Agriculturally Beneficial Endophytic Bacteria of Selected Medicinal Plants

Abstract

The exploration of endophytic bacteria in medicinal plants offers promising avenues for enhancing agricultural productivity and sustainability. This study investigates the potential of endophytic bacteria isolated from selected medicinal plants (Fagonia indica, Ajuga bracteosa, Berberis lycium, and Punica granatum) to promote plant growth and yield in agronomic crops. By identifying and characterizing these beneficial microbes, we aim to harness their plant growthpromoting properties and stress tolerance capabilities, thus contributing to more resilient and productive agricultural systems. The study is divided into three main parts: 1) exploring the endophytic bacteria for mitigating salt stress in tomato plants, 2) evaluating the plant growthpromoting potential of endophytic bacteria isolated from the roots of Berberis lycium Royle, and 3) Exploring the endophytic bacteria of Punica granatum L. as biocontrol agents. In the first part of the study, endophytic bacteria (n =9) associated with Fagonia indica Burm.f. and Ajuga bracteosa wall ex. Benth were investigated and validated to produce auxin, siderophores, solubilized phosphate, and lytic enzymes known to stimulate plant growth. Selected bacterial strains (n=4) namely Enterobacter hormaechei (MOSEL-FLS1), Stenotrophomonas maltophilia (MOSEL-FLS2), Bacillus subtilis (MOSEL-S8), and Staphylococcus epidermidis (MOSEL-S9) were assessed in pot experiment with salt stress for their capacity to stimulate plant growth. These endophytes mitigated the impacts of salt stress, promoting increased tolerance and growth in tomatoes. Additionally, at various salt concentrations (50-200mM), all tested strains enhanced the activity of antioxidant enzymes (SOD and POD), chlorophyll levels, and proline content at different salt concentrations compared to control groups. Moreover, the activity of antioxidant enzymes and their relative transcript levels depended on the salinity stress concentration. Results revealed that strains MOSEL-FLS2 and MOSEL-S8 exhibited the best results and showed promising PGP traits. In the second part of our study, we isolated bacterial strains from Berberis lyceum Royle and evaluated them for secondary metabolites and their role in plant growth promotion. Five bacterial strains identified as Bacillus sp. through 16S rRNA gene sequencing demonstrated significant plant growth-promoting (PGP) traits. Notably, Bacillus subtilis exhibited substantial root elongation in canola, indicating its efficacy as a PGP agent. In addition, methanolic extracts XVI from these strains were analyzed for total phenolic contents (TPC), total flavonoid contents (TFC), DPPH free radical scavenging activity, reducing power, and total antioxidant capacity (TAC). Variability was observed in flavonoid and phenolic contents among extracts from the studied endophytes, with Bacillus paramycoides showing high phenolic content (183.1 μg QA/mg) and Bacillus subtilis showing high flavonoid content (58.5 μg GAE/mg), respectively. In addition, GCMS analysis of Bacillus subtilis crude extract confirmed the presence of diverse fatty acids (C8 to C24), highlighting its complex lipid profile. In the third part of the study, five endophytic bacteria were isolated from Punica granatum L., which were identified using 16S rRNA gene sequencing, and assessed for PGB traits. Bacillus thuringiensis (PGS4) showed the most promising PGP traits and was further studied for secondary metabolite production and antifungal potential. In vitro bioassays demonstrated that extracts from Bacillus thuringiensis were efficient in inhibition of Fusarium oxysporium compared to controls. FTIR analysis of the crude extract identified amines, carboxylic acids, and alkenes crucial for metabolite stability. GC-MS confirmed lipopeptides with β-fatty acids (C14 to C21) in the crude lipopolysaccharide extract. In vivo experiment was performed by applying Bacillus thuringiensis (PGS4) to chickpea plants grown in pots, we observed it can significantly reduced Fusarium wilt disease incidence and enhanced chickpea growth by 9.95%. The present work suggests that endophytes promote plant growth in tomatoes, canola, and chickpeas. In addition, these bacterial isolates can play important role in mitigating salinity and Fusarium wilt disease. Thus, this research highlighted the potential role of endophytes as biofertilizers, and biocontrol agents for the maintenance of plant development for sustainable agriculture.