Can Bacterial Endophytes Be Used as a Promising Bio-Inoculant for the Mitigation of Salinity Stress in Crop Plants?-A Global Meta-Analysis of the Last Decade (2011-2020).

Muhammad Aammar Tufail,Taimoor Hassan Farooq,Awais Shakoor,Afzal Ahmed Dar,Muhammad Saleem Arif,Gerardo Puopolo,Ilaria Pertot,Ana Bejarano,Asif Naeem

doi:10.3390/microorganisms9091861

Abstract

Soil salinity is a major problem affecting crop production worldwide. Lately, there have been great research efforts in increasing the salt tolerance of plants through the inoculation of plant growth-promoting endophytic bacteria. However, their ability to promote plant growth under no-stress and salinity-stress conditions remains largely uncertain. Here, we carried out a global meta-analysis to quantify the plant growth-promoting effects (improvement of morphological attributes, photosynthetic capacity, antioxidative ability, and ion homeostasis) of endophytic bacteria in plants under no-stress and salinity-stress conditions. In addition, we elucidated the underlying mechanisms of growth promotion in salt-sensitive (SS) and salt-tolerant (ST) plants derived from the interaction with endophytic bacteria under no-stress and salinity-stress conditions. Specifically, this work encompassed 42 peer-reviewed articles, a total of 77 experiments, and 24 different bacterial genera. On average, endophytic bacterial inoculation increased morphological parameters. Moreover, the effect of endophytic bacteria on the total dry biomass, number of leaves, root length, shoot length, and germination rate was generally greater under salinity-stress conditions than no-stress conditions. On a physiological level, the relative better performance of the bacterial inoculants under the salinity-stress condition was associated with the increase in total chlorophyll and chlorophyll-b, as well as with the decrease of 1-aminocylopropane-1-carboxylate concentration. Moreover, under the salinity-stress condition, bacterial inoculation conferred a significantly higher increase in root K+ concentration and decrease in leaf Na+ concentration than under the no-stress condition. In SS plants, bacterial inoculation induced a higher increase in chlorophyll-b and superoxide dismutase activity, as well as a higher decrease in abscisic acid content, than in ST plants. Under salinity-stress, endophytic bacterial inoculation increased root K+ concentration in both SS and ST plants but decreased root Na+ concentration only in ST plants. Overall, this meta-analysis suggests that endophytic bacterial inoculation is beneficial under both no salinity-stress and salinity-stress conditions, but the magnitude of benefit is definitely higher under salinity-stress conditions and varies with the salt tolerance level of plants.

Highlights

Global land resources are adversely affected by a range of abiotic factors including soil salinity, which is one of the most relevant threats to agricultural production and food security [1]
Our meta-analysis showed that endophytic bacterial inoculation had a positive impact on biomass production, which is in accordance with previous findings [32]
Our analysis suggests that endophytic bacterial inoculation is beneficial under both no-stress and salinity-stress conditions, but the magnitude of benefit is definitely higher under salinity stress conditions

Summary

Introduction

Global land resources are adversely affected by a range of abiotic factors including soil salinity, which is one of the most relevant threats to agricultural production and food security [1]. Soil salinity negatively affects many morphological and physical processes of plants including nutrients uptake, seed germination, and overall plant growth. After exposure to salinity, plants face an osmotic stress, which is followed by ion toxicity and nutrient imbalance. This condition, similar to water deficit, leads to the formation of hypertonic conditions outside the cell and impedes the plants to take up water. Excessive amounts of Na+ and Cl− damage plant cell walls, disturbs the osmotic balance, and modifies ion homeostasis within the cell, which induce changes in transpiration rate, translocations of nutrients, photosynthesis, and other metabolic processes [8]. Saline soils are likely dominated by Proteobacteria, Actinobacteria, Bacteroidetes, and Gemmatimonadetes, and by Acidobacteria, Firmicutes, Nitrospirae, and Verrucomicrobia [9] Yet, a relative abundance of Bacteroidetes and Proteobacteria has been positively correlated, while the abundance of Acidobacteria has been negatively correlated with high levels of salt [10]

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Results

Conclusion