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 , two of, three strains of , one of, and one strain of In ZnO liquid solubilization assays, the presence of glucose clearly stimulated organic acid production, leading to medium acidification and ZnO solubilization. While solubilization by and 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 strains) and complexation processes (i.e., complexation with glutamic acid in cultures of ). 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. Barriers toward a better understanding of the mechanisms underlying zinc (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 prevents the need to screen numerous bacterial candidates (for which the ability to solubilize Zn is unknown) for recovering Zn-solubilizing bacteria (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 the production of different organic acids among bacterial strains. These processes were also clearly affected by growth conditions (i.e., solid versus liquid cultures and the presence and absence of glucose). Although highlighted mechanisms may have significant effects at the soil-plant interface, these should only be transposed cautiously to real ecological situations.

Plantibacter flavus, Curtobacterium herbarum, Paenibacillus taichungensis, and Rhizobium selenitireducens Endophytes Provide Host-Specific Growth Promotion of Arabidopsis thaliana, Basil, Lettuce, and Bok Choy Plants

“…These results provide evidence of direct plant growth by a Plantibacter species. Bacteria of the genus Plantibacter have been found in association with a variety of plants, including the phyllosphere of grasses (35), the endosphere of yarrow, goldenrod, dactylis, and clover (32), and the rhizosphere of wheat (36), maple sap (37), and rye flakes (38). Strains of Plantibacter have been seen to solubilize zinc in soil (35).…”

Rhizosphere microbiome: revisiting the synergy of plant-microbe interactions

“…Fluorescent pseudomonads have been found to promote iron nutrition via siderophores for Graminaceous as well as dicotyledonous plant species (Shirley et al 2011). Rhizosphere microorganisms (Curtobacterium, Plantibacter, Pseudomonas, Stenotrophomonas, Streptomyces) are also known to mobilize zinc (Zn) by acidification of medium via gluconic acid production (Costerousse et al 2018;Whiting et al 2001).…”

Possible Roles of Rhizospheric and Endophytic Microbes to Provide a Safe and Affordable Means of Crop Biofortification

“…Recently, the discovery of Zn nanoparticles from the cell-free culture filtrates of Pseudomonas, Bacillus, and Azospirillum strains suggests that these microbes may solubilize Zn by producing nanoparticles to mobilize nutrients in the rhizosphere [94]. Eight strains from the genera Curtobacterium, Plantibacter, Pseudomonas, Stenotrophomonas, and Streptomyces have been successfully identified from the wheat rhizosphere to be highly efficient in Zn solubilization as well as siderophore production [95]. A high level of mobilization of Cu and Zn was observed in spinach (Spinacia oleracea L.) and tomato (Lycopersicon esculentum L.), which indicates that the release of organic ligands improves the solubility of trace elements in soil.…”

“…Instead, the mechanism employed changes depending on the situation. For example, the presence of glucose in the liquid medium stimulated organic acid (such as gluconic, malonic, and oxalic acid) production by eight Zn-solubilizing bacterial strains [95], leading to the acidification of the medium and subsequent ZnO solubilization. In contrast, in the absence of glucose, ZnO dissolution resulted from proton extrusion (e.g., via ammonia consumption by Plantibacter strains) and complexation processes (e.g., complexation with glutamic acid in the culture of Curtobacterium sp.).…”