Systemic induction of secondary metabolites biosynthesis in medicinal aromatic plants mediated by rhizobacteria

Abstract

Plant growth-promoting rhizobacteria (PGPR) are increasingly accepted and important in agricultural production worldwide. There is rising demand for food produced without synthetic chemical products, for sustainable agricultural methods, and for a holistic vision of development associated with environmental protection. In this study, we found that the effects of PGPR inoculation and volatile organic compound (VOC) emission on various aromatic plant species (Origanum majorana [sweet marjoram], Origanum x majoricum [Italian oregano], Ocimum basilicum [sweet basil], Tagetes minuta [wild marigold], Mentha x piperita [peppermint]) varied depending on the inoculated strain (Pseudomonas fluorescens WCS417r, Azospirillum brasilense Sp7, Bacillus subtilis GB03, or their combination). PGPR-inoculated plant species display host response specificity. The responses we observed were diverse. In most cases, the growth parameters evaluated (leaf number, shoot fresh weight, root dry weight) were significantly increased by P. fluorescens inoculation. Essential oil (EO) yield was increased to varying degrees in O. majorana, O. x majoricum, and T. minuta inoculated with P. fluorescens. Monoterpene accumulation was increased ~2-fold in most cases, and 24-fold in O. majorana. The effects of VOCs emitted by P. fluorescens evaluated in M. piperita showed the same tendency that direct inoculation. In contrast, inoculation with B. subtilis showed different effects upon the affected plant: O. majoran and O. majoricum did not show any change in EO yield in contrast with O. basilicum. The same result was observed with the exposition to B. subtilis VOCs in O. basilicum but in M. piperita did not affect the total monoterpenes accumulation. Total EO yield was increased by direct root inoculation with A. brasilense in O. x majoricum but not in T. minuta. Our findings indicate that inoculation with certain PGPR causes systemic induction of monoterpene pathways in various aromatic plants species, suggesting that PGPR inoculation can significantly increase productivity and reduce the amount of fertilizer required for economically viable aromatic crop production.