Preferential Promotion of Lycopersicon esculentum (Tomato) Growth by Plant Growth Promoting Bacteria Associated with Tomato

A total of 74 morphologically distinct bacterial colonies were selected during isolation of bacteria from different parts of tomato plant (rhizoplane, phylloplane and rhizosphere) as well as nearby bulk soil. The isolates were screened for plant growth promoting (PGP) traits such as production of indole acetic acid, siderophore, chitinase and hydrogen cyanide as well as phosphate solubilization. Seven isolates viz., NR4, NR6, RP3, PP1, RS4, RP6 and NR1 that exhibited multiple PGP traits were identified, based on morphological, biochemical and 16S rRNA gene sequence analysis, as species that belonged to four genera Aeromonas, Pseudomonas,Bacillus and Enterobacter. All the seven isolates were positive for 1-aminocyclopropane-1-carboxylate deaminase. Isolate NR6 was antagonistic to Fusarium solani and Fusarium moniliforme, and both PP1 and RP6 isolates were antagonistic to F. moniliforme. Except RP6, all isolates adhered significantly to glass surface suggestive of biofilm formation. Seed bacterization of tomato, groundnut, sorghum and chickpea with the seven bacterial isolates resulted in varied growth response in laboratory assay on half strength Murashige and Skoog medium. Most of the tomato isolates positively influenced tomato growth. The growth response was either neutral or negative with groundnut, sorghum and chickpea. Overall, the results suggested that bacteria with PGP traits do not positively influence the growth of all plants, and certain PGP bacteria may exhibit host-specificity. Among the isolates that positively influenced growth of tomato (NR1, RP3, PP1, RS4 and RP6) only RS4 was isolated from tomato rhizosphere. Therefore, the best PGP bacteria can also be isolated from zones other than rhizosphere or rhizoplane of a plant.

Electronic supplementary material

The online version of this article (doi:10.1007/s12088-014-0470-z) contains supplementary material, which is available to authorized users.     

Native bacterial endophytes promote host growth in a species-specific manner; phytohormone manipulations do not result in common growth responses

Background

All plants in nature harbor a diverse community of endophytic bacteria which can positively affect host plant growth. Changes in plant growth frequently reflect alterations in phytohormone homoeostasis by plant-growth-promoting (PGP) rhizobacteria which can decrease ethylene (ET) levels enzymatically by 1-aminocyclopropane-1-carboxylate (ACC) deaminase or produce indole acetic acid (IAA). Whether these common PGP mechanisms work similarly for different plant species has not been rigorously tested.

Methodology/ Principal Findings

We isolated bacterial endophytes from field-grown Solanum nigrum; characterized PGP traits (ACC deaminase activity, IAA production, phosphate solubilization and seedling colonization); and determined their effects on their host, S. nigrum, as well as on another Solanaceous native plant, Nicotiana attenuata. In S. nigrum, a majority of isolates that promoted root growth were associated with ACC deaminase activity and IAA production. However, in N. attenuata, IAA but not ACC deaminase activity was associated with root growth. Inoculating N. attenuata and S. nigrum with known PGP bacteria from a culture collection (DSMZ) reinforced the conclusion that the PGP effects are not highly conserved.

Conclusions/ Significance

We conclude that natural endophytic bacteria with PGP traits do not have general and predictable effects on the growth and fitness of all host plants, although the underlying mechanisms are conserved.     

Bacterial endophytes: recent developments and applications.

Endophytic bacteria have been found in virtually every plant studied, where they colonize the internal tissues of their host plant and can form a range of different relationships including symbiotic, mutualistic, commensalistic and trophobiotic. Most endophytes appear to originate from the rhizosphere or phyllosphere; however, some may be transmitted through the seed. Endophytic bacteria can promote plant growth and yield and can act as biocontrol agents. Endophytes can also be beneficial to their host by producing a range of natural products that could be harnessed for potential use in medicine, agriculture or industry. In addition, it has been shown that they have the potential to remove soil contaminants by enhancing phytoremediation and may play a role in soil fertility through phosphate solubilization and nitrogen fixation. There is increasing interest in developing the potential biotechnological applications of endophytes for improving phytoremediation and the sustainable production of nonfood crops for biomass and biofuel production.     

Himalayan bacterial endophytes enhance microalgal cell numbers and chlorophyll content in synthetic co-culture

Endophytic bacteria associated with medicinal plants from Himalayan mountains possess great biotechnological potential. However, the influence of these Himalayan bacterial endophytes (HBE) on microalgal-promotion and metabolite production is still largely unknown. In this study, the interactions between two endophytic bacterial isolates of an endangered Himalayan medicinal plant and long-chain fatty acids accumulating green alga Micractinium sp. GA001 are characterized in synthetic co-culture systems. The endophytes Staphylococcus pasteuri PPE11 and Yersinia enterocolitica PPE118 significantly enhance microalgal cell numbers with 56% and 49% increase in total chlorophyll content, respectively. Co-culturing microalgae with these endophytes demonstrated distinct responses toward photosynthesis at different temperatures. Endophytes were metabolically active for an extended time (more than 28 days) in co-culturing. The findings were further complemented with genomics studies of endophytes which were subjected to multiple sequencing approaches to assemble and annotate their genomes, resulting in key genes involved in PGP activities, metabolites production and transportation being identified. This study expands the benefits and bioprocessing potential of endophytes of Himalayan medicinal plants.

     

Diversity and metabolic potential of culturable root-associated bacteria from Origanum vulgare in sub-Himalayan region

Study of rhizospheric bacteria from important plants is very essential, as they are known to influence plant growth and productivity, and also produce industrially important metabolites. Origanum vulgare is a perennial medicinal aromatic plant rich in phenolic antioxidants. Present study investigates the diversity of culturable root-associated bacteria from this plant in Palampur, India, which constitutes a unique ecosystem due to high rain fall, wide temperature fluctuations and acidic soil. Both root endophytes and rhizospheric soil bacteria were isolated, which resulted in a total of 120 morphologically different isolates. They were found to group into 21 phylotypes based on restriction fragment length polymorphism analysis. Growth medium composition had significant effect on the diversity of the isolated bacterial populations. The isolates were characterized for various metabolic, plant growth promoting (PGP) and other biotechnologically useful activities, based on which they were clustered into groups by principal component analysis. Majority of the isolates belonged to ??-Proteobacteria and Firmicutes. Pseudomonas and Stenotrophomonas were the most dominant species and together constituted 27.5?% of the total isolates. Many isolates, especially ??-Proteobacteria, showed very high PGP activities. Few isolates exhibited very high antioxidant activity, which may find potential applications in food and health industries. Firmicutes were catabolically the most versatile group and produced several hydrolytic enzymes. To the best of our knowledge, it is the first study describing rhizospheric microbial community of O. vulgare.     

Isolation and characterization of endophytic colonizing bacteria from agronomic crops and prairie plants

Endophytic bacteria reside within plant hosts without causing disease symptoms. In this study, 853 endophytic strains were isolated from aerial tissues of four agronomic crop species and 27 prairie plant species. We determined several phenotypic properties and found approximately equal numbers of gram-negative and gram-positive isolates. In a greenhouse study, 28 of 86 prairie plant endophytes were found to colonize their original hosts at 42 days postinoculation at levels of 3.5 to 7.7 log(10) CFU/g (fresh weight). More comprehensive colonization studies were conducted with 373 corn and sorghum endophytes. In growth room studies, none of the isolates displayed pathogenicity, and 69 of the strains were recovered from corn or sorghum seedlings at levels of 8.3 log(10) CFU/plant or higher. Host range greenhouse studies demonstrated that 26 of 29 endophytes were recoverable from at least one host other than corn and sorghum at levels of up to 5.8 log(10) CFU/g (fresh weight). Long-range dent corn greenhouse studies and field trials with 17 wild-type strains and 14 antibiotic-resistant mutants demonstrated bacterial persistence at significant average colonization levels ranging between 3.4 and 6.1 log(10) CFU/g (fresh weight) up to 78 days postinoculation. Three prairie and three agronomic endophytes exhibiting the most promising levels of colonization and an ability to persist were identified as Cellulomonas, Clavibacter, Curtobacterium, and Microbacterium isolates by 16S rRNA gene sequence, fatty acid, and carbon source utilization analyses. This study defines for the first time the endophytic nature of Microbacterium testaceum. These microorganisms may be useful for biocontrol and other applications.     

Beneficial Bacteria Isolated from Grapevine Inner Tissues Shape Arabidopsis thaliana Roots

We investigated the potential plant growth-promoting traits of 377 culturable endophytic bacteria, isolated from Vitis vinifera cv. Glera, as good biofertilizer candidates in vineyard management. Endophyte ability in promoting plant growth was assessed in vitro by testing ammonia production, phosphate solubilization, indole-3-acetic acid (IAA) and IAA-like molecule biosynthesis, siderophore and lytic enzyme secretion. Many of the isolates were able to mobilize phosphate (33%), release ammonium (39%), secrete siderophores (38%) and a limited part of them synthetized IAA and IAA-like molecules (5%). Effects of each of the 377 grapevine beneficial bacteria on Arabidopsis thaliana root development were also analyzed to discern plant growth-promoting abilities (PGP) of the different strains, that often exhibit more than one PGP trait. A supervised model-based clustering analysis highlighted six different classes of PGP effects on root architecture. A. thaliana DR5::GUS plantlets, inoculated with IAA-producing endophytes, resulted in altered root growth and enhanced auxin response. Overall, the results indicate that the Glera PGP endospheric culturable microbiome could contribute, by structural root changes, to obtain water and nutrients increasing plant adaptation and survival. From the complete cultivable collection, twelve promising endophytes mainly belonging to the Bacillus but also to Micrococcus and Pantoea genera, were selected for further investigations in the grapevine host plants towards future application in sustainable management of vineyards.     

Optimization of isolation and cultivation of bacterial endophytes through addition of plant extract to nutrient media

Many endophytes have beneficial effects on plants and can be exploited in biotechnological applications. Studies hypothesize that only 0.001–1% of all plant‐associated bacteria are cultivable. Moreover, even after successful isolations, many endophytic bacteria often show reduced regrowth capacity. This research aimed to optimize isolation processes and culturing these bacteria afterwards. We compared several minimal and complex media in a screening. Beside the media themselves, two gelling agents and adding plant extract to media were investigated to enhance the number and diversity of endophytes as well as the growth capacity when regrown after isolation. In this work, 869 medium delivered the highest numbers of cultivable bacteria, as well as the highest diversity. When comparing gelling agents, no differences were observed in the numbers of bacteria. Adding plant extract to the media lead to a slight increase in diversity. However, when adding plant extract to improve the regrowth capacity, sharp increases of viable bacteria occurred in both rich and minimal media.     

Functional abilities of cultivable plant growth promoting bacteria associated with wheat (Triticum aestivum L.) crops

In the pursuit of sustainable agriculture, bioinoculants usage as providers of a crop''s needs is a method to limit environmental damage. In this study, a collection of cultivable putative plant growth promoting (PGP) bacteria associated with wheat crops was obtained and this bacterial sample was characterized in relation to the functional diversity of certain PGP features. The isolates were obtained through classical cultivation methods, identified by partial 16S rRNA gene sequencing and characterized for PGP traits of interest. Functional diversity characterization was performed using Categorical Principal Component Analysis (CatPCA) and Multiple Correspondence Analysis (MCA). The most abundant genera found among the 346 isolates were Pseudomonas, Burkholderia, and Enterobacter. Occurrence of PGP traits was affected by genus, niche, and sampling site. A large number of genera grouped together with the ability to produce indolic compounds; phosphate solubilization and siderophores production formed a second group related to fewer genera, in which the genus Burkholderia has a great importance. The results obtained may help future studies aiming prospection of putative plant growth promoting bacteria regarding the desired organism and PGP trait.     

Diversity of Culturable Plant Growth-Promoting Bacterial Endophytes Associated with Sugarcane Roots and Their Effect of Growth by Co-Inoculation of Diazotrophs and Actinomycetes

Application of environmentally friendly agents to reduce the use of chemicals and to enhance growth of plants is an ultimate goal of sustainable agriculture. The use of plant growth-promoting endophytes has become of great interest as a way to enhance plant growth and additionally protect plants from phytopathogens. In this study, 135 isolates of endophytic bacteria including actinomycetes were isolated from roots of commercial sugarcane plants cultivated in Thailand and were characterized for plant growth-promoting (PGP) traits. Based on morphological and 16S rRNA sequence analysis, the endophytes were distributed into 14 genera of which the most dominant species belong to Bacillus, Enterobacter, Microbispora, and Streptomyces. Two strains of endophytic diazotrophs, Bacillus sp. EN-24 and Enterobacter sp. EN-21; and two strains of actinomycetes, Microbispora sp. GKU 823 and Streptomyces sp. GKU 895, were selected based on their PGP traits including 1-aminocyclopropane-1-decarboxylate deaminase, indole-3-acetic acid, nitrogen fixation, phosphate solubilization, and siderophore production for evaluation of sugarcane growth enhancement by individual and co-inoculation. Sixty days after co-inoculation by endophytic diazotrophs and actinomycetes, the growth parameters of sugarcane plants were significantly greater than that of individual and un-inoculated plants. The results indicated that these endophytes have high potential as PGP agents that could be applied to promote sugarcane growth and could be developed as active added value biofertilizers in the future.     

Isolation of ACC deaminase-producing habitat-adapted symbiotic bacteria associated with halophyte Limonium sinense (Girard) Kuntze and evaluating their plant growth-promoting activity under salt stress

Background and aims

Many plant growth-promoting endophytes (PGPE) possessing 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity can reduce the level of stress ethylene and assist their host plants cope with various biotic and abiotic stresses. However, information about the endophytic bacteria colonizing in the coastal halophytes is still very scarce. This study aims at isolating efficient ACC deaminase-producing plant growth-promoting (PGP) bacterial strains from the inner tissues of a traditional Chinese folk medicine Limonium sinense (Girard) Kuntze, a halophyte which has high economic and medicinal values grown in the coastal saline soils. Their PGP activity and effects on host seed germination and seedling growth under salinity stress were also evaluated.

Methods

A total of 126 isolates were obtained from the surface sterilized roots, stems and leaves of L. sinense (Girard) Kuntze. They were initially selected for their ability to produce ACC deaminase as well as other PGP properties such as production of indole-3-acetic acid (IAA), N₂-fixation, and phosphate-solubilizing activities and subsequently identified by the 16S rRNA gene sequencing. For selected strains, seed germination, seedling growth, and flavonoids production in axenically growth L. sinense (Girard) Kuntze seedlings at different NaCl concentrations (0–500 mM) were quantified.

Results

Thirteen isolates possessing ACC deaminase activity were obtained. The 16S rRNA gene sequencing analysis showed them to belong to eight genera: Bacillus, Pseudomonas, Klebsiella, Serratia, Arthrobacter, Streptomyces, Isoptericola, and Microbacterium. Inoculation with four of the selected ACC deaminase-producing strains not only stimulated the growth of the host plant but also influenced the flavonoids accumulation. All four strains could colonize and can be re-isolated from the host plant interior tissues.

Conclusions

These results demonstrate that ACC deaminase-producing habitat-adapted symbiotic bacteria isolated from halophyte could enhance plant growth under saline stress conditions and the PGPE strains could be appropriate as bioinoculants to enhance soil fertility and protect the plants against salt stress.     

Evaluation of bacteria isolated from rice rhizosphere for biological control of charcoal rot of sorghum caused by <Emphasis Type="Italic">Macrophomina phaseolina</Emphasis> (Tassi) Goid.

A total of 360 bacteria, isolated from the rhizospheres of a system of rice intensification (SRI) fields, were characterized for the production of siderophore, fluorescence, indole acetic acid (IAA), hydrocyanic acid (HCN) and solubilization of phosphorus. Of them, seven most promising isolates (SRI-156, -158, -178, -211, -229, -305 and -360) were screened for their antagonistic potential against Macrophomina phaseolina (causes charcoal rot in sorghum) by dual culture assay, blotter paper assay and in greenhouse. All the seven isolates inhibited M. phaseolina in dual culture assay, whereas six isolates solubilized phosphorous (except SRI-360), all seven produced siderophore, four produced fluorescence (except SRI-178, -229 and -305), six produced IAA (except SRI-305) and five produced HCN (except SRI-158 and -305). In the blotter paper assay, no charcoal rot infection was observed in SRI-156-treated sorghum roots, indicating complete inhibition of the pathogen, while the roots treated with the other isolates showed 49–76% lesser charcoal rot infection compared to the control. In the antifungal activity test (in green house on sorghum), all the isolates increased shoot dry mass by 15–23% and root dry mass by 15–20% (except SRI-158 and -360), over the control. In order to confirm the plant growth-promoting (PGP) traits of the isolates, the green house experiment was repeated but, in the absence of M. phaseolina. The results further confirmed the PGP traits of the isolates as evidenced by increases in shoot and root dry mass, 22–100% and 5–20%, respectively, over the control. The sequences of 16S rDNA gene of the isolates SRI-156, -158, -178, -211, -229, -305 and -360 were matched with Pseudomonas plecoglossicida, Brevibacterium antiquum, Bacillus altitudinis, Enterobacter ludwigii, E. ludwigii, Acinetobacter tandoii and P. monteilii, respectively in BLAST analysis. This study indicates that the selected bacterial isolates have the potential for PGP and control of charcoal rot disease in sorghum.     

Bacterial biosynthesis of 1-aminocyclopropane-1-caboxylate (ACC) deaminase,a useful trait to elongation and endophytic colonization of the roots of rice under constant flooded conditions

This study was conducted to investigate the role of 1-aminocyclopropane-1-carboxylate (ACC) deaminase in Pseudomonas fluorescens strain REN₁ and its ability to reduce ethylene levels produced during stress, endophytically colonize and promote the elongation of the roots of rice seedlings under gnotobiotic conditions. We isolated 80 bacteria from inside roots of rice plants grown in the farmers’ fields in Guilan, Iran. All of the isolates were characterized for plant growth promoting (PGP) traits. In addition, the colonization assay of these isolates on rice seedlings was carried out to screen for competent endophytes. The best bacterial isolate, based on ACC deaminase production, was identified and used for further study. 16S rDNA sequence analysis revealed that the endophyte was closely related to Pseudomonas fluorescens. The results of this study showed ACC deaminase containing P. fluorescens REN1 increased in vitro root elongation and endophytically colonized the root of rice seedlings significantly, as compared to control under constant flooded conditions. The trait of low amount of indole-3-acetic acid (IAA) production (<15 μg mL⁻¹) and the high production of ACC deaminase by bacteria may be main factors in colonizing rice seedling roots compared to other PGP traits (siderophore production and phosphate solubilization) in this study. Endophytic IAA and ACC deaminase-producing bacteria may be preferential selections by rice seedlings. Therefore, it may be suggested that the utilization of ACC as a nutrient gives the isolates advantages in more endophytic colonization and increase of root length of rice seedlings.     

Bacterial Biosynthesis of 1-Aminocyclopropane-1-Carboxylate (ACC) Deaminase and Indole-3-Acetic Acid (IAA) as Endophytic Preferential Selection Traits by Rice Plant Seedlings

In this study, bacteria were isolated from the rhizosphere and inside the roots and nodules of berseem clover plants grown in the field in Iran. Two hundred isolates were obtained from the rhizosphere (120 isolates), interior roots (57 isolates), and nodules (23 isolates) of clover plants grown in rotation with rice plants. Production of chitinase, pectinase, cellulase, siderophore, salicylic acid, hydrogen cyanide, indole acetic acid (IAA), 1-aminocyclopropane-1-carboxylate (ACC) deaminase, solubilization of phosphate, antifungal activity against various rice plant pathogen fungi, N₂ fixation, and colonization assay on rice seedlings by these strains was evaluated and compared (endophytic isolates vs. rhizosphere bacteria). The results showed both the number and the ability of plant growth-promoting (PGP) traits were different between endophytic and rhizosphere isolates. A higher percentage of endophytic isolates were positive for production of IAA, ACC deaminase, and siderophore than rhizosphere isolates. Therefore, it is suggested that clover plant may shape its own associated microbial community and act as filters for endophyte communities, and rhizosphere isolates with different (PGP) traits. We also studied the PGP effect of the most promising endophytic and rhizosphere isolates on rice seedlings. A significant relationship among IAA and ACC deaminase production, the size of root colonization, and plant growth (root elongation) in comparison with siderophore production and phosphate solubilization for the isolates was observed. The best bacterial isolates (one endophytic isolate and one rhizosphere isolate), based on their ability to promote rice growth and colonize rice roots, were identified. Based on 16S rDNA sequence analysis, the endophytic isolate CEN7 and the rhizosphere isolate CEN8 were closely related to Pseudomonas putida and Pseudomonas fluorescens, respectively. It seems that PGP trait production (such as IAA, ACC deaminase) may be required for endophytic and rhizosphere competence as compared to other PGP traits in rice seedlings under constant flooded conditions. The study also shows that the presence of diverse rhizobacteria with effective growth-promoting traits associated with clover plants may be used for sustainable crop management under field conditions.     

The contribution of nitrogen fixation to sugarcane (Saccharum officinarum L.), and the identification and characterization of part of the associated diazotrophic bacterial community

Background and aims

Rhizospheric, epiphytic and endophytic bacteria are associated with several non-legumes, colonizing their surface and inner tissues. Many of these bacteria are beneficial to their hosts, and are collectively termed plant growth-promoting rhizobacteria (PGPR). Recent interest has focused particularly upon PGPR that are endophytic (i.e. PGPE), and which have been reported to be associated with important crops such as rice, wheat and sugarcane. Different mechanisms are involved in bacteria-induced plant growth promotion (PGP), including biological nitrogen fixation (BNF), mineral solubilization, production of phytohormones and pathogen biocontrol. In Uruguay, sugarcane (Saccharum officinarum L.) is considered a strategic multipurpose crop, used for bioenergy, feed, sugar and bioethanol production. The aim of this work was to estimate the BNF contribution to Uruguayan sugarcane cultivars, as well as to identify and characterize the (culturable) putatively endophytic diazotrophic bacteria associated with these varieties.

Methods and results

Results using the ¹⁵N-dilution technique have shown that these sugarcane varieties obtain significant inputs of N from BNF (34.8–58.8% Ndfa). In parallel, a collection of 598 isolates of potentially endophytic diazotrophs was obtained from surface-sterilized stems using standard isolation techniques, and nifH ⁺ isolates from these were the subject of further studies. The bacteria were shown to belong to several genera, including Pseudomonas, Stenotrophomonas, Xanthomonas, Acinetobacter, Rhanella, Enterobacter, Pantoea, Shinella, Agrobacterium and Achromobacter. Additionally, some PGP features were studied in 35 selected isolates. The data obtained in this study represent the initial steps in a program aimed at determining the mechanisms of PGP of non-legume crops in Uruguay (such as sugarcane) with potentially beneficial plant-associated bacteria.     

Isolation and Characterization of Endophytic Colonizing Bacteria from Agronomic Crops and Prairie Plants

Endophytic bacteria reside within plant hosts without causing disease symptoms. In this study, 853 endophytic strains were isolated from aerial tissues of four agronomic crop species and 27 prairie plant species. We determined several phenotypic properties and found approximately equal numbers of gram-negative and gram-positive isolates. In a greenhouse study, 28 of 86 prairie plant endophytes were found to colonize their original hosts at 42 days postinoculation at levels of 3.5 to 7.7 log₁₀ CFU/g (fresh weight). More comprehensive colonization studies were conducted with 373 corn and sorghum endophytes. In growth room studies, none of the isolates displayed pathogenicity, and 69 of the strains were recovered from corn or sorghum seedlings at levels of 8.3 log₁₀ CFU/plant or higher. Host range greenhouse studies demonstrated that 26 of 29 endophytes were recoverable from at least one host other than corn and sorghum at levels of up to 5.8 log₁₀ CFU/g (fresh weight). Long-range dent corn greenhouse studies and field trials with 17 wild-type strains and 14 antibiotic-resistant mutants demonstrated bacterial persistence at significant average colonization levels ranging between 3.4 and 6.1 log₁₀ CFU/g (fresh weight) up to 78 days postinoculation. Three prairie and three agronomic endophytes exhibiting the most promising levels of colonization and an ability to persist were identified as Cellulomonas, Clavibacter, Curtobacterium, and Microbacterium isolates by 16S rRNA gene sequence, fatty acid, and carbon source utilization analyses. This study defines for the first time the endophytic nature of Microbacterium testaceum. These microorganisms may be useful for biocontrol and other applications.     

Functional and phylogenetic diversity of root-associated bacteria of Ajuga bracteosa in Kangra valley

Present study investigates the cultivable diversity of root-associated bacteria from a medicinal plant Ajuga bracteosa in the Kangra valley, in order to determine their plant growth promoting (PGP) and biotechnological potential. The plant was found to exhibit a positive rhizosphere effect of 1.3-1.5. A total of 123 morphologically different bacteria were isolated from the rhizospheric soil and roots of the plant. Medium composition was found to have significant effect on the composition of isolated bacterial populations. Majority of the rhizospheric soil isolates belonged to α- and γ-Proteobacteria, with Pseudomonas constituting the most dominant species. Endophytic bacterial community, on other hand, consisted almost exclusively of Firmicutes. Majority of the isolates showed PGP activity by producing siderophores and indole acetic acid. Several isolates were found to exhibit very high antioxidant activity in the culture medium. A significant proportion of isolates also demonstrated other ecologically important activities like phosphate solubilization, nitrogen fixation, and production of hydrolytic enzymes including amylase, protease, lipase, chitinase, cellulase, pectinase and phosphatase. Firmicutes were found to be metabolically the most versatile group and performed multiple enzyme activities. This is the first systematic study of culturable bacterial community from the rhizosphere of A. bracteosa, particularly in the Kangra valley region.     

Evidence of variability in the structure and recruitment of rhizospheric and endophytic bacterial communities associated with arable sweet sorghum (Sorghum bicolor (L) Moench)

Background and aims

Sorghum is the second most cultivated crop in Africa and is a staple food source in many African communities. Exploiting the associated plant growth-promoting bacteria (PGPB) has potential as an agricultural biotechnology strategy to enhance sorghum growth, yield and nutritional properties. Therefore this study aimed to evaluate factors that shape bacterial communities associated with sorghum farmed in South Africa, and to detect bacteria consistently associated with sorghum which may impart PGP activities.

Methods

Terminal-Restriction Fragment Length Polymorphism (T-RFLP) was used to assess factors that potentially shape rhizospheric (rhizosphere and rhizoplane) and endophytic (root, shoot, stem) bacterial communities associated with South African sorghum, and together with Denaturing Gradient Gel Electrophoresis (DGGE) to identify consistently sorghum-associated bacterial taxa.

Results

The sorghum rhizospheric communities were less variable than the endophytic ones. Geographical location was the main driver in describing bacterial community assemblages found in rhizospheric sorghum-linked niches, with total NO₃-N, NH₄-N, nitrogen, carbon, pH and, to a lesser extent, clay content identified as the main abiotic factors shaping sorghum-associated soil communities. Endophytic communities presented rather stochastic assemblages, with pH being the main variable explaining their structures. Despite community variations, specific bacterial taxa were consistently detected in sorghum-created rhizospheric and endophytic environments, irrespective of environmental factor effects.

Conclusions

Soil structure and composition, which are influenced by agricultural practices, played major roles in shaping sorghum-associated edaphic bacterial communities. In contrast, endophytic bacterial communities displayed more variation. Nevertheless, potentially agronomically relevant (cyano)bacterial taxa constantly associated with sorghum were identified which is suggestive of their deterministic recruitment.     

Induction of abiotic stress tolerance in plants by endophytic microbes

Endophytes are micro‐organisms including bacteria and fungi that survive within healthy plant tissues and promote plant growth under stress. This review focuses on the potential of endophytic microbes that induce abiotic stress tolerance in plants. How endophytes promote plant growth under stressful conditions, like drought and heat, high salinity and poor nutrient availability will be discussed. The molecular mechanisms for increasing stress tolerance in plants by endophytes include induction of plant stress genes as well as biomolecules like reactive oxygen species scavengers. This review may help in the development of biotechnological applications of endophytic microbes in plant growth promotion and crop improvement under abiotic stress conditions.

Significance and Impact of the Study

Increasing human populations demand more crop yield for food security while crop production is adversely affected by abiotic stresses like drought, salinity and high temperature. Development of stress tolerance in plants is a strategy to cope with the negative effects of adverse environmental conditions. Endophytes are well recognized for plant growth promotion and production of natural compounds. The property of endophytes to induce stress tolerance in plants can be applied to increase crop yields. With this review, we intend to promote application of endophytes in biotechnology and genetic engineering for the development of stress‐tolerant plants.     

Diversity of endophytic fungal community associated with Piper hispidum (Piperaceae) leaves

Tropical and subtropical plants are rich in endophytic community diversity. Endophytes, mainly fungi and bacteria, inhabit the healthy plant tissues without causing any damage to the hosts. These fungi can be useful for biological control of pathogens and plant growth promotion. Some plants of the genus Piper are hosts of endophytic microorganisms; however, there is little information about endophytes on Piper hispidum, a medicinal shrub used as an insecticide, astringent, diuretic, stimulant, liver treatment, and for stopping hemorrhages. We isolated the fungal endophyte community associated with P. hispidum leaves from plants in a Brazilian forest remnant. The endophytic diversity was examined based on sequencing of the ITS1-5.8S-ITS2 region of rDNA. A high colonization frequency was obtained, as expected for tropical angiosperms. Isolated endophytes were divided into 66 morphogroups, demonstrating considerable diversity. We identified 21 isolates, belonging to 11 genera (Alternaria, Bipolaris, Colletotrichum, Glomerella, Guignardia, Lasiodiplodia, Marasmius, Phlebia, Phoma, Phomopsis, and Schizophyllum); one isolate was identified only to the order level (Diaporthales). Bipolaris was the most frequent genus among the identified endophytes. Phylogenetic analysis confirmed the molecular identification of some isolates to genus level while for others it was confirmed at the species level.