title: Selenium hyperaccumulation by Astragalus (Fabaceae) does not inhibit root nodule symbiosis.
abstract: OBJECTIVE
  A survey of the root-nodule symbiosis in Astragalus and its interaction with selenium (Se) has not been conducted before. Such studies can provide insight into how edaphic conditions modify symbiotic interactions and influence partner coevolution. In this paper plant-organ Se concentration ([Se]) was investigated to assess potential Se exposure to endophytes. •
  
  
  METHODS
  Selenium distribution and molecular speciation of root nodules from Se-hyperaccumulators Astragalus bisulcatus, A. praelongus, and A. racemosus was determined by Se K-edge x-ray absorption spectroscopy. A series of greenhouse experiments were conducted to characterize the response of root-nodule symbiosis in Se-hyperaccumulators and nonhyperaccumulators. •
  
  
  RESULTS
  Nodules in three Se-hyperaccumulators (Astragalus crotalariae, A. praelongus, and A. preussii) are reported for the first time. Leaves, flowers, and fruits from Se-hyperaccumulators were routinely above the hyperaccumulator threshold (1,000 µg Se g(-1) DW), but root samples rarely contained that amount, and nodules never exceeded 110 µg Se g(-1) DW. Nodules from A. bisulcatus, A. praelongus, and A. racemosus had Se throughout, with a majority stored in C-Se-C form. Finally, an evaluation of nodulation in Se-hyperaccumulators and nonhyperaccumulators indicated that there was no nodulation inhibition because of plant Se tolerance. Rather, we found that in Se-hyperaccumulators higher levels of Se treatment (up to 100 µM Se) corresponded with higher nodule counts, indicating a potential role for dinitrogen fixation in Se-hyperaccumulation. The effect was not found in nonhyperaccumulators. •
  
  
  CONCLUSIONS
  As the evolution of Se hyperaccumulation in Astragalus developed, root-nodule symbiosis may have played an integral role.
authors.affiliation: Graduate Degree Program in Ecology, Colorado State University, Fort Collins, Colorado 80523, USA.
authors.forename: Elan R
authors.initials: ER
authors.lastname: Alford
authors.forename: Elizabeth A H
authors.initials: EA
authors.lastname: Pilon-Smits
authors.forename: Sirine C
authors.initials: SC
authors.lastname: Fakra
authors.forename: Mark W
authors.initials: MW
authors.lastname: Paschke
mesh_terms: D001130:Arizona; D027883:Astragalus Plant / Q000187:drug effects / Q000254:growth & development / Q000382:microbiology / Q000502:physiology*; D001419:Bacteria / Q000187:drug effects / Q000378:metabolism; D003120:Colorado; D060026:Endophytes / Q000187:drug effects / Q000378:metabolism; D007887:Fabaceae / Q000187:drug effects / Q000254:growth & development / Q000382:microbiology / Q000502:physiology; D009586:Nitrogen Fixation / Q000187:drug effects; D018517:Plant Roots / Q000187:drug effects / Q000378:metabolism / Q000382:microbiology; D012643:Selenium / Q000494:pharmacology*; D012987:Soil / Q000737:chemistry*; D013045:Species Specificity; D013559:Symbiosis / Q000187:drug effects*; D014861:Washington; D056928:X-Ray Absorption Spectroscopy
chemical_list: D012987:Soil; D012643:Selenium