## 1 Introduction  
The lack of an appropriate level of water is one of the most disturbing environmental features affecting crop yield. It influences plant health by modifying multiple factors related to the metabolism of the plant, generating osmotic and oxidative stress, and ultimately causing the death of the plant due to the increase of high levels of reactive oxygen species (ROS). In addition, plant specialized metabolites can be altered by biotic or abiotic stress. The effect of deficiency of water, in aromatic and medicinal plants, may cause in some cases positive effects on secondary metabolite yield and composition [1]. Although the quantity and components of the EOs differ among regions, as a result of differences in climate, soil composition, agronomic conditions, harvesting season, essential oil processing technique and the drying system applied [2], water deficit is found to be the main stress disturbing numerous processes in the plant chemistry [3].  
The essential oil (EO) production of aromatic plants is significantly connected with the glandular trichomes, which are specialized foliar organs where oil constituents are produced and stored [4]. Furthermore, phytohormones have been found to positively affect the amounts and flavor of EO, especially JAsmonic acid (JA), which control the biosynthesis of secondary metabolites, particularly terpenoids, and regulates secondary metabolite biosynthetic pathways [5, 6].  
_Mentha piperta_ is an aromatic plant from the Lamiaceae family and is of economic interest due to the production essential oils (EOs) and phenolic compounds [2]. It has long been consumed in traditional medicine for the treatment of several diseases, including gastrointestinal infectious and respiratory disease, such as anti-congestive and expectorants and as anti-spasmodic on the digestive and vascular systems [7, 8, 9]. Furthermore, it is currently one of the most important crops from the economic and pharmaceutical points of view. Its EOs have been shown to be insecticidal and demonstrate antimicrobial efficacy against bacterial and fungal plant pathogens [8].  
Plant growth promoting rhizobacteria (PGPR) are soil bacteria that in addition to favoring plants growth in some cases, are able to decrease the occurrence or intensity of plant diseases by the induction of systemic resistance (ISR), a resistance mechanism that generates structural or functional modifications related with plant defense [10; 11; 12]. Moreover, in the last few years, it has been described that certain PGPR enhance the tolerance to water deficiency stress [13; 14]. Our previous studies have shown that the inoculation of peppermint plants subjected to drought stress has the ability to improve various parameters of interest, such as the fresh biomass. In addition, drought effects were ameliorated by the reduction of MDA (malonyldialdehydedehyde) levels, thus avoiding the accumulation of ROS and increasing antioxidant enzyme activities and the antioxidant level, specifically in relation to the total phenolic compounds [14].  
Considering that the inoculation with PGPR promotes biomass and secondary metabolite biosynthesis, and taking into account that drought stress could also increase their production, we determined the effects of the application of both these conditions on EO yield. Thus, the present survey was designed to analyze the effect of PGPR inoculation on the biosynthesis and accumulation of EOs and in particular their emission in _M. piperita_. Additionally, we determined the amount of endogenous plant hormones and gene expression of enzymes implicated in the protein synthesis leading to bioactive terpen, in plants subjected to drought stress and PGPR, to try to get a deeper understanding of how the influence of drought stress and PGPR inoculation in combination modifies secondary metabolite biosynthesis in _M. piperita_.