Halotolerant Plant Growth-Promoting Rhizobacteria in Mitigating Salinity Stress: Comparison
Please note this is a comparison between Version 2 by Sirius Huang and Version 1 by Surya Kant.

Soil salinity is one of the major abiotic constraints in agricultural ecosystems worldwide. High salinity levels have negative impacts on plant growth and yield, and affect soil physicochemical properties. Salinity also has adverse effects on the distribution and abundance of soil microorganisms. Halotolerant plant growth-promoting rhizobacteria (HT-PGPR) secrete secondary metabolites, including osmoprotectants, exopolysaccharides, and volatile organic compounds. The importance of these compounds in promoting plant growth and reducing adverse effects under salinity stress has been widely recognised. HT-PGPR are emerging as effective biological strategies for mitigating the harmful effects of high salinity; improving plant growth, development, and yield; and remediating degraded saline soils. 

  • exopolysaccharides
  • osmoprotectants
  • growth hormones
  • soil microbes
  • volatile organic compounds

1. Introduction

Food insecurity is a growing problem globally, owing to population growth, changing climates, and declining agricultural land, which threaten sustainable agriculture [1,2][1][2]. By 2070, the world population is expected to rise from 8 billion to 10 billion people [3]. This will require more farmland to meet the increasing food demand, but environmental factors including rising temperatures, erratic rainfall patterns, drought and soil salinity already limit land suitable for agricultural production [4]. Soil salinization is a major contributor to the degradation of agricultural land and reductions in crop productivity, with salinity affecting over 800 million hectares of land or 6% of the total worldwide land area [5]. Crop growth is impeded by salinity due to the toxicity of certain ions, nutrient imbalances, and osmotic stress [6], and low levels of organic matter in the soil, all of which can amplify the unfavourable effects of salinization [7,8][7][8]. The replacement of the ions responsible for salinity, either chemically or through the addition of organic materials has been effective in the amelioration of saline soils [9]. The productivity of such saline soils can therefore be increased through the adoption of new sustainable approaches, such as the use of inorganic or organic soil amendments and salt-resistant crop varieties [10].
Based on their responses to saline conditions, plants can be separated into two categories, glycophytes and halophytes, with the majority of plant species being glycophytes, and therefore relatively intolerant to salinity [11,12][11][12]. Halophytes, salt tolerant species, have evolved specialised strategies for survival in these conditions, such as root and shoot salt exclusion, ion compartmenting into various organs, and the synthesis of compatible solutes. The metabolic capacity of plants to respond to salt stress can also be enhanced by microbial interactions [13]. Research has shown many instances where microorganism-based plant biotechnology has proven to be more effective than traditional plant breeding or genetic modification techniques, and even soil amelioration approaches in alleviating soil constraints [14]. Microbes such as halotolerant plant growth-promoting rhizobacteria (HT-PGPR) can use a wide range of metabolic and genetic strategies to assist plants in mitigating the effects of salt stress and other abiotic stresses caused by harsh environmental conditions [15,16][15][16]. HT-PGPR are able to produce a wide range of secondary metabolites that aid in plant protection and assist in maintaining growth under saline environments, for both the symbiotic plant and bacteria species [17,18][17][18]. Most of these metabolites are only formed under abiotic stress conditions and enable the plant to survive in extreme climatic conditions, by acting on vital survival mechanisms including ion transport systems and uptake of osmoprotectants (Figure 1) [19]. Associated organic compounds have recently been shown to support plants in better adapting to saline conditions [20]
Figure 1. Effects of salinity stress and associated tolerance mechanisms induced by halotolerant plant growth-promoting rhizobacteria (HT-PGPR) in both plant roots and shoots.

2. HT-PGPR: Diversity and Their Effect on Crop Production

The use of HT-PGPR has recently emerged as a viable solution to issues associated with increasing soil salinity in agricultural lands [21]. These halophilic and halotolerant microorganisms are already adapted to thrive in salty environments [22] and through symbiotic relationships with associated plant species, they can influence host plant survival, root development, and growth (Figure 2) [23]. HT-PGPR can not only recognise and react to signal molecules secreted by plant roots, but also secrete a diverse range of signalling molecules that influence plant behaviour. These microbes also synthesis beneficial molecules including siderophores, phytohormones, volatile organic compounds (VOCs), exopolysaccharides (EPS), and other metabolites, along with solubilising nutrients such as phosphorus (P), zinc (Zn), and potassium (K) (Figure 2) [14,15][14][15]. These metabolites can assist plants through a wide range of biochemical, physiological, and molecular responses, including preserving ionic homeostasis through Na+/K+ transporters, enhancing water capacity, and activating SOS (Salt-Overly Sensitive) genes [24[24][25],25], together with acting as osmoprotectants, antioxidants, and compatible solutes. This three-tiered interwoven action–cumulation association between HT-PGPR and plant salt stress responses includes the existence of the bacteria itself in hyperosmotic conditions, the induction of salt-resistance in crops, and the improvement of soil quality [26] (Figure 1). Crop salt stress has been found to be reduced by several HT-PGPR species, including Azospirillum, Arhrobacter, Burkholderia, Alcaligenes, Bacillus, Enterobacter, Klebsiella, Microbacterium, Streptomyces, Pseudomonas, Pantoea, and Rhizobium [27]. These species may occur naturally around crop species or can be isolated from native halophytic plants.
Figure 2. Effect of HT-PGPR on plant growth and salinity tolerance in glycophytic plants. The plate shown includes potential HT-PGPR isolated from native halophytes and consists of rhizobacteria and endophytes. Beneficial effects are shown in blue boxes, the underlying processes in orange boxes, and benefits plants in the last column.
Wheat inoculated with HT-PGPR Enterobacter cloacae, Pseudomonas putida, Pseudomonas fluorescens, and Serratia ficaria, then cultivated in naturally saline fields (ECe = 15 dSm−1), had increased germination rates, percentage, and index by 51%, 43%, and 123%, respectively, in comparison to the nontreated controls, along with increased yield [28]. Another study found that the shoot and root fresh and dry biomass weights of Brassica juncea, cultivated in saline conditions (ECe = 12 dSm−1), significantly increased after inoculation with salt-tolerant Pseudomonas azotoformans JMM15 and Pseudomonas argentinensis HMM57 strains [29]. Additionally, an F-11 halotolerant Staphylococcus jettensis F1 increased the dry biomass of Zea mays by a factor of three when plants were grown under 200 mM saline stress [30]. Pseudomonas putida performed best among strains tested under high salinity (15 dSm−1), increasing plant height by 52%, root length by 60%, grain output by 76%, 100-grain weight by 19%, and straw yield by 67%, in wheat compared to the uninoculated crop [31]. Although it has been shown that HT-PGPR may improve crop production (in salt-affected soil) and reduce salinity stress, there is still much to learn about the interactions and processes that take place between plants and microorganisms under multidimensional stresses like salinity.

3. HT-PGPR and Their Effects in Mitigating Salt Stress in Crops

HT-PGPR provide plants with resistance to salt stress through several key processes. One such process is the regulation of the salt overly sensitive (SOS) pathway, which is involved in salt influx/efflux across membranes, through metabolites and associated gene expression patterns. It has been shown that the SOS1 gene is directly regulated by metabolites such as EPS, VOCs, and suitable solutes (i.e., proline, glycine betaines, and trehalose) [25], which also direct stress regulation in SOS genes [3], HKT1 transporter (high-affinity K) expression [32], and other genes implicated in the reduction in salt stress, i.e., ethylene biosynthesis and antioxidant protein encoding genes [33,34][33][34]. Examples of the beneficial effects of HT-PGPR, along with how they help plants cope with saline conditions in different plants, are reviewed and presented in Table 1
Table 1. Effects of inoculating plant species with various halotolerant plant growth-promoting rhizobacteria (HT-PGPR) species. Table summarises genes involved, mechanisms to increase salt tolerance, and actual effects observed in different plants.

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