Evaluation of MALDI-TOF mass spectrometry for the competitiveness analysis of selected indigenous cowpea (Vigna unguiculata L. Walp.) Bradyrhizobium strains from Kenya
Cowpea N2 fixation and yield can be enhanced by selecting competitive and efficient indigenous rhizobia. Strains from contrasting agro-ecologies of Kilifi and Mbeere (Kenya) were screened. Two pot experiments were established consisting of 13 Bradyrhizobium strains; experiment 1 (11 Mbeere + CBA + BK1 from Burkina Faso), experiment 2 (12 Kilifi + CBA). Symbiotic effectiveness was assessed (shoot biomass, SPAD index and N uptake). Nodule occupancy of 13 simultaneously co-inoculated strains in each experiment was analyzed by matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry (MS) to assess competitiveness. Strains varied in effectiveness and competitiveness. The four most efficient strains were further evaluated in a field trial in Mbeere during the 2014 short rains. Strains from bacteroids of cowpea nodules from pot and field experiments were accurately identified as Bradyrhizobium by MALDI-TOF based on the SARAMIS™ database. In the field, abundant indigenous populations 7.10 × 103 rhizobia g−1 soil, outcompeted introduced strains. As revealed by MALDI-TOF, indigenous strains clustered into six distinct groups (I, II, III, IV, V and VI), group III were most abundant occupying 80% of nodules analyzed. MALDI-TOF was rapid, affordable and reliable to identify Bradyrhizobium strains directly from nodule suspensions in competition pot assays and in the field with abundant indigenous strains thus, its suitability for future competition assays. Evaluating strain competitiveness and then symbiotic efficacy is proposed in bioprospecting for potential cowpea inoculant strains.
KeywordsBradyrhizobium Cowpea Symbiotic effectiveness Nodule occupancy Protein profile Bacteroid
The authors thank Dr. Laurie Paule Schönholzer, Dr. Seher Bahar Aciksöz Özden, Monika Macsai, Carla Mosimann, Silvana Niedermann and Sämi Bickel for the technical support during experiments and Dr. Federica Tamburini for N analysis. Winnie Kimutai and Silas Kiragu are acknowledged for their support during field trials. Thanks to Dr. Abidine Traore for the cowpea nodules from which strain BK1 was isolated, and MEA for facilitating Biofix inoculant from which CBA (CB 1015) strain was isolated. Valuable input by the anonymous reviewers and the editor for improving this manuscript is also acknowledged.
This research was supported by funding from ETH Zurich Engineering for Development (E4D) scholarship through the Sawiris Foundation for Social development (Grant number 2-71060-13).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
This article does not contain any studies with human participants or animals performed by any of the authors.
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