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

Abstract

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 H2O2 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., LKT1, NHX1, SOS1, LePIP2, SlERF16, and SlWRKY39. Shoot tissues staining with light-excitable Na+ indicator Sodium GreenTM 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, this could be harnessed in the form of a consortium for salt stress alleviation. The present study established inter-genera colonization of O. tenuiflorum endophytes in tomato and revealed its potential in maintaining Na+/K+ balance, reducing ROS, and improving root architecture under elevated salinity.

ROS generated from biotic stress: Effects on plants and alleviation by endophytic microbes

“…Endophytes have been reported to maintain ROS homeostasis in both biotic ( 2020 ; Sahu et al., 2019 ; Singh et al., 2020 ) and abiotic stresses ( Sahu et al., 2021 ). Endophytes decrease ROS content in the plant cells by increasing scavenging via increased redox state of glutathione and ascorbate along with promoting antioxidant enzyme activities ( Sadeghi et al., 2020 ).…”

Salt-tolerant endophytic bacterium Enterobacter ludwigii B30 enhance bermudagrass growth under salt stress by modulating plant physiology and changing rhizosphere and root bacterial community

“…We found that the ratio of Na + (root) to Na + (leaf) markedly decreased in un-inoculated bermudagrass compared with E. ludwigii B30 inoculated bermudagrass under SS, indicating that E. ludwigii B30 inoculation restricted Na + translocation from the roots to aerial parts of bermudagrass, thereby restricting toxic Na + accumulation within photosynthetic tissues ( Porcel et al, 2016 ). Sahu et al (2021) showed similarly that endophytic bacteria inoculation restricted Na + translocation from the roots to shoot in salt-stressed tomato. Under SS, excess Na + ions will compete against K + (the other macronutrient), inducing nutritional and metabolic disturbance and exerting adverse impacts on plants.…”

Biosynthetic Mechanisms of Secondary Metabolites Promoted by the Interaction Between Endophytes and Plant Hosts

“…In recent years, researchers have been trying to explore the evolutionary relationship between host and endophytes, and trace the reciprocity mechanism of this specific evolutionary relationship, in order to provide theoretical basis for the commercial development of endophytes. At present, some endophytic bacteria and their active products have been successfully used in commercial production and obtained great benefits in the preparation of new drugs and agricultural protection ( Singh and Gaur, 2017 ; Mishra et al, 2018b ; Sahu et al, 2021 ). It is of great economic value to reveal the interactive mechanisms between plants and endophytes to explore and release the potential for the industrial development of endophytes.…”