فرارهای میکروبی به عنوان القا کننده های رشد گیاه Microbial volatiles as plant growth inducers

1. Introduction Currently, the high demand for food and the need for increasing both performance and quality of agricultural crops have led to the applications of large amounts of chemical products (i.e. mineral fertilizer and commercial phytohormones), which have been used primarily to increase nutrient availability and stimulate the growth of species grown under field and greenhouse conditions, respectively (Zaman et al., 2015). Nevertheless, their applications have caused serious environmental problems, resulting in loss of soil biological activity, erosion derived from runoff, and leaching from spray components of these products (Savci, 2012). In addition, the synthetic compounds applied in greenhouse conditions have caused food contamination associated with toxic substance accumulation (e.g. nitrosamine compounds in lettuce) (Ward, 2009). Therefore, the search for sustainable alternatives has been carried out in order to reduce the input of chemical products in crops and to produce chemical-free food, so rhizosphere microorganisms have emerged as potential growth inducers. Microorganisms, both bacteria and fungi, are found in high quantity and wide diversity in the rhizosphere zone, defined as “the narrow zone influenced by plant roots and characterized by their intense association with microbial activity” (Mendes et al., 2013; Dessaux et al., 2016; Van Dam and Bouwmeester, 2016). These microorganisms utilize root exudates, which contain ions, oxygen, water, enzymes, mucilage, and primary and secondary metabolites, representing between 20–40% of fixed carbon located in the underground root system (Philippot et al., 2013; Venturi and Keel, 2016). The plant exudates can determine or modify the microbial community along the root system (Badri et al., 2009). Meanwhile, microorganisms secrete diverse non volatile metabolites with beneficial effects to induce plant growth through direct and indirect pathways, which constitutes a traditional mechanisms studied to date (Dotaniya and Meena, 2015). Several studies conducted in the last decades indicate that direct pathways involve the release of phytohormones (i.e auxin, ethylene, and cytokinins) and organic substances (i.e organic acids) that contribute to growth stimulation and nutrient availability, respectively. Indirect pathways comprise substances that prevent pathogens attack through the production of hydrolytic enzymes, antibiotics, siderophores, and hydrogen cyanide (Goswami et al., 2016; Vejan et al., 2016). However, a new mechanism mediated by volatile organic compounds (VOCs) was reported for the first time by Ryu et al. (2003), who showed that volatiles released by Bacillus subtilis GB03 induced growth on Arabidopsis thaliana, being the first evidence that volatile organic compounds can modulate growth, stress, nutrition, and health processes in plants. To date, studies have achieved considerable progress in elucidating the mode of action of this type of compounds; however, it is still poorly understood.