Abstract
Nutritional factors exert significant influence on the growth of entomopathogenic fungi, one of the main agents employed commercially in the biological control of arthropods. Thus, the objective of this work is to optimize the culture medium and solid fermentation time for production of proteases and conidia of Metarhizium robertsii ARSEF 2575 and to evaluate the interference of riboflavin and salts on virulence and resistance to abiotic stress factors. In the first step, nine groups were separated: negative control, positive control, and seven supplementation groups: ammonium nitrate, ammonium chloride, potassium nitrate, sodium nitrate, ammonium sulfate, ammonium phosphate, urea. Sodium nitrate showed significant difference in protease production at the time of 20 days of solid fermentation. Then, different concentrations of sodium nitrate and riboflavin as supplement were evaluated. Response surface methodology demonstrated that riboflavin and sodium nitrate influence proteolytic activity and conidia production, but without synergism. Supplementation of the medium with the optimal concentration of sodium nitrate and riboflavin did not interfere with the germination of conidia without exposure to abiotic stress, but did increase the thermotolerance of conidia. The presence of riboflavin and sodium nitrate at optimal concentrations in the culture medium did not alter fungal virulence with and without exposure to heat stress, varying according to the presence or absence of the supernatant during exposure, evidencing that resistance to heat exposure is multifactorial and dependent on intra- and extracellular factors. Moreover, the supplementation increased the larvicidal activity of the supernatant against Aedes aegypti.
Highlights
The growing demand for sustainable methods of pest control has generated an increase in the consumption of organic products that act as biocontrollers, in particular microorganisms, especially entomopathogenic fungi
Studies related to the use of additives for M. robertsii are scarce, one of the most promising being riboflavin—a complex nitrogen source mainly related to cellular oxidative metabolism (Pereira-Junior et al 2018)
Conidia production on inorganic salt media The analysis of the amount of conidia produced by the fungus in rice supplemented with different inorganic salts showed no significant difference (P > 0.05) between the groups treated among themselves or when compared to the control groups
Summary
The growing demand for sustainable methods of pest control has generated an increase in the consumption of organic products that act as biocontrollers, in particular microorganisms, especially entomopathogenic fungi. The Metarhizium genus, in particular, represents a significant portion of bioproducts marketed in several countries (Mascarin et al 2019; Parra and Junior 2019). In the infection process, the fungus secretes enzymes that catalyze the hydrolysis of the cuticle and Ferreira et al AMB Express (2021) 11:166 allow hyphae to cross it and reach the hemocel (Guo et al 2017). Different compounds, such as inorganic salts and vitamins, can influence biomass formation and extracellular enzyme production in certain isolates (Shah et al 2005). Studies that determine in detail riboflavin as a potential supplement for M. robertsii culture media are lacking
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