2.1. Sample Collection

R. ellipticus plant samples were collected from a forest in Mawklot village, Upper Shillong, Meghalaya (N25^°33’07.9”, E91^°49’49.2”). A. riparia plant samples were collected from St. Edmund’s College campus, Shillong, Meghalaya (N25^°34’00.3”, E91^°53’48.2”). Entire plant along with the soil attached to the roots was carefully uprooted, stored in sterilized polythene bags, and brought to the laboratory. The loosely attached soil was shaken off and discarded. The more tightly bound soil was shaken more vigorously, aseptically collected, and treated as rhizosphere soil, and stored at 4 ± 1°C until use.

2.2. Determination of Soil pH and Organic Matter

Soil pH was measured by suspending 10 g fresh unsieved soil in distilled water in a ratio of 1:5 (w/v), stirred for 15 min, followed by another 15 min of settling down of soil particles, after which pH reading was taken. Soil organic matter was estimated using the method of Walkley and Black [19].

2.3. Pre-treatment and Isolation of Actinomycetota

Soil samples were air-dried at room temperature for 1 week, half of which were subsequently dry heated at 120 ± 2°C for 1 h. 1 g of both soil samples was suspended in 9 ml sterilized distilled water and serially diluted up to 10³ times. Three different selective media, namely, Actinomycetes isolation agar, starch casein agar, and Streptomyces agar, were inoculated with 100 ml of diluted soil suspension. These media were supplemented with cycloheximide (50 mg ml-¹) and rifampicin (20 mg ml-¹) to prevent the growth of contaminating fungi and bacteria. Inoculated media were incubated at 28 ± 2°C for a duration of 3–4 weeks and putative Actinomyces total colonies were selected and maintained in Bennett’s agar medium.

2.4. Morphological Characterization

The selected colonies were inoculated on coverslips obliquely inserted on ISP3 media by following the method of Cross [20], incubated at 28°C for 7 days, and observed under a microscope (Olympus CX21i) at ×400. Morphological characterization of the cultures was based on aerial and substrate mycelia and spore chains.

2.5. Molecular Characterization

Genomic DNA was isolated from the cultures by an enzymatic method of Chen et al. [21] and subsequently used for amplification of 16S rRNA gene. Five primer pairs were used depending on their success of amplification, namely, 27F-A3R, 27F-1492R, 27F-Sm5R [22], 27F-1392R [23], and 27F-1525R [24]. The 25 ml PCR mix consisted of PCR buffer with 1.5 mM MgCl₂, each dNTP of 0.2 mM, each primer of 0.5 mM, Taq DNA polymerase of 0.625 U, and 5 ml of genomic DNA. Reactions were performed at an initial denaturation temperature of 95°C for 5 min, 35 cycles of denaturation at 95°C for 45 s, annealing temperatures at 51°C, 49°C, 49°C, 50°C, and 48°C for 45 s for the above respective primer pairs, extension temperature at 72°C for 90 s, and final extension of 72°C for 7 min. PCR products were verified by agarose gel electrophoresis (1.2% w/v agarose added with 1 ml/10 ml v/v LabSafe™ Nucleic Acid Stain, G-Biosciences), followed by purification and sequencing at Eurofins Genomics, India. The obtained sequences were compared with sequences from NCBI database for genus identification.

2.6. Nucleotide Accession Numbers

The partial 16S rRNA gene sequences were deposited in GenBank against which the following accession numbers were assigned: MH424581-MH424592 and MH425315-MH425321.

2.7. Antimicrobial Activities

The culture filtrates were evaluated for antibacterial activity against Escherichia coli MTCC 1669, Pseudomonas aeruginosa MTCC 4673, Staphylococcus aureus MTCC 9886, Bacillus subtilis MTCC 1305, and anticandidal activity against Candida albicans MTCC 7253 by following the agar well diffusion method described by Syiemiong and Jha [25], except that the incubation was carried out under stationary condition at 30 ± 2°C for 30 days. The cultures were also evaluated for antagonistic activity by dual culture against phytopathogenic bacterial and fungal strains, namely, Xanthomonas campestris pv. campestris Pammel (ITCC BH0004), X. oryzae pv. oryzae Ishiyama (ITCC BB0013), Ralstonia solanacearum Smith (ITCC BI0001), Fusarium oxysporum f. sp. zingiberi Trujillo (ITCC 2698), and Aspergillus niger (MTCC 4325). Dual culture assay against bacterial phytopathogens was performed according to Vijayakumar et al. [26] and against fungal phytopathogens, according to Khamna et al. [27]. MTCC strains were obtained from IMTECH, Chandigarh, India, and ITCC strains were obtained from IARI, New Delhi, India.

2.8. Detection of Biosynthetic Gene Clusters (BGCs)

Detection of BGCs, namely, Type I polyketide synthase (PKS-I), type II polyketide synthase (PKS-II), and non-ribosomal peptide synthetase was carried out according to the method described by Syiemiong and Jha [25].

2.9. In vitro PGP Activities

Indole-3-acetic acid (IAA) production, phosphate solubilization, siderophore, and diazotrophic activities were evaluated according to the methods described by Syiemiong and Jha [28].

2.10. Tomato Seed Germination Assay and Seedling Growth

Tomato seeds (Rocky, Syngenta) were surface sterilized with 70% ethanol for 5 min, followed by 2% NaOCl for 3 min, and rinsed 5 times in sterilized distilled water. Ten surface-sterilized seeds were soaked in five selected 2-week Actinomycete suspension cultures (stationary condition; 105 CFU ml^-1) for 2 h and subsequently transferred into sterilized Petri dishes containing sterilized moistened filter paper and incubated at 25 ± 2°C until germination. Control seeds were surface sterilized as above, but soaked in uninoculated broth and incubated in the same way. The Petri dishes were regularly moistened with sterilized distilled water. Germination percentage was measured as number of seeds germinated out of the total number of seeds ×100. The length of the newly germinated seedlings was also measured 4 days post-germination. The vigor index of tomato seedlings was measured according to Tamreihao et al. [29].

2.11. PGP Assay on Tomato Plant under Different Soil Conditions

To study the influence of resident microorganisms on the PGP activity of the selected isolates, tomato plants were grown on sterilized and unsterilized soil conditions. To study the influence of organic matter, the tomato plants were grown on soil amended with and without organic manure. To perform the above investigations, the assay was conducted in four different soil conditions, namely, (1) sterilized soil, (2) unsterilized soil, (3) sterilized soil amended with 30% (w/w) sterilized organic manure, and (4) unsterilized soil amended with 30% (w/w) unsterilized organic manure. Commercially produced organic manure was used with the following composition: 30–40% moisture, 90% of the material pass through 4 mm sieve, bulk density (g/cm³) <1.0, total viable count (N, P, K, Zn bacteria or N, P bacteria or N, K bacteria) of 5.0 × 10⁶, total organic carbon of 14%, total nitrogen (N) of 0.8%, total phosphorus (P₂O₅) of 0.5%, total potassium (K₂O) of 0.8%, total NPK of 3%, conductivity (dsm^-1) <4, pH 6.5–8.0, C: N ratio <16, and heavy metal content (mg/kg) As2O3 10, Cd 5, Cr 50, Cu 300, Hg 0.15, Ni 50, Pb 100, and Zn 1000. Under sterilized soil condition, tomato seeds were surface sterilized as already described, followed by sowing on germination trays filled with sterilized peat. Germinated seedlings were transplanted into plastic cups containing soil as in (1) or (3) above. Under unsterilized soil condition, the germinated seedlings were transplanted into cups containing soil as in (2) and (4) above. Immediately after transplantation, the seedlings were soil drenched by the selected 2-week Actinomycete suspension cultures (105 cfu ml^-1) around the base of the stem. Control plants were drenched with uninoculated broth. The second drenching was done 1 week later and the plants were finally uprooted 2 weeks later for the measurement of plant fresh weight. All treatments were conducted in five replicates. All the plants were kept in a completely randomized fashion under a net house (50% shade) with daily mid-day temperatures ranging from 21.23 ± 0.77°C to 27.90 ± 0.91°C and relative humidity from 54.33 ± 4.67% to 99.00 ± 0.00%.

2.12. Statistical Analysis

Multiple comparisons between sample means and control means were performed by ANOVA and Tukey’s HSD test at P < 0.05. Statistical analysis of the data was done using SPSS.

3.1. Morphological and Molecular Characterization

Fifty-one putative colonies from the rhizosphere of R. ellipticus and 42 colonies from that of A. riparia were selected, out of which 16 from R. ellipticus and 11 from A. riparia were verified as Actinomycetota according to the micromorphological characteristics. Twelve isolates from the rhizosphere of R. ellipticus and seven from that of A. riparia could be identified using 16S rRNA gene sequence. The remaining isolates were mostly identified only on the basis of micromorphological characteristics due to their poor genomic DNA quality. Overall, Streptomyces was the dominant genus from both the rhizospheres, albeit Amycolatopsis was also prominent from the rhizosphere of R. ellipticus [Figure 1]. The isolates, their GenBank accession numbers, and their closest neighbors from the NCBI database are given in Table 1. The high frequency of isolation of Streptomyces, as shown in this study, has also been reported in other rhizospheres of medicinal and crop plants from India [30-32] and other parts of the world [27,33,34]. Streptomyces is found to be able to easily adapt to various nutritional conditions and availability [35] due to its metabolic versatility, and this could be one of the reasons for its fast growth and colonization on artificial media during isolation. Apart from Streptomyces, Amycolatopsis has also been reported as a frequent genus isolated from soils and rhizospheres of numerous plant species [33,36-39].

Figure 1: Actinomycete genera from rhizospheres of A. riparia and R. ellipticus. [Click here to view]

Table 1: Identified actinomycete isolates from rhizospheres of A. riparia and R. ellipticus.

3.2. Antimicrobial Activities

Agar well diffusion assay of the culture filtrates showed nine isolates from the rhizosphere of R. ellipticus with antimicrobial potential against the bacterial and fungal strains, while there were almost no isolates from the rhizosphere of A. riparia that showed antimicrobial activity. Dual culture assays against bacterial and fungal phytopathogens also showed more numbers of isolates from the rhizosphere of R. ellipticus with antagonistic activity. However, looking at their genetic potential for antimicrobial properties, the distribution of BGCs related to the synthesis of antimicrobial compounds was more or less the same from the isolates of the two rhizospheres. Despite the similarity in BGC profile, the phenotypic difference in expressing antimicrobial activity between the isolates of the two rhizospheres could be due to the difference in organic matter of the soils where the plants were growing. It was previously found that composts can shift the microbial community structure of the rhizosphere, thereby increasing the incidence of bacteria antagonistic against soil-borne root pathogens [40]. In this study, the soils from the two sampling sites had significantly different levels of organic matter. The forest soil from where R. ellipticus plants were collected had more organic matter (158.31 ± 14.20 mg g^-1 soil) than that of the college campus soil from where A. riparia plants were collected (96.06 ± 1.07 mg g^-1 soil). The higher organic matter of the forest soil could have changed its microbiome structure, thereby increasing the incidence of isolation of antagonistic Actinomycetota. There was also a difference in pH of the soils from the two sites, with the forest soil being acidic (pH 4.52 ± 0.03) and the college campus soil being alkaline (8.07 ± 0.09). The reason for the better recovery of antimicrobial-producing strains from acidic soil in this work is not clear, but the previous studies have also reported on antimicrobial-producing Actinomycetota from acidic soils [41,42]. Among the antagonistic isolates from this study, they were mostly of the genus Amycolatopsis and a few from Streptomyces. Streptomyces is already a well-known antibiotic-producing genus of Actinomycetota. The genus Amycolatopsis has also been reported to be a potent antibiotic producer [33,43] and several bioactive compounds have been characterized from it [44-46]. The isolates which showed antagonistic potential in vitro were, however, not tested in vivo for biocontrol activity against disease-causing phytopathogens, and this evaluation needs to be looked into.

3.3. In vitro PGP Activities

In vitro PGP assays of the isolates in terms of IAA production, phosphate solubilization, siderophore, and diazotrophic activities, however, showed that the isolates from the rhizosphere of A. riparia showed better PGP properties as compared to those from the rhizosphere of R. ellipticus. The difference in PGP activities among the isolates from these two rhizospheres could again be due to reasons with respect to soil pH and organic matter. The alkaline nature of the soil from the college campus as already mentioned, combined with its scanty vegetation, could have been responsible for its low organic matter and hence its low nutrient availability [47]. This could have probably assembled PGP microorganisms to enhance nutrient availability. Other workers have also reported on the isolation of plant growth promoting rhizobacteria (PGPR) strains from nutrient-limited soils such as from desert and semi-desert conditions [48,49]. The stark difference in PGP properties among the isolates from the two rhizospheres in this study could also be due to the plants themselves assembling specific microbial communities with specific PGP activities due to the differences in the root exudates and chemical signals secreted by them [50]. Almost all the isolates with PGP property belonged to the genus Streptomyces. PGP Streptomyces strains have also been routinely isolated from rhizospheres and endospheres of numerous plants by previous workers. Al-Dhabi et al. isolated Streptomyces strains from the rhizosphere of tomato plants which could produce IAA, solubilize phosphate, and produce siderophores [51]. Barman and Dkhar isolated numerous PGP Streptomyces strains from the endosphere of several medicinal plants from Meghalaya [52,53].

3.4. Tomato Seed Germination Assay and Seedling Growth

Based on overall PGP activities assayed in vitro, broth cultures of four isolates from the rhizosphere of A. riparia (Amycolatopsis AR-11, Streptomyces AR-23, Streptomyces AR-24, and Streptomyces AR-37) and one from the rhizosphere of R. ellipticus (Streptomyces RE-29) were tested in vitro for their ability to increase seed germination and to enhance seedling growth in tomato. All the five isolates significantly increased seed germination and seedling growth, and hence increased the vigor of the tomato seedlings [Figure 2]. Three isolates, namely, Streptomyces AR-23, Streptomyces AR-37, and Streptomyces RE-29 showed maximum vigor which were not significantly different from each another. A tabulated comparison of the PGP activity of the tested isolates on seed germination percentage, seedling length, and vigor index of tomato is given in Table 2. Other workers have also reported on Streptomyces strains showing enhancing effects on seedling growth of plants. An endophytic Streptomyces strain BPSAC147 isolated from Rhynchotechum ellipticum in Mizoram was found to significantly enhance seedling growth of tomato in vitro [54]. Barman and Dkhar also reported endophytic Streptomyces strains isolated from medicinal plants in Meghalaya that could increase root and shoot length in rice plants [53]. Dochhil et al. reported on two endophytic Streptomyces strains isolated from Centella asiatica in Meghalaya that significantly enhanced seed germination and seedling growth in Phaseolus vulgaris [55].

Figure 2: (a) Vigor indices of tomato seedlings after treatment with broth cultures of the selected isolates; vertical bars = standard error; means with different letters above vertical bars are significantly different from each other at P < 0.05. B 4-day-old tomato seedlings after treatment with broth cultures of the selected PGP isolates. (b). 4-day-old tomato seedlings after treatment with broth cultures of the selected PGP isolates. [Click here to view]

Table 2: Effect of the selected isolates on seed germination, seedling length, and vigor index of tomato.

3.5. PGP Assay on Tomato Plant under Different Soil Conditions

To test the ability of the five selected isolates to promote plant growth in vivo, tomato plants were used as test plants. The tomato plants were grown in four different garden soil conditions as already described, namely, sterilized soil, unsterilized soil, sterilized soil and manure, and unsterilized soil and manure. The results of pH and organic matter analysis of the above soil conditions are given in Table 3. All the four soil conditions fall under the optimum pH range required for normal plant growth, which is usually from 6.1 to 7.8, a range associated with optimum availability of soil nutrients [47]. However, the organic matter was significantly more in the soils amended with manure. There were no significant differences in organic matter between sterilized and unsterilized soils.

Table 3: pH and organic matter of garden soil under different soil conditions.

Without the amendment of organic manure and under sterilized soil condition, none of the tested isolates could significantly enhance tomato plant growth. However, under unsterilized soil condition, three of the tested isolates, namely, AR-11, AR-23, and RE-29 could significantly enhance plant growth in terms of plant fresh weight [Table 4]. With the amendment of organic manure, one isolate AR-37 could significantly enhance tomato plant growth under sterilized soil condition and two more isolates AR-11 and AR-23 in addition to AR-37 could also significantly enhance plant growth under unsterilized soil condition [Table 5]. These results indicated a positive influence of resident soil microorganisms in enhancing PGP activity of the tested isolates under soils of both low and high organic matter. Hassani et al. [56] also reviewed on the existence of “helper” microbes which positively influenced or assisted other PGP microbes in promoting plant growth. Although the helper effect of the resident soil microorganisms on the tested PGP isolates were found to be significant under soils not amended with organic manure [Table 4], the helper effect was, however, not significant on soils amended with organic manure [Table 5]. The reason could be that the presence of abundant organic matter in the soil surpassed the requirement of the resident soil microorganisms in helping the PGP isolates to promote plant growth, and their requirement to help the PGP isolates becomes essential only under low nutrient conditions. Arif et al. [57] also showed that PGPR combined with nitrogen-enriched compost could significantly improve productivity and seed quality in sunflowers.

Table 4: Plant growth promotion of tomato plants by selected isolates under sterilized and unsterilized soil conditions without the amendment of organic manure.

Table 5: Plant growth promotion of tomato plants by selected isolates under sterilized and unsterilized soil conditions with the amendment of organic manure.

The influence of different treatments and soil conditions on PGP activity of tomato plants is shown in [Figure 3a and b], and 25-day-old tomato plants treated with Streptomyces AR-23 are shown in [Figure 3c]. Although some of the tested PGP isolates showed promising results on tomato plant growth, these results are still preliminary to be conclusive, and hence, further intensive investigations are needed under field conditions to assess their effects on tomato crop productivity on a larger scale.

Figure 3: (a-b) Comparison of growth of tomato plant under different treatments and soil conditions; vertical bars = standard error. (c) A 25-day-old tomato plant treated with Streptomyces AR-23 under different soil conditions [Click here to view]

1.  Munaweera TI, Jayawardana NU, Rajaratnam R, Dissanayake N. Modern plant biotechnology as a strategy in addressing climate change and attaining food security. Agric Food Secur 2022;11:26. [CrossRef]

2.  Saeed Q, Xiukang W, Haider FU, Kucerik J, Mumtaz MZ, Holatko J, et al. Rhizosphere bacteria in plant growth promotion, biocontrol, and bioremediation of contaminated sites:A comprehensive review of effects and mechanisms. Int J Mol Sci 2021;22:10529. [CrossRef]

3.  Matilla MA, Krell T. In:Egamberdieva D, Ahmad P, editors. Plant growth promotion and biocontrol mediated by plant-associated bacteria. In:Plant Microbiome:Stress Response. Microorganisms for Sustainability. Vol. 5. Singapore:Springer;2018. 45-80. [CrossRef]

4.  Oren A, Garrity GM. Valid publication of the names of forty-two phyla of prokaryotes. Int J Syst Evol Microbiol 2021;71:005056. [CrossRef]

5.  Goodfellow M. In:Goodfellow M, Kämpfer P, Busse HJ, Trujillo ME, Suzuki KI, Ludwig W, et al, editors. Phylum XXVI. Actinobacteria phyl. nov. Bergey's Manual of Systematic Bacteriology. The Actinobacteria. Vol. 5. 2^nd ed. New York Dordrecht Heidelberg London:Springer;2012. 33. [CrossRef]

6.  Genilloud O. Actinomycetes:Still a source of novel antibiotics. Nat Prod Rep 2017;34:1203-32. [CrossRef]

7.  Wang Z, Solanki MK, Yu ZX, Anas M, Dong DF, Xing YX, et al. Genome characteristics reveal the biocontrol potential of actinobacteria isolated from sugarcane rhizosphere. Front Microbiol 2021;12:797889. [CrossRef]

8.  Trinidad-Cruz JR, Rincón-Enríquez G, Evangelista-Martínez Z, Guízar-González C, Enríquez-Vara JN, López-Pérez L, et al. Actinobacteria from avocado rhizosphere:Antagonistic activity against Colletotrichum gloeosporioides and Xanthomonas sp. Terra Latinoam 2021;39:1-9.

9.  Laassami A, Yekkour A, Meklat A, Djemouai N, Zitouni A, Mokrane S, et al. Actinobacteria associated with vineyard soils of Algeria:Classification, antifungal potential against grapevine trunk pathogens and plant growth-promoting features. Curr Microbiol 2020;77:2831-40. [CrossRef]

10.  Roy S, Santra HK, Banerjee D. In:Yadav AN, Rastegari AA, Yadav N, Kour D, editors. Diversity and biotechnological potential of culturable rhizospheric actinomicrobiota. In:Advances in Plant Microbiome and Sustainable Agriculture:Microorganisms for Sustainability. Vol. 19. Singapore:Springer Nature;2020. 159-87. [CrossRef]

11.  Anusree T, Bhai RS, Shabeer TP, Oulkar D. Streptomyces spp from black pepper rhizosphere:A boundless reservoir of antimicrobial and growth promoting metabolites. J Biol Act Prod from Nat 2019;9:1-23. [CrossRef]

12.  Köberl M, Schmidt R, Ramadan EM, Bauer R, Berg G. The microbiome of medicinal plants:Diversity and importance for plant growth, quality, and health. Front Microbiol 2013;4:400. [CrossRef]

13.  Djemouai N, Meklat A, Gaceb-Terrak R, Youcef KO, Nacer A, Saadi SA, et al. Streptomyces species from the rhizosphere of the medicinal plant Artemisia herba-alba Asso:Screening for biological activities. Biologia 2022;77:2281-99. [CrossRef]

14.  Janatiningrum I, Lestari Y. Enzyme production, antibacterial and antifungal activities of actinobacteria isolated from Ficus deltoidea rhizosphere. Biodiversitas 2022;23:1950-7. [CrossRef]

15.  Devi S, Sharma P, Rana A, Pal J, Kumari A. Diversity and plant growth-promoting potential of actinomycetes associated with the rhizosphere of Arnebia euchroma from Himachal Pradesh (India). J Environ Biol 2021;42:964-72. [CrossRef]

16.  Khaniya L, Bhattarai R, Jan HA, Hussain W, Abbasi AM, Bussmann RW, et al. In:Kunwar RM, Sher H, Bussmann RW, editors. Rubus ellipticus Sm. Rubus foliolosus Weihe and Nees Rubus fruticosus L. Rubus irritans Focke ROSACEAE. Ethnobotany of the Himalayas:Ethnobotany of Mountain Regions. Cham:Springer Nature Switzerland;2021. 1717-33. [CrossRef]

17.  Sengupta R, Dash SS. Invasion status of three non-native species from family Asteraceae in Mizoram. Nelumbo 2020;62:27-39. [CrossRef]

18.  Wahyuono S, Vairappan CS, Mohd. DM, Fauziati V, Purwantiningsih, Marchaban. Major metabolites of methylripariochromene-A, a bioactive substance in the leaves of Eupatorium riparium Reg. In male rats urine. Maj Farm 2018;14:11-9. [CrossRef]

19.  Walkley A, Black IA. An examination of the degtjareff method for determining soil organic matter, and a proposed modification of the chromic acid titration method. Soil Sci 1934;37:29-38. [CrossRef]

20.  Cross T. In:Holt JG, Sneath PH, Krieg NR, editors. Growth and Examination of Actinomycetes:Some Guidelines. Bergey's Manual of Determinative Bacteriology. 9^th ed. Baltimore:Williams and Wilkins;1994. 605-9.

21.  Chen X, Jiang Y, Li Q, Han L, Jiang C. In:Dhanasekaran D, Jiang Y, editors. Molecular phylogenetic identification of Actinobacteria. Actinobacteria:Basics and Biotechnological Applications. Rijeka Croatia:InTech;2016. 141-74. [CrossRef]

22.  Monciardini P, Sosio M, Cavaletti L, Chiocchini C, Stefano D. New PCR primers for the selective amplication of 16S rDNA from different groups of actinomycetes. FEMS Microbiol Ecol 2002;42:419-29. [CrossRef]

23.  Farris MH, Olson JB. Detection of actinobacteria cultivated from environmental samples reveals bias in universal primers. Lett Appl Microbiol 2007;45:376-81. [CrossRef]

24.  Mingma R, Pathom-Aree W, Trakulnaleamsai S, Thamchaipenet A, Duangmal K. Isolation of rhizospheric and roots endophytic actinomycetes from Leguminosae plant and their activities to inhibit soybean pathogen, Xanthomonas campestris pv. Glycine. World J Microbiol Biotechnol 2014;30:271-80. [CrossRef]

25.  Syiemiong D, Jha DK. Antibacterial potential of Actinobacteria from a limestone mining site in Meghalaya, India. J Pure Appl Microbiol 2019;13:789-802. [CrossRef]

26.  Vijayakumar R, Vaijayanthi G, Panneerselvam A, Thajuddin N. In:Dhanasekaran D, Thajuddin N, Panneerselvam A, editors. Actinobacteria:A Predominant Source of Antimicrobial Compounds. Antimicrobials:Synthetic and Natural Compounds, Boca Raton, Florida:CRC Press, Taylor and Francis Group;2015. 117-41. [CrossRef]

27.  Khamna S, Yokota A, Lumyong S. Actinomycetes isolated from medicinal plant rhizosphere soils:Diversity and screening of antifungal compounds, indole-3-acetic acid and siderophore production. World J Microbiol Biotechnol 2009;25:649-55. [CrossRef]

28.  Syiemiong D, Jha DK. Search for plant growth promoting actinobacteria from a limestone mining spoil soil in Meghalaya. Res J Life Sci Bioinformatics Pharm Chem Sci 2019;5:1024-36.

29.  Tamreihao K, Nimaichand S, Chanu SB, Devi KA, Lynda R, Jeeniita N, et al. Acidotolerant Streptomyces sp. MBRL 10 from limestone quarry site showing antagonism against fungal pathogens and growth promotion in rice plants. J King Saud Univ Sci 2016;30:143-52. [CrossRef]

30.  Jog R, Nareshkumar G, Rajkumar S. Plant growth promoting potential and soil enzyme production of the most abundant Streptomyces spp. From wheat rhizosphere. J Appl Microbiol 2012;113:1154-64. [CrossRef]

31.  Thampi A, Bhai RS. Rhizosphere actinobacteria for combating Phytophthora capsici and Sclerotium rolfsii, the major soil borne pathogens of black pepper (Piper nigrum L.). Biol Control 2017;109:1-13. [CrossRef]

32.  Momin MD. Characterization of Rhizospheric Actinomycetes of Major Crop Plants and their Plant Growth Promoting Properties Under Jhum fields of Mizoram. Aizawl:Mizoram University;2020.

33.  Ostash B, Gren T, Hrubskyy Y, Tistechok S, Beshley S, Baranov V, et al. Cultivable actinomycetes from rhizosphere of birch (Betula pendula) growing on a coal mine dump in Silets, Ukraine. J Basic Microbiol 2014;54:851-7. [CrossRef]

34.  Palaniyandi SA, Yang SH, Damodharan K, Suh JW. Genetic and functional characterization of culturable plant-beneficial Actinobacteriaassociated with yam rhizosphere. J Basic Microbiol 2013;53:985-95. [CrossRef]

35.  Maciejewska M, Adam D, Martinet L, NaôméA, Ca?usinska M, Delfosse P, et al. A phenotypic and genotypic analysis of the antimicrobial potential of cultivable Streptomyces isolated from cave moonmilk deposits. Front Microbiol 2016;7:1455. [CrossRef]

36.  Tan HM, Cao LX, He ZF, Su GJ, Lin B, Zhou SN. Isolation of endophytic actinomycetes from different cultivars of tomato and their activities against Ralstonia solanacearum in vitro. World J Microbiol Biotechnol 2006;22:1275-80. [CrossRef]

37.  Alekhya G, Gopalakrishnan S. Exploiting plant growth-promoting Amycolatopsis sp. In chickpea and sorghum for improving growth and yield. J Food Legum. 2016;29:225-31.

38.  Thawai C. Amycolatopsis rhizosphaerae sp. nov., isolated from rice rhizosphere soil. Int J Syst Evol Microbiol 2018;68:1546-51. [CrossRef]

39.  Ningthoujam DS, Lynda RK, Tamreihao K, Chanu SB, Aruna KH, Jeeniita N. Isolation and characterization of Amycolatopsis sp. Strain CRJ2-11 with biocontrol and plant growth promoting potential from upland rice rhizosphere in Manipur, India. Elyns J Microbes 2016;1:104. [CrossRef]

40.  De Brito AM, Gagne S, Antoun H. Effect of compost on rhizosphere microflora of the tomato and on the incidence of plant growth-promoting rhizobacteria. Appl Environ Microbiol 1995;61:194-9. [CrossRef]

41.  Guo X, Liu N, Li X, Ding Y, Shang F, Gao Y, et al. Red soils harbor diverse culturable actinomycetes that are promising sources of novel secondary metabolites. Appl Environ Microbiol 2015;81:3086-103. [CrossRef]

42.  Sharma P, Thakur D. Antimicrobial biosynthetic potential and diversity of culturable soil actinobacteria from forest ecosystems of Northeast India. Sci Rep 2020;10:4104. [CrossRef]

43.  Nimaichand S, Devi AM, Tamreihao K, Ningthoujam DS, Li WJ. Actinobacterial diversity in limestone deposit sites in Hundung, Manipur (India) and their antimicrobial activities. Front Microbiol 2015;6:413. [CrossRef]

44.  Bauermeister A, Calil FA, Pinto FD, Medeiros TC, Almeida LC, Silva LJ, et al. Pradimicin-IRD from Amycolatopsis sp. IRD-009 and its antimicrobial and cytotoxic activities. Nat Prod Res 2019;33:1713-20. [CrossRef]

45.  Hashizume H, Sawa R, Yamashita K, Nishimura Y, Igarashi M. Structure and antibacterial activities of new cyclic peptide antibiotics, pargamicins B, C and D, from Amycolatopsis sp. ML1-hF4. J Antibiot (Tokoyo) 2017;70:699-704. [CrossRef]

46.  Izuta S, Kosaka S, Kawai M, Miyano R, Matsuo H, Matsumoto A, et al. Dipyrimicin A and B, microbial compounds isolated from Amycolatopsis sp. K16-0194. J Antibiot (Tokoyo) 2018;71:535-7. [CrossRef]

47.  Msimbira LA, Smith DL. The roles of plant growth promoting microbes in enhancing plant tolerance to acidity and alkalinity stresses. Front Sustain Food Syst 2020;4:106. [CrossRef]

48.  De la Torre-Hernández ME, Salinas-Virgen LI, Aguirre-Garrido JF, Fernández-González AJ, Martínez-Abarca F, Montiel-Lugo D, et al. Composition, structure, and PGPR traits of the rhizospheric bacterial communities associated with wild and cultivated Echinocactus platyacanthus and Neobuxbaumia polylopha. Front Microbiol 2020;11:1424. [CrossRef]

49.  He A, Niu S, Yang D, Ren W, Zhao L, Sun Y, et al. Two PGPR strains from the rhizosphere of Haloxylon ammodendron promoted growth and enhanced drought tolerance of ryegrass. Plant Physiol Biochem 2021;161:74-85. [CrossRef]

50.  Tkacz A, Cheema J, Chandra G, Grant A, Poole PS. Stability and succession of the rhizosphere microbiota depends upon plant type and soil composition. ISME J 2015;9:2349-59. [CrossRef]

51.  Al-Dhabi NA, Esmail GA, Ghilan AK, Arasu MV. Composting of vegetable waste using microbial consortium and biocontrol efficacy of Streptomyces sp. Al-Dhabi 30 isolated from the Saudi Arabian environment for sustainable agriculture. Sustainability 2019;11:6845. [CrossRef]

52.  Barman D, Dkhar MS. Plant growth-promoting potential of endophytic bacteria isolated from Costus speciosus in tropical deciduous forest of Eastern Himalaya. Proc Natl Acad Sci India Sect B Biol Sci 2019;89:841-52. [CrossRef]

53.  Barman D, Dkhar MS. Seasonal variation influence endophytic actinobacterial communities of medicinal plants from tropical deciduous forest of Meghalaya and characterization of their plant growth-promoting potentials. Curr Microbiol 2020;77:1689-98. [CrossRef]

54.  Passari AK, Upadhyaya K, Singh G, Abdel-Azeem AM, Thankappan S, Uthandi S, et al. Enhancement of disease resistance, growth potential, and photosynthesis in tomato (Solanum lycopersicum) by inoculation with an endophytic actinobacterium, Streptomyces thermocarboxydus strain BPSAC147. PLoS One 2019;14:e02149014. [CrossRef]

55.  Dochhil H, Dkhar MS, Barman D. Seed germination enhancing activity of endophytic Streptomyces isolated from indigenous ethno-medicinal plant Centella asiatica. Int J Pharma Bio Sci 2013;4:256-62.

56.  Hassani MA, Durán P, Hacquard S. Microbial interactions within the plant holobiont. Microbiome 2018;6:58. [CrossRef]

57.  Arif MS, Shahzad SM, Riaz M, Yasmeen T, Shahzad T, Akhtar MJ, et al. Nitrogen-enriched compost application combined with plant growth-promoting rhizobacteria (PGPR) improves seed quality and nutrient use efficiency of sunflower. J Plant Nutr Soil Sci 2017;180:464-73. [CrossRef]