Strong demand for food requires specific efforts by researchers involved in the agricultural sector to develop means for sufficient production. While, agriculture today faces challenges such as soil fertility loss, climate change and increased attacks of pathogens and pests. The production of sufficient quantities in a sustainable and healthy farming system is based on environmentally friendly approaches such as the use of biofertilizers, biopesticides and the return of crop residues. The multiplicity of beneficial effects of soil microorganisms, particularly plant growth promotion (PGP), highlights the need to further strengthen the research and its use in modern agriculture. Rhizobia are considered as PGP comes in symbiosis with legumes taking advantage of nutrients from plant root exudates. When interacting with legumes, rhizobia help in increased plant growth through enriching nutrients by nitrogen fixation, solubilizing phosphates and producing phytohormones, and rhizobia can increase plants’ protection by influencing the production of metabolites, improve plant defense by triggering systemic resistance induced against pests and pathogens. In addition, rhizobia contain useful variations to tolerate abiotic stresses such as extreme temperatures, pH, salinity and drought. The search for rhizobium tolerant strains is expected to improve plant growth and yield, even under a combination of constraints. This chapter summarizes the use of rhizobia in agriculture and its benefits.
- induced resistance
- stress tolerance
Agricultural productivity is significantly affected by nitrogen and phosphorus deficiencies, which are essential for plant growth. In addition, it is related to the physical and biological properties of the soil, pest and disease attacks and abiotic stresses. For sustainable agriculture, it would be interesting to carry out an efficient management of nitrogen in the environment. This usually involves the use of microorganisms biologically fixing nitrogen that is used directly by the plant and is, therefore, less susceptible to volatilization, denitrification and leaching. In agricultural settings, perhaps 80% of this biologically fixed N comes from symbiosis involving leguminous plants and one of the Rhizobia species . Legumes are able to establish a symbiotic
2. Plant growth promotion by rhizobia inoculation
Rhizobia can enhance plant growth promotions by both direct and indirect ways. Several mechanisms are involved in the plant growth promotions by rhizobia, different mechanisms involved are discussed as follows.
2.1. Direct plant growth promotions
2.1.1. Biological nitrogen fixation
Nitrogen is a vital element for plant growth; it is required for synthesis of macromolecules such as amino acids, nucleic acids and chlorophyll. In agriculture, fertilization with nitrogen products is practiced to increase the production yield of food [6, 7]. About 78% of the atmospheric air is N, this gaseous substance cannot be used in this form by most living organisms until it has been fixed, that is, reduced (combined with hydrogen) to ammonia. Biological nitrogen fixation (BNF) accounts for about 60% of nitrogen used in agriculture. Significant growth in fertilizer-N usage has occurred in both developed and developing countries . The requirements for fertilizer-N are predicted to increase further in the future ; however, the use of high doses of fertilizers has a negative and unpredictable impact on the environment and contaminates the soil, water and natural areas. These effects are considered a threat to human and animal health affecting the quality of life. In addition, developing countries must use cheaper and environmentally friendly alternative methods. Legumes are BNF capable and meet their own needs. The use of legume crops substantially reduces the N requirement from external sources . For more than 100 years, BNF has commanded the attention of scientists concerned with plant mineral nutrition, and it has been exploited extensively in agricultural practice . However, its efficiency varies, and depends on the host genotype, rhizobial efficiency, soil conditions and climatic factors . Currently, the use of microorganisms capable of fixing atmospheric nitrogen is of great practical importance because it makes it possible to bridge the limits of chemical fertilization, which has resulted in unacceptable levels of water pollution . In addition to pollution problems, especially in the water supply, the application of chemical fertilizers is carried out in excess, which becomes very expensive for farmers, whereas the biological fixation of nitrogen through microorganisms can be adapted to the needs of the plant . In legume-
2.1.2. Phosphate solubilization
Phosphorus (P) is the most limiting element for plant growth after nitrogen. There are several forms that are inorganic (bound, fixed or labile) and organic (bound), and the concentration depends on the source. The concentration ranges from 140 ppm in carbonate rocks to over 1000 ppm in volcanic materials . The majority of P applied as fertilizer enters into the immobile pools through precipitation reaction with highly reactive Al3+ and Fe3+ in acidic soils, and with Ca2+ in calcareous soils [19, 20]. The availability of phosphorus for plants is influenced by several conditions such as soil pH, aeration, temperature, texture and organic matter, extent of root systems of plants and secretions of root exudates and microbes. Soil microorganisms play a key role in soil P dynamics and subsequent availability of P to plants . Although chemical fertilizer supplies plants with P requirements, excessive application of P fertilizers is costly for the farmer and harmful to the environment. The content of phosphorus in plants varies from 0.2 to 0.8% dry weight, but only 0.1% of this phosphorus is available to plants . The main source of P for the plant remains in the soil solution. The P content values of agricultural soil solutions are generally very low and remain unsuitable for the needs of the host plant. With the ability to solubilize phosphate, the microbial system can compensate for the amounts of P required for growth of the host plant . Several rhizobia species may solubilize phosphorus, including
2.1.3. Siderophore formation
Iron is considered an essential micronutrient of plants and is present in the soil with a significantly different distribution. Iron can be present in different forms, either in divalent (ferrous or Fe2+) or trivalent (ferric or Fe3+) states. Soil pH and Eh (redox potential) and the availability of other minerals determine the state of iron in the soil . In aerobic environments, iron exists as insoluble hydroxides and oxyhydroxides, which are not available to plants and microbes . In general, bacteria have the ability to synthesize siderophores, low molecular weight compounds capable of sequestering Fe3+. These siderophores have a high affinity for Fe3+, making iron available to plants. Siderophores are soluble in water and exist in extracellular and intracellular environments. Fe3+ ions are reduced to Fe2+ and released into cells by Gram-positive and -negative rhizobacteria. This reduction leads to the destruction/recycling of siderophores . Siderophores can also form a stable complex with heavy metals such as Al, Cd, Cu, and so on and with radionuclides including U and NP . Thus, plant inoculation by siderophore-producing bacteria protects them from stress caused by heavy metals and helps them absorb iron. Several rhizobial species nodulating various legumes are known for their production of siderophores .
2.1.4. Phytohormone production
Substances that stimulate plant growth at low concentrations, less than or equal to micromolar concentrations are called phytohormones. These molecules include indole-3-acetic acid (IAA) (auxin), cytokinins, gibberellins and abscisic acid.
2.2. Indirect plant growth promotions
2.2.1. Biological control of plant disease
In addition to their plant growth promoting effects,
2.2.2. Antagonistic effects of rhizobia to pathogens and pest
Antagonism of pest and pathogen populations by
Several studies on the mode of action of
2.2.3. Induction of plant defense by rhizobia against pests and diseases
Rhizobium populations may also promote plant health by stimulating the plant host. The presence of
2.2.4. Resistance of rhizobia to abiotic stress factors
In the Rhizobium-legume symbiosis, which is a N2-fixing system, the physiological state of the host plant is a determining factor in the process of atmospheric nitrogen fixation. Therefore, limiting agents do not allow the tolerant and competitive rhizobium strains to express its full nitrogen-binding capacity, which affects the vigor of the host legume. In Tunisia, several factors may limit the symbiotic nitrogen fixation, particularly drought, especially since Tunisia is located in semiarid, arid and Saharan climatic zones where annual rainfall ranges from 100 to 300 mm . Drought affected the crop yields of pulses in Tunisia, which led farmers to abandon this crop in some areas. In addition to drought, legume crops are affected by salinity, soil pH, nutrient deficiency, mineral toxicity, extreme temperatures, diseases and pests .
2.2.5. Soil salinity
Salinity is considered a limiting factor in nodulation and nitrogen fixation in legume-Rhizobium associations, which can adversely affect the yield of legume crops . Rhizobia can tolerate high concentrations unlike legume plants. The growth of certain strains is inhibited by 100 mM NaCl [57, 58], whereas other strains such as
2.2.6. Water deficiency and drought
Water deficiency is a major limiting factor of symbiotic nitrogen fixation in many arid regions of the Mediterranean basin. One of the immediate responses of rhizobia to water deficiency concerns the morphological changes [62, 63]. Water stress allows the reduction of legume root infection by rhizobia, hence the reduction of nodulation. In addition, the water deficit also restricts the development and function of nodules [59, 64]. The development of effective nodules in desert soils highlights that some strains can tolerate extreme conditions in soils with limited moisture levels [65, 66, 67].
2.2.7. High temperature and heat stress
In temperate regions, the free life and symbiotic life of rhizobia is affected . The optimal temperature range for growth of rhizobial strains varies from 28 to 31°C. Some rhizobial strains cannot grow at 38°C, while others that survive heat stress can lose their nodulation power due to alteration of compounds involved in the infective process such as plasmid hardening or alterations of cellular polysaccharides . Nodules formed at high soil temperature (35–40°C) are usually ineffective formation; however, some strains of rhizobia, such as R.
2.2.8. Acid soils and soil acidification
Acid soils constrain agricultural production in worldwide , with the scope of the problem likely to increase as the result of acid rain, long-term N fertilization and legume N2 fixation. Legumes are particularly affected, acidity limiting both survival and persistence of nodule bacteria in soil, and the process of nodulation itself . The absence of nodules has been noted in legumes grown in acidic soils, particularly in soils with a pH below 5. The susceptibility of certain rhizobial strains to these conditions is a cause of inhibition of nodule formation [73, 74, 75]. Nodules are absent even when a viable population of Rhizobium can be demonstrated [76, 77]. Some researchers have observed that nodulation of
Rhizobia produce multiple beneficial effects on plant growth stimulation, host defense against disease and survival under stress with many other unknown benefits. This chapter describes the potential of rhizobia for the promotion of plant growth and highlights the different mechanisms of growth stimulation and the spectrum of resistance available against various abiotic stresses in several crops. In sustainable agriculture, the biological fixation of nitrogen is an important process, particularly in the legume farming system. To benefit from leguminous crops, it would be interesting to select symbiotic pairs adapted to severe conditions and to fix considerable quantities of nitrogen. The importance of the
Conflict of interest
Authors confirm there are no conflicts of interest.