Abstract
The fungal entomopathogens Metarhizium spp. have been developed as environmentally friendly mycoinsecticides. However, heat stress severely reduces the viability of Metarhizium conidia in the field, which is an important obstacle to the successful use of these mycoinsecticides. Heat treatment induces rapid accumulation of pyruvate, which timely scavenges heat-induced ROS (reactive oxygen species) in hyphal cells of M. robertsii. However, in heat-treated conidia, pyruvate accumulation occurs later than the rapid production of ROSs, which could harm the conidial cells. In the present study, a transgenic M. robertsii strain was constructed with the pyruvate kinases gene overexpressed during conidiation. Two independent transformants of the transgenic strain produced conidia under optimal conditions with elevated pyruvate concentration. This inhibits the rapid heat-induced ROS production and prevents the collapse of mitochondrial membrane potential, thereby increasing conidial tolerance to heat stress. In conclusion, the tolerance of M. robertsii conidia to heat stress was improved by increasing the conidial pyruvate concentration, which could be translated into a more effective pest control.
Highlights
The entomopathogenic fungi Metarhizium spp. are being developed as environmentally friendly mycoinsecticides (Li et al, 2010)
We found that heat treatment induced pyruvate accumulation in M. robertsii by upregulating the pyruvate kinase gene (MAA_06851) for pyruvate production and by downregulating many genes involved in pyruvate consumption, including MAA_02871 and MAA_08787 (Zhang et al, 2017)
To increase the pyruvate concentration in the conidia that were grown under the optimal conditions [on potato dextrose agar (PDA) at 26°C], we overexpressed the pyruvate kinase gene (MAA_06851) or knocked down using the antisense RNA method the two genes (MAA_02871 and MAA_08787) that are involved in pyruvate consumption
Summary
The entomopathogenic fungi Metarhizium spp. are being developed as environmentally friendly mycoinsecticides (Li et al, 2010). In spite of Metarhizium being adaptable to many environments, abiotic stresses, such as heat and UV radiation, severely reduce the viability of conidia in the field, and these stressors are important obstacles to the successful use of Metarhizium spp. Conidial tolerance to abiotic stresses can be improved by selecting the optimal growth substrate and conditions for conidial production (Rangel et al, 2015). Overexpressing genes that encode a SOD (superoxide dismutase) and a bacterial thioredoxin increases heat tolerance in the entomopathogenic fungus Beauveria bassiana (Xie et al, 2010; Ying and Feng, 2011). Overexpressing HSP25 increases the growth of M. robertsii under heat
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