Keywords

9.1 Introduction

Soil salinity constitutes a major obstacle for agriculture in arid and semi-arid regions. Twenty to thirty percent of the world’s cultivated area and almost half of the world’s irrigated soils are severely affected by the lack of water in prolonged drought conditions, and by increased salinity (Bartels and Sunkar 2005). Furthermore, increases in aridity and salinity seem unavoidable in many countries. Hence, the development and sustainability of profitable agricultural systems are seriously threatened (Cordovilla et al. 1994; Nabti et al. 2007, 2010). Furthermore, all global change scenarios predict a substantial raise of the sea level in the next decades due to global warming. In consequence, salinization of large agricultural land areas at fertile coastal areas, which are also highly populated, is to be expected.

It is well known, that the performance of agriculture under less favorable conditions, such as semiarid or saline conditions, presents already major challenges in many countries. In addition to a proper supply of nutrients, dominating constraints for plant growth are the lack of water and the increasing salinity of soils. Sustainable and cost-effective plant growth becomes even more important due to the need of energy efficient plant growth for biomass and bio-energy production, especially in soils of lower quality, in addition to food production (Nabti et al. 2007).

The natural occurrence of halotolerant bacteria in saline soils opens up a possible important role of these microorganisms also in the interaction with plants under these stress conditions. However, the diversity of these bacteria and the ecology of their interactions with plants are still under investigation and not fully understood. General strategies existing for growth and survival of prokaryotes in environments with elevated osmolarity are well documented. The compatibility of high salt concentrations within the cell cytoplasm requires extensive structural and enzymatic modifications, which is restricted mainly to members of the Halobacteraceae. All other prokaryotes have evolved the accumulation of a specific group of molecules of low molecular mass, termed compatible solutes , as general mechanism to cope with environments of elevated osmolarity. High intracellular concentrations of these osmolytes balance out high external osmolarity and are perfectly compatible or even protect cellular processes (Sleator and Hill 2001). To overcome the salt problem, improvement of management practices by using suitable salt-tolerant plant cultivars is under development (Vinocur and Altman 2005). However, the inoculation with osmotolerant bacteria or rhizobacteria could be a more ready utilizable and sustainable solution to mitigate the detrimental salt-effects on plants by the modulation or amelioration the salt stress (Dodd and Pérez-Alfocea 2012). Some osmotolerant, plant growth promoting bacterial strains (e.g., Pseudomonas , Rhizobium , Azospirillum , Bacillus and Planococcus) are being developed on these lines (Creus et al. 1998; Mayak et al. 2004; Bartels and Sunkar 2005; Egamberdiyeva 2005, 2011; Rabie et al. 2005; Sapsirisopa et al. 2009; Rajput et al. 2013).

Recent studies about the utilization of halophilic and halotolerant plant growth promoting bacteria in mitigation of the deleterious effects of salt stress have been performed. There are numerous and diverse, process of mitigation of salt stress in wheat seedlings by halotolerant bacteria (Nabti et al. 2012; Ramadoss et al. 2013), application of osmotolerant PGPR to ameliorate sodium chloride stress on tomato plants (Tank and Saraf 2010; Shen et al. 2012), restoration of growth of salt-affected maize and soybean plants by application with osmotolerant rhizobacteria (Aly et al. 2003; Naz et al. 2009), the improvement of nutrient elements in sunflower under high salinity (Shirmardi et al. 2010), and the increase of halotolerant growth of barley by the inoculation with novel halotolerant rhizobacteria (Cardinale et al. 2014).

9.2 Effect of Salinity

Salinity is considered as a major threat in ecosystem, because this phenomenon increases progressively and quickly, then disturbs and influences the biotic and abiotic soil components.

9.2.1 Saline Stress and Its Effects on Soil Properties

Soil salinity constitutes a natural situation in ecosystems in arid and semi-arid regions. The term salt-affected soil is reserved for soils that are saline or sodic. Saline soils have an excess of soluble salts in the solution within the micro- and macropore space structure. Sodic soils have a surplus of sodium associated with the negatively charged clay particles (Bianco and Defez 2011).

Accumulation of salt in the soil is due to inadequate leaching and drainage of irrigation water. Most of salts present in irrigation water are chlorides, sulfates, carbonates, and bicarbonates of calcium, magnesium, sodium and potassium. Salinity affects the soil structure and adversely affects plant growth and crop yields (Frenkel et al. 1978; Ghadiri et al. 2004). Three major types of salinity based on soil and groundwater processes are known: groundwater associated salinity, transient salinity and irrigation salinity (Rengasamy 2006). Salinity is regarded as the most severe environmental stress, which affects both the soil system and living organisms. Soil structure is defined as the arrangement of the solid particles and aggregates and of the micro-and macropore space between them. Pores are the main soil physical features because most soil processes that have immediate consequences for soil biological activity or soil conservation occur either within the pore space or on the surfaces of the particles that form the pores (Kay 1990; Rengasamy and Olsson 1991; Allotey et al. 2008). It is well known that high salinity leads to negative effects on soil structure: elevated sodium concentrations cause soil dispersion and clay platelets to swell and aggregate. Thus, the forces involved in the binding of clay particles are disrupted under the influence of sodium ions. Soil dispersion causes clay particles to plug soil pores. Therefore, soil permeability for water and air is reduced and surface crusting does occur (Abu-Sharar et al. 1987). It is documented that the presence of water in soil leads to the swelling of soil particles with high smectite clay content and to the hydratation of some minerals as a result of the reduction of the cross-sectional area of soil pores. This process is occurring under high sodium or low salt concentrations and causes dispersion and movement of fine particles within the pores. The particles will be caught in smaller pores and therefore, water and air will be blocked within the soil structure (Roiston et al. 1984).

9.2.2 Effect of Salinity on Plant Growth

Saline soil becomes a problem when salt is highly accumulated in the root zone. As a result, plant growth is affected negatively, because plant roots are inhibited to withdraw water from the surrounding soil and thus the water supply is reduced (Rengasamy 2010a). Plant growth is also directly and efficiently inhibited by high salinity of the water. This is due to the high osmotic potential of the soil solution and nutrient uptake deficiencies caused by the abundance of Na+ and Cl− in the soil solution (Marschner 1995); Sairam and Tyagi 2004). The presence of these ions in high concentrations decreases the activity of other essential elements in soil and can lead to a reduction in the accessibility and uptake of essential mineral nutrients by plants (Bianco and Defez 2011).

Also, high salinity causes an inhibition and deterioration of several stages of plant life such as seed germination, biosynthesis of phytohormones and other plant growth stimulating factors, photosynthesis by increasing photorespiration and alerting the normal homoeostasis of cells which generates an increased generation of reactive oxygen species. Other affected processes are the maturation of cell structures and plant morphology, root and stem growth, ion and organic solute transport, nutrient uptake as well as general enzymatic activities (Xiong and Zhu 2002). Osmotic stress reduces stem height, root length and dry weight. Under high environmental NaCl concentrations, sodium uptake into roots raises as well, while N, P, K and Mg-uptake decrease and the intracellular ionic equilibrium is disturbed (Gunes et al. 1996; Abdel-Ghaffar et al. 1998). Plants become less capable to take up sufficient water for growth in the presence of high level of dissolved salts in the root zone (Rengasamy 2010b). Under these conditions, plants expend more energy on osmotic adjustment by accumulating compatible solutes (Robinson and Jones 1986; Kempf and Bremer 1998).

Numerous studies showed the negative effects of high salinity of plant growth from the seed germination to maturity (François et al. 1986). The effects of salt stress on wheat and barley were often and profoundly studied by different authors (Sarin and Narayanan 1968; Qureshi et al. 1980; Rachidai et al. 1994; Sharma et al. 2005; Mahmoodzadeh et al. 2013). Other plants were also investigated under stress conditions : pepper (Gunes et al. 1996), sorghum (Gill et al. 2002), alfalfa (Fougere et al. 1991), beans (Cordovilla et al. 1994), maize (Azevedo et al. 2004), lentil (Golezani and Yengabad 2012), soybean (Ahmed and Sandhu 1988), sunflower (Naz and Bano 2013), tomato (Tank and Saraf 2010; Kahlaoui et al. 2011; Jan et al. 2014) cucumber etc. (Ibekwe et al. 2010).

9.3 Strategy of Osmoregulation in Halotolerant Bacteria

When bacterial cells are exposed to high osmolarity or salt stress, the water activity decreases in their cytoplasm (Epstein 1986) and most of their proteins and other macromolecules as well as essential functions are impaired (Bakker et al. 1987). Also, abrupt plasmolysis blocks different physiological processes, such as nutrient uptake or inhibition of DNA -replication and macromolecule biosynthesis (Kogut and Russell 1987; Bartels and Sunkar 2005).

Cellular adaptation to osmotic stress is a fundamental biological process protecting organisms against the lethal effects of dehydratation. Osmoregulation is of great significance in agriculture, since water is the major limiting factor in crop productivity (Le Rudulier et al. 1984). Two strategies of osmoadaptation are adopted in bacteria: at first, maintenance of osmotic equilibrium by keeping cytoplasmic KCl concentrations similar to the surrounding environment and in parallel some physiological modifications to protect vital metabolic functions are key mechanisms; secondly the accumulation of compatible solutes (organic osmolytes) in the cytoplasm (Galinski and Trüper 1982; Galinski and Trüper 1994; Galinski 1995) so as to balance the high osmotic potential in the environment in a physiological compatible manner.

The adaptation of rhizospheric bacteria to high salinity was reviewed in detail by Miller and Wood (1996). Osmoregulation is a general process in soil microorganisms, because the presence of variable salt concentrations in soil would lead to the disruption of numerous processes. For example, a disturbance in the interaction of plants with bacteria is caused by alterations of proteins involved in the initial attachment steps (adsorption and anchoring ) of bacteria to plant roots in symbiotic interactions as well as by the inhibition of nodulation and nitrogen fixation activities. Furthermore, a disturbance of the molecular structure of exopolysaccharide (EPS) and lipopolysaccharide (LPS) of bacterial cells surfaces cuts off the molecular signal exchange between bacteria and plant host, bacterial chemotaxis and finally root colonization of surface-associated or endophytic PGPB (Jofré et al. 1998).

Osmoregulation in the nitrogen fixing rhizobacteria of the genus Azospirillum is well documented (Hartmann 1988; Riou and Le Rudulier 1990; Hartmann et al. 1991, 1992; Riou et al. 1991; Tripathi et al. 1998). Bacteria of the genus Azospirillum accumulate compatible solutes such as gulatamate , proline , glycine betaine , trehalose in order to adapt to the fluctuations in soil salinity. The understanding of the osmoadaptation mechanisms in Azospirillum spp. can contribute towards the long-term goal of improving plant-microbe interactions in salinity affected crop productivity (Tilak et al. 2005). Other substances like poly-β-hydroxybutyrate (PHB) may also be involved in the resistance of Azospirillum brasilense towards the osmotic stress and root colonization (Kadouri et al. 2003). PHB is an energy and carbon storage material accumulated in response to the limitation of an essential nutrient. The effect of different salt concentrations on growth and PHB accumulation of four different Sinorhizobium strains was examined by Arora et al. (2006) suggested that minimum PHB content was accumulated at low or zero salinity, maximum was observed by the salt-tolerant strains at higher salt concentrations which, define key role of PHB in cell protection in saline conditions.

Certain osmotolerant strains of the genus Rhizobium showed a different utilization of osmoprotective substances such as dimethylsulfoniopropionate , dimethylsulfonioacetate , glycine betaine, proline, ectoine and choline (Le Rudulier et al. 1983; Bernard et al. 1986; Le Rudulier and Bernard 1986; Talibart et al. 1994; Pichereau et al. 1998; Boncompagni et al. 1999; Zahran 1999; Bouhmouch et al. 2001). Mechanisms of osmotolerance with different pathways are present in the genus Sinorhizobium (Shamseldin et al. 2006; Abolhasani et al. 2010). Sinorhizobium meliloti takes up osmolytes like trehalose, maltose, cellobiose, gentibiose, turanose and palatinose as cytosolic osmolytes under salt-stress in an early growth phase. These compounds are catabolized during later growth phases and thus contribute to an increase in the cytosolic concentrations of two endogenously-synthesized osmolytes: glutamate and N-acetyl glutaminylglutamine amide (Gouffi and Blanco 2000). Many other osmotolerant PGPB, like Pseudomonas spp., Bacillus spp., and e. g. Streptomyces spp., were isolated and examined for their capacities to challenge osmotic stress in detail (Halverson et al. 2000; Upadhyay et al. 2012).

9.4 Plant Growths Under Salinity Stress Conditions

Under saline or sodic stress, both halophytes and halotolerant rhizospheric and plant-associated bacteria are especially adapted to the particular stress conditions. The functional interactions between the two partners can reduce the effect of the salt pressure (Biro et al. 2002). Previous studies reported that plant growth promotion and amelioration of salinity stress in crop plants by salt-tolerant bacteria could involve different mechanisms such as antioxidant enzymes , phosphate solubilization, siderophores and secretion of various phytohormones (Alizadeh and Parsaeimehr 2011; Chakraborty et al. 2011). Furthermore, the application of halotolerant PGPB reduced the negative effects of saline stress by increasing the leaf’s relative water content and enhancing photosynthetic pigment production in both stress and normal conditions (Saghafi et al. 2013). Additionally, the majority of halophilic or halotolerant PGPB are characterized by their capability to induce antioxidant enzymes involved in the tolerance of plants towards severe salt stress (Del Rio et al. 2003; Kohler et al. 2009).

Rabie and Almadini (2005) showed the positive effect of the dual inoculation of the arbuscular mycorrhizal fungus Glomus clarum and the diazotroph Azospirillum brasilense on salt-tolerance in cowpea plants. They revealed that different vegetative plant growth parameters, protein content, nitrogenase, and phosphatase activities as well as nutrient elements (N, P, K, Ca, Mg) are significantly improved in the presence of A. brasilense. Ghorai et al (2013) showed the potential effect of some high salt tolerant PGPB on groundnut seedling under saline conditions. The experiment revealed significant improvement of shoot height, root length and biomass of groundnut seedling after 7 days under salinity stress. The application of halotolerant PGPB is not only restricted to legumes and cereals, but is also performed on trees such as banana and mangrove forest growing under hard environmental conditions caused by salinity (Maziah et al. 2009; Das et al. 2011). Furthermore, Shukla et al. (2012) demonstrated an increase of NaCl-stress inhibited growth of peanut (Arachis hypogaea) after inoculation with the diazotrophic rhizosphere bacterium Brachybacterium saurastrense (Gontia et al. 2011) and other halotolerant isolates from the halophyte Salicornia (Jha et al. 2012).

The use of Rhizosphere bacteria possessing the traits of PGP under saline stress is becoming prevalent worldwide to achieve sustainable agriculture along with soil reclamation through phytoremediation as well as bioremediation (Tank and Saraf 2010). Additionally, many studies were carried out to highlight the possibility to restore cereal growth under saline conditions by using osmotolerant bacteria. A restoration of salt effected growth of (Triticum durum) was sown after inoculation of seeds with the osmotolerant PGPR Azospirillum brasilense NH isolated from salt-affected soil was reported by Nabti et al. (2010). The addition of extracts of marine algae Ulva increased the protecting effect on salt-stressed plants due to the content of osmolytes in the algae. Chickpea plants irrigated with saline water were inoculated with Azospirillum brasilense strain Cd, and a noticeable growth-restoration of nodulation, root and shoot and crop yield occurred under saline conditions (Hamaoui et al. 2001). Similarly, Rojas-Tapias et al (2012) reported significant effects of inoculation with two strains of Azotobacter sp. on the amelioration of growth of (Zea mays) under high NaCl concentration. The experiment revealed a significant restoration of plant biomass (length and weight), exclusion of Na+ and K+, improvement of polyphenol and chlorophyll contents and maintenance of nitrogen fixation and phosphate solubilization activities under saline stress conditions. Concurrent with accumulation of proline, which is regarded as potent osmolyte and indicator of osmotic stress, was greatly diminished.

Other studies revealed the necessity of application of osmotolerant strains of Rhizobium to improve common bean production in the Mediterranean area (Bouhmouch et al. 2001). In fact certain Rhizobium sp. are able to respond to the challenge to soil salinity by a natural selection imposed by certain legumes, which play an important role in the remediation of this kind of soils (Yadav and Agarwal 1961). Some salt tolerant pseudomonas (P. fluorescens, P. aeruginosa and P. stutzeri) (<6 % NaCl) of PGPB were selected which showed a significant effect on tomato plant growth under sodium chloride stress due to phosphate solubilization, siderophores production; ethylene reduction and IAA production.

Most recently, research was conducted on the utilization of marine bacteria as salt tolerant PGPB to mitigate the effect of stress on inoculated plants (Kim et al. 2014). Schnell and colleagues isolated a diversity of novel halotolerant bacteria that mitigate the adverse plant growth under salt-stress conditions (Suarez et al. 2014a, b; Cardinale et al. 2014). Upon inoculation of these bacteria to Hordeum vulgare cv. Propino was, a remarkable restoration of salt-affected growth was achieved (Schnell, personal communication).

9.4.1 ACC-Deaminase and Halotolerant PGPB

Ethylene is a volatile phytohormone and is known to play an important role in plant growth regulation at very low concentrations such as development of different vegetative plant parts, nodulation, or rooting of cuttings (Davis 2004). It is also involved in the transduction of a signal for the recognition of salt stress (Selvakumar et al. 2012). Plants growing in contaminated soils are often subjected to the combined stress of nutritional deficiency and chemical toxicity resulting. This results in the production of stress ethylene, which leads to growth inhibition of plants and decreases in plant biomass (Maheshwari 2011) .

One of the strategies for increasing the efficacy of phytoremediation is based on the use of contaminant–tolerant plant species in combination with PGPB (Gerhardt et al. 2006). These soil microorganisms producing 1-aminocyclopropane-1-carboxylate deaminase (ACC) are capable to promote plant growth by cleaving plant-produced ACC, a precursor in the biosynthesis of ethylene, which decreases the level of ethylene in plants and thereby relieves the blockage in plant growth (Honma and Shimomura 1978; Glick 2005; Aamir et al. 2013). Ethylene production is regulated by different parameters such as light, temperature and IAA. When plant growth is subjected to an environmental stress, ethylene is produced at high concentrations as response to harsh conditions (Glick 2005). In addition, the production of ethylene is indispensable to ensure normal plant growth and development. However, high concentrations of ethylene induced by stressful conditions proved harmful, because it provokes defoliation and inhibits seed germination and other cellular processes involved in normal growth and development. The involvement of ACC-deaminase positive PGPB degrades ACC in the ethylene biosynthesis pathway in the root system. However, the ACC-deaminase level should be at least 100–1,000-fold greater than the ACC-oxidase level. This is likely to be the case, provided that the expression of ACC- oxidase is not induced (Glick et al. 1998). This ACC-deaminase enzymatic activity is regarded as a very important mechanism (Glick and Holgan 2003; Mayak et al. 2004), since decreasing elevated ethylene levels in roots during salinity stress leads to improved plant development (Fig. 9.1) as observed by Blaha et al. (2006). Bacteria containing ACC deaminase have been used successfully in many cases, both in laboratory experiments and in the field, to ameliorate some of the stress experienced by plants used in phytoremediation (Gerhardt et al. 2006).

Fig. 9.1
figure 1

Diagram of ACC-deaminase in bacteria following the model of Glick et al. (1998). The ACC-deaminase in bacteria reduce the ethylene level (inhibition of root elongation) in plants by conversion of ACC to ammonia and α-ketobutyrate

Full size image

Many rhizobacteria are described as ACC-deaminase-positive such as (Azospirillum, Pseudomonas, Rhizobium and Bradyrhizobium) . They hydrolyze the ethylene precursor ACC as nutrient source for C and N by converting it to ammonia and α-ketoglutarate (Honma and Shimomura 1978; Glick et al. 1998; Saleem et al. 2007). Various halotolerant bacteria isolated from the rhizosphere of halophytic plants in the vicinity of the Yellow Sea harbour ACC-deaminase activity in saline medium (0,85 M NaCl) (Siddikee et al. 2010).

Kausar and Shahzad (2006) determined the positive effect of ACC deaminase in P. putida and P. fluorescens which reused to restore growth of maize under saline conditions. Bacteria containing ACC-deaminase with a potential effect on growth of maize under saline conditions were also isolated and characterized by Hussain et al. (2013). Montero-Calasanz et al (2013) confirmed, that the inoculation of Pantoea sp. AG9, Chryseobacterium sp. AG13, Chryseobacterium sp. CT348, Pseudomonas sp. CT364 and Azospirillum brasilense Cd containing ACC-deaminase significantly induced the rooting in semi-hardwood cuttings of Arbequina, Hojiblanca and Picual cultivars of olive (Olea europea L.). Similar results were reported by Husen et al (2011), who were able to restore and enhance soybean growth under acidic and low fertility conditions in field soil by using ACC-deaminase producing Pseudomonas sp. strains. Nadeem and coworkers reported, that several halotolerant rhizosphere strains containing ACC-deaminase increased plant height, root length, total biomass and grain, N-, P-, and K-contents, chlorophyll- and carotenoid-concentrations and yields in maize under salt stress (Nadeem et al. 2006a, b, 2007). The authors showed the potential effect of some rhizospheric bacteria on ground-nut seedlings using ACC-deaminase containing P. fluorescens strain TDK1. Under high salt conditions, salt-affected plant growth was restored using the wild type P. fluorescens, while seedlings inoculated with a ACC-deaminase negative mutant or non-inoculated seedlings were not rescued (Saravanakumar and Samiyappan 2007).

PGPB containing ACC-deaminase in combination with rhizobia improve the growth and nodulation in plants by suppressing the endogenous level of ethylene (Zafar-ul-Hey et al. 2013). Ameliorative effects of some bacteria containing ACC-deaminase were clearly shown after inoculation of squash plants under salt stress (Yildirim et al. 2006). However, some osmotolerant bacteria increasing plant growth under salt stress were shown to lack the ACC deaminase gene, for e.g. in the slightly halophilic rhizospheric bacterium Azospirillum brasilense NH which, nevertheless, is able to restore salt-affected growth of wheat plants (Nabti et al. 2010).

9.4.2 Phytohormone Production of PGPB

Plant hormones or phytohormones are a group of naturally occurring, organic substances which influence physiological processes of plants at low concentrations. These hormones affect differentiation and development of plant growth through the regulation of diverse processes. These hormones are also involved in signaling systems of stress (Davis 2004). Plant hormones are divided into different groups such as: indole acetic acid (IAA), gibberelins (GA), cytokinins (CK), abscisic acid (ABA), ethylene and the cofactor pyrroquinoline quinone (PQQ) (Davis 2004; Jha et al. 2013).

It is well documented that the application of salt tolerant PGPB supports plant growth and renders them more tolerant to salt stress by improving their antioxidant status and physiological response by the intervention with several growth promoting substances like IAA, GA3 and ABA mediated by PGPB (Perrig et al. 2007). More recently, spermidine was described as new protector molecule against stress (Alavi et al. 2013). In their study regarding salt effects on soybean seedlings, Asim et al. (2013) showed a remarkable decrease in IAA content in leaves of stressed plants. They found major changes in endogenous level of phytohormones to occur between 48 and 96 h after inoculation with PGPB. IAA causes morphological changes of roots and the root system leading to improved shoot growth and yields when present at optimal concentrations. Naz and co-workers (2009) isolated and characterized halotolerant bacteria by a high production of proline and the plant hormones IAA, GA3, trans-zeatin riboside and ABA. Inoculation of soybean plants by these halotolerant strains showed a significant improvement in shoot and root length and dry weight under salt stress (20 dS/m NaCl). Thus, these bacteria appear promising as potential bio-fertilizers to that of saline soils.

Nabti et al. (2007, 2010) revealed the restoration of salt-affected growth of durum wheat by the osmotolerant PGPB A. brasilense NH, isolated from salt-affected Algerian soil close to the Mediterranean coast. This strain was able to produce high amount of IAA in saline medium (200 mM NaCl), while IAA-production in the salt-sensitive reference strain A. brasilense Sp7 was almost completely inhibited at this NaCl-concentration. It is well demonstrated that IAA play a considerable role in plant resistance to salt stress (Ali and Abbas 2003; Kaya et al. 2010, 2013; Khalid et al. 2013; Kang et al. 2014). Many strains of halophilic bacteria belonging to the genus Halomonas sp. isolated from the rhizosphere of Salicornia plants were characterized and tested for PGP-features under high salt concentration, where a noticeable quantity of IAA was observed (Mapelli et al. 2013). Ul-Hassan and Bano (2014) reported in their study that the level of IAA and ABA increased in leaves of wheat growing in saline soil after a dual inoculation with isolates of Pseudomonas sp. and Bacillus cereus. Furthermore, Jha and Subramanian (2013) showed clearly the direct and potential effect of some osmotolerant bacteria on germination of paddy seeds under saline conditions. This is probably due to the intervention and supplementation of IAA by the inoculated bacteria which plays an important role in the germination of seeds. Additionally, Raza and Faisal (2013) studying the growth promotion of maize by desiccation tolerant Micrococcus luteus chp 37 showed an involvement of IAA and HCN in the restoration and protection of growth of maize under harsh conditions. Inoculation with halotolerant PGPB improved plant growth under saline conditions by the augmentation of auxin (IAA) which caused a reduction of the uptake of toxic ions by plants (Zhang et al. 2008; Chakraborty et al. 2011). Recently, the impact of plant hormones like IAA, ABA and GA3 on amelioration of salinity stress in crop plant growth as well as the direct implication of phytohormones was investigated (Jha et al. 2013) in restoration and growth enhancement of common bean (Phaseolus vulgaris) under high salinity conditions after inoculation with two halotolerant strains of P. extremorientalis and P. chlororaphis (Egamberdieva 2011). In their study on the utilization of osmotolerant PGPB so as to mitigate deleterious effects of salinity and osmotic stress in Cucumis sativus, Kang et al (2014) revealed that stressed plants showed an upregulation of stress-responsive abscisic acid which is not occurring when PGPB were inoculated, whereas salicylic acid and GA were highly produced in PGPB-inoculated plants.

9.4.3 Minerals Uptake, Nitrogen Fixation, Siderophores Production, Phosphate Solubilization and Root Colonization by Halotolerant PGPB

Mineral disturbances under salinity stress reduce plant growth by affecting the availability, transport, and partitioning of nutrients. Salinity can differentially affect the mineral nutrition of plants and cause nutrient deficiencies or imbalances, due to the competition of Na+ and Cl– with nutrients such as K+, Ca2+ and NO3 - (Jouyban 2012). Nevertheless, halotolerant bacteria can help to overcome the turbulence of nutrient uptake by restoration of phosphorus availability, and enrichment of rhizosphere with NO3 - N (Ul-Hassan and Bano 2014). These bacteria also influence the uptake of nutrient elements (N, P, and K, Ca, Mg, Na Cl, Fe, Zn, Cu Mn) (Shirmardi et al. 2010; Yildirim et al. 2011; Jha and Subramanian 2013). The effects of co-inoculation with P. fluorescens and R. meliloti strains on nodulation and mineral uptake in alfalfa (Medicago sativa) clearly showed restoration of growth and only moderated negative effects of salt and significantly increased plant growth by elevation of P and N and diminution of Na+ in planta (Younesi et al. 2013a). Similar effects were reported by Han and Lee (2005) during their study on the effect of soil salinity on the antioxidant status, photosynthesis, mineral uptake and growth of lettuce. In this experiment, the P, K and N concentrations were increased in lettuce plants; in contrast, Na+ was decreased. Some observations revealed that the capacity of bacteria to mitigate salt stress is due to the production of polysaccharides binding Na+ in the root zone (Ashraf et al. 2006; Awad et al. 2012). Sarathambal and Ilamurugu (2013) showed the detailed PGP-features of some salt-tolerant diazotrophic PGPR, which were used to restore growth of paddy under saline conditions. Nutrient uptake (N, P and K) were increased, and in parallel, Na+ level in plant tissues were reduced. In addition to these characters, these rhizobacteria showed the potential to fix nitrogen and solubilize phosphate under high salinity conditions (Sarathambal and Ilamurugu 2013). Osmotolerant rhizobacteria are known to enhance nutrient uptake in plants, nitrogen fixation, phosphate solubilization and siderophore biosynthesis in saline soils (Bashan et al. 2000; Ibekwe et al. 2010; Chakraborty et al. 2011; Shookietwattana and Maneewan 2012; Aamir et al. 2013; Jha and Subramanian 2013; Younesi et al. 2013b). In addition, the inoculation with osmotolerant PGPB to alleviate the deleterious effects of salt stress conditions on growth and yield of strawberry plants under salinity conditions was examined by Karlidag et al. (2013). Improvement of vegetative growth of plants, chlorophyll content, nutrient element, and yield of strawberry plants under saline stress was highly pronounced. Electrolyte leakage of plants was found reduced and the contents of nutrient elements in leaves and roots were considerably increased – except of sodium (Na+) and chlorine (Cl-). Nitrogen fixation, potassium, phosphorus, calcium, magnesium, sulfur, manganese, copper, and iron concentrations were increased besides of N, P, K, Ca, Mg, S, Mn, Cu, and Fe in leaves and roots. The neutralizing effect on salinity and drought stress in plants was explained by ameliorative effects of PGPB on increased water potential and decreased electrolytic leakage in plants. In parallel, Na+ concentration was decreased, while potassium and phosphate achieved a high level in comparison to the non-inoculated plants (Kang et al. 2014). Some species of halotolerant Rhizobium are capable to nodulate under high NaCl concentration. This phenomenon is mainly due to the induction of the nod genes in the absence of flavonoid inducers (Guasch-Vidal et al. 2013). A co-inoculation of alfalfa with Rhizobium sp. and P. fluorescens under saline conditions resulted in a synergetic effect, which was more significant than upon inoculation with each strain separately. This is due to the combination between the nodulating rhizobacteria (Rhizobium) and the phosphate-solubilizing bacteria (Pseudomonas fluorescens) (Younesi et al. 2013a). Similar results were obtained in the co-inoculation of bean plants with both of the bacterial strains of Rhizobium and Pseudomonas (Ahmed et al. 2013). Pandey and Maheshwari (2007a, b) studied combination of rhizobacteria for growth promotional activity in Cajanus cajan using two species consortia and multispecies consortia and observed exceptional increase in seedling growth in mixed-species, co-inoculated consortium.

Efficient root colonization of plants by PGPB is a central parameter for the enhancement of plant growth in presence or absence of salinity. Bacterial root colonization is fundamentally influenced by the presence of specific bacterial traits required for attachment and subsequent establishment. However, additional abiotic and biotic factors play an important role in root colonization too. When a microbe colonizes a root, the process must be in accordance with an array of external parameters including water content, temperature, pH, soil types (texture, organic matter, micro-aggregate stability, presence of nutrients such as N, P, K, and Fe), composition of root exudates, and the presence of other microorganisms (Ahmad et al. 2011; Maheshwari 2012).

Some endophytic bacterial members of the plant microbiome have multiple functions as they have potentials for biological control, nitrogen fixation and plant growth promotion. This extraordinary feature was found for example in the Gram-negative nitrogen–fixing bacteria of the genus Azospirillum living in close association with plant roots (Broek et al. 1998). Within the species A. brasilense , an endophytic colonization is known for the strain Sp245, which colonize the root cortex area of wheat plant roots (Rothballer et al. 2003; Schloter and Hartmann 1998). Endophytic colonization of plants by microorganisms is only possible, when the plant is not recognizing the bacterial cells as enemy and does not mobilize defense reactions against it. On the other hand, the bacterium has abilities to defend itself, enter and thrive within a plant successfully, exerting its beneficial effects with less competition with the abundant rhizosphere microbiota (Nabti et al. 2010). Puentea et al. (1999) showed a potential root colonization of mangrove seedlings by strains of A. brasilense and A. halopraeferens , irrigated with seawater. A difference of the colonization pattern was observed between the two strains, where A. brasilense cells were anchored to the root surfaces and to themselves by a network of fibrillar material. However, A. halopraeferens yielded mainly single cells embedded in a thick sheath. The author suggested the feasibility of using terrestrial PGPB for the inoculation of marine plants. An analogous experiment was performed with slightly halophilic A. brasilense NH showing an efficient root colonization of durum wheat at 200 mM NaCl (Fig. 9.2). However, the same strain did not show this capacity in the absence of NaCl (Nabti et al. 2010).

Fig. 9.2
figure 2

Confocal laser scanning microscopic images of wheat roots inoculated with A. brasilense NH in axenic conditions after 4 weeks of growth under saline (200 mM NaCl) and non saline conditions: The roots were fixed in 4 % PFA, FISH-analysis was performed using the probes EUB- 338-I, II, III, labelled with FLUOS and the specific probe Abras-1420, labelled with Cy3. A. brasilense NH cells are stained in yellow (combination of both fluorescent signals [Cy3=red, FLUOS=green]). Orthogonal optical sections of a three dimensional confocal image created from a z-stack of xy-scans. (a) Shows the root surface colonization by A. brasilense NH at 200 mM NaCl. (b) No surface root colonization by A. brasilense NH in absence of NaCl

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In recent years, many studies are concerned with isolation of salt tolerant endophytic bacteria and their application in saline stress alleviation (Chookietwattana and Maneewan 2012; Damodaran et al. 2013; Kannan et al. 2014; Cardinale et al. 2014).

9.4.4 Biocontrol and Soil Remediation

In addition to deleterious effects of salinity stress on plant growth, phytopathogenic microbes often successfully attack salt-stressed plants probably due to a weakened defense system or lesions in their surface structures. Environmental conditions have major influences on the pathogenicity, because the epiphytic phase of pathogens is strongly influenced by water availability, temperature and surface wetness (Agrios 1997). Thus, to cope with this problem, farmers frequently use chemical pesticides and fungicides. Unfortunately, continuous and increasing application of chemical substances generated pathogen resistance to antimicrobial agents as well as environment pollution (Compant et al. 2005). In addition, more and more chemical pesticides are no longer allowed to be in use and therefore alternatives are badly needed (Maheshwari 2013).

As an alternative, specific salt-tolerant or salt-sensitive PGPB, so-called biological control agents, are getting in use to protect plants from pathogen attack. In these cases, plant growth stimulation occurs through suppression of phytopathogens by the production of siderophores, antibiotics or other substances. Other mechanisms are also involved in the antifungal activity such as allelochemicals , biocidal volatiles , hydrolytic and detoxification enzymes (Sturz and Christie 2003; Chakraborty et al. 2011).

Furthermore, plants are capable to acquire an induced systemic resistance (ISR) towards phytopathogens after inoculation with PGPB. When these bacteria are associated with plant roots, they stimulate a protection of the plant host and confer a resistance against a large spectrum of different pathogenic agents (Van Hulten et al. 2006). The importance of bacteria in growth promotion and their ability to elicit the induced systemic tolerance also against abiotic stress conditions, such as e.g. UV-radiation and desiccation stress, has been documented in detail (Damodaran et al. 2013). Many salt-tolerant PGPB were studied for their capability to eliminate phyto-pathogenic fungi in number of rhizosphere (Wolf et al. 2002; Belimov et al. 2005). Bhakthavatchalu et al. (2013) demonstrated the potential effects of volatile and diffusible metabolites produced by the salt tolerant P. aeruginosa FP6 in the biocontrol. Among biocontrol rhizobacteria, bacteria belonging to the genera Pseudomonas and Bacillus are the most important ones (Van Peer et al. 1990; Cook 1993; Paul et al. 2006; Bhakthavatchalu et al. 2013; Saravanan et al. 2013).

It is well known that continuous spreading of heavy metal into the environments occurs, e.g. as contaminants of household compost into soils or via air pollution. This constitutes a significant environmental pollution and its negative impact on human health and agriculture. Rhizosphere, as an important interface of soil and plant, plays a significant role in the phytoremediation of soils contaminated with heavy metals and organic xenobiotics. Microbial populations are known to affect heavy metal mobility and availability to the plant through chelating mechanism, acidification, phosphate solubilization and redox changes. Therefore, they can enhance phytoremediation processes (Jing et al. 2007).

The most common heavy metal contaminants are Cd, Cr, Cu, Hg, Pb and Ni. The level of these toxic elements increases daily in soils and cause a considerable and accelerated pollution. This is the backside consequence of industrialization with its huge production of gas, fuel, fertilizer, sewage and pesticide (Kabata-Pendias 2011). However, phytoremediation is an alternative biological approach of treating the polluted areas and reducing the phytotoxicity in polluted soils (Weyens et al. 2010; Nanda and Abraham 2013). Phytoremediation is defined as the utilization of plants in the elimination of toxic metal and organics. To improve these activities, PGPR that facilitate the proliferation of various plants especially under environmentally stressful conditions may be used to lower the level of growth inhibiting stress ethylene within the plant and also to provide the plant with iron from the soil (Glick 2003).

Few studies showing the utilization of PGPR to reduce the negative effects of Lead (Pb), Chromium (Cr) and other toxic heavy metals on plant growth were reported (Janhmohammadi et al. 2013; Khan et al. 2013; Nakbanpote et al. 2014). Field and laboratory experiments have also been performed to show the positive effect of bacteria of the genus Pseudomonas in the bioremediation system for decontamination of petroleum and salt-affected soils (Greenberg et al. 2007). It is important to mention the relationship between the mechanisms involved in mitigation of salt stress and bioremediation, such as the reduction of ethylene, which is increased by several stress conditions – high salinity/osmolarity and contamination by heavy metals .

9.5 Conclusion

In summary, the successful restoration of plant growth under salinity conditions after inoculation with halotolerant or halophilic PGPB provides the basis for a suitable alternative of a successful formulation to improve crop growth and yield in saline soils. Additionally, halotolerant PGPB are able to enhance plant growth under saline conditions by various mechanisms. These mechanisms are similar to those of salt-sensitive PGPB plant interactions. The major difference resides in the osmoregulatory adaptation of halotolerant PGPB and some specific traits to relief deficiencies in salt-stressed plants.