The endophytic role and growth-promoting ability of the nonpathogenic strain Fusarium oxysporum FO12 in herbaceous hosts

The nonpathogenic strain Fusarium oxysporum FO12 has been characterized as an effective biological control agent against Verticillium dahliae in olive. Nevertheless, the endophytic role of FO12 in herbaceous hosts is still uncertain. Thus, the aim of this study was to demonstrate the nonpathogenic character of FO12 in eight V. dahliae-susceptible herbaceous hosts and to evaluate its ability to promote plant growth. To this end, plants of each host were grown in pots filled with sterile peat moss and inoculated by irrigation with a conidial suspension of FO12 (106 conidia ml−1) two times—at sowing and at the emergence of the first true leaves. Many plants irrigated with sterile distilled water were included as controls. Sampling was conducted at 7, 14, 21, and 28 days after the second inoculation. The fresh weight and length of the aerial parts, roots, and whole plants were determined at each sampling time. Fungal colonization was assessed via quantitative and qualitative methods, including microbiological isolation from roots and stems and confocal laser scanning microscopy (CLSM) imaging at each sampling time. FO12 exhibited the ability to endophytically colonize the roots and stems of the inoculated plants. However, the effect on growth and the degree of colonization varied depending on the host. FO12 did not exert any evident pathogenic effects or negative effects on plant growth or development. The growth of the eggplant, lettuce, and tomato plants in the FO12 treatment group was similar to that in the control group, whereas the relative area under the growth progress curve was significantly greater than that in the control group for plant length in cotton and cucumber; for root fresh weight in pepper, sunflower, and wheat; and for plant fresh weight in sunflower and wheat. These results highlight the need to continue researching the interaction between FO12 and V. dahliae in a wider range of plant species of agronomic interest.