Effectiveness of Glutamicibacter sp. against the salt stress in plants

Salinity is a widespread abiotic constraint which has serious adverse effects on plant growth and crop productivity. Halotolerant PGPR (Plant growth-promoting rhizobacteria) that lives in close association with the root zones of saline soils is good alternative to mitigate salinity stress. Therefore, this study aimed to assess the effectiveness of Glutamicibacter sp. strain to enhance the establishment and growth of the common reed (Phragmites australsis) under saline conditions. After a period of a non-saline treatment, P. australsis plants with and without Glutamicibacter sp. were grown for twenty days under salinity levels that were achieved using 0 and 300 mM NaCl under greenhouse conditions.

The results indicated that inoculation of this isolate to P. australsis plant significantly increased plant growth in terms of various growth parameters such as number of leaves and thallus (56%–33%), shoot fresh weight/dry weight (40–56%), and root fresh weight (56%), under saline conditions. The bacterium strain also elevated the content of polyphenols (21%), whereas decreased the level of total soluble sugar and proline. In addition, it decreased the Na+ accumulation in shoot by 20%, and increased the K+ uptake in both shoot and root in the range of 30%, thereby increasing their K+/Na+ ratio.  Furthermore, it increased the electrical conductivity of saturation paste extract (ECe) from 4.11 to 5.35 dS m-1 and the soluble sodium concentration from 3.1to 5.49 g.kg-1 in the soil under salt conditions. Therefore, we concluded that Glutamicibacter sp. could serve as a useful tool that mitigates the deleterious effects of salinity in plants.

Salt stress is one of the major abiotic stressors that undermine plant growth and limit crop productivity in arid and semi-arid regions around the world (Porcel et al. 2012). Elevated soil salt concentration causes ion imbalance, osmotic stress, and leads to oxidative damage as a secondary stress (Zhu 2001). These changes may result in plant growth cessation or even death (Chinnusamy et al. 2006). To address saline soils and improve plant salt tolerance, many approaches are employed to combat salt stress; of these approaches, the addition of salt tolerant microorganisms to the soil has been considered as a cost-effective alternative to alleviate salinity stress in plants (Dodd and Pérez-Alfocea 2012; Egamberdieva et al. 2019).

Furthermore among the microorganisms colonized plant roots, plant growth-promoting rhizobacteria (PGPRs) have been effective at improving plant stress tolerance (Etesami and Beattie 2017; Etesami 2018). In the last decade, researchers documented that bacteria belonging to various genera including Rhizobium,

Bacillus, Pseudomonas, Azotobacter, Azospirillum and Enterobacter exhibited excellent outcomes to host plants against different abiotic stress environments such as salinity (Grover et al. 2011; Upadhyay et al. 2009; Sarkar et al., 2018). PGPRs naturally protect plants from the adverse effects of soil salinity by reprogramming the stress induced physio-chemical changes in plants that called induced systemic tolerance (IST) (Yang et al. 2009, Chen et al. 2016; Etesami and Beattie 2017; Etesami 2018).

Many researchers have reported that PGPR may increase the plant’s capacity to handle stress through phosphorus solubilisation, nitrogen fixation, iron sequestration, production of plant growth-promoting hormones (mainly auxins), ACC (1-aminocyclopropane-1-carboxylic acid) deaminase synthesis that degrade the precursor of ethylene, biofilm formation and exopolysaccharides (EPSs) production (Ullah et al. 2015).  Additionally, PGPR is able to activate plant antioxidant defence machinery to alleviate oxidative damage in plants suffering from abiotic stress (Kohler et al. 2009; Bano and Fatima 2009; Kumar et al. 2018), produce volatile organic compounds (VOCs) to regulate the Na+ content in plants (Zhang et al. 2008; Yang et al. 2009) and to maintain a favourable ratio of K+/Na+ ions amenable for plant growth and induce of transcription factors under stress responses (Gupta et al. 2012).

In summary, organic compounds produced by PGPR regulate plant photosynthesis, hormone homeostasis, osmotic homeostasis, cell membrane integrity, detoxification and ionic balance to confer plant salt tolerance (Radhakrishnan and Baek 2017; Chen et al. 2017) It appears that PGPR could be an important group of beneficial rhizosphere microbes that can be exploited to alleviate salt stress and confer tolerance of host plants (Chen et al. 2016; Sharma et al. 2016; Singh and Jha. 2016; Chen et al. 2017; Ilangumaran and Smith 2017; Vimal et al. 2018; Ma et al. 2019). Therefore, inoculation with plant growth-promoting halotolerant bacteria could be a rapid and efficient alternative to improve plant stress tolerance of host plants.

The isolation and identification of moderately halophilic PGPRs strains can be exploited to develop effective bio-inoculants to enhance common read (Phragmites australsis) production in salt affected areas. There are few reports on application of PGPR inoculants to mitigate salt stress, particularly the role of Glutamicibacter sp.  on P. australsis plants.  Hence, the present study was conducted to investigate the effectiveness of salt tolerant PGPR strain (Glutamicibacter sp.)  isolated from saline soils to improve plant growth of P. australsis plant under salt stress. Glutamicibacter sp.  was selected on the basis of its in vitro PGP activity and 16S rRNA gene sequencing for application.