Greener Journal of Agricultural Sciences
ISSN: 2276-7770; ICV: 6.15
Vol. 4 (4), pp. 117-129, May 2014
Copyright ©2017, the copyright of this article is retained by the author(s)
Research Article (DOI: http://dx.doi.org/10.15580/GJAS.2014.4.010314003)
Impact of Pesticides on PGPR Activity of Azotobacter sp. Isolated from Pesticide Flooded Paddy Soils
Chennappa G.1, Adkar-Purushothama C.R.2#, Naik M.K.3, Suraj U.4, Sreenivasa M.Y.1*
1Department of Studies in Microbiology, University of Mysore, Manasagangotri, Mysore- 570 006, Karnataka, India.
2Laboratory of Plant Pathology, Faculty of Agriculture and Life Science Hirosaki University, Bunkyo-cho 3, Hirosaki 036-8561, Aomori Ken, Japan #Present address: Pavilion for Applied Cancer Research, Department of Biochemistry
Faculty of Medicine and Health sciences, Sherbrooke University 3201, Rue Jean-Mignault, Sherbrooke, Quebec J1E 4K8, Canada.
3Department of Plant Pathology, College of Agriculture, University of Agricultural Sciences, Raichur – 584104, Karnataka, India.
4NRC on DNA finger printing, NBPGR, PUSA Campus, New Delhi - 110012 India.
Article No.: 010314003
Azotobacter strains were isolated from paddy soils by serial dilution agar plate method. The colonies were glistening; smooth, slimy, brown to black in morphology on Jenson’s N-free agar plates and the cells were Gram- negative bacteria. Biochemically, they were positive for indole production, citrate, catalase and Voges Proskauer test. Further, Nif gene sequence results revealed the presence of seven Azotobacter species. The effect of 1, 3 and 5% pesticides concentration viz., pendimethalin, chloropyrifos, glyphosate and phorate on nitrogen fixation, indole acetic acid, gibberllic acid, phosphate solubilization and bioassay of Azotobacter sp. were studied. Among all the species, GVT-1 strain was found to fix nitrogen at a maximum of 30µgN2 ml-1 day-1, produced highest quantity gibberllic acid (10µg25ml-1) and able to solubilize the phosphate at the rate of 9.8cm by forming the halo zone which was supplemented with 5% phorate. Similarly, GVT-1 strain produced a highest amount of indole acetic acid (31.8µgml-1) in 5% pendimethalin. Further bioassay activities of GVT-1 strain found efficient in increasing the root, shoot length and vigor of the plant. From these results it is clear that the Azotobacter sp. not only produces plant growth promoting substances but is also resistant to different pesticides and is not affected by the bacterial activity.
E-mail: sreenivasamy @gmail.com
Azotobacter; Nif gene; PGP activity; Bioassay; Pesticides
Rice is one of the major agricultural crops and cultivation is found throughout India under various cultivation practices. In the last two decades the production and utilization of rice was increased with the population to maintain food security. To improve the production of rice, several cultivation practices have been adopted. Among all the practices, chemical pesticides are in the race and used excessively for the control of plant diseases and pests. Some of the pesticides have direct effect on soil microbiological aspects, environmental pollution and health hazards (Martin et al., 2011). These pesticides may cause everlasting changes in the soil microflora (Aleem et al., 2003), adverse effect on soil fertility and crop productivity, inhibition of nitrogen (N2) fixing bacteria, (Sachin, 2009), interference with ammonification (Reinhardt et al., 2008). Several toxic and carcinogenic pesticides have banned in many of the countries including India. Several species are known to tolerate many of pesticides and some of them are known to degrade several pesticides in different conditions (Tejera et al., 2005). The pesticide resistant and degrading bacterial group includes many species such as Pseudomonas, Flavobacteria, Azotobacter, Acetobacter, Arthrobacter, Alcaligenes, Bacillus, Enterobacter and Klebsiella. Among all bacteria, plant growth promoting rhizobacteria (PGPR) have more influence on soil physiological properties. PGPR can helps to replace chemical fertilizer nitrogen for the sustainable cultivation by fixing the atmospheric N2 and producing growth promoting substances (Ahmad et al. 2005). Many species of Pseudomonas, Bacillus and Azotobacter can grow and survive at extreme environmental conditions (Jimenez et al. 2011). Among them, the genus Azotobacter are common nitrogen fixing bacterium found in agricultural soils playing different beneficial roles by secreting vitamins, amino acids, sidephores and auxins (Aquilanti et al., 2004; Sharma et al., 2009). Azotobacter produces indole acetic acid (IAA), gibberllic acid (GA) which are important plant growth hormones and these hormones will helps in seed germination and plant growth considerably (Asma et al., 2012). Several species of Azotobacter are isolated and identified in all the soil conditions and well known species are A. vinelandii and A. chroococcum (Page and Shivprasad, 1991). Several pesticides are being used for the cultivation of rice viz., pendimethalin, phorate, glyphosate, simazine, malathion, endosulfan etc, which are completely degraded by A. chroococcum and are greatly studied since last two decades because of its PGPR activity for sustainable agriculture (Moneke et al., 2010). Azotobacter species can tolerate and survive in extreme environmental condition by producing cysts. Pesticide tolerant Azotobacter species was isolated from paddy soils and are known to produce IAA in media supplemented with 5% pesticides (Chennappa et al., 2013). Similarly neutral effect of endosulfan (2mgL-1) was also reported on IAA production and nitrogen fixation activity of A. chroococcum respectively and which is used as a sulphur source (Castillo et al. 2011). Hexachlorohexane, phorate and carbofuran haveincreased the bacterial population under invitro conditions and no inhibitory effects was recorded (Das and Mukharjee, 2000). Special interest was taken in the present study because in Northern parts of Karnataka, for cultivation of paddy high level of pesticide are being used to control the pests and diseases of paddy. Present investigation reveals that the Azotobacter species can fix N2, produces IAA, GA and phosphate solubilization (PS) in the media containing different types of pesticides concentration and the investigation was carried with the following objectives; 1) Isolation of efficient PGPR Azotobacter species from paddy soils, 2) Molecular characterization of N2 fixing Azotobacter species, 3) Effect of pesticides on N2 fixation, IAA, GA production, PS and bioassay activities.
MATERIALS AND METHODS
Isolation and Identification of Azotobacter species
Soil samples were collected from 0 to 15 cm of different locations of paddy soils of Karnataka. Nitrogen fixing Azotobacter strains were isolated by serial dilution agar plate method and 1ml of serially diluted soil suspensions from 10-3, 10-4, 10-5 and 10-6 dilutions were inoculated onto Jenson’s N- free media (Ahmad et al., 2008; Sharma et al., 2009). The plates were maintained in triplicates for higher accuracy of the results. The plates were incubated at 28±2º C for 5 days, after incubation period, the discrete and well grown colonies were enumerated (Khan et al., 2008). Azotobacter cultures were identified and characterized based on the morphological characters and biochemical tests as described in the Bergey’s manual of bacteriology. Pure cultures were obtained by repeated sub-culturing on same media slants.
The soil pH was measured according to the protocol of Jimenez et al. (2011) and pH tolerance was carried out at different pH ranges. Different ranges of pH (5.5 to 9.5) Burk’s media were prepared by adjusting the media pH (NaOH or HCL); one set with pH 7.0 were maintained as control. The test bacterial cultures were inoculated by streak plate method and incubated at 28 ± 2º C for 7 days. The growths of bacterial colonies on media plates were enumerated and CFU were calculated.
DNA Extraction and Amplification of nif gene
The DNA was extracted from the pure cultures of Azotobacter using Genei DNA isolation kit and the purified DNA was amplified with specific primer Nif f (5'-GGTTGTGACCCCGAAAGCTA) and Nif r (5'-GCGTACATGGCCATCATCTC) for identification nif genes in Azotobacter. 25μl PCR reaction mixture contains 15.3μl of autoclaved distilled water, 2μl of purified DNA template, 2.5μl 1X Taq buffer, 2μl of MgCl2, 2μl of deoxyribonucleotide triphosphate mixture, 1μl of each primer, 0.2μl of Taq DNA polymerase (Genei). PCR amplification reaction was run in a thermocycler with following conditions (35 cycles consisting of 95º C for 1 min, 55º C for 1 min, 72º C for 1 min, and final extension at 72º C for 5 min) were done (Rajeswari and Kasthuri, 2009; Jimenez et al., 2011). The amplified were checked in 1.5% agarose gel for further confirmation.
Cloning, sequencing and cluster analyses
PCR products were cloned into pGEM®-T Easy Vector Systems (Promega) and transformed into competent E. coli strain DH5α by the following manufacturer instructions. Plasmid DNA was isolated and the presence of the insert was confirmed by restriction digestion of plasmid DNA with EcoRI restriction enzymes (Life Sciences, Canada). The clones of interest were sequenced (Genei, Bengaluru, India) and the sequences were confirmed with NCBI BLAST database for the identity of the isolates based on previously published database sequences (Chennappa et al., 2013). Online software MEGA 5.1 was used to construct the phylogenetic tree using Maximum Likelihood NJ method.
Effect of Pesticides on N2 fixation
The Azotobacter strains were inoculated onto a 100ml Jenson’s N-free broth in 250ml Erlenmeyer flask supplemented with 1, 3 and 5% of each pesticides viz., pendimethalin, chloropyrifos, glyphosate and phorate respectively. The flasks were incubated at 30±2º C for 10 days on a rotary shaker (120 rpm) under continuous airflow whereas, inoculated broth without any pesticides treatment maintained as control (Bano and Musarrat, 2003). The flasks were maintained in triplicates for higher accuracy of the results. The concentration of nitrogen in each liquid culture was measured by Kjeldahl method (Bremner, 1965). Nitrogen fixation was determined in terms of increase in total Kjeldahl nitrogen in comparison to control using micro Kjeldahl apparatus.
Effect of Pesticides on IAA production
The Azotobacter strains were inoculated onto a 100ml Jenson’s N free broth in 250ml Erlenmeyer flask supplemented with 1, 3 and 5% of each pesticides viz., pendimethalin, chloropyrifos, glyphosate and phorate respectively. The flasks were incubated at 30±2º C for 7 days on a rotary shaker (100 rpm) under continuous airflow whereas, inoculated broth without any pesticides treatment maintained as control (Bano and Musarrat, 2003). The test sets were maintained in triplicates for greater accuracy of the results. After incubation, IAA estimation was done according to the protocol of Ahmad et al. (2008) with Solawaski’s reagent. The development of pink color was read at 530nm using spectrophotometer (Genway 6506) and IAA produced was estimated by referring to a standard graph prepared with different levels of standard IAA (Beneduzi et al., 2008; Patil, 2011).
Effect of Pesticides on GA production
Azotobacter isolates were inoculated onto a sterilized 100ml nitrogen free malate broth in 250ml Erlenmeyer flask supplemented with 1, 3 and 5% of each pesticides viz., pendimethalin, chloropyrifos, glyphosate and phorate respectively and incubated at 30±2º C for 7 days on a rotary shaker (100 rpm) under continuous airflow whereas inoculated broth without any pesticides treatment maintained as control (Bano and Musarrat, 2003). After incubation, the absorbance of the samples was measured at 254 nm in UV spectrophotometer. The test sets were maintained in triplicates. The GA produced by the Azotobacter isolates was estimated as described by Upadhyay et al. (2009).
Effect of Pesticides on Phosphate solubilization
Azotobacter isolates were spot inoculated onto a Pikovskaya agar plates containing tri calcium phosphate (TCP) as a major phosphate source supplemented with 1, 3 and 5% of each pesticides viz., pendimethalin, chloropyrifos, glyphosate and phorate respectively. The plates were maintained in triplicates for higher accuracy of the results and incubated at 30±2º C for 7 days whereas inoculated plate without pesticides maintained as control. (Beneduzi et al., 2008; Sachin, 2009) and observed for the halo zone formation around the discrete colonies.
Effect of Pesticides on Bioassay of Potential Isolates
Bioassay was carried out for the potential Azotobacter isolates at different concentrations of culture filtrate (100, 500, 1000 and 2000 ppm) and pesticide concentration on paddy seed germination, root length, shoot length and vigor index test. The paddy seeds were soaked for an overnight in all the concentration of culture filtrate supplemented with 1, 3 and 5% of each pesticides viz., pendimethalin, chloropyrifos, glyphosate and phorate respectively. Whereas, one set untreated (without pesticides) seeds maintained as control. The seeds were spread on blotter paper and incubated at 32±2º C for 10 days in a growth chamber. After 10 days of incubation, the effects of different pesticide concentrations on seed germination, root, shoot length and vigor index were recorded over the control (Sachin, 2009). The experimental set up was maintained in triplicates for higher statistical accuracy of the results.
Nucleotide sequence accession number:
All the Azotobacter sequences were deposited in NCBI genbank along with location of the isolates and the allotted accession numbers are: KF470798, KF470799, KF470800, KF470801, KF470802, KF470803, KF470804, KF470805, KF470806, KF470807, KF470808 and KF470809.
Nif gene sequences of different Azotobacter species deposited in NCBI genbank isolated from paddy soil sample and the accession numbers are KF881088, KF881089, KF881090, KF881091, KF881092, KF881093, KF881094, KF881095, KF881096, KF881097, KF881098, KF881099, KF881100, KF881101, KF881102, KF881103, KF881104, KF881105, KF881106, KF881107, KF881108, KF881109, KF881110, KF881111, KF881112, KF881113, KF881114, KF881115, KF881116, KF881117, KF881118, KF881119, KF881120 and KF881121.
All the Azotobacter strains sequences were deposited in NCBI genbank along with location of the isolates. Accession numbers are: JX262171, JX262167, JX262163, JX262175, JX262166, JX262173, JX262172, JX262174, JX262168, JX262165, JX262169, JX262170, JX262164 and JX262176 (Chennappa et al., 2013).
RESULTS AND DISCUSSION
Isolation and identification of Azotobacter
A total of 35 Azotobacter strains were isolated from saline paddy soils of Karnataka by serial dilution spread plate method on Jenson’s N- free media. All the isolates produced melanin like water- soluble brown, light brown, pale yellow, milky white to black pigments and formed sticky colonies. These isolates were Gram –ve, some of them were produced cysts and they were positive for indole production, citrate utilization catalase production and negative for methyl red and starch hydrolysis (Table 1). The production of sticky pigments and cyst formation is one of the typical identification characters of the genus Azotobacter. Similarly Aquilanti et al. (2004) reported three different isolation methods for rhizosphere soils of Azotobacter and identified viz., A. chroococcum, A. vinelandii, A. paspali, A. beijerinckii, A. salinestris, A. armeniacus, A. brasilense, A. insignis, A. agilis and A. nigricans were genetically differ from each other but not with morphological characters. In reference to this study, Azotobacter species are aerobes and the activity will be more in non-water logged soils. Akhter et al. (2012) reported that the soil salinity and pH was increased because of the high inputs of the pesticides for the cultivation of paddy. Zhan and Sun (2010) reported the diversity of free living nitrogen fixing microorganisms from soils of copper mining waste lands and found different uncultured microorganisms which are efficient nitrogen fixing and are closely related to diazotrophic group. Tejera et al. (2005) reported the similar kind of morphological and biochemical characteristic features of Azotobacter species isolated from sugarcane rhizosphere soils. All these reports clearly indicate the distribution of Azotobacter species and their activities in different soil ecosystem.
The pH of collected soil samples were in the range of 6.0 to 9.2 and pH 6.5 to 7.5 ranges is suitable for the growth of Azotobacter species. Among all the strains of Azotobacter KOP-5 strain was survived at a pH of 9.5 and did not observed any inhibition of growth at higher pH range. DVD-7 strain was sensitive to the pH of 8.2 and no growth was observed above 8.2 pH value (Fig 1.). The genus Azotobacter is ubiquitous in nature and they can grow with pH ranging from 6.0 to 9.0 of different climatic temperatures (Jimenez et al., 2011). Paddy soil salinity was increased because of the maximum use of pesticides and chemical fertilizers for the control of pests and diseases of paddy under irrigated conditions. Similarly Chennappa et al. (2013) reported the species of Azotobacter are tolerant to pesticides of 1 to 5% concentration and were isolated from saline paddy soils. This result clearly indicates the species of Azotobacter are resistant to higher pH values and can tolerate up to pH value of 9.5 in the presence of different pesticide concentration.
Amplification and sequence analyses of nif genes
Based on preliminary morphological identification, all 35 isolates were selected for nif gene studies. After amplification of nif genes of Azotobacter, amplicons size was around 350bp (Fig 2). BLAST data result confirmed that they were matched 98 to 100 per cent in homology and all the sequences were similar to published NCBI database sequences of nif genes Azotobacter species viz., A. vinelandii (RCR-4), A. salinestris (GVT-1), A. chroococcum (SND-4), A. sp. (DVD-7), A. tropicalis (KOP-11), A. nigricans subsp nigricans (YG-7) and A. armeniacus (GVT-11) respectively (Table 1). Similarly Zehr et al. (2003) and Bhatia et al. (2009) clearly reported the major clusters of nif genes in Azotobacter and many of diazotrophic nitrogen fixing bacterial species isolated from cotton soil samples and found similar nif genes in the present study. After BLAST data result, A. tropicalis and A. armeniacus and A. salinestris were found new to the paddy soils; till now no such data is available on A. tropicalis, A. armeniacus and A. salinestris species and are equally beneficial to the soil ecosystem.
Nif gene cluster analyses
Based on the BLAST data analyses, phylogenetic tree of Azotobacter nif gene reveals that it formed different cluster nodes and produced individual clusters with Azotobacter nif gene sequences. Only GVT-3, RCR-11, YG-7 and RCR-8 Sequences produced entirely different nodes and but all the other species were identical to one another with respect to whole genome sequences data (Fig 3). Microorganisms catalyze biological nitrogen fixation with the enzyme nitrogenase, which has been highly conserved through evolution (Zehr et al., 2003). Steenhoudt and Vanderleyden (2000) reported and concluded that nitrogen fixing ability of A. vinelandii and nif HDK genes clusters. Nitrogen fixation in Azotobacter is complicated by the presence of three biochemically and genetically distinct nitrogenase enzymes synthesized under different conditions of metal supply. All the amplified nif genes were similar to the reference nif gene of Azotobacter and Pseudomonas. Martensson et al. (2009) concluded that nif gene can be used as a molecular marker for the diazotrophic diversity study. Wu et al. (2011) isolated the nif gene from different diazotrophic bacteria from surface sea sediments. Soni and Goel (2011) reported the nif genes isolated directly from saline soil samples and these results are similar to the present results of nif gene of Azotobacter species. Upadhyay et al. (2009) isolated nitrogen- fixing bacteria from wheat rhizosphere and concluded that nifH gene was responsible for nitrogen fixation. High levels of saline concentration were not hampered by the nitrogen fixing activity and on nif gene. This reports indicates that the divergence of nif genes in the ecosystem with respect to different physiological conditions. Till now no reports were found on nif gene of A. salinestris, A. tropicalis, and armeniacus species, and majority of the work was done on A. vinelandii and A. chroococcum nif gene clusters. Using different 16S rDNA methods for microbial diversity analyses and based on 16S rDNA analyses have given more accurate and confirmative results as compared but not with traditional methods.
Effect of Pesticides on N2 fixation
Based on nif gene studies, five different strains were selected and used to study the effect of pesticides on nitrogen fixation. Among 5 isolates, GVT-1 strain fixed highest amount of N2 (30μgNml-1day-1) supplemented with 5% phorate followed by pendimethalin (17.5μgNml-1day-1) in 100ml of Jenson’s media (Table 2). The recommended dosages of the present study are, Chloropyrifos (1.75ml/l), phorate (5kg/ acre), pendimethalin (6.25ml/l) and glyphosate (5ml/l). All the 5 strains were fixed a maximum amount of N2 at 5% phorate and this concentration did not affect the nitrogen fixing activity of Azotobacter strains. Among all the pesticides, 5% glyphosate has negative effect on N2 fixation as compared to other concentration and fixed least amount of N2 (Azt-16). Pendimethalin 3% concentration did not affect the metabolic activity of the strain and above 3% pesticide concentration is lethal to bacterial respiration and reduced the N2 fixation. Mrkovacki (2002) stated that glyphosate not only inhibited the N2 fixation but also reduced A. chroococcum respiration rate by 60%. Castillo et al. (2011) reported that endosulfan concentration inhibited the nitrogenase activity of A. chroococcum 94 and 96 % at 2- 10mgL-1 of endosulfan but A. chroococcum has degraded endosulfan completely. Similarly in all the 3% pesticide treatments nitrogen fixation was recorded and this concentration does not have negative effect on metabolic activity. Among all the pesticides concentration, KOP-11 isolate showed maximum reduction of N2 fixation in 5% glyphosate and a maximum resistance was showed by GVT-1 isolate at 5% phorate concentration. Das and Mukharjee et al. (2000) reported, the commonly used insecticides (phorate, HCH and carbofuran) will have advantages for the growth of bacteria using as a carbon and sulphur source but few of them have showed effects on metabolic activities.
Effect of Pesticides on IAA production
Among all the strains, GVT-1 strain produced maximum quantity of IAA (31.8µgml-1) supplemented with 5% pendimethalin over the control (33µgml-1). All the isolates were produced IAA in the range of 10 - 31.8µgml-1 at 5% different pesticide concentrations. Among them, Azt-16 was very sensitive towards the production of IAA supplemented with 5% glyphosate and GVT-1 was resistant to 5% concentration of all the pesticides. Among all pesticides, pendimethalin did not affect the IAA production of Azotobacter and this strain is resistant to 5% pendimethalin as compared to the other strains but upto 3% pesticide concentration did not affected the metabolic activity. Castillo et al. (2011) concluded that no endosulfan effect was observed on IAA production of A. chroococcum and very negligible variations were noticed on IAA production. Similarly Asma et al. ( 2012 ) reported the effect of endosulfan on IAA production of Azotobacter and found that even 50ppm of endosulfan concentration inhibited the IAA production. In the present study above 3% pesticide concentration slightly reduced the IAA production and lethal to bacterial respiration with reference to IAA production (Table 2). The production of IAA and activity vary from species to species and the environmental conditions. Similarly, Mrkovacki et al. (2002) reported that pendimethalin was efficiently degraded by A. chroococcum up to 55% in 20 days and did not shown any negative effect on IAA production. All the strains were isolated from paddy soils and proved the PGP potential with respect to A. chroococcum. Ahmad et al. (2008) reported a maximum of 24.8µgml-1 of IAA produced by Azotobacter species which was supplemented with 2mg of tryptophan. Further Patil (2011) reported the highest amount (32.80µgml-1) of IAA was produced by Azotobacter supplemented with 5mg of tryptophan. The addition of tryptophan has increased the IAA production and influences the metabolic activity of the bacteria. Similarly Upadhyay et al. (2009) isolated PGPR strains at different salinity soil conditions and all the strains able to produce IAA at the rate 3.79 to 119. 63µgml-1.
This report clearly indicates that the diazotrophic bacteria have the same type of metabolic activities in the bacterial diversity.
Effect of Pesticides on GA production
All the Azotobacter isolates produced GA in the range of 5 to 10µg25ml-1 supplemented with 5% pesticides concentration. GVT-1 strain produced the highest amount of GA (10µg25ml-1) in 5% pendimethalin and phorate as compared to the other strains (Table 3). GA is also one of the important plant growth promoting substances produced by the different species of PGPR including Azotobacter species. Among all the isolates RCR-4 produced least amount (5µg25ml-1) of GA which was supplemented with 5% glyphosate and showed sensitive towards the GA production. Similarly Asma et al. (2012) reported the effect of endosulfan on GA production of Azotobacter and 50ppm of endosulfan concentration inhibited the GA production. All the isolates were resistant to 3% pendimethalin concentration as compared to the glyphosate. This result clearly indicates that above 1% glyphosate concentration has inhibited the GA production ability of the Azotobacter similarly to endosulfan and this concentration is lethal to the metabolic activity of the Azotobacter. Among all the concentrations, none of the isolates showed resistant to above 1% pesticide concentration over the control. Latifi et al. (2012) discussed the biodegradation of chlorpyrifos by Pseudomonas putida and strain has degraded the chlorpyrifos in 9 days of period. Similarly Upadhayay et al. (2009) reported the production of gibberellins by Arthrobacter (130.74µgmg-1) and Bacillus species (7.67µgmg-1) isolated from wheat rhizosphere under 1 to 8% of saline conditions. The GA production variation due to NaCl concentration has influenced bacterial growth rate, and all PGPR’s will not produce equal quantity of GA and it varies from species to species and geographical conditions respectively.
Effect of Pesticides on P- Solubilization
The formation of halo zone around the colony indicates the P solubilization of the available TCP to Phosphates. After 7 days of incubation, the diameters of halo zone from edge of the colony were 7 to 9.8cm in all the 5% pesticide concentration. A. salinestris (GVT-1) formed large halo zone (9.8cm) in plates containing 5% phorate concentration as compared to other strains (Table 3). In control plates the P solubilization was ranged from 13 to 14.2 cm in 7 days. All the isolates were equal in phosphate solubilization and 3% of all the pesticides showed negative effect on bacterial growth on plates. All the isolates were grown in 1% of pesticides in 4 days of incubation and 1% concentration did not affect the bacterial activity but in 3% concentration growth was inhibited by pesticides; definitely has showed bacteriostatic effect on Azotobacter species. Moneke et al. (2010) reported that the species of Azotobacter and other species viz., Pseudomonas, Escherichia, Acetobacter etc., can tolerate and degrade glyphosate herbicides. This report clearly indicates that all the isolates were resistant to 1, 3 and 5 % of pesticides and but the bacterial activity was inhibited as compared to the control. Previously Garg et al. (2001) reported phosphate- solubilizing activity of Azotobacter under invivo conditions and similar results were recorded in the present study. Kumar et al. (2001) reported the P solubilizing activity of A. chroococcum under green house condition of wheat and P solubilization has increased the total vigor of the plant and found similar results in the present study. Upadhyay et al. (2009) isolated P solubilizing PGPR strains at 1 to 8% of NaCl conditions and all the strains able to solubilize P at the rate 4.5 to 8.0mm as compared to the present work A. salinestris solubilized double with 5% glyphosate concentration. All P solubilizing species will not have same efficiency rate and GVT-1 strain found more efficient than the previous. A. salinestris, A. tropicalis, A. chroococcum and A. armeniacus were able to solubilize equal amount of phosphate as compared to previously isolated A. chroococcum and A. vinelandii strains. These strains can be used as alternate P solubilizer as compared to the existing strains.
Effect of Pesticides on Bioassay activity
Among 5 isolates, GVT-1 was selected for bioassay activity based on NF, IAA, GA production and PS results. Among all the culture filtrate, 2000ppm has increased the paddy seed vigor index or growth (1715, 1684 and 1635) and seed germination rate upto 95 - 98% as well as root (8.3cm) and shoot length (9.3cm) in 1, 3 and 5% phorate concentration over the control (1770) respectively. Initially in 4 days not much variation were observed on the seed germination and seedling growth but after 7 days of seed germination containing 100, 500, 1000 and 2000ppm concentration increased per cent of growth rate in 1, 3 and 5% pesticide concentration. The percent of paddy seed germination (96-100%) was increased over the control (91%) samples. The culture filtrate concentrations on paddy seeds has increased root and shoot length and vigor index at a maximum of 50% growth (Fig 4a, 4b, 4c and 4d) in 1 and 3% phorate concentration. This report clearly indicates that the isolates has capacity to increase the vigor of the plant and resistant to all the concentrations of pesticides. Sharma et al. (2009) reported A. chroococcum strain increased the litchi root and shoot length by 81.39% and was isolated from orchard soils . Similarly , Patil ( 2011 ) reported the impact of Azotobacter culture on root and shoot length of groundnut (38.88 to 49.88) and gram (27.65 to 36.35) seeds. Sachin (2009) studied IAA production and phosphate solubilizing activities of A. chroococcum and concluded that the PGPR strain increased the seed germination, root and shoot length of maize respectively. Ahmad et al. (2008) reported the impact of Azotobacter isolates on root elongation in Sesebania aculeate and Vigna radiata plants supplemented with 1mg of tryptophan and found that all the isolates increased root length in Sesebania aculeate (4.7cm); Vigna radiate (5.5cm) over the control (3.8 and 3.5cm). A. salinestris strains significantly increased seed germination, root, shoot length as well as vigor of plant as compared to the A. chroococcum strain respectively. This proves the potentiality of the strains with respect to the N2 fixation, IAA, GA, and P solubilizing activity these isolates were equally beneficial in bioassay activities. Limited work has been done on bioassay activities of Azotobacter species and the effect of pesticides on NF, IAA, GA production and PS for agriculturally important crops including paddy. The present work will initiate the potentiality of the strains isolated from native soils and can be used as alternate PGP strains.
From the present study, Azotobacter species have been isolated and identified viz., A. vinelandii, A. chroococcum, A. sp. A. salinestris, A. armeniacus and A. tropicalis. All the isolates were distinct from other Azotobacter species in the phylogenetic diversity and were resistant to 4 different pesticides which are generally used for the paddy cultivation. Among them A. salinestris were fixed a maximum amount of N2 , produced IAA , GA and solubilize the P supplemented with different concentration of pesticides. The strains can withstand different pH values and the pesticide concentration under water logged soils and bioassay activity of the isolates proved the efficiency in increasing the growth of the plant respectively. Isolates were important in the field of agriculture as a PGPR to maintain the soil fertility and to improve food production with eco-friendly management.
For isolation, identification and the instrumentation facility was supported Dr. M. K. Naik, professor and Head Dept. of Plant Pathology, College of Agriculture, UAS, Raichur, Karnataka and valuable suggestions and laboratory facilities during the research work.
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Cite this Article: Chennappa G, Adkar-Purushothama CR, Naik MK, Suraj U, Sreenivasa MY, 2014. Impact of Pesticides on PGPR Activity of Azotobacter sp. Isolated from Pesticide Flooded Paddy Soils. Greener Journal of Agricultural Sciences. 4(4):117-129, http://dx.doi.org/10.15580/GJAS.2014.4.010314003.