Plant growth-promoting rhizobacterium Pseudomonas PS01 induces salt tolerance in Arabidopsis thaliana
Plant growth-promoting rhizobacteria (PGPR) may contribute to sustainable crop production by improving plant growth and/or plant tolerance to abiotic stresses. Soil salinity, which limits the productivity of crop plants, is one of the major concerns of modern agriculture, especially in countries heavily affected by climate change as Vietnam. Currently, only a few reports have studied local PGPR isolated in Vietnam, particular Pseudomonas. Therefore, our study aimed to isolate and identify a region-specific Pseudomonas strain and evaluate the effects of this strain on germination, growth promotion and gene expression of Arabidopsis thaliana under salt stress.
The Pseudomonas named PS01 was isolated from maize rhizosphere collected in Ben Tre province, Vietnam. This strain was identified as a member of the Pseudomonas putida subclade. Pseudomonas PS01 could improve the germination rate of Arabidopsis seeds in 150 mM NaCl. A. thaliana plants inoculated with Pseudomonas PS01 survived under salt stress conditions up to 225 mM NaCl, while all non-inoculated plants were dead above 200 mM NaCl. The transcriptional levels of genes related to stress tolerance showed that only LOX2 was up-regulated, while APX2 and GLYI7 were down-regulated in inoculated plants in comparison to the non-inoculated controls. In turn, RD29A and RD29B did not show any significant changes in their expression profiles.
KeywordsArabidopsis thaliana Plant growth-promoting rhizobacteria (PGPR) Pseudomonas PS01 Salt stress tolerance
- A. thaliana
days after sowing
Glyoxalase I 7
- MS ½
half-strength Murashige and Skoog medium
Relative to Dessication A
Relative to Dessication B
reactive oxygen species
relative transcript level
Soil salinity is a widespread problem that limits crop yield and cultivation throughout the world, including the Mekong Delta, Vietnam . Salinity creates ion imbalance and generates highly reactive oxygen species (ROS) in plants, which causes ion toxicity and oxidative stress [2, 3]. This, in turn, leads to plant growth inhibition, slower development, senescence and death. To improve plant salinity tolerance, several strategies, such as the use of fertilizers, traditional breeding and genetic engineering, have been extensively studied for decades . The application of plant growth-promoting rhizobacteria (PGPR) is one of the most promising alternative approaches to improve crop production in saline soils [2, 4, 5]. Various salt-tolerant PGPR including Azospirillum, Burkholderia, Rhizobium, Pseudomonas, Acetobacter and Bacillus have been successfully applied or tested for plant growth promotion under salt stress [6, 7]. The fluorescent Pseudomonas is considered an important model to assess beneficial plant–bacteria interactions, including plant growth promotion under abiotic stress [8, 9]. Inoculation of plants with Pseudomonas was found to alleviate salinity effects on plant development by reducing the uptake of toxic ions, inducing systemic resistance, producing phytohormones, increasing nutrient uptake and establishing root colonization [10, 11, 12, 13, 14]. Pseudomonas-induced salt tolerance has been mainly studied at the physiological and biochemical levels in plants. However, little is known about the transcriptional changes of plant salt-responsive genes in an interactive process .
To cope with salt stress, early plant responses include synthesis of ROS scavengers, detoxification of ROS and abscisic acid (ABA) signaling [15, 16, 17]. The glyoxalase pathway, which degrades methylglyoxal, is one of the main detoxification pathways . In the ABA response, the expression of RD29 (Responsive to Desiccation) genes including RD29A and RD29B is induced by salt stress . PGPR may significantly enhance plant antioxidant activities, and thus protect plants from salt toxicity, by increasing the expression of enzymes such as ascorbate peroxidase (APX), superoxide dismutase (SOD) and catalase [16, 19, 20]. Interestingly, A. thaliana inoculated with PGPR such as Burkholderia phytofirmans PsJN and Enterobacter spp. EJ01 showed an enhanced tolerance to salt stress that involved transcriptional changes of genes related to early stress responses [16, 19].
It is necessary to identify native microbial strains which can be used in regional crops as potential plant growth promoters to achieve desirable yields . The application of indigenous PGPR will provide more advantages for regional crops since PGPR can easily acclimatize to the local environmental conditions and enhance the plant–microbe interactions . In previous study, we successfully isolated and identified some Pseudomonas spp. capable of enhancing plant growth in Vietnam . Therefore, in this study, we aimed to isolate a Pseudomonas strain that can increase salt stress tolerance and to investigate the underlying molecular mechanisms.
Materials and methods
Rhizobacteria were isolated from maize (Zea mays L.) rhizosphere collected in Ben Tre province, Vietnam. A soil suspension was obtained by shaking roots with adhered soil in phosphate-buffered saline for 10 min at 180 rpm. The suspension was serially diluted, spread onto King’B medium (KB)  and incubated at 30 °C for 24 h. A single colony was picked up and re-cultured a few times on solid KB to obtain a pure culture. The presence of fluorescent Pseudomonas was examined under UV light. Total isolates were grown on different concentration NaCl (0–10%) to evaluate salt tolerance property.
For phenotyping, the bacterial strain was identified according to morphological and chemotaxonomic characters based on the Bergey’s Manual of Determinative Bacteriology. For genotyping, bacterial genomic DNA was extracted and purified using the Wizard Genomic DNA purification kit (Promega, USA). The complete 16S rDNA was amplified by using PCR with the universal bacterial primers 27F and 1492R (Additional file 1: Table S1) . The PCR product was sequenced and analyzed with BLASTn to identify the strain genus. Simultaneously, a fragment of rpoD gene was amplified by using the primers rpoD 70F and rpoD 70R (Additional file 1: Table S1) [26, 27]. The sequences of related species and genera were obtained from GenBank. Multiple sequence alignments were performed by ClustalX; the phylogenetic analysis was determined by employing the neighbor-joining method. The phylogenetic tree was constructed with MEGA version 6 Software .
Plant growth conditions and treatments
To define whether the bacteria had an effect on the germination of Arabidopsis in normal and saline condition, sterilized and synchronized seeds were inoculated with bacterial suspension (106 CFU/ml), or MgCl2 solution as a control or nongrowth-promoting strain Escherichia coli DH5α as a negative control, and germinated on solid, half-strength Murashige and Skoog medium (MS ½) plates with or without 150 mM NaCl. Plates were placed in a growth chamber at 22 °C with a photoperiod of 16/8 h (light/dark). Four days after sowing (DAS), the seed germination percentage was determined.
To investigate the effect of Pseudomonas PS01 on Arabidopsis salt tolerance under different NaCl concentrations, 4-day-old seedlings were transferred to solid MS ½ supplemented with different concentrations of this salt. Plates were placed vertically in a growth chamber at 22 °C with a photoperiod of 16/8 h (light/dark). After 7 days, the plant survival rate was determined.
RNA extraction and real-time PCR (RT-PCR) analyses
RNA extraction was performed on plantlets after being transplanted to MS ½ alone or supplemented with 150 mM NaCl for 24 h. RNA was obtained by using the Trizol (Invitrogen™, USA) method. RT-PCR was performed by using the Luna Universal One-Step RT-PCR Kit (New England Biolabs, USA). PCR primers are shown in Additional file 2: Table S2. The relative transcript level (RTL) was calculated by normalizing to ACT2 as follows: RTL = 2∆∆Ct, where ∆∆Ct = ∆Ct(gene) − ∆Ct(ACT2). All experiments were performed with three biological and two technical replicates.
For comparison between treatments, ANOVA was performed with Graphpad Prism 7.0.
Identification of bacteria
Pseudomonas PS01 enhances seed germination rate in salt stress conditions
Pseudomonas PS01 enhances salt stress tolerance
To address the effects of NaCl and PS01 on A. thaliana salt tolerance in vitro, the survival rates (%) of PS01-inoculated and non-inoculated plants grown on media supplemented with NaCl concentrations ranging from 75 mM to 225 mM were evaluated. On the 7th day after being transferred to the salt stress media, Arabidopsis seedlings were observed and photographed to identify the number of surviving plants.
PS01 root colonization was shown to increase the survival of plants exposed to saline concentrations ranging from 75 to 225 mM NaCl (Additional file 5: Fig. S3). For instance, all plants inoculated with PS01 could survive the 175 mM NaCl treatment as opposed to only 30–40% of the controls (Fig. 2b). These results suggest that Pseudomonas PS01 may enhance A. thaliana survival under salt stress.
Pseudomonas PS01 induces transcriptional changes in salt-stressed A. thaliana plants
To investigate the molecular mechanisms of PS01-induced salt stress tolerance in Arabidopsis, some genes related to early salt stress responses such as ROS scavenging (APX2), detoxification (GLYI7), ABA signaling (RD29A and RD29B) and JA synthesis (LOX2) were chosen for quantitative RT-PCR analysis. The gene expression profiles of these genes were obtained from 4 different samples: seedlings inoculated or non-inoculated with PS01 and grown in MS ½ alone or transferred to MS ½ supplemented with 150 mM NaCl.
PS01 is the first Pseudomonas strain isolated in Vietnam which alleviates the effect of salinity on plant growth. PS01 belongs to the Pseudomonas putida subclade based on the rpoD gene tree (Fig. 1), being closely related to P. taiwanensis (GenBank accession number HE577796.1). Interestingly, P. putida was already known to reduce the detrimental effect of salinity on germination and plant growth [7, 31].
In this study, Pseudomonas PS01 improved the germination rate of A. thaliana on salt stress medium (MS ½ supplemented with 150 mM NaCl). Compared to previous studies focusing on the effect of bacteria on plant growth, the germination process was accelerated and its period was also extended . For example, P. putida R4 and P. chlororaphis R5 improved cotton seed germination in response to NaCl stress up to 64 and 73%, respectively . In contrast, Pseudomonas spp. PDMZnCd2003 negatively affected rice germination, and reduced seedling shoot and root length . We have shown that PS01 helps improve salt stress tolerance in A. thaliana seedlings. Under salt stress conditions, the transcriptional level increase of GLYI7 and APX2 in seedlings inoculated with PS01 was lower than that of non-inoculated seedlings. Glyoxalase can be considered as an alarm in the abiotic stress response in plants . In turn, ascorbate peroxidases catalyze the conversion of H2O2 into H2O. The down-regulation of GLYI7 and APX2 suggests that inoculation with PS01 may reduce stress level in plants. Therefore, the mechanisms contributing to the survival of inoculated plants during salt stress can be related to other molecular pathways. Alternatively, the production of biofilms by the bacteria could be beneficial to plant survival under stress conditions as shown in recent reports, in which bacterial exopolysaccharide (EPS) and biofilm formation stimulated plant growth under salt stress by reducing Na+ uptake by the plant [20, 35]. However, further evidences for EPS production or other putative mechanisms participating in the PS01-mediated salt tolerance of plants need to be verified.
Jasmonate (JA) is a positive regulator of the salt stress response that accumulates rapidly when plants are submitted to stress [20, 36]. LOX2 encodes a lipoxygenase that constitutes an essential component of the JA synthesis pathway . Compared to non-inoculated seedlings in response to salt stress, the increase of LOX2 expression in PS01 inoculated seedlings in our study is in agreement with Cho et al.  and Poupin et al. , who also reported that PGPR such as Pseudomonas chlororaphis O6 and Burkholderia phytofirmans PsJN mediated systemic resistance against abiotic stress by increasing the expression of defense genes regulated by the JA pathway [14, 38]. The phenotypic changes, transcriptional profile and spatiotemporal responses under salt stress of plants inoculated with PS01 will be analyzed at different plant growth stages to clarify this pathway.
Our study has shown that Pseudomonas PS01 can improve Arabidopsis thaliana germination and survival rate under salt stress.
Although, mechanism underlying PS01-Arabidopsis interaction to enhance plant salt tolerance has not been fully discovered, further studies have been conducting in our group to provide new insights into this interaction including whole genome analysis and transposon mutant library screening of PS01, as well as long-term transcriptomic analysis of plants inoculated with this isolate. PS01 will be inoculated with maize under salt stress and be developed as a biofertilizers in the future prospect.
TNC, BTHT and MTTH designed this research, performed the experiments, analyzed the data and wrote the manuscript. LVB reviewed of the literature and provided technical assistance to TNC. All authors read and approved the final manuscript.
We would like to thank Dr. Stéphane Mari (BPMP Montpellier, France) for providing Arabidopsis thaliana Col-0 seeds. We also thank Dr. Tran Hong Nha Nguyen (University of Science, VNU-HCMC, Vietnam), Dr. Tuan Minh Tran (Nanyang Technological University, Singapore) and Dr. Ngoc Bui (Department of Molecular Oncology, H. Lee Moffitt Cancer Center, Tampa, Florida) for critical reading, editing and valuable suggestions for this manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
The data supporting the conclusions of this article is included within the article and its additional files.
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This research was funded by Vietnam National University-Ho Chi Minh City under Grant Number (C2018-18-19). The World Academy of Sciences (TWAS) supported the equipment and publication fee under research Grant Award numbers 16-142 RG/BIO/AS_I–FR3240293339. The funding bodies had no role in the design, collection, data analysis, interpretation and reporting of data or in writing the manuscript.
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