Fusarium is a genus that includes the most devastating soil-borne fungal agents. Examples are F. verticillioides (Fv) and F. oxysporum (Fo) that can infect maize and ortive plants, respectively. Fv is the main contaminant of maize seeds stocks and it produce mycotoxins that are highly cancerogenic for animals. The most important Fv mycotoxins are fumonisins due to the high cancerogenic potential. The EU compiled several laws to regulate the concentration of fumonisins in maize seed stocks. The role of mycotoxins is elusive. Our results show that mutants of Fv in the FUM1 gene, the key enzyme in fumonisins production, display no difference in necrosis severity in stalk maize assay. These data suggest that the necrosis is not generated by the effect of the mycotoxins. F. musae (Fm) is a sister species of Fv causing postharvest disease on bananas. Although the two species are genetically similar, they have distinct host ranges with Fm not being able to colonize maize.
Similarly to Fv and Fm, different forma specialis of F. oxysporum have distinct hosts. For example, Fo f.sp. lycopersici (Fol) is able to colonize tomato but not melon while the Fo f.sp. melonis (Fom) infect melon and not tomatoes. As in Fv, the role of mycotoxins in Fo is elusive but there is a lot of information on the role of effectors. The best studied effectors in Fol are the SIX effectors. These effectors are well characterized and play a crucial role in full virulence of Fol in tomato. Some of these effectors are pathogenicity factors. Recently, a single Fom gene was identified to be essential for the host specificity. These results show that a single gene can be responsible for host specificity in Fo species.
The aim of this project is to use comparative genomics to identify specific effectors of Fv essential for the virulence and/or the pathogenicity on maize and to identify the pathogenicity factor that makes Fo f. sp. melongena pathogenic on Solanum melongena when compared to Fol or Fom.
Pathogenic fungi often carry conditionally dispensable (CD) chromosomes that are involved in increased virulence and pathogenicity (Covert et al 1998; Soyer et al 2018). CD chromosomes are responsible for pathogenicity, but their evolution has not been elucidated yet. The aim of our project is to better understand the role of CD in Fusarium. On one side, we will investigate effector genes involved in host specificity and on the other we will study the origin of Fusarium spp dispensable chromosomes.
It has also been demonstrated that transfer of pathogenicity chromosomes from melon infecting species in non-pathogenic Fo isolates, confer the ability to infect melon plant while transfer of pathogenicity chromosome from Fo. f.sp. radicis cucumerinum into Fo isolates confer virulence on melon, cucumber and watermelon (Li et al. 2020). Moreover, the same author evidenced how independent Forc transformants with a putative effector gene from Fom showed strongly reduced or no pathogenicity towards cucumber, while retaining pathogenicity towards melon and watermelon. This suggests that the protein encoded by this gene is recognized by an immune receptor in cucumber plants (Li et.al 2020). The evidence which comes from these data suggest that despite a pattern of CD chromosomes which characterize each ff.spp., single elements may have the same contribution to modulate virulence. Furthermore, it has been recently shown how a few pairs of effectors are able to influence the ability to infect Arabidopsis and Cabbage in Fo species. SIX8-PSE1 are effectors gene present in Fo and they confer virulence against Arabidopsis while gene loss or the presence of their homologs SIX8-PSL1 gives the inability to infect Brassicaceae (Ayukawa et al. 2021). Similarly, to Fo-melon interaction, Fo specificity was found in other crops. Fo f.sp. melongenae it is known to be able to infect eggplants but effectors require for full infection are still unknown. A GWAS approach will be used to identify crucial genes required for infection on eggplants. This study wants to contribute to undercover new effectors genes to increase effector profiles for Fusarium species not yet well characterized.
Furthermore, we would like to characterize the role of effectors in Fv-maize interaction and the role of CD effectors. Our aim is to understand how and if single CD effector genes can influence virulence within Fv. Mycotoxins are produced by some species of fungi and cause poisoning because of their permanence on foodstuffs even after the death of the fungus. The most known ones are fumonisins, produced by Fusarium species, by Fusarium verticillioides and aflatoxins produced by Aspergillus. As well as in Fv it has been observed the presence of presumed mycotoxins also in Fusarium oxysporum, fusaric acid is the most described, even though its biological role has not been completely identified yet. Recent discoveries identified new biological roles of mycotoxin. In one case has been demonstrated how mycotoxin can also manipulate soil-microbiome and moreover acts as inhibitor of fungi growth (Snelders et al 2021). For this reason we want to investigate more on the function of fumonisins produced by Fv and elucidate the function of the fusaric acid produced by Fo including metagenomic techniques to look at the effect of these molecules against other fungi species and bacteria.
The results coming from this research will elucidate the fundamental role of CD in different plant pathogens and will shade some light on a marginal pathogen like Fo f. sp. melongenae that is of crucial importance in Italy were eggplants are cultivated. The identification of the key effector genes involved in plant colonization will tell us more on how to generate resistant eggplants to Fo f. sp. melongenae. Moreover, we will understand the role of mycotoxins in the interaction between Fv and maize
Ayukawa, Y., Asai, S., Gan, P. et al. (2021) A pair of effectors encoded on a conditionally dispensable chromosome of Fusarium oxysporum suppress host-specific immunity. Commun Biol 4, 707 .
Covert, S. F. (1998). Supernumerary chromosomes in filamentous fungi. Curr. Genet. 33, 311¿319.
Li, J., Fokkens, L., Conneely, L. J., & Rep, M. (2020). Partial pathogenicity chromosomes in Fusarium oxysporum are sufficient to cause disease and can be horizontally transferred. Environmental microbiology, 22(12), 4985¿5004.
Li, J, Fokkens, L, Rep, M. A single gene in Fusarium oxysporum limits host range. Mol Plant Pathol. 2021; 22: 108¿ 116.
Ma, LJ., van der Does, H., Borkovich, K. et al. (2010) Comparative genomics reveals mobile pathogenicity chromosomes in Fusarium. Nature 464, 367¿373.
Snelders, N.C., Rovenich, H., Petti, G.C. et al. Microbiome manipulation by a soil-borne fungal plant pathogen using effector proteins. Nat. Plants 6, 1365¿1374 (2020).