Fusaric acid (FA) is amongst the oldest identified secondary metabolites produced by Fusarium species, known for a long time to display strong phytotoxicity and moderate toxicity to animal cells; however, the cellular targets of FA and its function in fungal pathogenicity remain unknown. Here, we investigated the role of FA in Fusarium oxysporum, a soil-borne cross-kingdom pathogen that causes vascular wilt on more than 100 plant species and opportunistic infections in humans. Targeted deletion of fub1, encoding a predicted orthologue of the polyketide synthase involved in FA biosynthesis in F. verticillioides and F. fujikuroi, abolished the production of FA and its derivatives in F. oxysporum. We further showed that the expression of fub1 was positively controlled by the master regulator of secondary metabolism LaeA and the alkaline pH regulator PacC through the modulation of chromatin accessibility at the fub1 locus. FA exhibited strong phytotoxicity on tomato plants, which was rescued by the exogenous supply of copper, iron or zinc, suggesting a possible function of FA as a chelating agent of these metal ions. Importantly, the severity of vascular wilt symptoms on tomato plants and the mortality of immunosuppressed mice were significantly reduced in fub1Δ mutants and fully restored in the complemented strains. Collectively, these results provide new insights into the regulation and mode of action of FA, as well as on the function of this phytotoxin during the infection process of F. oxysporum.

Fusarium verticillioides, which causes ear, kernel and stem rots, has been reported as the most prevalent species on maize worldwide. Kernel infection by F. verticillioides results in reduced seed yield and quality as well as fumonisin contamination, and may affect seedling traits like germination rate, entire plant seedling length and weight. Maize resistance to Fusarium is a quantitative and complex trait controlled by numerous genes with small effects. In the present work, a Genome Wide Association Study (GWAS) of traits related to Fusarium seedling rot was carried out in 230 lines of a maize association population using 226,446 SNP markers. Phenotypes were scored on artificially infected kernels applying the rolled towel assay screening method and three traits related to disease response were measured in inoculated and not-inoculated seedlings: plant seedling length (PL), plant seedling weight (PW) and germination rate (GERM). Overall, GWAS resulted in 42 SNPs significantly associated with the examined traits. Two and eleven SNPs were associated with PL in inoculated and not-inoculated samples, respectively. Additionally, six and one SNPs were associated with PW and GERM traits in not-inoculated kernels, and further nine and thirteen SNPs were associated to the same traits in inoculated kernels. Five genes containing the significant SNPs or physically closed to them were proposed for Fusarium resistance, and 18 out of 25 genes containing or adjacent to significant SNPs identified by GWAS in the current research co-localized within QTL regions previously reported for resistance to Fusarium seed rot, Fusarium ear rot and fumonisin accumulation. Furthermore, linkage disequilibrium analysis revealed an additional gene not directly observed by GWAS analysis. These findings could aid to better understand the complex interaction between maize and F. verticillioides.

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Fusarium verticillioides and Fusarium proliferatum cause a wide range of maize diseases.  These fungi produce dangerous mycotoxins, such as fumonisin B1, which are important threats to humans and animals. Given this predicament, the present study aimed to identify the fungi both molecular-morphologically and also investigate the pathogenicity variation and mating type of 41 Fusarium strains in maize (Zea mays L.) samples with sifting their fumonisin contents.  Furthermore, species-specific primers for the molecular identification of distinct strains amplified 2 fragments of 578 and 800 bp in Fusarium verticillioides, while a single 585 bp band was amplified in Fusarium proliferatum.  Accordingly, 24 isolates out of 41 were identified as F. verticillioides, and 13 isolates were identified as F. proliferatum.  The fumonisin-producing and non-producing Fusarium strains were identified using the VERTF-1/VERTF-2 primers.  A total of 24 isolates of F. verticillioides were positively scored based on the amplification of a single 400 bp fragment.  The highest and lowest fumonisin content, as measured using an enzyme-linked immunosorbent assay (ELISA), belonged to strains MS1 and MG3, respectively, and ranged from 960-12673 and 4.07-23 ppm, respectively.  Additionally, the mating type test showed that the sexual form of the studied Fusarium species could possibly belong to the A and D mating populations.  In vivo and in vitro pathogenicity tests revealed a high susceptibility.