Napier grass can be used as feed for livestock and possibly for bioenergy production. However, the stimulation of the growth of Napier grass by plant growth-promoting bacteria (PGPB) has been rarely found. Thus, this study was performed to investigate the ability of Streptomyces spp. PB5, SRF1, St8, STRM104, and STRM302 to support the growth of Napier grass (Pennisetum purpureum × Pennisetum americanum cultivar Pak Chong 1) under a low water system. Among the five bacterial isolates, Streptomyces sp. St8 was the most suitable bacterial inoculant to stimulate the growth of plants grown under a low water system. Napier grass grew under a low water system and inoculated with Streptomyces sp. St8 had the highest shoot and root weight compared to the other inoculated isolates. The shoot and root fresh weights of plants grown under a low water system were 21.3 ± 1.53 g and 4.29 ± 0.77 g when inoculated with Streptomyces sp. St8. Moreover, Streptomyces sp. St8 also stimulated the growth of plants grown under a normal water system: the highest shoot length (61.3 ± 5.67 cm), shoot fresh weight (26.9 ± 4.07 g), and root fresh weight (4.84 ± 0.54 g) were found in plants inoculated with this bacterial isolate. Furthermore, the plant’s root-to-shoot ratios grown under a low water system were inoculated with each isolate of Streptomyces sp. (PB5, SRF1, St8, STRM104, and STRM302) were lower than for plants grown in the control pots. It means that bacterial inoculation under a low water system could protect the efficiency of roots from producing shoot biomass in the plants. Based on the results found in this study, Streptomyces sp. St8, a microbial inoculant, can be used with Napier grass cropping to produce feed for livestock or bioenergy production.

• Ahmad, F., Ahmad, I., & Khan, M. S. (2008). Screening of free-living rhizospheric bacteria for their multiple plant growth promoting activities. Microbiological Research, 163(2), 178-181. https://doi.org/10.1016/j.micres.2006.04.001

• Calvelo Pereira, R., Monterroso, C., & Macías, F. (2010). Phytotoxicity of hexachlorocyclohexane: Effect on germination and early growth of different plant species. Chemosphere, 79(3), 326–333. https://doi.org/10.1016/j.chemosphere.2010.01.035

• Chandra, D., Srivastava, R., Glick, B. R., & Sharma, A. K. (2018). Drought-tolerant Pseudomonas spp. improve the growth performance of finger millet (Eleusine coracana (L.) Gaertn.) under non-stressed and drought-stressed conditions. Pedosphere, 28(2), 227-240. https://doi.org/10.1016/S1002-0160(18)60013-X

• Chukwuneme, C. F., Babalola, O. O., Kutu, F. R., & Ojuederie, O. B. (2020). Characterization of actinomycetes isolates for plant growth promoting traits and their effects on drought tolerance in maize. Journal of Plant Interaction, 15(1), 93-105. https://doi.org/10.1080/17429145.2020.1752833

• de Souza, R., Ambrosini, A., & Passaglia. L. M. P. (2015). Plant growth-promoting bacteria as inoculants in agricultural soils. Genetics and Molecular Biology, 38(4), 401-419. https://doi.org/10.1590/S1415-475738420150053

• Deblonde, P. M. K., & Ledent, J. F. (2001). Effects of moderate drought conditions on green leaf number, stem height, leaf length and tuber yield of potato cultivars. European Journal of Agronomy, 14(1), 31-41. https://doi.org/10.1016/S1161-0301(00)00081-2

• Fahsi, N., Mahdi, I., Mesfioui, A., Biskri, L., & Allaoui, A. (2021). Phosphate solubilizing rhizobacteria isolated from jujube Ziziphus lotus plant stimulate wheat germination rate and seedlings growth. PeerJ, 9, e11583 https://doi.org/10.7717/peerj.11583

• Goswami, D., Vaghela, H., Parmar, S., Dhandhukia, P., & Thakkera, J. N. (2013). Plant growth promoting potentials of Pseudomonas spp. strain OG isolated from marine water. Journal of Plant Interactions, 8(4), 281-290. https://doi.org/10.1080/17429145.2013.768360

• Haegele, T., Bunnom, T., Khumhom, S., Braeuchler, C., Liplap, P., & Arjharn, W. (2017). Expanding the farming potential of Napier grass (Pennisetum purpureum SCHUMACH.) under low-fertile conditions. Suranaree Journal of Science and Technology, 24(2), 137-151.

• Huang, X., El-Alawi, Y., Penrose, D. M., Glick, B. R., & Greenberg, B. M. (2004). Response of three grass species to creosote during phytoremediation. Environmental Pollution, 130(3), 453-363. https://doi.org/10.1016/j.envpol.2003.12.018

• Khan, N., Bano, A., & Babar, M. A. (2017). The root growth of wheat plants, the water conservation and fertility status of sandy soils influenced by plant growth promoting rhizobacteria. Symbiosis, 72(3), 195-205. https://doi.org/10.1007/s13199-016-0457-0

• Kumar, A., Singh, S., Gaurav, A. K., Srivastava, S., & Verma, J. P. (2020). Plant growth-promoting bacteria: Biological tools for the mitigation of salinity stress in plants. Frontiers in Microbiology, 11, 1216. https://doi.org/10.3389/fmicb.2020.01216

• Kumar, B. L., & Gopal, D. V. R. S. (2015). Effective role of indigenous microorganisms for sustainable environment. 3 Biotech, 5, 867-876. https://doi.org/10.1007/s13205-015-0293-6

• Lakshminarayanan, V., Ponnuswamy, R., & Rengaraju, B. (2015). Screening, purification and characterization of ß-glucan from a novel strain Bacillus cereus LVK13 (KC 898956). International Journal of ChemTech Research, 8(3), 1156-1162.

• Li, X., Geng, X., Xie, R., Fu, L., Jiang, J., Gao, L., & Sun, J. (2016). The endophytic bacteria isolated from elephant grass (Pennisetum purpureum Schumach) promote plant growth and enhance salt tolerance of hybrid Pennisetum. Biotechnology for Biofuels, 9, 190. https://doi.org/10.1186/s13068-016-0592-0

• Machado, S., & Paulsen, G. M. (2001). Combined effects of drought and high temperature on water relations of wheat and sorghum. Plant and Soil, 233(2), 179-187. https://doi.org/10.1023/A:1010346601643

• Mei, C., Amaradasa, S., Sikaroodi, M., Zhang, X., Gillevet, P., Nowak, J., & Lowman, S. (2021). Chapter 7 - Potential application of plant growth promoting bacteria in bioenergy crop production. In J. White, A. Kumar, & S. Droby (Eds.), Microbiome stimulants for crops (pp. 109-123). Woodhead Publishing. https://doi.org/10.1016/B978-0-12-822122-8.00014-5

• Nantasaksiri, K., Charoen-Amornkitt, P., & Machimura, T. (2021). Land potential assessment of Napier grass plantation for power generation in Thailand using SWAT model. Model validation and parameter calibration. Energies, 14(5), 1326. https://doi.org/10.3390/en14051326

• Negawo, A. T., Teshome, A., Kumar, A., Hanson, J., & Jones, C. S. (2017). Opportunities for Napier grass (Pennisetum purpureum) improvement using molecular genetics. Agronomy, 7(2), 28. https://doi.org/10.3390/agronomy7020028

• Niu, S., Gao, Y., Zi, Z., Liu, Y., Liu, X., Xiong, X., Yao, Q., Qin, Z., Chen, N., Guo, L., Yang, Y., Qin, P., Lin, J., & Zhu, Y. (2022). The osmolyte-producing endophyte Streptomyces albidoflavus OsiLf-2 induces drought and salt tolerance in rice via a multi-level mechanism. The Crop Journal, 10(2), 375-386. https://doi.org/10.1016/j.cj.2021.06.008

• Odiyi, B. O., & Oludare, P. A. (2015). Impact of simulated salinity gradient on growth indices of Pennisetum purpureum Schumach. Jordan Journal of Agricultural Sciences, 11(3), 661-667. https://journals.ju.edu.jo/JJAS/article/view/10315/4651

• Osman, N. A., Roslana, A. M., Ibrahima, M. F., & Hassana M. A. (2020). Potential use of Pennisetum purpureum for phytoremediation and bioenergy production: A mini review. Asia Pacific Journal of Molecular Biology and Biotechnology, 28(1), 14-26. https://doi.org/10.35118/apjmbb.2020.028.1.02

• Pereira, N. C. M., Galindo, F. S., Gazola, R. P. D., Dupas, E., Rosa, P. A. L., Mortinho, E. S., & Teixeira Filho, M. C. M. (2020). Corn yield and phosphorus use efficiency response to phosphorus rates associated with plant growth promoting bacteria. Frontiers in Environmental Science, 8, 40. https://doi.org/10.3389/fenvs.2020.00040

• Sade, N., Galkin, E., & Moshelion, M. (2015). Measuring Arabidopsis, tomato and barley leaf relative water content (RWC). Bio-Protocol, 5(8), e1451. https://doi.org/10.21769/BioProtoc.1451

• Sangdee, A., Kornphachara, S., & Srisawat, N. (2016). In vitro screening of antagonistic activity of soil Streptomyces against plant pathogenic fungi and assessment of its characters. International Journal of Agricultural Technology, 12(1), 173-185.

• Somtrakoon, K., Sabutong, B., Srinoi, P., Chaiyasit, R., Sangdee A., & Chouychai W. (2021). Pattern of Streptomyces sp. culture filtrate application on seedling growth of rice cv. RD6 cultivated under fluorene or phenanthrene contamination. Journal of Agricultural Research and Extension, 38(3), 15-27.

• Somtrakoon, K., Sangdee, A., Chouychai, W. (2019a). Roles of plant growth promoting bacteria on growth of ornamental plants grown in anthracene-spiked soil. Journal of Agricultural Research and Extension, 36(2), 11-22.

• Somtrakoon, K., Sripasa, N., Ladsena, S., Sangdee, A., & Chouychai, W. (2019b). Optimum conditions for indole-3-acetic acid production by Streptomyces and its stimulation on seed germination of rice cv. KDML105. Journal of Agricultural Research and Extension, 36(3), 12-22.

• Videira, S. S., de Oliveira, D. M., de Morais, R. F., Borges, W. L., Baldani, V. L. D., & Baldani, J. I. (2012). Genetic diversity and plant growth promoting traits of diazotrophic bacteria isolated from two Pennisetum purpureum Schum. genotypes grown in the field. Plant Soil, 356, 51-66. https://doi.org/10.1007/s11104-011-1082-6

• Waramit, N., & Chaugool, J. (2014). Napier grass: A novel energy crop development and the current status in Thailand. Journal of the International Society for Southeast Asian Agricultural Sciences, 20(1), 139-150.

• Xu, Z., Mei, X., Tan, L., Li, Q., Wang, L., He, B., Guo, S., Zhou, C., & Ye, H. (2018). Low root/shoot (R/S) biomass ratio can be an indicator of low cadmium accumulation in the shoot of Chinese flowering cabbage (Brassica campestris L. ssp. chinensis var. utilis Tsen et Lee) cultivars. Environmental Science and Pollution Research, 25, 36328–36340. https://doi.org/10.1007/s11356-018-3566-x

• Yandigeri, M. S., Meena, K. K., Singh, D., Malviya, N., Singh, D. P., Solanki, M. K., Yadav, A. K., & Arora, D. K. (2012). Drought-tolerant endophytic actinobacteria promote growth of wheat (Triticum aestivum) under water stress conditions. Plant Growth Regulation, 68, 411-420. https://doi.org/10.1007%2Fs10725-012-9730-2

• Zhang, S., Xu, X., Sun, Y., Zhang, J., & Li, C. (2018). Influence of drought hardening on the resistance physiology of potato seedlings under drought stress. Journal of Integrative Agriculture, 17(2), 336–347. https://doi.org/10.1016/S2095-3119(17)61758-1