Evaluation of fluorescent pseudomonads isolated from Satureja rechingeri rhizosphere for the pyoverdine production and plant growth improvement under field conditions

Abstract

Background and objectives: In recent years, the efficiency of native and compatible isolates of fluorescent pseudomonads in promoting the growth of various medicinal plants has been noticed. Fluorescent pseudomonads are a group of bacteria belonging to the genus Pseudomonas that produce a fluorescent green siderophore called pyoverdine under iron-deficiency conditions. These bacteria may also naturally enhance plant growth through other mechanisms such as fixing atmospheric nitrogen, synthesizing plant hormones, solubilizing phosphate, and suppressing plant pathogens and pests. Among the native medicinal plants of Iran, Reshingari savory (Satureja rechingeri Jamzad.) as a critically endangered species is of great importance. This species is utilized in traditional medicine as an antioxidant, digestion facilitator, anti-inflammatory, diuretic, sedative, and disinfectant, as well as in teas and spices. In this way, it is necessary to attempt to domesticate, promote cultivation, and enhance the growth of Reshingari savory using eco-friendly methods. The purpose of this study was to identify the most effective native and compatible isolates of fluorescent pseudomonads in terms of producing the pyoverdine siderophore. Additionally, the present study aimed to investigate the stimulating effects of these isolates on the growth parameters of Reschingari savory under field conditions.

Materials and methods: In this study, sampling was performed randomly from the depth of 25 cm of Satureja rechingeri roots and rhizospheric soil in the natural habitats of Mehran County in Ilam Province, Iran (n=3). Screening of the fluorescent pseudomonads’ isolates were conducted based on the production of fluorescent green pigment on King's B medium. Among 22 isolates of the fluorescent pseudomonads, isolates PF4, PF11, and PF19 showed the highest production of fluorescent green pigments around their colonies and introduced as the superior isolates. These isolates were identified based on several biochemical tests such as oxidase, arginine dihydrolase, KOH string test, oxidative-fermentative test, hypersensitive response (HR), levan, nitrate reduction, catalase, pectinase, lecithinase, gelatinase, starch hydrolysis, metabolism of carbohydrates (glucose, fructose, galactose, sucrose, trehalose, xylose, arabinose, sorbitol, adonitol, meso inositol, ethanol, and glycerol), etc. The pyoverdine amount produced by the superior isolates was evaluated using succinate medium and optical spectrometry method set at 400 nm based on a completely randomized experiment with three replicates. Reshingari savory seeds were also collected from its natural habitats located in Mehran County. Two months after planting the seeds in the seedling trays under greenhouse conditions, the plant’s 16-leaf seedlings were transferred to the research farm of the Research Institute of Forests and Rangelands which is located in Tehran province. Each two-year-old plant was treated with a suspension (107 CFU/ml) of the superior isolate by adding to the soil based on a randomized complete blocks design (P≤0.05). Seven months after applying the treatments, which coincided with the full flowering stage in the third year of planting, several vegetative traits such as the plant height, canopy diameter, the total number of branches per plant, the number of flowering branches, the shoot fresh and dry weights, the root fresh and dry weights, the root length and diameter were measured. Three replicates were randomly selected from three blocks for each treatment. The plots were 2 x 2 meters with a distance of two meters between them, and there was a 3-meter distance between the blocks.

Results: According to the results, isolates PF4, PF11, and PF19 belonged to biovars II, III, and V of Pseudomonas fluorescens, respectively. The highest concentration of siderophore pyoverdine (0.42 mg/ l) was significantly produced by isolate PF11 (P<0.05). The highest plant height (56.4 cm), canopy diameter (61.6 cm), total number of branches per plant (14.7), number of flowering branches (6.33), shoot fresh weight (161.67 g) and dry weight (114.3 g), root fresh weight (11.21 g) and dry weight (7.22 g) were observed in PF11 treatment and the highest root length (14.53 cm) and diameter (0.45 cm) were belonged to PF4 treatment.

Conclusion: According to the findings of the present study, the treatment of Reshingari plants with the growth-promoting rhizobacterium PF11 belonging to the group of fluorescent pseudomonads with the ability to produce the microbial siderophore pyoverdine improved the plant growth parameters under field conditions. It was also found that the efficiency of the bacterial isolates belonging to the P. fluorescens species can be significantly different from each other in terms of influencing the plant growth parameters. Therefore, it is recommended to apply the native and compatible isolate PF11 as an alternative to the use of chemical fertilizers in the cultivation of Reshingari savory.