Flexible metabolism in Metarhizium anisopliae and Beauveria bassiana: role of the glyoxylate cycle during insect pathogenesis

Abstract

Insect pathogenic fungi such as Metarhizium anisopliae and Beauveria bassiana have an increasing role in the control of agricultural insect pests and vectors of human diseases. Many of the virulence factors are well studied but less is known of the metabolism of these fungi during the course of insect infection or saprobic growth. Here, we assessed enzyme activity and gene expression in the central carbon metabolic pathway, including isocitrate dehydrogenase, aconitase, citrate synthase, malate synthase (MLS) and isocitrate lyase (ICL), with particular attention to the glyoxylate cycle when M. anisopliae and B. bassiana were grown under various conditions. We observed that ICL and MLS, glyoxylate cycle intermediates, were upregulated during growth on 2-carbon compounds (acetate and ethanol) as well as in insect haemolymph. We fused the promoter of the M. anisopliae ICL gene (Ma-icl) to a marker gene (mCherry) and showed that Ma-icl was upregulated when M. anisopliae was grown in the presence of acetate. Furthermore, Ma-icl was upregulated when fungi were engulfed by insect haemocytes as well as during appressorium formation. Addition of the ICL inhibitor 3-nitroproprionate delayed conidial germination and inhibited appressorium formation. These results show that these insect pathogenic fungi have a flexible metabolism that includes the glyoxylate cycle as an integral part of germination, pathogenesis and saprobic growth.

Analysis of Whitefly Transcriptional Responses to Beauveria bassiana Infection Reveals New Insights into Insect-Fungus Interactions

“…As an acid-induced virulent factor [68], phosphoenolpyruvate carboxykinase together with pyruvate carboxylase is likely involved in the glucose metabolic process [70]. Isocitrate lyase is responsible for the glyoxylate cycle which may allow B. bassiana to utilize a variety of carbon sources during the insect pathogenesis [67], [68], [69], [71], [72]. The 22.4 fold up-regulation of isocitrate lyase probably indicated its very important role in the infection process.…”

“…This suggests a possibility that B. bassiana utilize this strategy to escape from recognition by the whitefly immune system. The highly up-regulated enzymes associated with carbon metabolism also indicate a large amount of energy consumed for successful fungal infection [67], [68], [69], [71], [72]. Also, special attention needs to be paid to the four putative secretion proteins and their roles in regulating host defense responses.…”

Metabolic Effect of an Exogenous Gene on Transgenic Beauveria bassiana Using Liquid Chromatography–Mass Spectrometry-Based Metabolomics

“…It is noteworthy that the glyoxylate cycle has been shown to be required for germination, pathogenesis, and saprobic growth in insect pathogenic fungi. 29 Therefore, it is likely that increased levels of glyoxylate and beauverolides are one reason why the transformant is more toxic than wild isolate.…”

Targeting of insect epicuticular lipids by the entomopathogenic fungus Beauveria bassiana: hydrocarbon oxidation within the context of a host-pathogen interaction

“…Isocitrate lyase (ICL) and malate synthase (MLS) are not only up-regulated during growth on two-carbon compounds including acetate and ethanol, but also during growth in insect hemoplymph (Padilla-Guerrero et al, 2011). In M. anisopliae , icl is up-regulated during the initial infection stage of appressoria formation as well as during late host growth events when the fungi are engulfed by insect heamocytes, highlighting the contribution of the glyoxylate cycle in pathogenesis (Padilla-Guerrero et al, 2011).…”

“…Isocitrate lyase (ICL) and malate synthase (MLS) are not only up-regulated during growth on two-carbon compounds including acetate and ethanol, but also during growth in insect hemoplymph (Padilla-Guerrero et al, 2011). In M. anisopliae , icl is up-regulated during the initial infection stage of appressoria formation as well as during late host growth events when the fungi are engulfed by insect heamocytes, highlighting the contribution of the glyoxylate cycle in pathogenesis (Padilla-Guerrero et al, 2011). Production of metabolic acids by entomopathogenic fungi, potentially resulting from cuticular hydrocarbon recognition and assimilation cues, can also directly participate in insect virulence; and citrate, formate, and oxalate have been shown to contribute to B. ba ssiana virulence (Bidochka and Khachatourians, 1991; Kirkland et al, 2005).…”