Agriculture is the most important resource in the world markets, and ecological systems face many challenges. The global consumption of chemical fertilizers is growing and plays a vital role in crop yields1. However, plants grown in this way do not have enough time to grow and mature properly and therefore do not acquire excellent plant qualities2. In addition, very harmful toxic compounds can accumulate in the human body and soil3. Therefore, there is a need to develop environmentally friendly and sustainable solutions to reduce the need for chemical fertilizers. Beneficial microorganisms can be an important source of biologically active natural compounds4.
Endophytic communities in leaves vary depending on the host plant species or genotype, plant growth stage, and plant morphology. 13 Several studies have reported that Azospirillum, Bacillus, Azotobacter, Pseudomonas, and Enterobacter have the potential to promote plant growth. 14 Furthermore, Bacillus and Azospirillum are the most intensively studied PGPB genera in terms of improving plant growth and yield. 15 Studies have shown that co-inoculation of Azospirillum brasiliensis and Bradyrhizobium in legumes can enhance the yield of maize, wheat, soybean, and kidney bean. 16, 17 Studies have shown that inoculation of Salicornia with Bacillus licheniformis and other PGPBs synergistically promotes plant growth and nutrient uptake. 18 Azospirillum brasiliensis Sp7 and Bacillus sphaericus UPMB10 improve root growth of sweet banana. Similarly, fennel seeds are difficult to grow due to poor vegetative growth and low germination, especially under drought stress conditions20. Seed treatment with Pseudomonas fluorescens and Trichoderma harzianum improves early growth of fennel seedlings under drought stress conditions21. For stevia, studies have been conducted to evaluate the effects of mycorrhizal fungi and plant growth promoting rhizobacteria (PGPR) on the ability of the organism to grow, accumulate secondary metabolites, and express genes involved in biosynthesis. According to Rahi et al.22, inoculation of plants with different PGPRs improved their growth, photosynthetic index, and accumulation of stevioside and stevioside A. On the other hand, inoculation of stevia with plant growth-promoting rhizobia and arbuscular mycorrhizal fungi stimulated plant height, stevioside, mineral, and pigment contents.23 Oviedo-Pereira et al.24 reported that the irritant endophytes Enterobacter hormaechei H2A3 and H5A2 increased SG content, stimulated trichome density in leaves, and promoted accumulation of specific metabolites in trichomes, but they did not promote plant growth;
GA3 is one of the most important and biologically active gibberellin-like proteins31. Exogenous treatment of stevia with GA3 can increase stem elongation and flowering32. On the other hand, some studies have reported that GA3 is an inducer that stimulates plants to produce secondary metabolites such as antioxidants and pigments, and is also a defense mechanism33.
Phylogenetic relationships of isolates in relation to other strain types. GenBank accession numbers are given in parentheses.
Amylase, cellulase and protease activities are shown as clear bands around the colonies, while white precipitates around the colonies indicate lipase activity. As shown in Table 2, B. paramycoides SrAM4 can produce all hydrolases, while B. paralicheniformis SrMA3 can produce all enzymes except cellulase, and B. licheniformis SrAM2 produces only cellulase.
Several important microbial genera have been associated with increased secondary metabolite synthesis in medicinal and aromatic plants74. All enzymatic and non-enzymatic antioxidants were significantly increased in S. rebaudiana Shou-2 compared to the control. The positive effect of PGPB on TPC in rice was also reported by Chamam et al.75; Furthermore, our results are consistent with the results of TPC, TFC, and DPPH in S. rebaudiana, which was attributed to the combined action of Piriformospora indica and Azotobacter chroococcum76. TPC and TFC77 were significantly higher in basil plants treated with microorganisms compared to untreated plants. Moreover, the increase in antioxidants may occur for two reasons: hydrolytic enzymes stimulate the induced plant defense mechanisms in the same way as pathogenic microorganisms until the plant adapts to bacterial colonization78. Secondly, PGPB may act as an initiator of the induction of bioactive compounds formed via the shikimate pathway in higher plants and microorganisms 79 .
The results showed that there was a synergistic relationship between leaf number, gene expression and SG production when multiple strains were co-inoculated. On the other hand, double inoculation was superior to single inoculation in terms of plant growth and productivity.
Hydrolytic enzymes were detected after inoculation of bacteria on agar medium containing indicator substrate and incubation at 28 °C for 2–5 days. After plating bacteria on starch agar medium, amylase activity was determined using iodine 100 solution. Cellulase activity was determined using 0.2% aqueous Congo red reagent according to the method of Kianngam et al. 101 . Protease activity was observed through clear zones around colonies plated on skim milk agar medium as described by Cui et al. 102 . On the other hand, lipase 100 was detected after inoculation on Tween agar medium.
Post time: Jan-06-2025