Effect of endophyte Rhizobium sp. on disease control, seed germination and growth of basil microgreens

Microgreens are immature vegetables with green leaves that can be harvested and consumed shortly after the appearance of the first true leaves. Basically, the part that appears above the root is used as a fresh vegetable for salads. They are more nutritious than their adult plants. In this study, basil (Ocimum basilicum) seeds were used to produce microgreens. Pythium spp. such as Pythium ultimum is one of the diseases that limits the cultivation of many microgreens.

Ten endophytic bacteria were isolated from the roots of some adult plants of the Lamiaceae family (including basil, Phlomis aucheri and rosemary). Basil seeds were disinfected with 1% sodium hypochlorite for two minutes and then washed five times with distilled water and dried at room temperature. For each treatment, 20 seeds with three replications were placed in two layers of sterile filter paper inside petri dishes and transferred to an incubator with a temperature of 21 ° C and a humidity of 65 to 70%. In the initial pretest to select the best bacteria in terms of growth stimulus characteristics, three traits of germinated seeds percentage, seedling height and wet weight at a common bacterial concentration (5×107 cfu/ml) were evaluated. Antifungal activity against P. ultimum was performed in mix of PDA and NA medium. Bacterium was detected using 16S rRNA region primers: 27F and 1492r. Several growth factors and biochemical changes including total phenol, peroxidase and catalase were investigated.

None of the bacteria showed a direct antifungal effect against P. ultimum in the laboratory. Among the bacteria, isolate GF1 showed more effects of growth stimulation than other bacteria. All growth traits showed a significant increase in the level of 5% probability in both bacterial isolate GF1 concentrations compared to the control, so that the least observed changes in the increase of each growth factor was above 50%. The highest increase in both concentrations was related to seed length vigor and germination vigor. In some cases, a significant difference was observed between the two concentrations of GF1 isolate. None of the bacteria showed a direct antifungal effect against P. ultimum in the laboratory. Sequencing of 16S region of ribosomal genes showed that the bacterium is Rhizobium sp. . This bacterium (in two concentrations of 5×107 and 5×106) had a significant effect of improving the weight loss of microgreens, the disease severity and disease incidence caused by P. ultimum in comparison with the control. Biochemical analysis showed that total phenol, catalase and peroxidase were significantly increased in microgreens inoculated with this bacterium. The results related to changes in total phenol, catalase and peroxidase after 14 days and at the same time with microgreen harvest showed that the bacteria in both concentrations were able to increase the amount of total phenol and defense enzymes in the microgreens. The concentrations used by the bacteria showed a significant difference in all cases, but the low concentration was also able to increase the amount of total phenol and enzymes compared to the control. Changes in total phenol, catalase and peroxidase in almost all cases from highest to lowest included the following treatments: high concentration of bacteria and fungus, low concentration of bacteria and fungus, high concentration of bacteria, low concentration of bacteria, fungus alone and the control which did not contain any microbial treatment. However, in the case of catalase, the changes resulting from inoculation of the pathogenic fungus alone and the changes resulting from the use of bacteria at lower concentrations were almost equal and did not show a significant difference.

In this study, unlike many similar studies that aim at biological control. The initial screening of bacteria with useful potential in agriculture was not based on antifungal properties. Rather, the primary goal was to increase germination and microgreen growth factors. Since the goal of biological control is ultimately to increase efficiency of crop yield by disease control, it makes sense to prioritize growth-promoting characteristics that directly affect the efficiency. This study showed that the endophytic bacterium, Rhizobium sp. GF1, without direct antifungal effect using inducing resistance mechanism and increasing the plant's defense compounds is able to significantly control the disease. In this study, bacterium was obtained that have a dual useful ability in the fields of plant protection and vegetable growing.