Abstract
Isaria cateniannulata is a very important and virulent entomopathogenic fungus that infects many insect pest species. Although I. cateniannulata is commonly exposed to extreme environmental temperature conditions, little is known about its molecular response mechanism to temperature stress. Here, we sequenced and de novo assembled the transcriptome of I. cateniannulata in response to high and low temperature stresses using Illumina RNA-Seq technology. Our assembly encompassed 17,514 unigenes (mean length = 1,197 bp), in which 11,445 unigenes (65.34%) showed significant similarities to known sequences in NCBI non-redundant protein sequences (Nr) database. Using digital gene expression analysis, 4,483 differentially expressed genes (DEGs) were identified after heat treatment, including 2,905 up-regulated genes and 1,578 down-regulated genes. Under cold stress, 1,927 DEGs were identified, including 1,245 up-regulated genes and 682 down-regulated genes. The expression patterns of 18 randomly selected candidate DEGs resulting from quantitative real-time PCR (qRT-PCR) were consistent with their transcriptome analysis results. Although DEGs were involved in many pathways, we focused on the genes that were involved in endocytosis: In heat stress, the pathway of clathrin-dependent endocytosis (CDE) was active; however at low temperature stresses, the pathway of clathrin-independent endocytosis (CIE) was active. Besides, four categories of DEGs acting as temperature sensors were observed, including cell-wall-major-components-metabolism-related (CWMCMR) genes, heat shock protein (Hsp) genes, intracellular-compatible-solutes-metabolism-related (ICSMR) genes and glutathione S-transferase (GST). These results enhance our understanding of the molecular mechanisms of I. cateniannulata in response to temperature stresses and provide a valuable resource for the future investigations.
Highlights
Entomopathogenic hyphomycete fungi, such as Beauveria bassiana and Metarhizium spp., are very common and abundant in ecosystems and many are well known to control insects, nematodes, and plant pathogens [1, 2]
The 17,514 unigenes were accurately annotated by interrogating seven databases (Table 3): 65.34% (11,445) of unigenes could be annotated by BLASTx using the non-redundant protein sequences (Nr) database, 17.62% (3,087) by the nucleotide collection (Nt) database, 16.58% (2,905) using the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, 37.41% (6,552) by the Swiss-Prot protein database, 45.22% (7,920) by the Protein family (Pfam) databases, 49.94% (8,748) according to the gene ontology (GO) database, and 26.18% (4,586) according to the KOG database
We suggest that the cell may synthesize more β-1,3-glucan to enhance cell wall tensile strength against cold temperature. β-1,3-glucan has a coiled spring-like structure that confers a degree of elasticity and tensile strength to the cell wall [11]. β-1,3-glucan synthase (MaFKS)-RNAi transformants of entomopathogenic fungus M. acridum are more sensitive to agents that disturb the cell wall or cell membrane and to hyperosmotic stress in comparison with the wild type [12]
Summary
Entomopathogenic hyphomycete fungi, such as Beauveria bassiana and Metarhizium spp., are very common and abundant in ecosystems and many are well known to control insects, nematodes, and plant pathogens [1, 2] They are very susceptible to extreme temperatures (heat and cold) [3,4,5]. Two GPI (glycosylphosphatidylinositol)-anchored protein Ecm orthologous genes, Bbecm in B. bassiana and Mrecm in M. robertsii, have been reported to be essential for cell wall integrity and multi-stress tolerance [13] Both heat shock proteins (Hsps) and intracellular compatible solutes (e.g., trehalose, mannitol, glycerol) are important anti-stress agents [14, 15]. The ribosome pathway, endocytosis pathway and proteasome pathway are active in M. anisopliae under conditions of heat shock stress [16]
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