Plant growth is usually limited by biotic and abiotic stresses. Abiotic stress includes various environmental stresses, such as drought, temperature, salinity, air pollution, heavy metals, pesticides and soil pH. Biotic refers to living organisms that cause diseases, such as bacterial and fungal pathogens, pests, insects, viruses, and nematodes. Symbiotic relationships with endophytes and mycorrhizal fungi have been shown to increase stress tolerance in host plants (Gibert and Hazard 2011).

Abiotic Stress Tolerance

Drought is one of the most wide spread and common abiotic stresses and causes economically important losses in agriculture and forestry crops every year. The mutualistic symbiosis between bacterial or fungal endophytes or AM fungi and host plants could enhance host plant drought tolerance. For example, Japanese Bitter Orange (Poncirus trifoliate) seedlings

inoculated with AM fungus Glomus mosseae enhanced plant height and

increased relative water and chlorophyll contents when seedlings were subjected to three days of water depletion (Fan and Liu 2011). Similar results were observed when AM fungus inoculated rice plants were under drought conditions, with increased levels of protective compounds, such as ascorbate and proline, produced in the plants (Ruiz-Sanchez et al. 2011). The evergreen tree Theobroma cacao infected with the endophytic fungus Trichoderma hamatum isolate DIS 219b exhibited delayed drought stress by changes in stomatal conductance, water potential, and net photosynthesis (Bae et al. 2009).

In grasses, endophytic associations also increased drought tolerance as some accessions of the perennial ryegrass (Lolium perenne) infected by N. lolli showed more tillers, greater tiller length and higher biomass than non-infected plants (Kane 2011). Endophytic inoculation of Epichloe festucae in Fetusca eskia enhanced seedling survival under drought conditions (Gibert and Hazard 2011). A perennial bunchgrass Achnatherum sibiricum infected with endophytic fungi showed a higher root/shoot ratio and net photosynthetic rate than non-inoculated plants under drought conditions (Han et al. 2011). The symbiosis between Agrotis hyemalis and Epichloe amarillans produced 40% more inflorescences, earlier flowering and greater seed mass than non-inoculated plants under drought conditions (Davitt et al. 2011). However, when Panicum rigidulum plants were subjected to drought conditions, endophyte Balansia benningsiana infected plants did not show any advantages over control plants during drought stress but endophyte infection helped rapid leaf regrowth during recovery (Ren and Clay 2009).

Cultivated soils are becoming more saline due to excessive fertilizer use, the use of wastewater from urban and peri-urban areas and agricultural drainage as well as the desertification processes (Bashan and de-Bashan

2010) . Plant growth promoting bacteria offer the potential to reduce the impact of this stress. For instance, cucumber plants inoculated with Paecilomyces formosus showed increased shoot length compared with that of non-inoculated plants under high salinity conditions (Khan et al. 2012). In studies with Salicornia brachiata, the most salt-tolerant plant species among Salicornia spp., Brachybacterium saurashtrense and Pseudomonas sp. bacterial endophytes significantly increased plant growth under salt stress conditions. The bacteria Pseudomonas putida and P. pseudialcaligens inoculation increased plant growth of chickpeas under saline conditions in pot experiments (Patel et al. 2012). Inoculation of AM fungi Glomus mosseae, G. deserticola and Gigaspora gergaria enhanced the growth of wheat (Triticum aestivium) under high salinity conditions as well as increased nutrient uptake (potassium, nitrogen, phosphorus and magnesium), proline levels, acid and alkaline phosphatase activities, and total soluble protein content (Abdel-Fattah and Asrar 2012).

Phytoremediation is the process by which plants can uptake, accumulate, or metabolize toxic compounds, such as heavy metals, from contaminated soil (Kumar et al. 1995). The plant-endophyte association has been used at phytoremediation sites to degrade toxic compounds for practical use (Van Aken et al. 2004). Brassica juncea inoculated with a plant growth promoting bacterium strain A3R3 showed increased plant growth when grown in soil at different concentrations of nickel, with the increases of fresh and dry weights by 50 and 45%, respectively at 450 mg nickel/kg soil compared with non-inoculated plants (Ma et al. 2011). Many plant growth — promoting endophytes could alleviate plant stress from contaminants by degrading such contaminants, and in return, could provide the products for plant use (Weyens et al. 2009a, b). For phytoremediation of toxic metals, endophytes may have a metal-resistant or sequestration system and could reduce metal toxicity and influence metal translocation to the aboveground plant parts. Metal-resistant endophytic bacteria have been found in the genera Pseudomonas, Methylobacterium, Microbacterium and Burkholderia. In tall fescue (Lolium arundinaceum) grown under greenhouse conditions in a solution contaminated with cadmium, endophytic fungus (Neotyphodium coenophialum) infection enhanced cadmium accumulation and increased cadmium transport from roots to the shoots (Ren and Gao 2011). In Festuca arundinacea and Festuca pratensis grasses, grown under high cadmium conditions, results showed higher biomass production and higher levels of cadmium accumulation in the roots and shoots of endophyte-infected plants versus uninfected plants (Soleimani et al. 2010). Under greenhouse conditions, the seedlings of guinea grass (Panicum maximum) cultivars inoculated with Pantoea spp. Jp3-3 exhibited significant alleviation from the negative effect caused by the stress of 300 pM copper (Huo et al. 2012). Switchgrass and two other grasses, bahia grass (Paspalum notatum) and Johnson grass (Sorghum halepenese), were inoculated with two AM fungi, Glomus mosseae and G. intraradices, and results showed that the aboveground biomass of these three grasses contained 26.3 to 71.7% of the total amount of 137Cs, and 23.8-88.7% of the total amount of 90Sr (Entry et al. 1999). The proportion of contaminant removal from the soils by these plant species was significantly increased, possibly due to root colonization by mycorrhizal fungi and the high density of roots (Entry et al. 1999).

AM fungi also have the ability to boost switchgrass plant growth under acidic soils. Of the AM fungi tested, Glomus clarum and G. diaphanum aided to increase the dry matter of plants on soils at pHca 4 and pHca 5 compared with the non-inoculated plants (Clark et al. 1999a). The benefits of AM fungi could be attributed to an increase in acquisition of mineral nutrients such as phosphorus and a decrease of the toxic elements ferrous, boron, aluminum and manganese (Clark et al. 1999b), which are present in acidic soils.