GIBBERELLIN PRODUCING RHIZOBACTERIA Pseudomonas koreensis MU2 ENHANCE GROWTH OF LETTUCE (Lactuca sativa) AND CHINESE CABBAGE (Brassica rapa, chinensis)

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October – November 2019, vol. 9, no. 2
pages: 166-170
Article type: Microbiology of Microbiology
DOI: 10.15414/jmbfs.2019.9.2.166-170
Abstract: Microbial biofertilizers are considered environmentally safe tool for the healthy production of a plant. Massive application of synthetic pesticides and fertilizers in agriculture has resulted detrimental effect on nature and human health. In the current study, we isolated several strains of rhizoshpheric bacteria through screening from the diversed agricultural soil of Daegu, South Korea. The growth promoting ability of the isolated strains was checked on gibberellin-deficient rice dwarf mutant Waito-C, lettuce var. cheongchima and chinese cabbage var. wangmat. The strain having the higher ability to promote the Waito-c growth was selected and further investigation was done. The selected isolate was identified as Pseudomonas koreensis MU2 through 16s rDNA gene sequence analysis. The cultural filtrate analysis revealed that the isolate could produce endogenous phytohormone gibberellic acid (GA1 and GA3) and organic acids, such as citric acid, malic acid, and tartaric acid. Pot experiment revealed that the inoculation of Pseudomonas koreensis MU2 significantly increased shoot length, root length, fresh biomass, and dry biomass of chinese cabbage and lettuce. These results suggest that the Pseudomonas koreensis MU2 might be the possible candidate for bio-fertilizer as a plant growth-promoting rhizobacteria in plant.
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GIBBERELLIN PRODUCING RHIZOBACTERIA Pseudomonas koreensis MU2 ENHANCE GROWTH OF LETTUCE (Lactuca sativa) AND CHINESE CABBAGE (Brassica rapa, chinensis)


AUTHORS

Sang-Mo Kang, Arjun Adhikari, Ko-Eun Lee, Yeon-Gyeong Park, Raheem Shahzad, In-Jung Lee

ABSTRACT

Microbial biofertilizers are considered environmentally safe tool for the healthy production of a plant. Massive application of synthetic pesticides and fertilizers in agriculture has resulted detrimental effect on nature and human health. In the current study, we isolated several strains of rhizoshpheric bacteria through screening from the diversed agricultural soil of Daegu, South Korea. The growth promoting ability of the isolated strains was checked on gibberellin-deficient rice dwarf mutant Waito-C, lettuce var. cheongchima and chinese cabbage var. wangmat. The strain having the higher ability to promote the Waito-c growth was selected and further investigation was done. The selected isolate was identified as Pseudomonas koreensis MU2 through 16s rDNA gene sequence analysis. The cultural filtrate analysis revealed that the isolate could produce endogenous phytohormone gibberellic acid (GA1 and GA3) and organic acids, such as citric acid, malic acid, and tartaric acid. Pot experiment revealed that the inoculation of Pseudomonas koreensis MU2 significantly increased shoot length, root length, fresh biomass, and dry biomass of chinese cabbage and lettuce. These results suggest that the Pseudomonas koreensis MU2 might be the possible candidate for bio-fertilizer as a plant growth-promoting rhizobacteria in plant.


KEYWORDS

Chinese cabbage, gibberellin, lettuce, organic acid, Pseudomonas koreensis MU2

INTRODUCTION

Plant root system is the habitat of numerous microbial communities that regulate plant physiology and metabolism (Vurukonda et al., 2016). Plant growth-promoting rhizobacteria act as i) biofertilizer, ii) rhizoremediator, iii) phytostimulator, and iv) stress controller (Lugtenberg and Kamilova, 2009). Bacteria, like Bacillus and Pseudomonas spp., are considered predominant (Podile and Kishore, 2007). Pseudomonas spp., and Bacillus spp., have been widely reported to be involved in the production of enzymes and hormones that alleviate stress, control pathogens, and promote growth and quality yield production (Lucy et al., 2004). Pseudomonas koreensis is a Gram-negative, motile, non-spore-forming,  yellow-white, multiple polar flagellated,  rod bacterium isolated from farming soil in Korea (Kwon et al., 2003). The type strain is LMG 21318. There is a wide range of plant growth promoting rhizobacteria (PGPR) species like Agrobacterium, Azospirillum, Arthrobacter, Azotobacter, Burkholderia, Caulobacter, Erwinia, Chromobacterium, Flavobacterium, Micrococcous,  and Serratia that was reported to improve plant growth and development. (Ahemad and Kibret, 2014).

PGPR has been extensively reported to be involved in endogenous phytohormone production (Maheshwari, 2011) such as gibberellins (Bloemberg and Lugtenberg, 2001; Bottini et al., 2004; GutiérrezMañero et al., 2001). Endogenous hormones possess various plant growth promoting characteristics like flowering initiation, increase in plant height, enhancing seed germination, and mitigating stresses (Kang et al., 2017). Other organic compounds exuded by PGPR include organic acids, amino acids, nucleotides, phenolics, fatty acids, sterols, sugars, vitamins and plant growth regulators (Lugtenberg and Kamilova, 2009). These secondary metabolites, especially organic acids, are involved in metal solubilization (Muleta et al., 2013; Zaidi et al., 2009).

The beneficial impact of plant-microbe interaction has been reported by several authors, like Pseudomonades sp., Bacillus lentus,  and  Azospirillum improved photosynthetic pigments and antioxidant of basil plant under water stress (Heidari and Golpayegani, 2012). Likewise, Mesorhizobium sp. resist pesticides [herbicides (metribuzin and glyphosate), fungicides (hexaconazole, metalaxyl, and kitazin) insecticides and (imidacloprid and thiamethoxam)] and could promote growth of chickpea (Ahemad and Khan, 2012). Pseudomonas spp., and  P. chlororaphis solubilize phosphate and promote the growth of Coffea arabica L (Muleta et al., 2013). Besides these, PGPR also play a key role in mitigating various biotic and abiotic stress-like pathogens (Park et al., 2017), drought (Lim and Kim, 2013), heavy metal (Islam et al., 2014), salt stress (Karlidag et al., 2013), and heat stress (Park et al., 2017). Moreover, the mechanism that involves the beneficial role of PGPR are well documented in the literature (Ahemad and Kibret, 2014; Beneduzi et al., 2012; Bhattacharyya and Jha, 2012; Kuiper et al., 2004; Mittal et al., 2017; Ryu et al., 2005).

The plant-microbe interaction is the fundamental determinant of soil fertility and plant health (Heidari and Golpayegani, 2012). The changing lifestyle and the essential dietary concern of the people considered fresh vegetables as an important diet in their daily consumption to maintain good health (Huxley et al., 2004). The trend of fresh vegetable consumption has grown stronger around the world (López-Gálvez et al., 2009). However, the safety concerns with regard to fungi, bacteria and other pathogens are emerging issues (Forghani and Oh, 2013; Seymour et al., 2002). Although, there exist several strain of pathogenic bacteria, Pseudomonas spp. have been widely reported as a beneficial species that are involved in healthy production of the plant (Ahemad and Kibret, 2014). However, to date, there is no report regarding the beneficial role of particularly P. koerensis MU2 strain. Therefore, the aim of the present study was to investigate the functional role of P. koreensis in order to evaluate growth and production of Chinese cabbage and lettuce.

MATERIALS AND METHODS

Screening of the microbes

The soil from different agricultural land was collected from Daegu, South Korea to isolate plant growth-promoting rhizospheric microbes. One gram of the soil sample was suspended in 9 mL of saline (0.85% NaCl), diluted 5-folds, and spread on a petri dish containing Luria-Bertani (LB; Difco, USA) agar medium from each diluted solution. The plated sample was cultured in an incubator at 30°C for 4 days. The colonies of different microbial strains were distinguished and an individual strain from the different colonies was streaked on new LB plates and cultured in LB broth by incubating in a shaking incubator. Gibberellins and organic acid production ability of these strains were checked, and the ones with higher ability to produce organic acid and gibberellins’ were selected, identified and employed for inoculation on plant experiment.

Screening for gibberellins (GAs) detection

The bioassay of the isolated microorganism to detect the absence or presence of GA was performed through the application of culture on a dwarf mutant waito-C that lacks gibberellins. For these, rice seeds were surface sterilized with 2.5% sodium hypochlorite for 30 min, rinsed with distilled water and incubated for 24 h with 20-ppm uni- conazole to obtain equally germinated seeds. After attaining two leaves stage, the CF 100 ul of MU2 was applied in rice seedlings. After 10 days, the rice shoot length, shoot fresh weight and shoot dry weight was recorded.

Organic acids quantification

The method described by Kang et al., (2015) was followed to quantify the organic acid. Briefly, bacterial culture was filtered through 0.22 μm Millipore filter and 10 μL of filtrates were injected to HPLC (Model: Waters 600E) equipped with a Refractive Index Detector (Model: Waters 410). Column: RSpak KC-811(8.0 x 300mm), Eluent: 0.1% H3PO4/H2O, Temperature: 40℃ and Flow rate: 1.0 ml/min.

Extraction and quantification of gibberellins produced by bacteria

A protocol described by (Lee et al., 2015) was followed to extract and quantify GA content in bacterial culture. Briefly, an isolated bacterial strain MU2 was cultured in LB media in a shaking incubator for 5 days at 30℃. The culture was centrifuged and the filtrate (100 ml) was analyzed using GA extraction protocol (Lee et al. 1998). Three major ions of the supplemented [2H2] GA internal standards (obtained from Prof. Lewis N. Mander, Australian National University, Canberra, Australia) and the exogenous GA were monitored simultaneously. Retention time was determined by using the hydrocarbon standards to calculate the Kovats retention indices value (Gaskin and MacMillan 1991). GAs was detected by using gas chromatography with a mass spectrometer (6890N network GC system, and 5973 network mass selective detector; Agilent Technologies).

Identification and phylogenetic analysis of bacterial isolate MU2

An isolated bacterial strain MU2 was identified on the basis of partial 16S ribosomal rDNA sequence. The chromosomal DNA was isolated and the 27F primer (5′-AGAGTTTGATC(AC)TGGCTCAG-3′) and 1492R primer (5′-CGG (CT) TACCTTGTTACGACTT-3′) were used for PCR amplification. The BLAST search program (http://www.ncbi.nlm.nih.gov/BLAST/) was used to find the nucleotide sequence homology of this bacterial isolate. The closely related nucleotide sequences were aligned by ClustalW and MEGA (version 6.0) software, and the neighbor-joining tree was generated. Bootstrap (1000 replications) was used for statistical support for the nodes in the phylogenetic tree.

Experiment location, method, and design

The experiment was conducted at the green house of Kyungpook National Universtiy, Daegu, Korea located at longitude of 128.587655° E and latitude of 35.857655° N. The bacterial culture of MU2 was incubated for 5 days at 30 °C on a shaking incubator at 200 rpm in broth medium. The bacterial suspension was diluted in sterile distilled water to a final concentration of 108 CFU/ml. Chinese cabbage and lettuce seeds were purchased from Seminis Korea Co. (Korea), surface sterilized with sodium hypochlorite (5%) for 10 min, and thoroughly rinsed with autoclaved double distilled water (DDW). Seeds were sown in plastic tray containing horticultural soil and grown under the controlled greenhouse conditions (30±2°C). The composition of horticultural soil was as follows: peat moss (13–18%), perlite (7–11%), coco-peat (63–68%) and zeolite (6–8%), with macro-nutrients being NH4+~90 mg/kg; NO3~205 mg/kg; P2O5~350 mg/kg and K2O~100 mg/kg (autoclaved three times). Two-week-old Chinese cabbage and lettuce seedlings (50 per treatments) were transplanted to the pot and treated with 5 ml of bacterial culture 100 ppm. After 10 days, the growth attributes were recorded.

Statistical analysis

The present study was conducted in a completely randomized design (CRD) and the experiment was designed as Control and Treatment (P. koreensis) for both the crops, in which each treatment had 10 replication. The data were statistically analyzed with SAS 9.4 software (SAS Institute, Cary NC, USA). The mean values among treatments were compared  using Duncan’s multiple range test (DMRT) at p ≤ 0.05.

RESULTS AND DISCUSSION

Isolation, selection, and identification

Through screening of soil sample from diverse agricultural land, various PGPR were isolated. The isolates were investigated based on the preliminary test of secondary metabolites production. The isolate that had the innate ability to produce the organic acid, and that promoted the growth of GA-deficient dwarf rice mutant Waito-C was selected for further investigation. The phylogenetic analysis revealed that the sequence obtained by 16s rDNA represent the microbes Pseudomonas koreensis. The isolate was registered in an NCBI with an accession number MU2KP 676116. The phylogenetic tree was constructed with gene sequence obtained in Blast Search Mega 6. Version(1000 bootstrap) Figure 1.

 

Figure 1 Phylogenetic tree based on the sequence obtained from 27F and 1492R primers of 16S rDNA of MU2(KP676116) and those of related bacteria. Percentage confidence levels generated from 1000 bootstrap trees are indicated at each node.

Gibberellins (GAs) detection

The production of gibberellins through bacteria is widely reported (Hedden and Sponsel, 2015). The gibberellin acts as a signal molecule and also promotes plant growth (Bottini et al., 2004). In our study, the inoculation of the Pseudomonas koreensis MU2 culture significantly increased the shoot length by 27%, shoot fresh weight  by 29% and shoot dry weight by 33% of GA deficient mutant waito-c (Table 1). These results confirmed the ability of the microbes to produce GA.

Table 1 Effect of Pseudomonas koreensis on plant growth promoting characteristics of gibberellins deficient dwarf rice mutant waito-c.

  Shoot Length (cm) Shoot Fresh Weight (g) Shoot Dry Weight (g)
Control 5.76±0.32b 0.54±0.03b 0.060±0.006b
MU2 7.32±0.33a 0.70±0.07a 0.082±0.005a

Results are expressed as mean ±SD (n=10) and significantly different at a p <0.05. Means sharing different superscript letter in a column indicate significant difference determined by DMRT (p ≤ 0.05)

Gibberellins (GAs) quantification and analysis

In the present study, it was revealed that the bacteria were able to produce the gibberelins GA1 (1.92 ng/100 mL) and GA3 (9.82 ng/100 mL), in which GA3 was found significantly higher as compared to GA1 (Figure 2). Moreover, our results are consistent with the evidence that bacteria like Azospirillum brasilense, Azospirillum lipoferum, Acetobacter diazotropicus, Bacillus pumilus, Bacillus licheniformis are able to produce GA1 and GA3 (MacMillan, 2001).

Figure 2 Quantification of gibberellins (GAs) content produced by Pseudomonas koreensis MU2 on the cultural filtrate. Bars represent means SD (n=3). Means followed by different letter indicate significant difference determined by DMRT (p ≤ 0.05)

Organic acid analysis

Pseudomonas spp have been widely reported for their organic acid production ability (Berg, 2009; Rodrı́guez and Fraga, 1999; Vyas and Gulati, 2009). Our study revealed that the Pseudomonas koreensis could produce mallic acid (112.6 ug/mL), citric acid (308.4 ug/mL) , and tartaric acid (87.6 ug/mL) among which citric acid was found significantly higher as compared to malic acid and tartaric acid (Figure 3). It has been reported that numerous species of PGPR have the ability to produce organic acids (Berg, 2009; Compant et al., 2005). These organic acids include citric acid, malic acid, oxalic acid, fumaric acid, acetic acid, butyric acid, succinic acid,  valeric acid, piscidic acid, glycolic acid, formic acid, lactic acid, aconitic acid, pyruvic acid, malonic acid, tetronic acid, aldonic acid, glutaric acid, and erythronic acid (Ahemad and Kibret, 2014; Dakora and Phillips, 2002).

Figure 3 Quantification of Organic acid content produced by Pseudomonas koreensis MU2 on the cultural filtrate. Bars represent means SD (n=3). Means followed by different letter indicate significant difference determined by DMRT (p ≤ 0.05)

Effect of Bacterial culture on plant growth promoting attributes

The Pseudomonas spp. exert a wide range of beneficial characteristics like high-stress tolerance ability, detoxification of inorganic pollutants through immobilization/mobilization/oxidation/reduction/bioaccumulation and degradation of the xenobiotic compound and root colonization (Rajkumar et al., 2017). In our study, the inoculation of P. koreensis significantly increased the shoot length, root length, fresh biomass, dry biomass, and chlorophyll content of both the crops ( lettuce and Chinese cabbage) (Table 2). It have been reported that the Pseudomonas improved yield in wheat (Weller and Cook, 1986) and promoted growth in radish and potato (Kloepper et al., 1980). Pseudomonas (BA-8) improved the yield of sugarbeet (Çakmakçı et al., 2001). Similarly, the pseudomonas florescens improved growth in various crops like potato (Burr et al., 1978), Winter wheat (Weller and Cook, 1986), Tomato (Gagné et al., 1993),  Highlush Blueberry(de Silva et al., 2000). Moreover, Pseudomonas spp. improved growth in lettuce, maize, barley, and wheat (Lucy et al., 2004). Furthermore, our results are strongly agreed by previous reports (Ullah et al., 2014 ;Kang et al., 2012; Joo et al., 2005;Joo et al., 2004), where gibberellin producing microorganism promoted growth in various plants.

Microorganisms are considered as the main source of organic acid content in soil (Adeleke et al., 2017). A Pseudomonas excrete a huge amount of organic acids which dissolve in the rhizosphere that solubilizes the insoluble phosphate and make available for plant uptake (Rajkumar et al., 2017). Organic acids play a significant role in mineralization and metal detoxification (Adeleke et al., 2017)  as well as nutrient assimilation (Kashyap et al., 2017). Moreover, organic acid produced by Pseudomonas spp is involved in organic matter degradation through hydrolysis, acidogenesis, acetogenesis and methanogenesis (Adeleke et al., 2017). The organic acid produced by P. koreensis might have played role in metal solubilization, organic matter degradation, and nutrient  assimilation to promote the growth in the plant.

Up to date, altogether 136 kinds of GAs have been reported to be isolated from bacteria, fungi and plants, out of which only GA1, GA3 and GA4, have been reported to be involved in the regulation of plant physiology and growth promotion (Bottini et al., 2004). As our results indicate that MU2 could produce the biologically active gibberelins GA1 and GA3, and organic acids like malic acid, citric acid, and tartaric acid, these secondary metabolites produced by microbes might have played a key role in the growth promotion of dwarf mutant Waito-C, Chinese cabbage and lettuce. Thus, Pseudomonas koreensis MU2 might be one of the efficient strains for commercial biofertilizer production in order to implement organic production.

Table 2 Effect of Pseudomonas korensis MU2 on growth promoting attributes on chinese cabbage and lettuce.

Treatments Shoot length

(cm/plant)

Root length

(cm/plant)

Fresh biomass

(g/plant)

Dry biomass

(g/plant)

Chlorophyll

(SPAD)

Chinese cabbage
Control 11.28±0.66b 10.10±0.34 b 4.02±0.16 b 0.41±0.01 b 31.88±1.45 a
P. koreensis MU2 12.82±0.38 a 10.96±0.59 a 4.62±0.43 a 0.47±0.06 a 32.28±1.49 a
 

Lettuce

Control 16.82±1.09 b 16.46±0.57 b 6.06±0.74 b 0.57±0.10 b 30.68±1.06 a
P. koreensis MU2 18.84±1.24 a 17.66±0.63 a 7.06±1.09 a 0.64±0.11 a 32.76±2.16 a

Results are expressed as mean ±SD (n=10). Means sharing different superscript letter in a column indicate significant difference determined by DMRT (p ≤ 0.05)

CONCLUSION

The anthropogenic activities leading to global climate change have been a serious problem in the current agricultural practice. Moreover, excessive use of pesticides and fertilizers has an adverse effect on the human as well as the ecological cycle. Application of plant growth promoting bacteria is considered an environmental friendly approach for sustainable and healthy production.  The endogenous hormones and organic acid production by microbes play a key role in plant growth and development. In the present study, Pseudomonas koreensis MU2 was able to produce the biologically active gibberelline and organic acids and promoted the  plant growth. Thus, it might be a cost-effective biofetilizer for the healthy growth and  quality yield. Furthermore, our findings could lead to a better understanding of the plant-microbe interaction for future studies.

Acknowledgment: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1D1A3B03931629).

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