Identification of candidate gene associated with maize northern leaf blight resistance in a multi-parent population.

KEY MESSAGE: QTL mapping combined with genome-wide association studies, revealed a potential candidate gene for  resistance to northern leaf blight in the tropical CATETO-related maize line YML226, providing a basis for marker-assisted selection of maize varieties Northern leaf blight (NLB) is a foliar disease that can cause severe yield losses in maize. Identifying and utilizing NLB-resistant genes is the most effective way to prevent and control this disease. In this study, five important inbred lines of maize were used as parental lines to construct a multi-parent population for the identification of NLB-resistant loci. QTL mapping and GWAS analysis revealed that QTL qtl_YML226_1, which had the largest phenotypic variance explanation (PVE) of 9.28%, and SNP 5-49,193,921 were co-located in the CATETO-related line YML226. This locus was associated with the candidate gene Zm00001d014471, which encodes a pentatricopeptide repeat (PPR) protein. In the coding region of Zm00001d014471, YML226 had more specific SNPs than the other parental lines. qRT-PCR showed that the relative expressions of Zm00001d014471 in inoculated and uninoculated leaves of YML226 were significantly higher, indicating that the expression of the candidate gene was correlated with NLB resistance. The analysis showed that the higher expression level in YML226 might be caused by SNP mutations. This study identified NLB resistance candidate loci and genes in the tropical maize inbred line YML226 derived from the CATETO germplasm, thereby providing a theoretical basis for using modern marker-assisted breeding techniques to select genetic resources resistant to NLB.

Evaluation of Maize Hybrids for Identifying Resistance to Northern Corn Leaf Blight in Northeast China.

Northern corn leaf blight (NCLB) caused by Setosphaeria turcica is one of the most devastating foliar diseases in maize (Zea mays L.), resulting in great economic losses worldwide. The mutation of the pathogen exacerbates the occurrence and harmfulness of NCLB in China. Therefore, there is an urgent need for evaluating and cultivating resistant hybrids. Here, the response of 239 maize hybrids approved in Northeast China to NCLB was evaluated during 2019 and 2020. The results showed that 92 (38.49%) and 75 (31.38%) hybrids were rated as moderately resistant and resistant, respectively, which together constituted the predominant resistant categories. We observed that maize hybrids from different certified sources had different levels of resistance to NCLB, whose disease parameter values varied significantly (P < 0.05) among 52 main cultivated hybrids. In 2019 and 2020, the average size of the lesions increased from 21.02 to 21.06 cm2, the average lesion density decreased from 1.36 to 1.33 lesions/100 cm2, and more than 30% of the hybrids registered final disease severity scores between 10 and 30%. The area under the disease progress curve of the main cultivated hybrids ranged from 57.96 to 986.86 cm2 in 2019 and from 50.75 to 1,028.65 cm2 in 2020. Correlation analysis revealed a significant relationship (P < 0.0001) among four disease parameters. Current research has shown that many maize hybrids in Northeast China are resistant to NCLB, suggesting that the large-scale cultivation of susceptible hybrids has led to the occurrence and prevalence of the disease. This study should assist growers in purposefully selecting resistant commercial hybrids to contribute to the management of NCLB.

Inheritance of resistance against northern leaf blight of maize using conventional breeding methods.

Maize (Zea mays L.) is one of the important cereal crops along with wheat and rice worldwide. The purpose of this study was to use classical genetic approaches to assess the resistance of various maize parents and hybrids to the northern corn leaf blight (NCLB) disease in two different locations in Egypt. Eight parents, 28 F1, and 2 check hybrids were evaluated. The analysis of variance showed high significant variations between maize parents and their hybrids for the studied parameters and NCLB disease, besides there are significant variations between both locations. Results of maize parents showed that Sids 63, Giza 602, and Giza 628 cultivars exhibited the highest values and were resistant to NCLB in both locations comparing with Nubaria 39 and Gemmiza 18 that were susceptible to NCLB disease. Concerning the maize hybrids, analysis of variance and mean squares of growth characters in both locations indicated high significant variations between the maize hybrids including the check hybrids. When combined between the two locations for current parameters against NCLB, the data pointed that the Sakha location values for maize hybrids were much closed to the combining data in parents and the hybrids detected high resistance to this disease comparing with Nubaria location. All tested maize lines (38 lines), including parents and hybrids were classified as follows, two lines were rated as 1 (highly resistant), three were rated as 2 (resistant), sixteen were rated as 3 (moderate resistant), eight were rated 4 (moderately susceptible) and nine were rated 5 (susceptible). The data explaining that the crossing between high resistant maize cultivars produced high levels of resistance to NCLB disease. Therefore, our results verified that classical breeding could efficiently increase the resistance levels of maize germplasm against NCLB disease by developing new cultivars with superior performance in terms of grain yield, disease resistance and grain quality.

Unraveling genomic complexity at a quantitative disease resistance locus in maize.

Multiple disease resistance has important implications for plant fitness, given the selection pressure that many pathogens exert directly on natural plant populations and indirectly via crop improvement programs. Evidence of a locus conditioning resistance to multiple pathogens was found in bin 1.06 of the maize genome with the allele from inbred line "Tx303" conditioning quantitative resistance to northern leaf blight (NLB) and qualitative resistance to Stewart's wilt. To dissect the genetic basis of resistance in this region and to refine candidate gene hypotheses, we mapped resistance to the two diseases. Both resistance phenotypes were localized to overlapping regions, with the Stewart's wilt interval refined to a 95.9-kb segment containing three genes and the NLB interval to a 3.60-Mb segment containing 117 genes. Regions of the introgression showed little to no recombination, suggesting structural differences between the inbred lines Tx303 and "B73," the parents of the fine-mapping population. We examined copy number variation across the region using next-generation sequencing data, and found large variation in read depth in Tx303 across the region relative to the reference genome of B73. In the fine-mapping region, association mapping for NLB implicated candidate genes, including a putative zinc finger and pan1. We tested mutant alleles and found that pan1 is a susceptibility gene for NLB and Stewart's wilt. Our data strongly suggest that structural variation plays an important role in resistance conditioned by this region, and pan1, a gene conditioning susceptibility for NLB, may underlie the QTL.

Chromosomal locations of the maize (Zea mays L.) HtP and rt genes that confer resistance to Exserohilum turcicum

We used 125 microsatellite markers to genotype the maize (Zea mays L.) near isogenic lines (NIL) L30HtPHtPRtRt and L30htphtpRtRt and the L40htphtprtrt line which contrast regarding the presence of the recently described dominant HtP and the recessive rt genes that confer resistance to Exserohilum turcicum. Five microsatellite markers revealed polymorphisms between the NIL and were considered candidate linked markers for the HtP resistance gene. Linkage was confirmed by bulked segregant sample (BSS) analysis of 32 susceptible and 34 resistant plants from a BC1F1 population derived from the cross (L30HtPHtPRtRt x L40htphtprtrt) x L40htphtprtrt. The bnlg198 and dupssr25 markers, both located on maize chromosome 2L (bin 2.08), were polymorphic between bulks. Linkage distances were estimated based on co-segregation data of the 32 susceptible plants and indicated distances of 28.7 centimorgans (cM) between HtP and bnlg198 and 23.5 cM between HtP and dupssr25. The same set of susceptible plants was also genotyped with markers polymorphic between L30HtPHtPRtRt and L40htphtprtrt in order to find markers linked to the rt gene. Marker bnlg197, from chromosome 3L (bin 3.06), was found linked to rt at a distance of 9.7 cM. This is the first report on the chromosomal locations of these newly described genes.