title: Friend or Foe-Light Availability Determines the Relationship between Mycorrhizal Fungi, Rhizobia and Lima Bean (Phaseolus lunatus L.).
abstract: Plant associations with root microbes represent some of the most important symbioses on earth. While often critically promoting plant fitness, nitrogen-fixing rhizobia and arbuscular mycorrhizal fungi (AMF) also demand significant carbohydrate allocation in exchange for key nutrients. Though plants may often compensate for carbon loss, constraints may arise under light limitation when plants cannot extensively increase photosynthesis. Under such conditions, costs for maintaining symbioses may outweigh benefits, turning mutualist microbes into parasites, resulting in reduced plant growth and reproduction. In natural systems plants commonly grow with different symbionts simultaneously which again may interact with each other. This might add complexity to the responses of such multipartite relationships. We experimented with lima bean (Phaseolus lunatus), which efficiently forms associations with both types of root symbionts. We applied full light and low-light to each of four treatments of microbial inoculation. After an incubation period of 14 weeks, we quantified vegetative aboveground and belowground biomass and number and viability of seeds to determine effects of combined inoculant and light treatment on plant fitness. Under light-limited conditions, vegetative and reproductive traits were inhibited in AMF and rhizobia inoculated lima bean plants relative to controls (un-colonized plants). Strikingly, reductions in seed production were most critical in combined treatments with rhizobia x AMF. Our findings suggest microbial root symbionts create additive costs resulting in decreased plant fitness under light-limited conditions.
authors.affiliation: Department of Biology, Portland State University, Portland, Oregon, 97201, United States of America.
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Ballhorn
authors.affiliation: Helmholtz-Centre for Environmental Research, Dept. Community Ecology, 06120, Halle, Germany.|German Centre for Integrative Biodiversity Research Halle-Jena-Leipzig (iDiv), Deutscher Platz 5e, 04103, Leipzig, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Schädler
authors.affiliation: Department of Biology, Portland State University, Portland, Oregon, 97201, United States of America.
authors.forename: Jacob D
authors.initials: JD
authors.lastname: Elias
authors.affiliation: Department of Biology, Portland State University, Portland, Oregon, 97201, United States of America.
authors.forename: Jess A
authors.initials: JA
authors.lastname: Millar
authors.affiliation: Department of Biology, Portland State University, Portland, Oregon, 97201, United States of America.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Kautz
mesh_terms: D008027:Light; D038821:Mycorrhizae / Q000502:physiology*; D027805:Phaseolus / Q000378:metabolism / Q000382:microbiology* / Q000528:radiation effects; D018517:Plant Roots / Q000378:metabolism / Q000382:microbiology / Q000528:radiation effects; D012231:Rhizobium / Q000502:physiology*; D053204:Root Nodules, Plant / Q000378:metabolism / Q000382:microbiology / Q000528:radiation effects; D013559:Symbiosis / Q000528:radiation effects*