimensions of symbiotic root interactions that are overlooked because plant mineral nutrition is the foundation and, perhaps too often, the sole explanation through which we view these relationships? In this paper we investigate how the root nodule symbiosis in selenium (Se) hyperaccumulator and nonaccumulator Astragalus species influences plant selenium (Se) accumulation.
  
  
  METHODS
  In greenhouse studies, Se was added to nodulated and nonnodulated hyperaccumulator and nonaccumulator Astragalus plants, followed by investigation of nitrogen (N)-Se relationships. Selenium speciation was also investigated, using x-ray microprobe analysis and liquid chromatography-mass spectrometry (LC-MS).
  
  
  RESULTS
  Nodulation enhanced biomass production and Se to S ratio in both hyperaccumulator and nonaccumulator plants. The hyperaccumulator contained more Se when nodulated, while the nonaccumulator contained less S when nodulated. Shoot [Se] was positively correlated with shoot N in Se-hyperaccumulator species, but not in nonhyperaccumulator species. The x-ray microprobe analysis showed that hyperaccumulators contain significantly higher amounts of organic Se than nonhyperaccumulators. LC-MS of A. bisulcatus leaves revealed that nodulated plants contained more γ-glutamyl-methylselenocysteine (γ-Glu-MeSeCys) than nonnodulated plants, while MeSeCys levels were similar.
  
  
  CONCLUSIONS
  Root nodule mutualism positively affects Se hyperaccumulation in Astragalus. The microbial N supply particularly appears to contribute glutamate for the formation of γ-Glu-MeSeCys. Our results provide insight into the significance of symbiotic interactions in plant adaptation to edaphic conditions. Specifically, our findings illustrate that the importance of these relationships are not limited to alleviating macronutrient deficiencies.
authors.affiliation: Graduate Degree Program in Ecology, Colorado State University, Fort Collins, Colorado 80523 USA Department of Forest and Rangeland Stewardship, Colorado State University, Fort Collins, Colorado 80523 USA.
authors.forename: Élan R
authors.initials: ÉR
authors.lastname: Alford
authors.affiliation: Biology Department, Colorado State University, Fort Collins, Colorado 80523 USA.
authors.forename: Stormy D
authors.initials: SD
authors.lastname: Lindblom
authors.affiliation: Biology Department, Colorado State University, Fort Collins, Colorado 80523 USA.
authors.forename: Marco
authors.initials: M
authors.lastname: Pittarello
authors.affiliation: Biology Department, Colorado State University, Fort Collins, Colorado 80523 USA.
authors.forename: John L
authors.initials: JL
authors.lastname: Freeman
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720 USA.
authors.forename: Sirine C
authors.initials: SC
authors.lastname: Fakra
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720 USA.
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Marcus
authors.affiliation: Proteomics and Metabolomics Facility, Colorado State University, Fort Collins, Colorado 80523 USA.
authors.forename: Corey
authors.initials: C
authors.lastname: Broeckling
authors.affiliation: Graduate Degree Program in Ecology, Colorado State University, Fort Collins, Colorado 80523 USA Biology Department, Colorado State University, Fort Collins, Colorado 80523 USA.
authors.forename: Elizabeth A H
authors.initials: EA
authors.lastname: Pilon-Smits
authors.affiliation: Graduate Degree Program in Ecology, Colorado State University, Fort Collins, Colorado 80523 USA Department of Forest and Rangeland Stewardship, Colorado State University, Fort Collins, Colorado 80523 USA.
authors.forename: Mark W
authors.initials: MW
authors.lastname: Paschke
mesh_terms: D027883:Astragalus Plant / Q000378:metabolism* / Q000382:microbiology; D018533:Biomass; D003545:Cysteine / Q000031:analogs & derivatives / Q000378:metabolism; D016566:Organoselenium Compounds / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism / Q000382:microbiology; D055170:Plant Root Nodulation; D018517:Plant Roots / Q000378:metabolism / Q000382:microbiology; D012231:Rhizobium / Q000502:physiology*; D012643:Selenium / Q000378:metabolism*; D012987:Soil; D013559:Symbiosis; D056928:X-Ray Absorption Spectroscopy
chemical_list: D016566:Organoselenium Compounds; D012987:Soil; C424943:glutamyl-Se-methylselenocysteine; D012643:Selenium; D003545:Cysteine
keywords: Astragalus; Fabaceae; hyperaccumulation; legume; nodulation; selenium; symbiosis; x-ray absorption spectroscopy; γ-glutamyl-methylselenocysteine