The fungus Fusarium verticillioides can infect maize ears, contaminating the grain with mycotoxins, including fumonisins. This global public health threat can be managed by breeding maize varieties that are resistant to colonization by F. verticillioides and by sorting grain after harvest to reduce fumonisin levels in food systems. Here, we employed two F. verticillioides inoculation techniques representing distinct infection pathways to dissect ear symptomatology and morphological resistance mechanisms in a diverse panel of maize inbred lines. The "point" method involved penetrating the ear with a spore-coated toothpick and the "inundative" method introduced a liquid spore suspension under the husk of the ear. We evaluated quantitative and qualitative indicators of external and internal symptom severity as low-cost proxies for fumonisin contamination, and found that kernel bulk density was predictive of fumonisin levels (78 to 84% sensitivity; 97 to 99% specificity). Inundative inoculation resulted in greater disease severity and fumonisin contamination than point inoculation. We also found that the two inoculation methods implicated different ear tissues in defense, with cob morphology being a more important component of resistance under point inoculation. Across both inoculation methods, traits related to cob size were positively associated with disease severity and fumonisin content. Our work demonstrates that (i) the use of diverse modes of inoculation is necessary for combining complementary mechanisms of genetic resistance, (ii) kernel bulk density can be used effectively as a proxy for fumonisin levels, and (iii) trade-offs may exist between yield potential and resistance to fumonisin contamination.
Popcorn is a popular snack food with higher grain value compared with “field corn” maize (Zea mays L.). In general, popcorn germplasm is less improved than common maize cultivars, with less favorable agronomic performance and greater susceptibility to pests, diseases, and lodging, especially in tropical environments. One approach to expanding and improving popcorn germplasm is to introduce favorable alleles for agronomic performance from field corn. If markers tagging quantitative trait loci (QTLs) associated with popping expansion were identified, they could be selected with markers to maintain popping expansion in breeding populations derived from crosses between field and popcorn germplasm. The objectives of this study were to identify single nucleotide polymorphism (SNP) markers and putative candidate genes associated with higher popping expansion in a diverse sample of popcorn and field corns relevant to Brazilian breeding programs. We applied a comprehensive genome‐wide association study (GWAS) for popping expansion, using 165,089 SNP markers in 183 inbred lines with 2 yr of trial data. Four SNPs were significantly associated with popping expansion, three of which were colocalized to previously reported meta‐QTLs, and one that was novel. Annotated genes closely linked to the associated SNPs were identified with functions related with starch content, which plays an important role in popping expansion quality. If these associations can be validated independently, they can be useful for breeders to select agronomically superior genotypes with greater popping expansion.
Fusarium ear rot (FER) disease of maize (Zea mays L.) is caused by Fusarium verticillioides (Sacc.) Nirenberg, which produces fumonisin (FUM), a mycotoxin linked to human and animal health risks. Extensive field trials, laborious inoculations and ear evaluations, and expensive antibody assays are required to reliably assess resistances to FER and FUM contamination in breeding populations.To evaluate the potential utility of genomic selection (GS) to improve FER and FUM in maize, we genotyped 6086 single nucleotide polymorphisms (SNPs) on 449 S 0:1 lines from a recurrent selection population. Two different partitions of the S 0:1 evaluation data were made to test the ability of models trained on 251 or 201 lines evaluated at three locations in 2014-2015 to predict FER and FUM of 198 or 248 different lines evaluated at three locations in 2016. Single-stage univariate and multivariate genomic best linear unbiased predictor (GBLUP) models and two-stage GBLUP, Bayes Cπ, Bayesian LASSO, and extreme gradient boosting models were compared for prediction. Maximum prediction accuracy for untested lines in a new year was 0.46 for FER and 0.67 for FUM. Bayesian models optimized for predicting traits influenced by major-effect loci were best for FUM in one set, despite no evidence for significant individual SNP-trait associations from genome-wide association study (GWAS) in the training sets; otherwise, GBLUP models were best. These results suggest that GS can help improve resistance to FER and FUM contamination in an applied breeding program. INTRODUCTIONFusarium ear rot is a fungal disease of maize common across most corn growing areas of the United States, and
Breeding for resistance to Fusarium ear rot (FER; Fusarium verticillioides Nirenberg) and fumonisin (FUM) contamination in maize (Zea mays L.) is labor intensive, time consuming, and expensive. The objective of this study is to explore three possible shortcuts to improve the efficiency of breeding for resistance to FER and FUM: relying on natural instead of artificial inoculation of Fusarium verticillioides, selecting inbred lines per se instead of topcross hybrids, and using grain test weight (TW) as an indirect selection criterion. We selected the 27 most resistant and 26 most susceptible S 0:1 lines from the third cycle of a broad-based recurrent selection population and topcrossed them to a common inbred line. The resulting topcross hybrids were evaluated in three North Carolina environments under artificial and natural inoculation of F. verticillioides. The entry-mean heritabilities for FER and FUM are considerably reduced under natural conditions compared to artificial inoculation; therefore, artificial inoculation should be practiced for efficient selection for resistance. We found a high correlation between FER and FUM content of S 0:1 lines per se and their topcross hybrids, suggesting that selection among early generation inbred lines per se is an efficient method to improve resistance in their topcross hybrids. The TW of inoculated S 0:1 lines per se was strongly and negatively correlated with FER and FUM of their topcross hybrids, suggesting that TW can be utilized as an indirect selection criterion to improve resistance to FER and FUM contamination.
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