ABSTRACT
Abstract. Excessive salt accumulation in soils is a major ecological and agronomical problem, in particular in arid and semiarid areas. While important physiological insights about the mechanisms of salt tolerance in plants have been gained, the transfer of such knowledge into crop improvement has been limited. The identification and exploitation of soil microorganisms (especially rhizosphere bacteria and mycorrhizal fungi) that interact with plants by alleviating stress opens new alternatives for a pyramiding strategy against salinity as well as
new approaches to discover new mechanisms involved in stress tolerance. Considering the kingdom of fungi, arbuscular mycorrhizal fungi (AMF) stand out as the most significant and widespread group of plant growth-promoting microorganisms. Ectomycorrhizal fungi (EMF) are also important symbionts of particular relevance for many woody plants. Considering the kingdom of bacteria, a wide range of microorganisms including different species and strains of Bacillus, Burkholderia, Pseudomonas, and the well-known nitrogen-fixing organisms
15.1 Introduction / 268 15.2 Strategies Used by Rhizosphere Microbes to Confer Plant Salt Tolerance / 270
15.2.1 Improving Plant Growth and Productivity / 270 15.2.2 Regulating of Water Uptake and Distribution to Plant Tissues by the Action of
Aquaporins / 271 15.2.3 Inducing the Synthesis and Accumulation of Compatible Solutes / 273
15.2.3.1 Soluble Sugars / 273 15.2.3.2 Proline / 274 15.2.3.3 Glycinebetaine / 274 15.2.3.4 Polyamines / 275
15.2.4 Regulating the Ionic Homeostasis / 275 15.2.5 Maintenance of Photosynthetic Capacity / 276 15.2.6 Improving Protein Synthesis / 277 15.2.7 Source-Sink Relations and Energetic Metabolism / 278 15.2.8 Altering Plant Hormonal Status / 279
15.2.8.1 Auxins / 279 15.2.8.2 Abscisic Acid / 280 15.2.8.3 Ethylene / 280
15.2.9 Enhancing Antioxidant Defense System / 281 15.3 Commercial/Agronomic Prospects / 282 15.4 Sustainable Intensification of Agriculture / 283 15.5 Conclusion / 284 References / 285
Rhizobium, Bradyrhizobium, Azotobacter, Azospirillum, and Herbaspirillum are classically regarded as important plant growth-promoting rhizobacteria (PGPR). Today, it is widely accepted that AMF, EMF, and PGPR promote plant growth and increase tolerance against stress conditions, at least in part, because they facilitate water and nutrient uptake and distribution as well as alter
plant hormonal status, and this ability has been attributed to various mechanisms. This chapter addresses the significance of soil biota in alleviation of salinity stress and their beneficial effects on plant growth and productivity. Moreover, it emphasizes new perspectives and challenges in physiological and molecular studies on salt stress alleviation by soil biota.
