Exogenous application with plant growth promoting rhizobacteria (PGPR) or proline induces stress tolerance in basil plants (Ocimum basilicum L.) exposed to water stress

In aromatic plants, growth and essential oil production are affected by various environmental factors, such as water stress (Mahajan and Tuteja, 2005). Basil (Ocimum basilicum L.) is an annual plant belongs to the Lamiaceae family which has been grown for its essential oil. The essential oil of basil is used to flavor foods, dental and oral products in fragrances and in medicines. The essential oil content varies between 0.2-5.2 percent. (Penuelas and Munne-Bosch, 2005). Drought, one of the environmental stresses, is the most significant factor restricting plant growth and crop productivity in the majority of agricultural fields of the world (Tas and Tas, 2007). The drought phenomenon is a chemical - physical complex, intervene in the organization of a number of large and small bio-molecules, such as nucleic acids, proteins, carbohydrates, fatty acids, hormones, ions, and nutrients (Chaves et al., 2003; Dhanda et al., 2004). Khalid (2006) reported that fresh and dry weights of Ocimum sp. were significantly decreased due to water stress application. Meanwhile, essential oil percentage, proline and sugar content increased while nutrient content decreased. Fresh and dry weights of Ocimum basilicum L. decreased as plant water deficit increased (Simon et al., 1992). The essential oil yield and proline contents of basil (Ocimum sp.) increased by subjecting plants to water stress just before harvesting (Baeck et al., 2001). Plant tissues exposed to environments with water deficit have generally shown reduction in cell size, and increase in vascular tissue and cell wall thickness (Pitman et al., 1983; Guerfel et al., 2009). Drought stress increases the formation of reactive oxygen species (ROS) such as H O (hydrogen 2 2 peroxide), O2– (superoxide) and OH- (hydroxyl) radicals. Excessive ROS reduction can cause oxidative stress, which damages plants by oxidizing photosynthetic pigments, membrane lipids, proteins and nucleic acids. The accumulation of stress metabolites like poly-sugars, proline, glycinebetaine (GB), abscicic acid (ABA) as well as up-regulation in synthesis of enzymatic and non-enzymatic antioxidants like superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), ascorbic acid (AA), α-tocopherol and glutathione are some of the significant biochemical responses in higher plants under water stress (Ramachandra Reddy et al., 2004; Valliyodan and Nguyen, 2006; Cattivelli et al., 2008). A high degree of tolerance to drought stress is offered by constitutive anatomical features which are stable in higher plants and hence act as better-observed indicators (Rhizopoulou and Psaras, 2003; Kulkarniet al., 2008). comprehensive understanding of water stress tolerance mechanisms in higher plants, aspects of physiology and cellular biochemistry should be investigated in combination with morpho-anatomical traits in order to find out the subtle links leading to better drought resistance. Such integrated traits, expressed at a higher level of organization are suggested to be quintessential in crop improvement programs. 

Proline, a multifunctional amino acid that besides acting as an excellent osmolyte is also known for stabilizing sub-cellular structures such as proteins and cell membranes, scavenging free radicals, balancing cellular homeostasis and signaling events and buffering redox potential under stress conditions (Szabados and Savoure, 2009). 

Plant growth promoting rhizobacteria (PGPR) are naturally soil bacteria that aggressively colonize plant roots and benefit plants by providing growth promotion. PGPR are reported to influence the growth, yield, and nutrient uptake by an array of mechanisms. Some bacterial strains directly regulate plant physiology by mimicking synthesis of plant hormones, whereas others increase mineral and nitrogen availability in the soil as a way to augment growth (Yasmin et al., 2007). Some PGPR also elicit physical or chemical changes related to plant defense, a process referred to as ‘induced systemic resistance’ (ISR) (van Loon et al., 1998). However, fewer reports have been published on PGPR as elicitors of tolerance to abiotic stresses, such as drought. 

Several strategies have been proposed to alleviate the degree of cellular damage caused by water stress and to improve crop tolerance. Among them, exogenous application of plant growth promoting rhizobacteria (PGPR) or compatible osmolytes such as proline, glycinebetaine, trehalose, etc., had gained considerable attention in mitigating the effect of stress (Ashraf and Foolad, 2007; Zahedi and Abbasi, 2015). There for, the aim of this study was to explore effects of PGPR or proline on drought tolerance in basil and to determine the interactive impacts of water stress, PGPR or proline on growth, anatomical features, oil yield and WUE in addition to osmotic components, antioxidant system and their possible role in reducing water deficit in basil plants.