Comparative transcriptomic analysis of the haustoria of Gymnosporangium yamadae and G. asiaticum

Abstract

To provide a theoretical basis for controlling the spread of rust disease, cultivating disease-resistant varieties and reducing yield losses, we investigated the transcriptome differences between Gymnosporangium yamadae and Gymnosporangium asiaticum at the haustorial stage and revealed a specialized selection mechanism for Gymnosporangium species to infect host plants. We sequenced the transcriptomes of the haustoria in rust-infected leaves when basidiospores of G. yamadae and G. asiaticum infected their hosts, and obtained 21 213 and 13 015 unigenes, respectively. Real-time fluorescence quantitative PCR validation of five genes selected from G. yamadae and G. asiaticum, respectively, showed that their expression profiles were generally consistent with the results of transcriptome analysis, demonstrating the reliability of the transcriptome data. We used seven databases such as Nr, GO, KEGG, and KOG to perform gene function annotation and enrichment analysis, and found that the genes from both rusts were mainly enriched in cellular processes, translation, and metabolism-related pathways. Moreover, we used SignalP, TMHMM online website and other software such as dbCAN, BLSAT, HMMER to show that there were 343 (2.51%) and 175 (2.79%) candidate effector proteins containing 14 and 5 proteases and 10 and 3 lipases in the haustoria of G. yamadae and G. asiaticum, respectively. Furthermore, we used OrthoFinder, BLAST and KaKs Calculator software to analyze the evolutionary relationship of the two fungi. Among one-to-one homologous genes, gene pairs with > 82% alignment were considered to be under conservative selection, and 12.37% under positive selection. Five effectors of G. asiaticum were under positive selection, and one of which was a lipase. No significant differences were found in the enrichment of expressed genes between G. yamadae and G. asiaticum, indicating the biological processes involved in haustoria were relatively conserved, despite the typical host selectivity between species. The low protein similarity between the two species suggested that they were under greater host selective pressure and there was significant evolutionary divergence, which might be related to the host-specific selection mechanism. In the haustorial, the main purpose of the effectors might be to regulate physiological processes in the plants rather than attacking the host directly, and G. yamadae and G. asiaticum might use plant lipids as energy sources.