ORIGINAL RESEARCH published: 18 October 2021 doi: 10.3389/fmicb.2021.744733 Edited by: Puneet Singh Chauhan, National Botanical Research Institute (CSIR), India Reviewed by: Gholamreza Khaksar, Chulalongkorn University, Thailand Anandham Rangasamy, Tamil Nadu Agricultural University, India Kamrun Nahar, Sher-e-Bangla Agricultural University, Bangladesh *Correspondence: Pramod K. Sahu Pramod.Sahu@icar.gov.in; pramod15589@gmail.com Specialty section: This article was submitted to Microbe and Virus Interactions with Plants, a section of the journal Frontiers in Microbiology Received: 21 July 2021 Accepted: 06 September 2021 Published: 18 October 2021 Citation: Sahu PK, Singh S, Singh UB, Chakdar H, Sharma PK, Sarma BK, Teli B, Bajpai R, Bhowmik A, Singh HV and Saxena AK (2021) Inter-Genera Colonization of Ocimum tenuiflorum Endophytes in Tomato and Their Complementary Effects on Na + /K + Balance, Oxidative Stress Regulation, and Root Architecture Under Elevated Soil Salinity. Front. Microbiol. 12:744733. doi: 10.3389/fmicb.2021.744733 Inter-Genera Colonization of Ocimum tenuiflorum Endophytes in Tomato and Their Complementary Effects on Na + /K + Balance, Oxidative Stress Regulation, and Root Architecture Under Elevated Soil Salinity Pramod K. Sahu 1 * , Shailendra Singh 1 , Udai B. Singh 1 , Hillol Chakdar 1 , Pawan K. Sharma 1 , Birinchi K. Sarma 2 , Basavaraj Teli 2 , Raina Bajpai 2 , Arpan Bhowmik 3 , Harsh V. Singh 1 and Anil K. Saxena 1 1 ICAR-National Bureau of Agriculturally Important Microorganisms, Maunath Bhanjan, India, 2 Department of Mycology and Plant Pathology, Institute of Agricultural Science, Banaras Hindu University, Varanasi, India, 3 ICAR-Indian Agricultural Statistics Research Institute, New Delhi, India Endophytic bacilli of ethano-botanical plant Ocimum tenuiflorum were screened for salt stress-alleviating traits in tomato. Four promising O. tenuiflorum endophytes (Bacillus safensis BTL5, Bacillus haynesii GTR8, Bacillus paralicheniformis GTR11, and Bacillus altitudinis GTS16) were used in this study. Confocal scanning laser microscopic studies revealed the inter-genera colonization of O. tenuiflorum endophytes in tomato plants, giving insights for widening the applicability of potential endophytes to other crops. Furthermore, in a pot trial under 150 mM NaCl concentration, the inoculated endophytes contributed in reducing salt toxicity and improving recovery from salt-induced oxidative stress by different mechanisms. Reduction in reactive oxygen species (ROS) (sub-cellular H 2 O 2 and superoxide) accumulation was observed besides lowering programmed cell death and increasing chlorophyll content. Endophyte inoculation supplemented the plant antioxidant enzyme system via the modulation of enzymatic antioxidants, viz., peroxidase, ascorbate peroxidase, superoxide dismutase, and catalase, apart from increasing proline and total phenolics. Antioxidants like proline have dual roles of antioxidants and osmoregulation, which might also have contributed to improved water relation under elevated salinity. Root architecture, viz., root length, projection area, surface area, average diameter, tips, forks, crossings, and the number of links, was improved upon inoculation, indicating healthy root growth and enhanced nutrient flow and water homeostasis. Regulation of Na + /K + balance and water homeostasis in the plants were also evident from the modulation in the expression of abiotic stress-responsive genes, viz., LKT 1, NHX 1, SOS1, LePIP2, SlERF 16, and SlWRKY 39. Shoot tissues staining with light-excitable Na + indicator Sodium Green TM Tetra (tetramethylammonium) salt showed low sodium transport and accumulation in endophyte-inoculated plants. All four endophytes exhibited different mechanisms for stress alleviation and indicated complementary effects on plant growth. Furthermore, Frontiers in Microbiology | www.frontiersin.org 1 October 2021 | Volume 12 | Article 744733