## 1 Introduction  
In nature, plants produce a large plethora of organic bioactive compounds. Several of them that do not directly affect life's fundamental processes are referred as secondary metabolites (SMs) [1]. Plant SMs are widely used as a natural chemical source in pharmaceutics, cosmetics and nutraceuticalals, and there is an increased interest in the cultivation of plant species devoted to production of bioactivecompounds [2]. However, the optimization of agronomic management strategies for secondary metabolite accumulation in nutraceutical species is still challenging [1; 3]. Different studies reported that many environmental factors might affect the production and the accumulation of SMs in plants [1]. Plant SMs are often compounds that have no fundamental function in the maintenance of life processes in the plants, but they are important to interact with the environment for adaptation and defense [4; 5]. For example, several SMs are produced and modulated to allow adaptation of plants to the changing environment, thus mitigating the effects of biotic, i.e. herbivores and pathogens, and abiotic stresses [4; 5; 6] Among these, water deficit is one of the most significant stresses that plants face during their development, dramatically affecting growth and crop yield. It was indicated that, when exposed to drought, plants, in addition to molecules with a role in plant stress response such as the abscisic acid (ABA), accumulate higher concentrations of SMs, suggesting that water stress can be an easy-to-use elicitor of bioactive compounds [4]. On the other hand, bioaccumulation of SMs can be also achieved by application of exogenous plant growth regulators such as phytohormones. As enhancers of SM biosynthesis, synthetic phytohormones have been successfully used as elicitors to stimulate the production of terpen, phenols and alkaloids [7; 8]. In this context, one of the used plant growth regulators is ethephon (2-chloroethylphosphonic acid), a precursor of the phytohormone ethylene, which directly influences different physiological plant processes, such as senescence abscission, floral transition, fruit ripening and maturation [9]. The use of ethephon to influence SM accumulation was so far demonstrated for _Coffea arabica_ and _Thalictrum rugosum_[10]. In the last decade, it has been often reported that also the establishment of symbiosis with arbuscular mycorrhizal (AM) fungi, which colonize plant roots and help their host plants in nutrient uptake while in return receive essential carbon compounds [11], may also induce changes in SM content in different plant tissues [12; 13; 14; 15; 16; 17; 18]. The real application of root-associated microorganisms, such as AM fungi, is currently considered to have an enormous potential in the frame of innovative and sustainable agriculture, providing benefits to plants not only by enhancing their nutrition and conferring tolerance to abiotic and biotic threats [19; 20], but also by increasing the amount of useful SMs produced by host plant [21]. In this work, attention was focused on the production of one of the main SMs produced by _Trigonella fonenum-gracum_ (fenugreek), i.e., trigonelline. Fenugreek belongs to the Fabaceae family and is one of the species that is gaining attention by drug research and industry because of its nutraceuticalal and pharmaceutical properties. It is indeed one of the oldest medicinal leguminous plants grown and commercialized world-wide, showing resilience to semi-arid conditions [22; 23; 24]. Its seeds contain a plethora of bioactive compounds with high pharmaceutical value, e.g., the saponin diosgenine and the pyridine alkaloid trigonelline. Particularly, the latter may have a role in the treatment of many different human diseases, for instance diabetes, due to its hypoglyemic, neuroprotective, anti-invasive, estrogenic and antibacterial properties [25]. It is found in many plants, like coffee beans, and in significant quantity in fenugreek seeds [26].