QTL mapping and candidate genes for resistance to Fusarium ear rot and fumonisin contamination in maize.

BACKGROUND: Fusarium verticillioides is a common maize pathogen causing ear rot (FER) and contamination of the grains with the fumonisin B1 (FB1) mycotoxin. Resistance to FER and FB1 contamination are quantitative traits, affected by environmental conditions, and completely resistant maize genotypes to the pathogen are so far unknown. In order to uncover genomic regions associated to reduced FER and FB1 contamination and identify molecular markers for assisted selection, an F2:3 population of 188 progenies was developed crossing CO441 (resistant) and CO354 (susceptible) genotypes. FER severity and FB1 contamination content were evaluated over 2 years and sowing dates (early and late) in ears artificially inoculated with F. verticillioides by the use of either side-needle or toothpick inoculation techniques. RESULTS: Weather conditions significantly changed in the two phenotyping seasons and FER and FB1 content distribution significantly differed in the F3 progenies according to the year and the sowing time. Significant positive correlations (P < 0.01) were detected between FER and FB1 contamination, ranging from 0.72 to 0.81. A low positive correlation was determined between FB1 contamination and silking time (DTS). A genetic map was generated for the cross, based on 41 microsatellite markers and 342 single nucleotide polymorphisms (SNPs) derived from Genotyping-by-Sequencing (GBS). QTL analyses revealed 15 QTLs for FER, 17 QTLs for FB1 contamination and nine QTLs for DTS. Eight QTLs located on linkage group (LG) 1, 2, 3, 6, 7 and 9 were in common between FER and FB1, making possible the selection of genotypes with both low disease severity and low fumonisin contamination. Moreover, five QTLs on LGs 1, 2, 4, 5 and 9 located close to previously reported QTLs for resistance to other mycotoxigenic fungi. Finally, 24 candidate genes for resistance to F. verticillioides are proposed combining previous transcriptomic data with QTL mapping. CONCLUSIONS: This study identified a set of QTLs and candidate genes that could accelerate breeding for resistance of maize lines showing reduced disease severity and low mycotoxin contamination determined by F. verticillioides.

Protein profiling and tps23 induction in different maize lines in response to methyl jasmonate treatment and Diabrotica virgifera infestation.

Plant responses to herbivore insects involve direct and indirect defense with the production of signal molecules including jasmonic acid (JA) and its derivatives (e.g. methyl jasmonate, MeJA). In maize (Zea mays), root feeding by Diabrotica virgifera larvae activates an indirect defense mechanism, through enthomopathogenic nematodes that are recruited after Terpene Synthase 23 (tps23) upregulation and (E)-ß-caryophyllene root emission. In order to gain insight into the correlation between JA signaling and response to Diabrotica attack, we analyzed tps23 expression and protein profiles in maize roots in response to MeJA treatment and insect infestation. Similar to herbivore feeding, MeJA treatment was found to increase tps23 transcript accumulation, with consistent variations for both treatments in maize lines differing in (E)-ß-caryophyllene production. Analysis of root protein profiles showed specific alterations leading to the identification of three proteins that were induced by MeJA treatment. We focused on a peroxidase-like protein (Px-like) showing that the corresponding transcripts accumulated in all tested lines. Results show that exogenous application of MeJA upregulates tps23 expression and specifically alters protein patterns in maize roots. Parallel effects on tps23 transcript accumulation were observed upon hormone exposure and insect infestation in different maize lines. In contrast, Px-like transcript profiling showed differences between treatments. These results support the possible involvement of MeJA in mediating the upregulation of tps23 in response to Diabrotica attack.