Antifungal activities of a novel triazole fungicide, mefentrifluconazole, against the major maize pathogen Fusarium verticillioides.

Fusarium ear rot (FER) is a serious fungal disease occurring the late growth stage of maize. FER not only reduces the yield of maize but also causes mycotoxin contamination, which affects the quality of maize and threatens human and animal health. Fusarium verticillioides is the predominant causative pathogen of FER worldwide. At present, there is no registered fungicide for use against maize FER in China. The novel isopropyl alcohol-triazole fungicide mefentrifluconazole (MFZ) has been shown to be effective against several Fusarium spp., but little is known about its specific activity against F. verticillioides. MFZ exhibited strong antifungal activities against 50 strains of F. verticillioides collected from the major maize-growing areas in China. MFZ inhibited mycelial growth, conidium production, germination and germ tube elongation of F. verticillioides. MFZ treatment significantly reduced fumonisin production and the expression levels of fumonisin biosynthetic genes. Genome-wide transcriptional profiling of F. verticillioides in response to MFZ indicated that the expression of genes involved in ergosterol biosynthesis, including fungicide target genes (cyp51 genes), was significantly downregulated by MFZ. MFZ treatment resulted in reduced ergosterol production and increased glycerol and malonaldehyde production as well as relative conductivity in F. verticillioides. A 2-year field experiment showed a significant reduction in FER severity in maize after spraying with MFZ at the tasseling stage. This study evaluated the potential of MFZ to control FER in maize and provides insights into its antifungal activities and mechanism of action against F. verticillioides.

ZmSIZ1a and ZmSIZ1b play an indispensable role in resistance against Fusarium ear rot in maize.

Fusarium ear rot (FER) is a destructive fungal disease of maize caused by Fusarium verticillioides. FER resistance is a typical complex quantitative trait controlled by micro-effect genes, leading to difficulty in identifying the host resistance genes. SIZ1 encodes a SUMO E3 ligase regulating a wide range of plant developmental processes and stress responses. However, the function of ZmSIZ1 remains poorly understood. In this study, we demonstrate that ZmSIZ1a and ZmSIZ1b possess SUMO E3 ligase activity, and that the Zmsiz1a/1b double mutant, but not the Zmsiz1a or Zmsiz1b single mutants, exhibits severely impaired resistance to FER. Transcriptome analysis showed that differentially expressed genes were significantly enriched in plant disease resistance-related pathways, especially in plant-pathogen interaction, MAPK signalling, and plant hormone signal transduction. Thirty-five candidate genes were identified in these pathways. Furthermore, the integration of the transcriptome and metabolome data revealed that the flavonoid biosynthesis pathway was induced by F. verticillioides infection, and that accumulation of flavone and flavonol was significantly reduced in the Zmsiz1a/1b double mutant. Collectively, our findings demonstrate that ZmSIZ1a and ZmSIZ1b play a redundant, but indispensable role against FER, and provide potential new gene resources for molecular breeding of FER-resistant maize cultivars.

In Search of Resistance Against Fusarium Ear Rot: Ferulic Acid Contents in Maize Pericarp Are Associated With Antifungal Activity and Inhibition of Fumonisin Production.

Fusarium verticillioides is a fungal pathogen of maize that causes seedling blight, stem rot, and Fusarium ear rot. Fungal infestation of maize kernels and ears affects grain quality from the ensuing mycotoxin buildup. Among the mycotoxins produced by F. verticillioides, fumonisins accumulate to high levels in Fusarium-infected maize kernels, fumonisin B1 (FB1) being the most abundant in naturally infected maize. Achieving resistance to Fusarium ear rot has been challenging, as various environmental factors facilitate fungal infection. Among the maize grain components that contribute to resistance to F. verticillioides infection, the pericarp is the first barrier faced by the fungus and thus plays a key role. Phenolic acids are major constituents of maize pericarp, of which ferulic acid (FA) is the predominant molecular species. In this work, we explored the relationship between FA levels, fungal infection, and FB1 production in 51 maize genotypes and whether the antioxidant activity of FA might play a role. We confirmed that FA is a major component of the seed pericarp, whose levels as bound FA varied between 4.5 and 26.3 mg/g across maize genotypes. We selected two pools of five maize varieties, with contrasting FA contents: low FA (LFA; 6.14 ± 0.40 mg/g) and high FA (HFA; 15.49 ± 1.31 mg/g). In vitro, HFA extracts inhibited fungal growth with effects comparable to FA concentrations in the 0.25-0.50 mM range. We also established a kernel assay to study F. verticillioides colonization and FB1 production in the LFA and HFA genotypes. Fungal colonization was significantly lower in HFA genotypes relative to LFA genotypes, based on ergosterol levels. Moreover, FB1 production was also inhibited in the HFA genotypes. Importantly, the antioxidant activity of maize pericarp extracts was associated with FA contents, with HFA extracts exhibiting a greater antioxidant activity than LFA extracts. Overall, our results highlight the role of FA and its antioxidant activity on resistance to Fusarium ear rot and provide the basis of a phenotypic trait that can be deployed for breeding selection.

Streptomyces Strains Promote Plant Growth and Induce Resistance Against Fusarium verticillioides via Transient Regulation of Auxin Signaling and Archetypal Defense Pathways in Maize Plants.

Driven by climate change, Fusarium ear rot (FER) caused by Fusarium verticillioides occurs frequently in maize worldwide. In parallel, legislative regulations and increasing environmental awareness have spurred research on alternative FER biocontrol strategies. A promising group of bacterial control agents is Streptomyces species due to their metabolic versatility. However, insights into the molecular modes of action of these biocontrol agents are often lacking. This study aims at unraveling the biocontrol efficacy of Streptomyces rhizobacterial strains against F. verticillioides. We first assessed the direct antagonism of four Streptomyces strains ST02, ST03, ST07, and ST08. Then, a profile of 16 genes associated with intrinsic plant defense signaling was assessed in maize plants. Both in vitro and in vivo data showed that the biocontrol strain ST03 perfectly suppressed the growth of F. verticillioides. High inhibition efficacy was also observed for extracellular compounds in the supernatant secreted by this strain. Especially, for maize cobs, the biocontrol strain ST03 not only inhibited the proliferation of F. verticillioides but also significantly repressed fungal fumonisin production 7 days after inoculation. On maize plants, the direct antagonism was confirmed by a significant reduction of the fungal DNA level in soils when co-applied with F. verticillioides and strain ST03. In terms of its action on plants, strain ST03 induced downregulation of auxin responsive genes (AUX1, ARF1, and ARF2) and gibberellic acid (GA)-related gene AN1 even in the absence of F. verticillioides at early time points. In leaves, the biocontrol strain induced the expression of genes related to salicylic acid (SA), and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA)-mediated pathways, and pathogenesis-related proteins in the presence or absence of the pathogen. Interestingly, the biocontrol strain significantly promoted plant growth even in the presence of F. verticillioides. All of which demonstrated that the Streptomyces strain ST03 is a promising FER biocontrol and a growth-promoting candidate.

Genomics of Maize Resistance to Fusarium Ear Rot and Fumonisin Contamination.

Food contamination with mycotoxins is a worldwide concern, because these toxins produced by several fungal species have detrimental effects on animal and/or human health. In maize, fumonisins are among the toxins with the highest threatening potential because they are mainly produced by Fusarium verticillioides, which is distributed worldwide. Plant breeding has emerged as an effective and environmentally safe method to reduce fumonisin levels in maize kernels, but although phenotypic selection has proved effective for improving resistance to fumonisin contamination, further resources should be mobilized to meet farmers' needs. Selection based on molecular markers linked to quantitative trait loci (QTL) for resistance to fumonisin contamination or/and genotype values obtained using prediction models with markers distributed across the whole genome could speed up breeding progress. Therefore, in the current paper, previously identified genomic regions, genes, and/or pathways implicated in resistance to fumonisin accumulation will be reviewed. Studies done until now have provide many markers to be used by breeders, but to get further insight on plant mechanisms to defend against fungal infection and to limit fumonisin contamination, the genes behind those QTLs should be identified.