Phenotypic and Genetic Characterization of a Maize Association Mapping Panel Developed for the Identification of New Sources of Resistance to Aspergillus flavus and Aflatoxin Accumulation

Abstract

Maize (Zea mays L.) susceptibility to ear rot by Aspergillus flavus (Link:Fr) and aflatoxin accumulation causes global economic and human health damage. Host plant resistance is an ideal solution, but commercial varieties lack sufficient resistance to solve the problem. Due to general lack of resistant maize germplasm, no currently available association mapping panels are expected to contain significant variation for the trait. A new association mapping panel containing the majority of aflatoxin accumulation resistant maize lines at the time of compilation is presented here along with genetic and phenotypic characterization data. Phenotypic data from testcrossed lines include aflatoxin levels, days to silking, and corn earworm [Helicoverpa zea (Boddie)] damage measured in seven environments and fungal biomass of kernels measured in three. In addition to identifying previously reported aflatoxin resistant lines, new resistant lines were discovered, which may be used for breeding improved germplasm. The Mexican landrace Tuxpeño is the progenitor or one main contributor for most of the resistant lines and likely the source of resistance, but a few other sources may allow additional novel resistance to be pyramided into future breeding lines. Genetic characterization of kinship, genetic diversity, and substructure analysis presented here will allow this resource to be used for association mapping of aflatoxin and identification of factors responsible for this challenging quantitative trait.

Aflatoxin Resistance in Maize: What Have We Learned Lately?

“…These lines, displaying resistance in at least one field site, are in the process of further characterization. Second, an aflatoxin association mapping panel containing 300 maize lines has recently been publicly released, and 30 to 40 lines displaying good resistance in up to 7 environments are now available for use [33,34]. Several of these inbreds have already been included in a new joint USAID/USDA project, together with two CGIAR centers, to incorporate as many of these lines into ongoing genetics studies and breeding activities as possible, with the goal of creating resistant OPV and hybrid cultivars as quickly as possible.…”

“…Phenotyping of the panel revealed that the most resistant lines include the previously identified resistant germplasm and many new lines that may now be used for breeding and genetics studies. Pedigrees of a majority of the lines identified as resistant trace back, all or in part, to one Mexican maize landrace, Tuxpeño [33,34]. This landrace has been used extensively in the creation of many of the maize breeding pools and populations of the International Maize and Wheat Improvement Center (CIMMYT) because it is a high yielding, agronomically superior dent population with good GCA.…”

“…Genome-wide association studies (GWAS) have also been brought to bear on the aflatoxin problem in maize. Because most maize germplasms do not display resistance to A. flavus infection or aflatoxin accumulation, the association mapping panel developed by Warburton et al [33] contains resistant germplasm that has been genotyped via Genotype By Sequencing (GBS, [51]). Two studies based on this germplasm have now been completed.…”

“…The first, using some of the lines from the association mapping panel, found no genetic sequences associated with aflatoxin accumulation [52]. The second, using more SNPs for GWAS and the entire panel, found several gene sequences associated with maize grain aflatoxin levels, with a high level of probability [33,34]. Twenty-one genetic regions were associated with aflatoxin reduction in grain at < 10 −6 in more than one environment.…”

“…Quantitative PCR has confirmed the differential expression of genes and proteins, although their functions have not been characterized [47,60]. Finally, reduced levels of aflatoxin can be correlated with a specific enzyme activity or protein component from infected maize kernels, for example, an aldose reductase [49], pericarp wax components [61], and 1-a Cys peroxiredoxin (PER 1), confirmed to be more expressed in one resistant maize inbred line than a susceptible inbred by Chen et al [62] but not associated with aflatoxin resistance in an association mapping panel by Warburton et al [33]. Similar validation studies continue with gene or protein sequences identified by means other than the genetics, genomics, and proteomics methods described above.…”