Strategies to facilitate the bioprospection of properties of agronomic interest in microbial culture collections and a successful case of selection to mitigate drought stress in maize.

backgroundThe replacement of synthetic molecules with biological solutions has been a global goal with high priority in agriculture. Worldwide, there are several well-organized microbial collections holding high biodiversity and biotechnological potential. This study was based on the hypothesis that in vitro analyses can guide the selection of promising strains for subsequent in vivo evaluation. We selected 100 strains representative of the “Diazotrophic and Plant Growth-Promoting Bacteria Culture Collection of Embrapa Soja”, Brazil, to be evaluated in vitro for proteolytic and cellulolytic activities, production of 1-aminociclopropano-1-carboxilato deaminase (ACC-deaminase), siderophore, indolic compounds (indole-3-acetic acid, IAA), exopolysaccharides (EPS), biofilm, solubilization of nutrients, and ability to grow in medium with reduced water activity and high temperature. The 100 strains were also evaluated in a greenhouse on maize growing in sterile substrate to assess their ability to promote tolerance to drought.

resultsHydrolytic and proteolytic activities were highlighted in Paenibacillus, Pantoea, and Bacillus, and ACC-deaminase was widespread in 38 strains of several genera. Tolerance to drought and high temperature (40 ± 2 °C) was highly present in Bacillus. Outstanding results were obtained with Azospirillum for EPS, in Paraburkholderia, Pseudomonas, and Bacillus for biofilm, and in Chromobacterium for IAA. Regarding properties that could putatively help the uptake of nutrients, 30 strains synthesized siderophores, but only seven were able to solubilize calcium phosphate, five of which were classified as Pseudomonas. A high correlation was found between the ability to grow in vitro in medium with reduced water activity and tolerance to drought in vivo. From this initial greenhouse experiment, 15 strains were selected to confirm their potential to mitigate drought in a greenhouse experiment with non-sterile soil. Three outstanding strains, Bacillus velezensis CNPSo 2384, Bacillus subtilis CNPSo 2606, and Bacillus sp. CNPSo 2723 were identified as promising candidates to compose future bio-inputs aimed at increasing plant tolerance to drought.

conclusionSpeeding up strain selection programs is highly relevant, and certain properties of agronomic interest can be found more easily in specific bacterial genera. Additionally, the proof of concept for a preliminary in vitro evaluation was confirmed in vivo for plant tolerance to drought, thereby stimulating the validation of other important microbial properties.