Humic acids enrich the plant microbiota with bacterial candidates for the suppression of pathogens

Abstract Humic acids (HA) stimulate the growth of several plant species by regulating their hormonal and redox metabolisms. Nevertheless, studies on their relationship with the plant-associated microbiota are almost nonexistent. Here, we hypothesized that the HA effect occurs in parallel with the regulation of the plant-associated bacterial community. Rice was grown for 264 h in nutrient solution supplied or not with HA (80 mg L−1). Plant growth was evaluated every 24 h and samples for 16S rRNA gene sequencing were taken after 240 h. The plant's net assimilation rate and the absolute and relative growth rates increased from 100 h to 264 h after the first HA application. The root volume and other growth variables were stimulated at different times along the plant growth cycle. Metataxonomics revealed that bacterial community structure and composition were affected upon HA application. The interactions between members of the community increased in HA-treated roots, showing more connected bacterial communities. Chitinophaga and Mucilaginibacter were the predominant genera in HA-treated roots. These bacteria are reported to produce enzymes that degrade compounds present in the wall of fungi, oomycetes, and nematode eggs. Pseudomonas and Acidobacteria Gp 1, both siderophore-producers and plant-growth promoters were also enriched, although with lower abundances. Given these results, we suggest that plants recruited these microorganisms in response to the stress caused by the HA-root interaction. For the first time, our findings indicate that HA-stimulated plants may adopt the ecological strategy of recruiting members of the bacterial community that are candidates for the suppression of pathogens and, therefore, involved in plant defense.