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Science , — Galloway J. Biogeochemistry 70 , — Oldroyd G. Zahran H. Moreau D. Paungfoo-Lonhienne C. The pH of the solution was adjusted to 2. For clean up, hydrophilic-liphophilic bal- ance HLB polymer cartridges Oasis Waters and strong anion exchanger SAX cartridges Isolute were set up in tandem on a vacuum manifold system Phenomenex. Representative chromatograms of the study compounds The SAX cartridges were then removed and the HLB car- sulfadiazine [1], sulfathiazole [2], oxytetracycline [3], chlortetra- tridges were washed sequentially with washing buffer, 0.
The solution a , control soil b , spiked soil at a spiking level of 0. Interaction between soil and antibiotics Table 1. Characteristics of soil samples listed with mean values and standard deviations.
The mean rates of clay, silt and sand in the soil identify the composite factors accounting for the maxi- samples were These mum fraction of variance present in the original groups. The to identify a unique set of closely related original variables pH values of the soil samples were in the range of 6. The factors having eigenvalues greater 7. The OC content of the than one were retained, since a factor with an eigenvalue soil samples varied from 1.
Multiple stepwise regression allows for the examination of which independent variables in the combination most Table 2. Heavy metal levels of soil samples listed with mean correctly predict the group assignment.
The group assign- values and standard deviations. Multiple stepwise regression was cho- E3 0. A proba- F2 1. The OTC recovery Furthermore, the CTC recovery at a spiking level of 0.
For SAs, the lowest and highest mean recovery rates were Recovery rates of tetracyclines and sulfonamides in inves- OTC was detected in four samples with concentra- tions ranging from 0. The mean values of TP and TN were SAs could and Downloaded by [Bogazici University] at 26 February The heavy metal levels of soil samples are given in Table Statistics 2, and were found in the range from 0.
The combined Pearson and Spearman correlation matrix for the physicochemical properties of soils. Table 4 shows the results of the factor analysis, including the loadings, eigenvalues and rotation sums of squared loadings. Concentrations of the antimicrobial compounds detected in investigated soil samples.
This factor accounted for The third factor Factor 3 consisted of Downloaded by [Bogazici University] at 26 February inter-correlated soil parameters were clay, sand, pH, mois- three items Factor 4 loaded on variables Multivariate statistical methods were chosen to evalu- K, Cu, percentage of moisture, Fe and Zn Factor analysis results for physicochemical properties gression method in the factor analysis.
Stepwise multiple of the soils. The unbalanced utilization of manures, the use of saline water in the water system, and desertification increase the saltiness of cultivable soils [2]. The saltiness of arable terrains is a noteworthy issue in agribusiness.
Around 0. The saltiness obstructs root 4. Microorganisms , 9, The beneficial microbiota, plant growth promoting rhizobacteria PGPR , and arbuscular myc- orrhizal fungi AMF , which occur naturally in the soil and those introduced to combat salinity stress play a key role in the survival of plants [9,10].
Effect of Soil Saltiness on Plant Development Excessive salt concentrations in the soil affect plant survival by upsetting cell home- ostasis and uncoupling major biochemical and physiological processes [1]. An antagonistic impact of saltiness on complex associations among mor- phological, physiological, and biochemical procedures include seed germination, plant development, and water and supplement uptake [16,17].
Saltiness additionally influences the developmental proteins, lipid digestion, and photosynthesis [18]. Overall, nutrient deficiency, decrease in osmotic pressure, and reduced water uptake from the soil are the main consequences of soil saltiness.
Plant Growth Promoting Bacteria The tight zone of soil, encompassing the root framework, is known as the rhizo- sphere [19,20]. Rhizobacteria are important for maintaining the rich- ness of soil as they are fundamental specialists in reusing soil supplements [21]. They include evolutionarily diverse microbes that have the exceptional ability to enhance growth and yield of numerous crops and wild plants [23].
These helpful microorganisms colonize the rhizosphere of plants and increase plant growth and development through different mechanisms [2,24,25].
One potential approach to diminish negative ecological effects that occurred because of the utilization of concoction of manures, herbicides, and pesticides is to use PGPR. PGPR promote the development of plants, sequestration of substantial metals, and counteract the negative effects of pesticides, thereby helping in bioremediation of polluted soils [26,27]. The utilization of PGPR in agribusiness began in the s, and their formulations are available commercially as biofertilizers and biopesticides [28].
PGPR have provided better financial returns because of their capacity to improve seed germination rate and increase crop development and yield of crops [27,29]. Numerous studies have shown the different mechanisms of action of PGPR and their applications in agriculture [33]. The generation of phytohormones by PGPR enhances plant growth [34].
Figure 1. PGP traits of bacteria. Traits that have direct effects and those that have indirect effects suppression of diseases on plant growth are shown in the figure.
Plant diseases reduce plant growth and development under both normal conditions and abiotic stress. PGPR modify the endogenous hormonal status of the plant, thereby improving the salt resilience of plants [44—46].
Several examples where PGPR enhanced plant growth and yield- related parameters and biofortification under salt stress are shown in Table 1. Figure 2. PGPR-mediated salt tolerance by multiple rhizospheric interactions in soil. Microorganisms , 9, 4 of 19 Table 1. Wheat [52] and proline content Planococcus rifietoensis Wheat Enhanced growth and yield [53] Thalassobacillus, Bacillus, Halomonas, Increased the root and shoot length, and plant Wheat [54] Oceanobacillus, Zhihengliuella sp. Wheat [56] protein content of treated plants Enterobacter cloacae Maize Increased root and shoot growth [57] Enhanced nutrient, chlorophyll, and protein Staphylococcus sciuri Maize [58] content Increased seed germination, plant growth, and Phosphate solubilizing bacteria Maize and Peanut [59] P content Curtobacterium flaccumfaciens Barley Increased plant growth [60] P.
This is accomplished by increasing the production of plant metabolites such as betaine, proline, and trehalose, and antioxidant enzymes such as SOD and CAT that scavenge reactive oxygen species [69].
Although PGPR are used as inoculants for biostimulation, biocontrol, and biofertiliza- tion [71,72] to facilitate plant growth of many cereals and other important agricultural crops, they can also improve the growth and yield under saline conditions [73—76].
Likewise, ACCD can protect plants from pathogenic microorganisms and drought stress. ACCD is a multimeric enzyme with a monomeric subunit atomic mass of roughly 35—42 kDa.
ACCD uses pyridoxal 5-phosphate as a cofactor [81]. Pyridoxal phosphate is firmly bound to the protein with roughly one particle for every subunit resulting in pyridoxaldimine with absorbance at nm.
Their substrate ACC is plant- produced but the enzyme is located in the cytoplasm of the microorganism that produces it. Microorganisms , 9, 5 of 19 The microbes reduce plant ethylene levels, thereby enhancing plant growth and development, particularly under stressful conditions. This leads to an increase in the root surface area for efficient interaction with soil microscopic organisms and the release of exudates.
The exudates might be used by the rhizospheric microscopic organisms as a nutrient source. ACC is released along with other root exudates. Figure 3. Salt stress increases ethylene production, thereby reducing plant growth. Adapted from del Carmen Orozco—Mosqueda et al. The plants produce more ACC than needed and furthermore, invigorate the exudation of ACC from the plant, some of which may happen as an outcome of enhanced plant cell division brought about by bacterial IAA [38]. Accordingly, plant growth promoting mi- crobes are provided with a one-of-a-kind wellspring of nitrogen due to ACC that empowers them to multiply under conditions in which other soil microscopic organisms may not promptly thrive.
As ACC deaminase acts as a sink for ACC and brings down ACC levels inside the plant, the inhibition of plant growth and development by ethylene particularly amid times of stress including salinity stress is diminished, and these plants, for the most part, have longer roots and shoots and greater biomass. Microorganisms , 9, 6 of 19 Table 2. Proteamaculans Wheat Increased plant height, root length, and [91] grain yield P. Fluorescens Azospirillum strains Wheat Increased shoot dry weight and grain [93] yield Pseudomonas putida, Pseudomonas fluorescens, Wheat Enhanced germination rate and [94] Enterobacter cloacae, Serratia ficaria improved the nutrient status Bacillus, Hallobacillus Wheat Enhanced plant growth [2] Klebsiella sp.
Wheat Increased plant biomass and chlorophyll [95] content B. Maize Improved growth, yield, and nutrition [78] ferrugineum, P. Barley and Oats Enhanced root biomass [] Aneurinibacillus aneurinilyticus, Paenibacillus French bean Enhanced plant growth [] sp.
Paenibacillus mucilaginosus strain N3 Green gram Increased overall dry biomass [] Bacillus megaterium, Variovorax paradoxus Cucumber Increased growth [] Pseudomonas strain Groundnut Increased total yield [] Leclercia adecarboxylata Tomato Improved plant growth [] 4. AMF are endomycorrhizal fungi the hyphae of fungi penetrate the cell wall and invaginate the cell membrane that belong to the phylum Glomeromycota [].
AMF form vesicles, arbuscules, and hyphae in the associated roots, and produce spores and hyphae in the rhizosphere. The development of a hyphal network by the AMF, which is connected with plant roots, provides plants greater access to soil surface area, resulting in improved growth [,]. AMF boost plant nutrition by increasing the availability and translocation of various nutrients.
They secrete a proteinaceous compound, glomalin, which helps soil aggregation and stimulates nutrient cycling. AMF play a vital role in improving soil quality and, ultimately, plant health []. A number of research studies have reported the ability of AMF to improve plant growth and yield under salinity stress Table 3.
They are known to promote salinity tolerance by employing several mechanisms, such as enhancing water use efficiency and nutrient acquisition by producing plant growth hormones and regulators, improving photosynthetic rate, balancing ionic equilibrium, and producing antioxidants [16,—].
Table 3. Response of AMF on different plants against salinity stress. Increased absorption of P via the mycorrhizal fungi contributes most to improve plant growth under salt stress []. However, other metabolic processes such as enhanced N assimilation and absorption of other nutrients such as N, K, and Mg seem to be involved in alleviating the deleterious effects of salinity []. AMF-plant symbiosis has been demonstrated to increase salinity tolerance in various host plants such as wheat, alfalfa, maize, and tomato Table 3.
Enhanced Water Uptake AMF are known to improve the water absorption capacity of plants, due to the net- work expansion of extraradical hyphae in the soil that pulls more water, making it available to the plant. In addition, AMF induce major changes in the relative abundance of organic so- lutes by modifying the composition of carbohydrates and inducing accumulation of specific osmolytes such as proline, glycine, and betaine, thus facilitating osmotic adjustment [].
Furthermore, AMF are able to enhance the functioning of water channel proteins, aquapor- ins, by modulating their expression, thereby helping in the transport of water inside the cells and maintaining the cellular osmoregulation [,]. GintAQPF1 and GintAQPF2, the two aquaporin genes present in the AM fungus Glomus intraradices, were found to be overexpressed under osmotic stress conditions, making the fungus tolerant to stress and increasing water supply to the host plant [].
For instance, the AMF associated with Oryza sativa have been shown to regulate the expression of genes encoding transporters, i. Microorganisms , 9, 9 of 19 4. Phytohormone Synthesis The AMF produce auxins and cytokinins CKs that help in the growth and devel- opment of the plant and also stimulate the synthesis of these hormones in plants under stress []. Modulation of phytohormone synthesis by AMF confers drought and salt tolerance in plants [].
Improved Photosynthesis Salinity stress decreases photosynthesis by reducing chlorophyll content and photo- synthetic enzymes activity. This results in higher PSII efficiency and enhanced photosynthetic capacity. Glomus mosseae inoculation significantly increased leaf chlorophyll content in peanut plants under salinity stress []. Similarly, tomato plants treated with salt exhibited a higher amount of chlorophyll a and b, total chlorophyll content, and carotenoid content after inoculation with AMF [].
These enzymes help to allevi- ate the excess ROS and maintain the equilibrium of the formation and removal of ROS, providing the host plant better tolerance against oxidative stress. This synergistic effect is a result of positive interac- tions between PGPR and mycorrhizal fungi that help promote the growth of each other, which ultimately benefits the plant []. In addition, upregulation of sodium ion channels, ABA-signaling, and salt overly sensitive SOS pathway mediate superior plant performance under a saline environment [].
Microorganisms , 9, 10 of 19 Figure 4. Although an increase in plant growth and grain yield was observed when PGPR and AMF are used in combination, several factors such as environmental conditions, soil quality, and the micro- bial strains used, contribute to variable results. Co-inoculation of Rhizobium with AMF resulted in significant enhancement of yield, nodulation, leghemoglobin, nitrogenase activity, IAA synthesis, and nutrient uptake of alfalfa subjected to salinity stress [].
Inoculation of soybean with AMF improved various attributes as observed in alfalfa, but also conferred protection against membrane damage by reducing hydrogen peroxide and lipid peroxidation []. Morphological and genetic level approaches to study genes associated with metabolism, nitrogen fixation, and cell colonization events revealed the occurrence of nutritional exchanges between endobacteria, fungi, and plants.
Some AMF species produce metabolites such as organic acids, volatile compounds ethylene , and nonvolatile compounds that attract specific bacte- ria [].
The expression of phosphate transporter genes was also upregulated. Callose deposition under salt stress is mediated by Cys-rich receptor-like kinase 2 []. Microorganisms , 9, 11 of 19 Table 4. Glomus intraradices, G. Significant effect on chlorophyll index and Potato P and Bacillus [] fasciculatum phosphorus absorption Bacillus subtilis and B.
Pseudomonas mendocina Enhanced plant biomass [] 6. Conclusions Salinity stress is a major deterrent to agricultural production. It has devastating effects on plant growth and reproduction, resulting in reduced yield.
Plants have an inherent ability to respond to specific types of stress. PGPR play key roles in salt stress tolerance and plant growth promotion, with direct and indirect mechanisms.
The increase in N content in the rhizosphere of legumes considerably accounts for improvement in nodulation and N-fixing capacity, resulting from cooperative interaction of Rhizobium and AMF. PGPR and AMF can colonize the root—soil environment to enhance plant growth, yield, nutrient content, and soil health due to synergistic interactions.
This is achieved through the production of phytohormones and antioxidants, ionic homeostasis, and improved photosynthesis under salinity stress.
The exploitation of these microbial populations needs a systematic strategy to optimize their potential in enhancing plant tolerance to salt stress. The employment of PGPR and AMF in field conditions has certain limitations such as short shelf life, variability in performance, and effect on the diversity and abundance of soil microbiota based on short term studies.
Some signaling pathways are common to biotic and abiotic salinity stress stress. PGPR evade plant defense systems. These mechanisms, if transmitted to pathogens, can have deleterious effects on plants. Author Contributions: A. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest.
References 1. Etesami, H. Halotolerant plant growth-promoting bacteria: Prospects for alleviating salinity stress in plants. Ramadoss, D. Mitigation of salt stress in wheat seedlings by halotolerant bacteria isolated from saline habitats.
SpringerPlus , 2, 6. Parihar, P. Effect of salinity stress on plants and its tolerance strategies: A review. Egamberdieva, D. Salt-tolerant plant growth promoting rhizobacteria for enhancing crop productivity of saline soils. Reporting from:. Report wrong cover image. Your name. Your email. Send Cancel. Toggle navigation Menu. Help Need help? Chat with us limited to Stanford community Email a reference question Find a subject specialist Using SearchWorks Connection Connect to e-resources Report a connection problem If we don't have it Interlibrary borrowing Suggest a purchase limited to Stanford community System status Access Advanced search Course reserves Selections 0 Clear all lists.
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