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

Humic acids (HAs) stimulate the growth of several plant species by regulating their hormonal and redox metabolisms. Nevertheless, studies on the relationship of these substances with the plant-associated microbiota are almost nonexistent. Here, we hypothesized that the effect of HAs occurs in parallel with the regulation of the plant-associated bacterial community. Our results show the positive influence of HAs on the growth of rice and its stimulation of the root system. Metataxonomics revealed that the structure and composition of root bacterial communities were affected upon the application of HAs. Chitinophaga and Mucilaginibacter were the predominant genera in HA-treated roots. These bacteria produce enzymes that degrade compounds like those present in the wall of fungi, oomycetes, and nematode eggs. Pseudomonas and the Gp 1 group of Acidobacteria, both siderophore-producers and plant-growth promoters were also enriched, although with lower abundances. Given these results, we suggest that plants recruit these microorganisms in response to the stress caused by the HA-root interaction. For the first time, our findings indicate that HA-stimulated plants 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.