Gopal Selvakumar

Arbuscular Mycorrhizal Fungi Community Structure, Abundance and Species Richness Changes in Soil by Different Levels of Heavy Metal and Metalloid Concentration

Arbuscular Mycorrhizal Fungi (AMF) play major roles in ecosystem functioning such as carbon sequestration, nutrient cycling, and plant growth promotion. It is important to know how this ecologically important soil microbial player is affected by soil abiotic factors particularly heavy metal and metalloid (HMM). The objective of this study was to understand the impact of soil HMM concentration on AMF abundance and community structure in the contaminated sites of South Korea. Soil samples were collected from the vicinity of an abandoned smelter and the samples were subjected to three complementary methods such as spore morphology, terminal restriction fragment length polymorphism (T-RFLP) and denaturing gradient gel electrophoresis (DGGE) for diversity analysis. Spore density was found to be significantly higher in highly contaminated soil compared to less contaminated soil. Spore morphological study revealed that Glomeraceae family was more abundant followed by Acaulosporaceae and Gigasporaceae in the vicinity of the smelter. T-RFLP and DGGE analysis confirmed the dominance of Funneliformis mosseae and Rhizophagus intraradices in all the study sites. Claroideoglomus claroideum, Funneliformis caledonium, Rhizophagus clarus and Funneliformis constrictum were found to be sensitive to high concentration of soil HMM. Richness and diversity of Glomeraceae family increased with significant increase in soil arsenic, cadmium and zinc concentrations. Our results revealed that the soil HMM has a vital impact on AMF community structure, especially with Glomeraceae family abundance, richness and diversity.

Halotolerant bacteria with ACC deaminase activity alleviate salt stress effect in canola seed germination

Amelioration of salt stress effect on canola seed germination was investigated using 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase-producing plant growth promoting halotolerant bacteria. NaCl at 120xa0mM concentration reduced canola seed germination by 50xa0%. The inoculation of ACC deaminase-producing halotolerant Brevibacterium epidermidis RS15 and Bacillus aryabhattai RS341 at 120xa0mM NaCl significantly increased the seed germination with decreased seed ACC content. Notably, the hydrolytic enzymes activities like amylase, invertase, and protease also increased due to inoculation of RS15 and RS341 compared to uninoculated salt stress imposed germinating canola seeds. Ethylene emission of salt stress exposed eight-day-old canola seedlings was reduced by 35.4 and 41.1xa0% compared to uninoculated salt stressed control due to respective inoculation of RS341 and RS15. The amelioration of salt stress inhibitory effect on the canola seed germination was attributed to the inoculation of ACC deaminase-producing halotolerant bacteria modulating ethylene emission and inducing hydrolytic enzymes.

Alleviation of salt stress in maize plant by co-inoculation of arbuscular mycorrhizal fungi and Methylobacterium oryzae CBMB20

Arbuscular mycorrhizal fungi (AMF) and plant growth-promoting bacteria (PGPB) can have multiple impacts on phytohormone production and nutrient uptake to improve plant growth. Co-inoculation of AMF and PGPB can mitigate the effects of salinity in plants. This study was conducted to evaluate the efficacy of co-inoculation of AMF (Glomus etunicatum) and Methylobacterium oryzae CBMB20 to alleviate salt stress and improve maize growth. Single and co-inoculation of AMF and M. oryzae CBMB20 significantly increased dry biomass, AMF root colonization, and nutrient accumulation in maize plants under salt stress. The proline content and Na uptake were significantly reduced in co-inoculation of AMF and M. oryzae CBMB20. In addition, co-inoculation of AMF and M. oryzae CBMB20 (soil) showed higher AMF density and AMF colonization compared to single AMF treatment. Among all treatments, co-inoculation of AMF and M. oryzae CBMB20 (soil) was more effective to ameliorate salt stress and improve plant growth. These results demonstrate that co-inoculation of AMF and M. oryzae CBMB20 (soil application) may be an effective alternative to enhance plant growth under salinity stress.

Cold Stress Tolerance in Psychrotolerant Soil Bacteria and Their Conferred Chilling Resistance in Tomato (Solanum lycopersicum Mill.) under Low Temperatures

The present work aimed to study the culturable diversity of psychrotolerant bacteria persistent in soil under overwintering conditions, evaluate their ability to sustain plant growth and alleviate chilling stress in tomato. Psychrotolerant bacteria were isolated from agricultural field soil samples colleced during winter and then used to study chilling stress alleviation in tomato plants (Solanum lycopersicum cv Mill). Selective isolation after enrichment at 5°C yielded 40 bacterial isolates. Phylogenetic studies indicated their distribution in genera Arthrobacter, Flavimonas, Flavobacterium, Massilia, Pedobacter and Pseudomonas. Strains OS211, OB146, OB155 and OS261 consistently improved germination and plant growth when a chilling stress of 15°C was imposed and therefore were selected for pot experiments. Tomato plants treated with the selected four isolates exhibited significant tolerance to chilling as observed through reduction in membrane damage and activation of antioxidant enzymes along with proline synthesis in the leaves when exposed to chilling temperature conditions (15°C). Psychrotolerant physiology of the isolated bacteria combined with their ability to improve germination, plant growth and induce antioxidant capacity in tomato plants can be employed to protect plants against chilling stress.

Effect of Salinity on Plants and the Role of Arbuscular Mycorrhizal Fungi and Plant Growth-Promoting Rhizobacteria in Alleviation of Salt Stress

Of the world’s 1.5 billion hectares of cultivable land, 77 million hectares (5 %) do not favor good yields due to high salt content and 20 % of the irrigated agricultural land is adversely affected by salinity. Salt-affected soils are increasing steadily in all continents, in particular in arid and semiarid areas which cover more than 7 % of the total land surface on earth. Due to the severe impact of salinity to crop production, salt stress is considered a major limiting factor in crop production. Under saline conditions plant growth is severely reduced, mainly due to osmotic stress. Low water potential in the roots causes water deficit within the plant. Toxic concentrations of Na+ and Cl− ions in plants affect cell membrane functions. Moreover, high amounts of these ions also reduce metabolic activities in plants which lead to growth inhibition and injury of the foliage. Furthermore, low nutrient uptake potential of root due to salinity stress causes nutrient imbalance and oxidative stress in plants which impede proper growth and development. Due to the scope of salinity affected soils and the deleterious effects of salinity on crop production, methods to remediate and/or improve crop production in salinity impacted soils are necessary. Management of soil salinity through land reclamation or improved techniques of irrigation provides only short-term solution and often expensive. Other methods like plant breeding for salt tolerance have been difficult and slow. The nature of salt tolerance traits and problems in developing appropriate testing environment makes it difficult to obtain. Microorganisms present in the rhizosphere soil play a vital role in improving plant growth and soil fertility. Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs that form symbiotic or mutualistic associations with roots of about 80 % of plant species. AMF widely exist in salt-affected soils and they infect the plant root cortical cells. AMF form arbuscules inside the cortical cells and utilize them as nutrient exchange sites. These also form vesicles between the cortical cells where all nutrients are stored. Under soil saline conditions, AMF extend their extraradical hyphae into non-rhizosphere soil to uptake more nutrients. Symbioses of halotolerant bacteria with plant roots also help plants withstand salinity stress. Halotolerant bacteria possess plant growth-promoting characters which may improve plant growth and nutrient uptake. The utilization of microorganisms is an economical and environmental friendly approach to alleviate stress in plants cultivated in salt-impacted agricultural fields.

Genetic Diversity and Association Characters of Bacteria Isolated from Arbuscular Mycorrhizal Fungal Spore Walls.

Association between arbuscular mycorrhizal fungi (AMF) and bacteria has long been studied. However, the factors influencing their association in the natural environment is still unknown. This study aimed to isolate bacteria associated with spore walls of AMF and identify their potential characters for association. Spores collected from coastal reclamation land were differentiated based on their morphology and identified by 18S rDNA sequencing as Funneliformis caledonium, Racocetra alborosea and Funneliformis mosseae. Bacteria associated with AMF spore walls were isolated after treating them with disinfection solution at different time intervals. After 0, 10 and 20 min of spore disinfection, 86, 24 and 10 spore associated bacteria (SAB) were isolated, respectively. BOX-PCR fingerprinting analysis showed that diverse bacterial communities were associated to AMF spores. Bacteria belonging to the same genera could associate with different AMF spores. Gram positive bacteria were more closely associated with AMF spores. Isolated SAB were characterized and tested for spore association characters such as chitinase, protease, cellulase enzymes and exopolysaccharide production (EPS). Among the 120 SAB, 113 SAB were able to show one or more characters for association and seven SAB did not show any association characters. The 16S rDNA sequence of SAB revealed that bacteria belonging to the phyla Firmicutes, Proteobacteria, Actinobacteria and Bactereiodes were associated with AMF spore walls.

Mycorrhizal Symbiotic Efficiency on C3 and C4 Plants under Salinity Stress – A Meta-Analysis

A wide range of C3 and C4 plant species could acclimatize and grow under the impact of salinity stress. Symbiotic relationship between plant roots and arbuscular mycorrhizal fungi (AMF) are widespread and are well known to ameliorate the influence of salinity stress on agro-ecosystem. In the present study, we sought to understand the phenomenon of variability on AMF symbiotic relationship on saline stress amelioration in C3 and C4 plants. Thus, the objective was to compare varied mycorrhizal symbiotic relationship between C3 and C4 plants in saline conditions. To accomplish the above mentioned objective, we conducted a random effects models meta-analysis across 60 published studies. An effect size was calculated as the difference in mycorrhizal responses between the AMF inoculated plants and its corresponding control under saline conditions. Responses were compared between (i) identity of AMF species and AMF inoculation, (ii) identity of host plants (C3 vs. C4) and plant functional groups, (iii) soil texture and level of salinity and (iv) experimental condition (greenhouse vs. field). Results indicate that both C3 and C4 plants under saline condition responded positively to AMF inoculation, thereby overcoming the predicted effects of symbiotic efficiency. Although C3 and C4 plants showed positive effects under low (EC < 4 ds/m) and high (>8 ds/m) saline conditions, C3 plants showed significant effects for mycorrhizal inoculation over C4 plants. Among the plant types, C4 annual and perennial plants, C4 herbs and C4 dicot had a significant effect over other counterparts. Between single and mixed AMF inoculants, single inoculants Rhizophagus irregularis had a positive effect on C3 plants whereas Funneliformis mosseae had a positive effect on C4 plants than other species. In all of the observed studies, mycorrhizal inoculation showed positive effects on shoot, root and total biomass, and in nitrogen, phosphorous and potassium (K) uptake. However, it showed negative effects in sodium (Na) uptake in both C3 and C4 plants. This influence, owing to mycorrhizal inoculation, was significantly higher in K uptake in C4 plants. For our analysis, we concluded that AMF-inoculated C4 plants showed more competitive K+ ions uptake than C3 plants. Therefore, maintenance of high cytosolic K+/Na+ ratio is a key feature of plant salt tolerance. Studies on the detailed mechanism for the selective transport of K in C3 and C4 mycorrhizal plants under salt stress is lacking, and this needs to be explored.

Spore associated bacteria of arbuscular mycorrhizal fungi improve maize tolerance to salinity by reducing ethylene stress level

The role of spore associated bacteria of arbuscular mycorrhizal fungi (AMF) in improving plant growth and alleviating salt stress is a potential area to explore. In the present study, 22 bacteria isolated from the spore walls of AMF were identified to contain 1-aminocyclopropane-1-carboxylate deaminase. These were tested for their ability to improve seed germination and alleviate salt stress in the early growth of maize. Among the isolates, 19 bacteria that were able to grow at 4u2009% NaCl were used for germination assay. Two bacteria and seven bacteria significantly improved maize seed germination at 100xa0mM NaCl and 200xa0mM NaCl, respectively. Based on the presence of plant growth promoting (PGP) characters and the ability to improve seed germination, five strains were chosen for further experiments. At 0xa0mM NaCl, all the strains were able to increase maize shoot and root growth significantly. At 25xa0mM NaCl, except for Bacillus aryabhattai S210B15, all the strains were able to increase shoot and root growth significantly. At 50xa0mM NaCl, Bacillus aryabhattai S110B3 and B. aryabhattai S210B15 significantly improved shoot length, whereas, Pseudomonas koreensis S2CB35 and B. aryabhattai S210B15 significantly increased root length. Although salinity increased ethylene production in maize, bacterial inoculation significantly reduced the ethylene level at 0, 25 and 50xa0mM NaCl. Among the five strains, only P. koreensis S2CB35 showed the presence of PGP functional traits of nifH, acdS and nodA genes.

Spore associated bacteria regulates maize root K+/Na+ ion homeostasis to promote salinity tolerance during arbuscular mycorrhizal symbiosis

BackgroundThe interaction between arbuscular mycorrhizal fungi (AMF) and AMF spore associated bacteria (SAB) were previously found to improve mycorrhizal symbiotic efficiency under saline stress, however, the information about the molecular basis of this interaction remain unknown. Therefore, the present study aimed to investigate the response of maize plants to co-inoculation of AMF and SAB under salinity stress.ResultsThe co-inoculation of AMF and SAB significantly improved plant dry weight, nutrient content of shoot and root tissues under 25 or 50xa0mM NaCl. Importantly, co-inoculation significantly reduced the accumulation of proline in shoots and Na+ in roots. Co-inoculated maize plants also exhibited high K+/Na+ ratios in roots at 25xa0mM NaCl concentration. Mycorrhizal colonization significantly positively altered the expression of ZmAKT2, ZmSOS1, and ZmSKOR genes, to maintain K+ and Na+ ion homeostasis. Confocal laser scanning microscope (CLSM) view showed that SAB were able to move and localize into inter- and intracellular spaces of maize roots and were closely associated with the spore outer hyaline layer.ConclusionThese new findings indicate that co-inoculation of AMF and SAB effectively alleviates the detrimental effects of salinity through regulation of SOS pathway gene expression and K+/Na+ homeostasis to improve maize plant growth.

Trap Culture Technique for Propagation of Arbuscular Mycorrhizal Fungi using Different Host Plants

Arbuscular mycorrhizal fungi (AMF) spore propagation and long term maintenance is still a complicated technique for farmers. The use of AMF for their ability to promote plant growth and protect plants against pathogen attack and environmental stresses demands AMF propagation for large scale application. This study aimed to propagate AMF spores by trap culture technique and assess their ability to propagate with different host plants in a continuous plant cycle. Mycorrhizal inoculation by trap culture in maize resulted in longer shoots and roots than sudangrass plants. Increase in dry weight with higher percentage also was observed for maize plants. After first and second plant cycle, maize plants had the higher percentage of mycorrhizal response in terms of colonization and arbuscules than sudangrass. Maximum in spore count also achieved in the pots of maize plants. The results show that maize plant is more suitable host plant for AMF spore propagation and trap culture technique can be used effectively to maintain the AMF culture for long time.