An overview on improvement of crop productivity in saline soils by halotolerant and halophilic PGPRs

Abstract

Salinity of water and soil are of the most important factors limiting the production of crops. Moreover, with the increasing population of the planet and saline fields worldwide there is no choice but to use saline soil and water in the near future. Therefore, to increase plant growth under saline stress condition, provision of sustainable and environmentally friendly management for the use of saline water and soil resources is necessary. The development of saline resistant plants is a potent approach to solve this problem. Generally, soil salinity negatively affects the plant growth through ion toxicity, oxidative stress, osmotic stress and ethylene generation. In recent years, scientists through genetic engineering techniques, which are based on molecular and physiological characteristics of plants, have made salt tolerance plants. However, the validation of the present technique is restricted to laboratory condition and it is not easily applied in the agronomy research under field environment. Another option would be to isolate and utilize salinity resistant microorganisms from the rhizosphere of halophyte plants, namely plant growth-promoting rhizobacteria (PGPR). The mechanisms of these bacteria includes; ACCdeaminase and exopolysachared production, osmolite accumulation, antioxidant system activation, ion hemostasis and etc. In this review, we will discuss mechanisms of PGPR in producing tolerate plants under salt stress and how to improve the plant-microbe interactions in future for increasing agricultural productivity to feed all of the world's people.

When Salt Meddles Between Plant, Soil, and Microorganisms

“…In response to salinity stress, ST-PGPRs accumulate many metabolites called compatible (organic) solutes, among which amino acids and derivatives (e.g., glutamate, proline, peptides, and N-acetylated amino acids), quaternary amines (e.g., glycine betaine and carnitine), sugars (e.g., sucrose and trehalose), and tetrahydro pyrimidines (ectoines). These metabolites enhance the stability of protein conformation, the balance of cell redox condition, cytosolic pH, complex II electron transport, membrane integrity, and the activity of enzymes such as ribulose bisphosphate carboxylase/oxygenase (RUBISCO) (Saghafi et al, 2019). Moreover, the storage of osmolytes represents a successful stress response mechanism that helps the bacteria to limit water loss, increasing their cytoplasmic concentration of K + .…”

“…Halophytic crop species used in agriculture can be limited by the lack of available nitrogen often affecting saline soils. PGPRs can fix nitrogen through symbiotic and non-symbiotic mechanisms (Saghafi et al, 2019). The first method involves the formation of nodes in the host roots by bacteria which results in nitrogen content of approximately 65% of the total nitrogen assimilation by plants (Rajwar et al, 2013).…”

“…Moreover, at the rhizosphere level, it has been found differential regulation of the expression of genes involved in ion affinity transporters. PGPR often impact the mineral nutrient exchange of both macro and micronutrients as a strategy to react to nutrient imbalance due to a higher uptake of Na + and Cl − ions ( Saghafi et al., 2019 ). Both fungi and bacteria can help plants to keep cellular ion homeostasis and sustainable K + /Na + ratios in shoots.…”

Secondary Metabolites From Halotolerant Plant Growth Promoting Rhizobacteria for Ameliorating Salinity Stress in Plants

“…in minimizing osmotic stress, maintain high turgor pressure, and equalize ion efflux across the plasma membrane in their mutualistic partner plants (Dodd and Perez-Alfocea, 2012). These compounds also control stomatal opening, hydraulic conductance, and transpiration rate to alleviate the water deficiency in plants (Paul and Lade, 2014;Saghafi et al, 2019). Amino acids and their derivatives (e.g., proline, glutamate, glycine betaine and ectoine), polyols (e.g., glycerol, inositol, sorbitol and mannitol), and non-reducing sugars (e.g., trehaloses) are some of the major compatible solutes produced by HT-PGPR under saline conditions (Sleator and Hill, 2002).…”

Nodule and Root Zone Microbiota of Salt-Tolerant Wild Soybean in Coastal Sand and Saline-Alkali Soil

“…These beneficial soil microbes reside in the rhizosphere and, together with root exudates, can provide plants with nutrients, growth hormones, antioxidants, and systemic resistance, even under high salt concentrations (Ahmad et al, 2013;Li et al, 2014;Kong et al, 2015;Agler et al, 2016). Indeed, salt-tolerant PGPR and their metabolites isolated from halophyte species in saline soils can play key roles in mitigating salinity stress and enhancing crop yield (Bouhmouch et al, 2005;Albdaiwi et al, 2019;Chauhan et al, 2019;Chen et al, 2019;Saghafi et al, 2019). Different microbes have been associated with various plants and growth environments, suggesting the existence of specific microbe-host interactions (Perez-Jaramillo et al, 2019).…”