Research progress in pathogenic mechanism of Fusarium oxysporum f. sp. cubense

Abstract: Banana is the fourth largest food crop and the largest commercial fruit in the world. However, banana Fusarium wilt, caused by Fusarium oxysporum f. sp. cubense (Foc) , has been seriously threatening banana industry worldwide. To date, three physiological races of Foc have been identified worldwide, of which tropical race 4 (TR4) is the most destructive pathogen and has significant impacts on banana production, and it is also the main pathogen of banana Fusarium wilt in China. The genetic diversity of Foc is different among different physiological races, and Foc has more genetic diversity within race 1 isolates than within race 4 isolates. All TR4 isolates belong to VCG01213/01216 and were identified with the same genetic lineage. As the typical representative of F. oxysporum species complex (FOSC) and the model fungus of soil-borne disease, Foc has been studied by scientists all over the world. However, the research on the pathogenic mechanism of Foc is relatively backward compared with that of F. oxysporum f. sp. lycopersici. Most of the current results on pathogenicity mechanism in Foc were sumarized from the results of studies on other form a specialis of F. oxysporum. However, because of the complex genetic background of FOSC, even different races and different VCG isolates possess distinct pathogenicity differentiation, the results of the other forma specialis of F. oxysporum are not completely applicable to Foc. So, it's necessary to conduct correlated research work on Foc. The process of infection by F. oxysporum can be divided into several steps: recognition between pathogen and host, attachment to root surface, penetration, colonization of the root cortex and finally presentation of diseased symptom on host plant. Based on the infection process, recent research results in the molecular mechanism of Foc were reviewed. 1. Recognition between pathogen and host plant, the first step in pathogenesis, is important for pathogen invading host plant. Researches on Foc showed that α-1, 6-mannosyltransferase Och1 and β-1, 3-mannosyltransferase Gas1 were involved in fungal cell wall integrity, could affect the pathogen's attachment to the root of the host, then reduce the mycelia invasion and the virulence of Foc. Two mitogen-activated protein kinases (MAPK) signaling pathways represented by Fmk1 and Mpk1 were found to be highly correlated with cell wall integrity in F. oxysporum. Researches on Foc also confirmed that the gene (in the Mpk1 signaling pathway) deletion mutants (ΔFoBck1, ΔFoMkk2 and ΔFoSlt2) showed more sensitive to cell wall inhibitors, Congo red (CR) and Fluorescent white (CFW) , than that of wild type, indicating that the integrity of the mutants cell wall was destroyed.In addition, FoSlt2 (homologue of Mpk1) knockout mutant also showed decreased virulence in Cavendish banana which further verified the relationship between cell wall integrity and pathogenicity. These results suggest that cell wall integrity is an important factor for the attachment and invasion of Foc during the recognition process. 2. Cell wall degrading enzymes (CWDEs) , secreted by F. oxysporum, play an important role in invasion and colonization of pathogenesis. However, deletion of individual CWDEencoding genes did not affect the virulence possibly because of functional redundancy. It is suggested that the pathogenicity is the result of the synergism of several synthesis genes of CWDEs. A protein kinase Snf1 isolated from F. oxysporum was identified as a transcriptional regulator and affects the activity of CWDEs. A transcriptional coactivator FoHfi1 in Foc was reported, and the encoding gene is located in the upstream of Snf1 and affects the activity of CWDEs. 3. Fungal toxin plays an important role in colonization of pathogenesis. It was reported that fusaric acid (FA) was the main component of Foc crude toxin as well as beauverin and fumonisin. FA biosynthetic gene (FUB) cluster have been reported in F. oxysporum. The cluster was consisted of at least 12 genes. And among them nine genes (including two zinc finger transcription factors) were essential for the biosynthesis of FA. 4. Comparative genomics analysis has revealed that the genome of F. oxysporum is compartmentalized into two regions. One is the core genomic region which is responsible for essential functions such as basic metabolism and reproduction. And the other is accessory genomic region which is responsible for the host specialization and pathogen virulence. Genome compartmentalization provides new insight into how different strains adapted to different hosts which is also helpful for identification of new pathogenicity-related genes in Foc. 5. Some other pathogenecity factors have been reported in Foc, such as the G-protein subunits FGA1, FGA2 and FGB1 which may affect the virulence potentially via the cAMP-dependent protein kinase pathway. These results provide information for elucidating the role of G protein signaling pathway in the pathogenesis of F. oxysporum. In this paper, genetic diversity and molecular mechanism of pathogenesis in F. oxysporum f. sp. cubense were summarized. The deciphering of pathogenic mechanism will help us to identify new pathogenicity-related genes in Foc, and the identification of key pathogenic factors involved in pathogenesis of Foc will provide some targets for effectively controlling the banana Fusarium wilt.