Bacterial Communities in the Embryo of Maize Landraces: Relation with Susceptibility to Fusarium Ear Rot

Locally adapted maize accessions (landraces) represent an untapped resource of nutritional and resistance traits for breeding, including the shaping of distinct microbiota. Our study focused on five different maize landraces and a reference commercial hybrid, showing different susceptibility to fusarium ear rot, and whether this trait could be related to particular compositions of the bacterial microbiota in the embryo, using different approaches. Our cultivation-independent approach utilized the metabarcoding of a portion of the 16S rRNA gene to study bacterial populations in these samples. Multivariate statistical analyses indicated that the microbiota of the embryos of the accessions grouped in two different clusters: one comprising three landraces and the hybrid, one including the remaining two landraces, which showed a lower susceptibility to fusarium ear rot in field. The main discriminant between these clusters was the frequency of Firmicutes, higher in the second cluster, and this abundance was confirmed by quantification through digital PCR. The cultivation-dependent approach allowed the isolation of 70 bacterial strains, mostly Firmicutes. In vivo assays allowed the identification of five candidate biocontrol strains against fusarium ear rot. Our data revealed novel insights into the role of the maize embryo microbiota and set the stage for further studies aimed at integrating this knowledge into plant breeding programs.

Fusarium verticillioides induces maize-derived ethylene to promote virulence by engaging fungal G-protein signaling

Seed maceration and contamination with mycotoxin fumonisin inflicted by Fusarium verticillioides is major disease of concern for maize producers world-wide. Meta-analyses of QTL for Fusarium ear rot resistance uncovered several ethylene (ET) biosynthesis and signaling genes within them, implicating ET in maize interactions with F. verticillioides. We tested this hypothesis using maize knock-out mutants of the 1-aminocyclopropane-1-carboxylate (ACC) synthases, ZmACS2 and ZmACS6. Infected wild-type seed emitted five-fold higher ET levels compared to controls, whereas ET was abolished in the acs2 and acs6 single and double mutants. The mutants supported reduced fungal biomass, conidia and fumonisin content. Normal susceptibility was restored in the acs6 mutant with exogenous treatment of ET precursor, ACC. Subsequently, we showed that fungal G-protein signaling is required for virulence via induction of maize-produced ET. F. verticillioides Gβ subunit and two regulators of G-protein signaling mutants displayed reduced seed colonization and decreased ET levels. These defects were rescued by exogenous application of ACC. We concluded that pathogen-induced ET facilitates F. verticillioides colonization of seed, and in turn host ET production is manipulated via G-protein signaling of F. verticillioides to facilitate pathogenesis.

The impact of fusarium ear rot in Poland and methods to reduce losses caused by the disease

Maize has a significant economic impact all over the world. Fungi in the genus Fusarium that cause fusarium ear rot of maize have significant effect on the yeld quality and quantity. The main threat is the contamination of grain with the mycotoxins they produce - as these are harmful to humans and animals. Such mycotoxins are a group of secondary metabolites of varied structure, which belong mainly to trichothecenes, fumonisins and zearalenones. As no efficient chemical control read and pink rot in the field is possible, prevention relies on cultural practices and use resistant hybrids. Insects play an important role in the infection, which is why it is recommended to control their prevalence during growing season.

The Effect of northern corn leaf blight severity on Fusarium ear rot incidence of maize

Northern corn leaf blight (NCLB) caused by Exserohilum turcicum and Fusarium ear rot caused by Fusarium verticillioides, are economically important maize diseases in South Africa. The effect of induced plant stress by NCLB on F. verticillioides ear rot and fumonisin production is unknown. Four field trials were conducted during 2016/2017 and 2017/2018 (November and December planting dates) at the Agricultural Research Council – Grain Crops in Potchefstroom (South Africa). Three maize cultivars with varying resistance levels to NCLB were selected (IMP50-10B – susceptible, BG3292 – moderately susceptible, DKC 61-94BR – resistant). NCLB severities were created through eight treatments: TMT1 – maximum control (three fungicide applications); TMT2 – standard control (two fungicide applications) and TMT3 – natural control (not inoculated or sprayed). The remaining treatments were inoculated with a cocktail of five NCLB races (Race 3, 3N, 23, 23N and 13N): TMT4 (five weeks after planting / WAP); TMT5 (five and six WAP); TMT6 (five, six and seven WAP); TMT7 (six and seven WAP); and TMT8 (seven WAP). Maize ears were naturally infected with F. verticillioides. Fifteen random plants were labelled at dent stage and NCLB severity (%), area under the disease progress curve, ear rot diseased area, ear rot severity (%), ear rot incidence (%) and total fumonisins (FB1+FB2+FB3; ug/kg) were established. Low levels of cob rot severity and fumonisins were obtained in all four trials. NCLB severity did not affect ear rot related parameters measured. Mean fumonisin levels were below the South African tolerance levels. Fumonisin concentrations differed significantly between cultivars but was not affected by NCLB severity or the cultivar x treatment interaction.

QTL mapping of Fusarium ear rot resistance in maize

Ear rot is a globally prevalent class of disease in maize, of which Fusarium ear rot (FER) caused by the fungal pathogen Fusarium verticilloides, is the most commonly reported. In this study, three F2 populations, namely F2-C, F2-D and F2-J, and their corresponding F2:3 families were produced by crossing three highly FER-resistant inbred lines, Cheng351, Dan598, and JiV203 with the same susceptible line, ZW18, for quantitative trait locus (QTL) mapping of FER-resistance. The individual crop plants were inoculated by injecting spore suspension of the pathogen into the kernels of the maize ears. The broad-sense heritability (H2) for FER-resistance was estimated to be as high as 0.76, 0.81, and 0.78 in F2-C, F2-D and F2-J, respectively, indicating that genetic factors played a key role in the phenotypic variation. We detected a total of 20 FER-resistant QTLs in the three F2 populations, among which QTLs derived from the resistant parent Cheng351, Dan598 and JiV203 explained 62.89 to 82.25%, 43.19 to 61.51% and 54.70 to 75.77% of the phenotypic variation, respectively. Among all FER-resistant QTLs detected, qRfer1, qRfer10, and qRfer17 accounted for the phenotypic variation as high as 26.58 to 43.36%, 11.76 to 18.02%, and 12.02 to 21.81%, respectively. Furthermore, QTLs mapped in different F2 populations showed some extent of overlaps indicating potential resistance ‘hotspots’. The FER-resistant QTLs detected in this study can be explored as useful candidates to improve FER-resistance in maize by introducing these QTLs into susceptible maize inbred lines using molecular marker-assisted selection.