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Growth kinetic and nitrogen source optimization for liquid culture fermentation of Metarhizium robertsii blastospores and bioefficacy against the corn leafhopper Dalbulus maidis

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The cosmopolitan entomopathogenic and root endophytic fungus Metarhizium robertsii has a versatile lifestyle and during liquid fermentation undergoes a dimorphic transformation from hyphae to conidia or microsclerotia, or from hyphae to blastospores. In all cases, these processes are mediated by environmental and nutritional cues. Blastospores could be used in spray applications to control arthropod pests above ground and may serve as an attractive alternative to the traditional solid-grown aerial conidial spores of Metarhizium spp. found in commercial products. Nitrogen is a vital nutrient in cell metabolism and growth; however, it is the expensive component in liquid cultures of entomopathogenic fungi. Our goals in this study were to optimize nitrogen sources and titers for maximum production of M. robertsii blastospores cultured in shake flasks at highly aerated conditions and to further determine their virulence against the corn leafhopper Dalbulus maidis, an important vector of serious pathogens in maize crops worldwide. Our fermentation studies revealed that the low-cost corn steep liquor (CSL) was the most suitable nitrogen source to improve blastospore growth in M. robertsii. The growth kinetic assays determined the optimal titer of 80 g L−1 and a yield up to 4.7 × 108 cells mL−1 within 5 days of cultivation (3 days preculture and 2 days culture), at a total cost of US$0.30 L−1. Moreover, the blastospore growth kinetic was strongly dependent on glucose and nitrogen consumptions accompanied by a slight drop in the culture pH. Insect bioassays evidenced a high virulence of these blastospores, either as dried or fresh cells, to D. maidis adults fed on maize plants. Our findings provide insights into the nutritional requirements for optimal and cost-efficient production of M. robertsii blastospores and elucidate the potential of blastospores as an ecofriendly tool against the corn leafhopper.

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The authors are grateful to the technician Lenita Maria and Professor Marcos Yassuo Kamogawa from Exact Sciences Department of University of São Paulo/ESALQ for the help in the extraction and quantification of nitrogen in liquid culture media and Anderson Ramos for providing the insects for the bioassays. We also thank the Brazilian National Council for Scientific and Technological Development (CNPq) [Grant Number 421629/2016–9] for financial support. The first author received a fellowship from the São Paulo Research Foundation (FAPESP) [Grant Number 2016/20610-6] during her PhD project, and GMM received a grant from Embrapa [Grant #].

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NSI, GMM, and ID planned the research and designed the experiments. NSI and SGM performed the experiments. GMM and NSI analyzed data. NSI, ID, JE, and GMM wrote the manuscript. All authors discussed results and commented on the manuscript.

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Correspondence to Natasha Sant´Anna Iwanicki or Gabriel Moura Mascarin.

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This article does not contain studies with human participants or vertebrates performed by any of the authors.

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Iwanicki, N.S., Mascarin, G.M., Moreno, S.G. et al. Growth kinetic and nitrogen source optimization for liquid culture fermentation of Metarhizium robertsii blastospores and bioefficacy against the corn leafhopper Dalbulus maidis. World J Microbiol Biotechnol 36, 71 (2020).

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