The Influence of Plant Growth-Promoting Bacteria and Humic Substances on the Rooting of Black Poplar (Populus nigra L.) Cuttings

Plants of the genus Populus are among the most economically important woody plants and are an experimental model system used to study woody species. We studied the ability of plant growth-promoting bacteria (PGPR) and humic substances (HSs) to influence the rooting of cuttings, a process that plays an important role in vegetative propagation. We used strains of bacteria from the collection of microorganisms of the Ufa Institute of Biology: <i>Pseudomonas protegens</i> DA1.2 and <i>Enterobacter ludwigii</i> BLK. For the experiment, cuttings of black poplar (<i>Pópulus nigra</i> L.) were used. They were placed in aqueous solution with either the addition of bacteria, HSs, or their combination. After 15 days, the number and length of adventitious roots were measured, and they were sampled for hormone immunoassay analysis and the determination of gene expression. The contents of the auxin indole-3-acetic acid (IAA) and abscisic acid (ABA) were measured, since both substances have an antagonistic effect on rooting. It was shown that the activation of root formation was due, on the one hand, to increased auxin levels under the influence of IAA-producing bacteria, resulting in increased expression of the <i>PnRGF9</i> gene that encodes the root meristem growth factor. On the other hand, the activation of rooting was due to a decrease in ABA content resulting from its breakdown by bacteria of the DA1.2 strain, which catabolize this hormone. HSs, which can inhibit ABA synthesis, also played a role. As a result, the preparation for the rooting of black poplar cuttings with the greatest effectiveness was observed in the combined use of DA1.2 bacteria and HSs.