Plant tissues produce or regulate different hormones to respond to internal and external cues during practically every aspect of plant growth and development. Bacterial endophytes have the ability to produce plant hormones and regulate their balance as well. Auxin, a hormone associated with plant growth promotion, influences many plant cellular functions and is an important regulator of growth and development. Bacterial endophytes are commonly capable of producing auxin which, at the genetic level, may either be constituently expressed or inducible (Mattos et al. 2008). Auxin producing bacterial endophytes increased the number and length of lateral roots in wheat (Barbieri and Galli 1993). Increased root length, root surface area and the number of root tips were observed in hybrid poplar inoculated with auxin producing bacteria, resulting in enhanced uptake of nitrate and phosphorus and boosting biomass by 60% compared with non-inoculated plantlets (Taghavi et al. 2009). Furthermore, Pseudomona flourescen significantly increased the growth of maize plant radicles under laboratory conditions via the production of auxin (Montanez et al. 2012). To date, multiple auxin biosynthesis pathways have been identified in bacteria, and their regulation is controlled by several different genetic and environmental factors (Bertalan et al. 2009). The production of native auxin, indole-3-acetic acid (IAA) by bacteria has been documented in species such as Rhizobium, Pseudomonas, Azospirillum, Azotobacter and Bacillus (Hayat et al. 2010).

Cytokinins are a diverse range of compounds that, like other plant hormones, are involved in many activities of plant growth and development. As a group, they have been shown to regulate cell division, seed dormancy and germination, senescence, new bud formation, and leaf expansion. They also play roles in controlling plant organ development, mediating responses to various extrinsic factors and the response to biotic and abiotic stresses (Spichal 2012). Researchers have demonstrated that certain endophytic bacteria are able to produce cytokinins and promote lateral root growth (Senthilkumar et al. 2009). Zeatin, a native plant growth promotive hormone, belonging to the cytokinin family, has been found in significantly higher levels in the beneficial bacteria B. subtilis and P. putida (Sgroy et al. 2009).

Gibberellins are native plant growth promotive hormones. Many plant growth promoting endophytes also produce gibberellins to enhance host plant growth (Joo et al. 2009; Fernando et al. 2010). For example, one Penicillium citrinum isolate, IR-3-3 from the sand dune flora, produced higher physiologically active gibberellins and stimulated Waito-c rice and Atriplex gemelinii seedling growth (Khan et al. 2008). Gibberellic acid levels were also high in the plant associated bacteria Lysinibacillus fusiformis, Achromobacter xylosoxidans, Brevibacterium halotolerans, and Bacillus licheniformis (Sgroy et al. 2009).

Ethylene, a simple organic molecule (CH2=CH2), is commonly thought to be a growth inhibitive hormone. It is typically produced when plants are exposed to environmental stress, repressing plant growth and development until the stress disappears or the levels of ethylene decrease (Gamalero and Glick 2012). Ethylene inhibits stem elongation, promotes lateral swelling of stems, and causes stems to lose their sensitivity to gravi-trophic stimulation (Glick 2005). In biomass production as in agriculture generally, it is important to keep ethylene low in order to maximize yields. An enzyme, 1-aminocyclopropane-1-carboxylate (ACC) deaminase produced by bacteria, interferes with the physiological processes of the host plant by decreasing ethylene levels (Hardoim et al. 2008) via metabolizing ACC, a precursor to ethylene so ethylene levels are reduced in plants, and plant growth is promoted. Activity of ACC deaminase is a common feature found in plant-growth promoting bacteria such as Enterobacter, Pseudomonas and Burkholderia (Shah et al. 1998; Sessitsch et al. 2005; Govindasamy et al. 2008). Burkholderia phytofirmans strain PsJN stimulates growth of many plant species, including potato, tomato, grapevine, and switchgrass (Pillay and Nowak 1997; Nowak et al. 1998; Barka et al. 2002; Kim et al. 2012) and was reported to have a high activity of ACC deaminase (Sessitsch et al. 2005). Endophytes that produce ACC deaminase have also been shown to increase host plant growth in soils with high salinity (Egamberdieva 2012; Siddikee et al. 2012) and increase drought tolerance (Arshad et al. 2008; Belimov et al.

2009) . Pseudomonas sp. strain A3R3 showed higher ACC deaminase activity and increased plant growth in nickel contaminated soil (Ma et al. 2011).

Abscisic acid (ABA) is involved in responses to environmental stresses such as heat, drought, and salt, and is also produced by endophytes.

Endophytic bacterial strains SF2, SF3, and SF4 isolated from sunflowers (Helianthus annuus) had the ability to produce ABA and jasmonic acid, which increased under drought conditions (Forchetti et al. 2007), implying these endophytes enhance stress tolerance of host plants. Two strains of Azospirillum brasilensis, successfully used to increase the yield of maize and wheat in field conditions, were both able to produce different plant growth regulators such as IAA, gibberellic acid, zeatin and ABA (Perrig et al. 2007), highlighting the ability of endophytes to confer multiple mechanisms of growth promotion.