Bioregulation of adventitious root induction by metabolites secreted from plant growth promoting Pantoea agglomerans strains

Adventitious rooting (AR) is a crucial part of the agamic propagation to obtain a high-quality root system. The process is regulated by several factors like hormones, light, and mineral nutrition. Auxins play a central role and activate the pathways involved in the major morphological and molecular changes. Adventitious roots are mainly induced with IBA, a synthetic auxin, but there is interest in finding natural alternatives to this product. This study evaluated the effect of metabolites secreted by Pantoea agglomerans on the induction and initiation phase of AR in vitro in microcuttings of Pyrus communis ‘Dar Gazi’, using IBA as control (CK). P. agglomerans is a plant growth-promoting bacterium, which produces IAA in secreted form when grown in medium supplemented with tryptophan. Tests were performed enriching the rooting medium with secreted metabolites from IAA-producing P. agglomerans cultures to achieve a final auxin amount of 0.2, 0,4 and 1 mg L-1. Explants were sampled after 4 and 7 days from the start of treatment (DAT), between 8 and 12 DAT for Pantoeatreated explants at the time of adventitious root appearance (TARA) and at 35 days for control explants (TARA). The results indicate a different regulation of physiological and phenotypic events generated after bacterial treatment: reduced time of roots appearance, root origin directly from the stem tissue, and absence of callus development, associated with a decrease in root number formation, while resulted longer than those generated in IBA-treated explants. qRT-PCR results of WOX, IAA/Aux, and ARFs genes and miRNA156, 160, 167, confirmed that the bacterial metabolites play a positive role in the induction of the AR process, and that the time scale of gene expression was coherent with the reduction of time of root appearance.