Abstract
The global decrease in soil fertility leads to a new agricultural scenario where eco-friendly solutions play an important role. The plant growth promotion through the use of microbes, especially endophytes and rhizosphere microbiota, has been proposed as a useful solution. Several studies have shown that humic substances are suitable vehicles for the inoculation of plant growth promoting bacteria, and that this combination has an enhanced effect on the stimulation of plant development. In this work, cucumber plants grown hydroponically have been pre-treated with a sedimentary humic acid (SHA) with known plant growth-enhancing effects, and culturable bacterial endophytes have been isolated from these plants. The hypothesis was that this pre-treatment with SHA could lead to the isolation of certain endophytic taxa whose proliferation within the plant could have been promoted as a result of the effects of the treatment with SHA, and that could eventually reinforce a potential synergistic effect of a combined application of those endophytic bacteria and SHA. The culturable endophytes that have been isolated from humic acid-treated cucumber plants have been identified as members of four main phyla: Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes. Isolates were characterized according to the following plant growth-promoting traits: nitrogen fixation/scavenging, phosphate solubilization, siderophore production and plant hormone production. Most of the isolates were able to fix/scavenge nitrogen and to produce plant hormones (indole-3-acetic acid and several cytokinins), whereas few isolates were able to solubilize phosphate and/or produce siderophores. The most promising endophyte isolates for its use in futures investigations as plant growth-promoting bacterial inocula were Pseudomonas sp. strains (that showed all traits), Sphingomonas sp., Stenotrophomonas sp. strains, or some Arthrobacter sp. and Microbacterium sp. isolates.
Highlights
In the last decades the human population has grown exponentially, reaching 7,600 million people in 2018, and as the Food and Agriculture Organization (FAO) has predicted, in 2050 the world population will be near to 10,000 million1
The number of viable endophytic bacterial isolates obtained from three different plants of cucumber previously grown in the presence of 100 ppm C sedimentary humic acid (SHA) in the nutrient solution was 72
Our results showed that the isolated endophytic bacteria produced mainly iP, and small amounts of cZ, while iPR was transformed/consumed from the medium
Summary
In the last decades the human population has grown exponentially, reaching 7,600 million people in 2018, and as the Food and Agriculture Organization (FAO) has predicted, in 2050 the world population will be near to 10,000 million1 This fact involves an increasing pressure over global food production and the land surface dedicated to that purpose. One approach is the use of biofertilizers containing living microorganisms (Dastager et al, 2010; Bhardwaj et al, 2014; Canellas et al, 2015; Pérez et al, 2016; Suhag, 2016; Dias et al, 2017) This strategy has recently gained relevance with the development of a new generation of gene sequencing techniques, which have allowed the assessment of microbe-plant relationships and the development of a new evolutionary model, the holobiontic theory (Rosenberg and Zilber-Rosenberg, 2016). This model proposes that microbiota would evolve over time to improve the fitness of the plant under changing environmental conditions such as drought, salinity, nutrient deficiency, or soil contamination (Murphy et al, 2015; Fidalgo et al, 2016; Soussi et al, 2016; Kumar et al, 2017)
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