Culturable bacterial endophytes from sedimentary humic acid treated plants: Potential use in plant growth promotion.. PGP endophytes SHA-cucumber

The global decrease in soil fertility leads to a new agricultural scenario where eco-friendly solutionsplay an essential role. The plant growth promotion through the use of microbes, especiallyendophytes and rhizosphere microbiota, has been proposed as a useful solution. The main plant growth-promoting features of these microorganisms suggest an effect in plant fitness similar to the effectproduced by humic substances. In this work, a sedimentary humic acid from leonardite has been usedto promote the growth of cucumber grown in a hydroponic system. The pre-treatment with this sortof humic substances has been suggested to modify specific endophytic taxa; because of that isbelieved that it may exist a complementarity between endophytes and humic substances actions. Inthis work, cultivable endophytes have been isolated and identified as members of four main phyla:Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes. Isolates were characterized accordingto some plant growth-promoting traits: nitrogen fixation/scavenge, phosphate solubilization,siderophore production, and plant hormone production (IAA and Cks). Most of the isolates showed plantgrowth-promoting traits, being the most common the nitrogen fixation/scavenge and plant hormoneproduction (both, IAA and Cks). Only a few isolates were able to solubilize phosphate and/or producesiderophores. The most promising isolates for future use as inoculum were Pseudomonas sp. strains,that showed all traits, but also Sphingomonas sp. strains or some Arthrobacter sp. andMicrobacterium sp. isolates were found as good plant growth promoters. That result suggests that combined use of humic substances and endophytes could improve plant growth due to the increase inmineral nutrient acquisition and the stimulation of plant growth with hormonal regulators. Futureworks must be done to study in-depth the combined use of sedimentary humic acids and endophytesin order to develop new agricultural solutions to soil fertility decrease.