Endophytic fungi: A potential source of bioactive compounds against avocado pathogens

Symbiotic relationship between plant species and their associated microbiome provides hosts (plants) with greater resistance to biotic and abiotic stress (Jia et al., 2016). Previous researches have shown that endophytic fungi are able to produce metabolites to inhibit the growth of a wide variety of microorganisms which are responsible of human and crop diseases. In fact, endophytic fungi are considered as a promising source of diverse and structurally novel bioactive compounds nowadays.

In this study, we evaluated the antifungal activity of endophytic fungal extracts against two avocado crop pathogens. For this, leaves and secondary roots were collected from avocado crops located in Montes de María (Bolívar-Colombia), making a selection of trees that survived to adverse sanitary conditions. The plant material was superficial disinfected using the protocol described by Waheeda and Shyam (2017). Subsequently, the samples were cut into 5 mm2 squares and disposed in groups of four segments on different solid culture media supplemented with chloramphenicol at 400 ppm (V8 agar, MEA, YEA, PDA). The process allow us to obtain 326 preliminary isolates and 133 of these showed different morphological characteristics when were grown on PDA.

In a second stage, in vitro antagonism tests were carried out on Fusarium solani and Fusarium equiseti using the endophyte-pathogen dual culture methodology described by Terhonen et.al. (2016). 42 strains of endophytes showed pathogen mycelial growth inhibition zones. These selected strains were aseptically fermented in sterile yeast extract broth for 14 days at 150 rpm and 21 ° C. Then, ultrasound-assisted extractions were performed using ethyl acetate (AcOEt). After that, the biomass was removed from the culture broth and subsequently, the filtrate was lyophilized and extracted with butanol (BuOH). Preliminary antifungal activity of endophytic fungal extracts was evaluated against F. solani and F. equiseti at 1.00 mg/ mL, using the agar dilution method described by Cosoveanu et.al., (2012); eight AcOEt extracts displayed mycelial growth inhibition greater than 50% and were incorporated into the culture medium at 8 different concentrations between 0.05- 2.00 mg/mL to calculate the median inhibitory concentration (IC50) with GraphPad software by means of a regression curve. The strongest antifungal activities were showed by the AcOEt extracts of 4 endophytic Diaporthe spp. (Phomopsis spp.) with IC50 values between 0.29-0.68 mg/mL. Our results showed that this genus is a rich source of bioactive metabolites wich could be used in biocontrol of avocado pathogens.

References

Cosoveanu, A., et.al. (2012). Journal of Horticulture, Forestry and Biotechnology, 16, 87-90.
Jia, M., Chen, L., Xin, H., Zheng, C., Rahman, K., & Han, T. (2016). Frontiers in Microbiology, 7, 1–14.
Terhonen, E., Sipari, N., & Asiegbu, F. O. (2016). Biological Control, 99, 53–63.
Waheedam, K., & Shyam, K. (2017). Journal of Plant Pathology & Microbiology, 8: 2.