Stress physiology in Azospirillum and others PGPRs

GALLARATO LUCAS ANTONIO; PAULUCCI NATALIA SOLEDAD; CHIAPPERO JULIETA; CESARI ADRIANA B; REGUERA YANINA B; VICARIO JULIO CESAR; DARDANELLI MARTA SUSANA

Handbook for Azospirillum

Springer

Lugar: Berlín; Año: 2014;

Improvement in agricultural sustainability requires optimal use and management of soil fertility and soil physical properties, and relies on soil biological processes and soil biodiversity (Choudhary et al. 2011). The continues use of chemical fertilizers and manures to enhance soil fertility and crop productivity often results in unexpected harmful environmental effects, including leaching of nitrate into ground water, surface run-off of phosphorus and nitrogen run-off, and eutrophication of aquatic ecosystems. All plants are known to perceive and respond to stress signals such as drought, heat, salinity, herbivory, and pathogens (Hirt, 2009). Soil grown plants are immersed in a sea of microbes and diverse beneficial microorganisms such as plant-growth promoting bacteria (PGPB) as well as plant-growth-promoting fungi (PGPF) can stimulate plant growth and/or confer enhanced resistance to biotic and abiotic stresses (Lugtenberg and Kamilova 2009). Soil microorganisms may comprise mixed populations of naturally occurring microbes that can be applied as inoculants to increase soil microbial diversity. Investigations have shown that the inoculation of efficient microbial community to the soil ecosystem improves soil quality, soil health, growth, yield and quality of crops. These microbial populations may consist of selected species including plant growth promoting bacteria, N2-fixing microorganisms, plant disease suppressive bacteria and fungi, actinomycetes and other useful microbes. Microbial inoculants are promising components for integrated solutions to agroenvironmental problems because inoculants possess the capacity to promote plant growth, enhance nutrient availability and uptake, and support the health of plants (Adesemoye and Kloepper 2009). Bacteria belonging to different genera including Rhizobium, Bacillus, Pseudomonas, Pantoea, Paenibacillus, Burkholderia, Achromobacter, Azospirillum, Microbacterium, Methylobacterium, Enterobacter, among others, provide tolerance to host plants under different abiotic stress environments (Egamberdieva and Kucharova 2009). Rhizobia (Azorhizobium, Bradyrhizobium, Mesorhizobium, Rhizobium and Sinorhizobium), and can also be considered as a soil bacteria with PGPR activity, where root colonization and growth promotion of rice, cereals, and other non-legumes have been reported (Chabot et al. 1996). Use of these bacteria (rhizobacteria) per se can alleviate stresses in an agriculture thus opening a new and emerging application of microorganisms (Choudhary et al. 2011). Rhizobacteria are constantly faced with environmental stimuli stresses and should be responding to a wide range of factors through signal transduction pathways that convert extracellular information into intracellular forms. The rhizosphere, the soil zone influenced by plant roots is dynamic. Its extent and properties are influenced by soil physical and chemical properties, weather and plant-induced changes in soil water content, the composition and density of soil microbial populations, and the metabolic activities of plants and microbes (Miller and Wood 1996). To survive in different conditions, rhizobacteria need to adapt by the accumulation or the release of specific solutes and changes in their membranes.Rhizobia are distinguished by their existence as both free-living soil bacteria and as nitrogen-fixing root endosymbionts. Among the rhizobia, the osmoadaptive mechanisms of the relatively salt-tolerant species Sinorhizobium meliloti have been examined in great detail (Miller and Wood 1996; Vriezen et al 2012). Although Azospirillum is also a nitrogen fixing gram-negative bacterium found in the rhizosphere, it does not induce differentiated structures on the roots of its plant hosts. Instead, this bacterium colonizes the mucigel sheath layer or spaces between the root epidermis and cortex, thus forming an ?associative symbiosis? with a wide variety of plants (Okon 1985, Miller and Wood 1996). Among azospirilla, the osmoadaptive mechanisms of Azospirillum brasilense have received the greatest attention. The main purpose of the chapter is to analyze how different types of abiotic stress affect the physiology of Azospirillum and plant growth promoting rhizobacteria (PGPRs) and how to study the system