Identification of Heterotrophic Zinc Mobilization Processes among Bacterial Strains Isolated from Wheat Rhizosphere (Triticum aestivum L.)

Abstract

Soil and plant inoculation with heterotrophic zinc-solubilizing bacteria (ZSB) is considered a promising approach for increasing zinc (Zn) phytoavailability and enhancing crop growth and nutritional quality. Nevertheless, it is necessary to understand the underlying bacterial solubilization processes 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 on 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 halos around the colonies. Eight strains were selected based on their Zn solubilization efficiency and siderophore production capacity. These included one strain of Curtobacterium, two of Plantibacter, three strains of Pseudomonas, one of Stenotrophomonas, and one strain of Streptomyces. In ZnO liquid solubilization assays, the presence of glucose clearly stimulated organic acid production, leading to medium acidification and ZnO solubilization. While solubilization by Streptomyces and Curtobacterium was attributed to the accumulated production of six and seven different organic acids, respectively, the other strains solubilized Zn via gluconic, malonic, and oxalic acids exclusively. In contrast, in the absence of glucose, ZnO dissolution resulted from proton 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 the literature, this study goes beyond and shows that solubilization mechanisms vary among ZSB and are strongly affected by growth conditions.