3 years ago

Identification of heterotrophic zinc mobilization processes among bacterial strains isolated from wheat rhizosphere (Triticum aestivum L).

Emmanuel Frossard, Cécile Thonar, Benjamin Costerousse, Laurie Schönholzer-Mauclaire
Soil and plant inoculation with heterotrophic Zinc Solubilizing Bacteria (ZSB) is considered as a promising approach for increasing zinc (Zn) phytoavailability and enhance crop growth and nutritional quality. It is nevertheless necessary to understand the underlying bacterial solubilization processes in order to predict their repeatability in inoculation strategies. Acidification via gluconic acid production remains the most reported process. In this study, wheat rhizosphere soil serial dilutions were plated onto several solid microbiological media supplemented with scarcely soluble Zn oxide (ZnO) and 115 putative Zn solubilizing isolates were directly detected based on the formation of solubilization halo around the colonies. Eight strains were selected based on their Zn solubilization efficiency and siderophore production capacity. These included one Curtobacterium, two Plantibacter, three Pseudomonas, one Stenotrophomonas and one strain of Streptomyces In ZnO liquid solubilization assays, the presence of glucose clearly stimulated organic acid production, leading to media acidification and ZnO solubilization. While solubilization by Streptomyces and Curtobacterium was attributed, respectively, to the accumulated production of seven and eight different organic acids, the other strains solubilized Zn via gluconic, malonic and oxalic acid, exclusively. In contrast, in absence of glucose, ZnO dissolution resulted from protons extrusion (e.g. via ammonia consumption by Plantibacter strains) and complexation processes (i.e. complexation with glutamic acid in cultures of Curtobacterium). Therefore, while gluconic acid production was described as a major Zn solubilization mechanism in literature, this study goes beyond and shows that solubilization mechanisms are various among ZSB and strongly affected by growth conditions.IMPORTANCE Barriers towards a better understanding of the mechanisms underlying Zn solubilization by bacteria include the lack of methodological tools for isolation, discrimination and identification of such organisms. Our study proposes a direct bacterial isolation procedure, which prevent the need of screening numerous bacterial candidates (i.e. which ability to solubilize Zn is unknown) for recovering ZSB. Moreover, we confirm the potential of Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry [MALDI-TOF MS] as a quick and accurate tool for the identification and discrimination of environmental bacterial isolates. This work also describes various Zn solubilization processes used by wheat rhizosphere bacteria, including proton extrusion and production of different organic acids among bacterial strains. These processes were also clearly affected by growth conditions (i.e. solid versus liquid cultures, presence and absence of glucose). Although highlighted mechanisms may have significant effects at the soil-plant interface, these should only be transposed to real ecological situations cautiously.

Publisher URL: http://doi.org/10.1128/AEM.01715-17

DOI: 10.1128/AEM.01715-17

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