Arabian Journal for Science and Engineering https://doi.org/10.1007/s13369-017-2999-8 RESEARCH ARTICLE - BIOLOGICAL SCIENCES Isolation and Characterization of Indigenous Plant Growth-Promoting Rhizobacteria and Their Effects on Growth at the Early Stage of Thai Jasmine Rice (Oryza sativa L. KDML105) Thanakorn Saengsanga 1 Received: 31 July 2017 / Accepted: 19 November 2017 © King Fahd University of Petroleum & Minerals 2017 Abstract Current exploitation of rhizobacteria as plant growth regulators for application in sustainable agriculture has attracted much attention. The objective of this study was to characterize the effects of indigenous plant growth-promoting rhizobacteria (PGPR) isolated from rice rhizosphere on growth at the early stage of Thai jasmine rice (Oryza sativa L. cv. KDML105) plants. Five isolates with the ability to produce indole acetic acid (IAA) were obtained and identified by 16S rDNA sequence analysis. These strains were identified as Enterobacter sp. NRRU-N13, NRRU-N20, NRRU-N21, NRRU-D47, and Bacillus sp. NRRU-D40. In the presence of tryptophan precursor, these IAA-producing isolates produced high IAA concentrations ranging from 37.92 to 46.97 μg mL −1 , with the highest IAA production observed for Enterobacter sp. NRRU-N13. In addition, four of the IAA-producing isolates exhibited phosphate solubilizing activities of > 400 mg L −1 , with the highest activity of 422.50 mg L −1 observed for Enterobacter sp. NRRU-N13. Maximum growth of rice seedlings measured in terms of root and shoot lengths and dry weights, and biomass was achieved when NRRU-N13 was employed as inoculants, thereby indicating that this isolate was the most promising PGPR. This study has highlighted the beneficial effects of PGPR isolates on growth of rice seedlings, with the isolate NRRU-N13 being the most promising one. As with the obtained results, this isolate could be used as plant growth-stimulating agents to increase crop production and yield. Keywords Indole acetic acid · Oryza sativa L. · Phosphate solubilization · Plant growth-promoting rhizobacteria · 16S rDNA sequence analysis 1 Introduction Rhizobacteria, which are root-colonizing microorganisms that are able to stimulate plant growth and control plant diseases, are recognized as plant growth-promoting rhizobac- teria (PGPR) [1]. Consequently, PGPR have been widely developed as plant growth regulators due to their benefi- cial effects. Several significant mechanisms of PGPR have been documented to enhance plant growth and development, which include nitrogen fixation, phosphate solubilization, phytohormone production (particularly indole acetic acid (IAA), which is an important natural phytohormone among B Thanakorn Saengsanga thanakorn.s@nrru.ac.th 1 Environmental Science Program, Faculty of Science and Technology, Nakhon Ratchasima Rajabhat University, Nakhon Ratchasima 30000, Thailand others essential for plant growth and development), and antibiotic and antifungal metabolite secretion [2]. Some PGPR not only produce plant growth regulators but also can convert atmospheric nitrogen into ammonia, which can then be taken by plants [3]. According to the afore- mentioned fact, exploring nitrogen-fixing rhizobacteria with IAA production activities with the purpose to utilize these microorganisms as a substitute to chemical fertilizers has drawn increasing interest in the scientific community [4]. Moreover, the literature has revealed that PGPR have the potential to alleviate the stress-induced damages to plants through 1-aminocyclopropane-1-carboxylate (ACC) deam- inase secretion [5] and metabolic compound synthesis for controlling and suppressing pathogens [2]. Many studies have shown that inoculation of plants with PGPR is a promising approach to increase plant growth and yield [6–10]. For instance, treatment of rice seeds with Enter- obacter cloacae was found to significantly improve seedling growth and yield of rice plants [11]. Besides, drenching of 123