Characterization and Molecular Mapping of Two Novel Genes Resistant to Pythium Stalk Rot in Maize

Exploiting genomic tools for genetic dissection and improving the resistance to Fusarium stalk rot in tropical maize

KEY MESSAGE

A stable genomic region conferring FSR resistance at ~250 Mb on chromosome 1 was identified by GWAS. Genomic prediction has the potential to improve FSR resistance. Fusarium stalk rot (FSR) is a global destructive disease in maize; the efficiency of phenotypic selection for improving FSR resistance was low. Novel genomic tools of genome-wide association study (GWAS) and genomic prediction (GP) provide an opportunity for genetic dissection and improving FSR resistance. In this study, GWAS and GP analyses were performed on 562 tropical maize inbred lines consisting of two populations. In total, 15 SNPs significantly associated with FSR resistance were identified across two populations and the combinedPOP consisting of all 562 inbred lines, with the P-values ranging from 1.99 × 10-7 to 8.27 × 10-13, and the phenotypic variance explained (PVE) values ranging from 0.94 to 8.30%. The genetic effects of the 15 favorable alleles ranged from -4.29 to -14.21% of the FSR severity. One stable genomic region at ~ 250 Mb on chromosome 1 was detected across all populations, and the PVE values of the SNPs detected in this region ranged from 2.16 to 5.18%. Prediction accuracies of FSR severity estimated with the genome-wide SNPs were moderate and ranged from 0.29 to 0.51. By incorporating genotype-by-environment interaction, prediction accuracies were improved between 0.36 and 0.55 in different breeding scenarios. Considering both the genome coverage and the threshold of the P-value of SNPs to select a subset of molecular markers further improved the prediction accuracies. These findings extend the knowledge of exploiting genomic tools for genetic dissection and improving FSR resistance in tropical maize.

A dual-subcellular localized β-glucosidase confers pathogen and insect resistance without a yield penalty in maize

Maize is one of the most important crops for food, cattle feed and energy production. However, maize is frequently attacked by various pathogens and pests, which pose a significant threat to maize yield and quality. Identification of quantitative trait loci and genes for resistance to pests will provide the basis for resistance breeding in maize. Here, a β-glucosidase ZmBGLU17 was identified as a resistance gene against Pythium aphanidermatum, one of the causal agents of corn stalk rot, by genome-wide association analysis. Genetic analysis showed that both structural variations at the promoter and a single nucleotide polymorphism at the fifth intron distinguish the two ZmBGLU17 alleles. The causative polymorphism near the GT-AG splice site activates cryptic alternative splicing and intron retention of ZmBGLU17 mRNA, leading to the downregulation of functional ZmBGLU17 transcripts. ZmBGLU17 localizes in both the extracellular matrix and vacuole and contribute to the accumulation of two defence metabolites lignin and DIMBOA. Silencing of ZmBGLU17 reduces maize resistance against P. aphanidermatum, while overexpression significantly enhances resistance of maize against both the oomycete pathogen P. aphanidermatum and the Asian corn borer Ostrinia furnacalis. Notably, ZmBGLU17 overexpression lines exhibited normal growth and yield phenotype in the field. Taken together, our findings reveal that the apoplastic and vacuolar localized ZmBGLU17 confers resistance to both pathogens and insect pests in maize without a yield penalty, by fine-tuning the accumulation of lignin and DIMBOA.

Evaluation of Resistance Resources and Analysis of Resistance Mechanisms of Maize to Stalk Rot Caused by Fusarium graminearum

Stalk rot is one of the most destructive and widely distributed diseases in maize plants worldwide. Research on the performance and resistance mechanisms of maize against stem rot is constantly improving. In this study, among 120 inbred maize lines infected by Fusarium graminearum using the injection method, 4 lines (3.33%) were highly resistant to stalk rot, 28 lines (23.33%) were resistant, 57 lines (47.50%) were susceptible, and 31 lines (25.84%) were highly susceptible. The inbred lines 18N10118 and 18N10370 were the most resistant and susceptible with disease indices of 7.5 and 75.6, respectively. Treatment of resistant and susceptible maize inbred seedlings with F. graminearum showed that root hair growth of the susceptible inbred lines was significantly inhibited, and a large number of hyphae attached and adsorbed multiple conidia near the root system. However, the resistant inbred lines were delayed and inconspicuous, with only a few hyphae and spores appearing near the root system. Compared with susceptible inbred lines, resistant maize inbred line seedlings treated with F. graminearum exhibited elevated activities of catalase, phenylalanine ammonia-lyase, polyphenol oxidase, and superoxide dismutase. We identified 153 genes related to disease resistance by transcriptome analysis. The mitogen-activated protein kinase signaling and peroxisome pathways mainly regulated the resistance mechanism of maize inbred lines to F. graminearum infection. These two pathways might play an important role in the disease resistance mechanism, and the function of genes in the two pathways must be further studied, which might provide a theoretical basis for further understanding the molecular resistance mechanism of stalk rot and resistance gene mining.

Genome-wide association study of maize resistance to Pythium aristosporum stalk rot

Stalk rot, a severe and widespread soil-borne disease in maize, globally reduces yield and quality. Recent documentation reveals that Pythium aristosporum has emerged as one of the dominant causal agents of maize stalk rot. However, a previous study of maize stalk rot disease resistance mechanisms and breeding had mainly focused on other pathogens, neglecting P. aristosporum. To mitigate crop loss, resistance breeding is the most economical and effective strategy against this disease. This study involved characterizing resistance in 295 inbred lines using the drilling inoculation method and genotyping them via sequencing. By combining with population structure, disease resistance phenotype, and genome-wide association study (GWAS), we identified 39 significant single-nucleotide polymorphisms (SNPs) associated with P. aristosporum stalk rot resistance by utilizing six statistical methods. Bioinformatics analysis of these SNPs revealed 69 potential resistance genes, among which Zm00001d051313 was finally evaluated for its roles in host defense response to P. aristosporum infection. Through virus-induced gene silencing (VIGS) verification and physiological index determination, we found that transient silencing of Zm00001d051313 promoted P. aristosporum infection, indicating a positive regulatory role of this gene in maize's antifungal defense mechanism. Therefore, these findings will help advance our current understanding of the underlying mechanisms of maize defense to Pythium stalk rot.

Identification of Pathogens and Evaluation of Resistance and Genetic Diversity of Maize Inbred Lines to Stalk Rot in Heilongjiang Province, China

Maize stalk rot, caused by multiple pathogens, is a serious soilborne disease worldwide. Composition of pathogens causing maize stalk rot and resistance of maize inbred lines in Heilongjiang Province, China, are not well understood. In this study, 138 fungal isolates were collected from different maize-producing areas in Heilongjiang Province, which were identified as Fusarium graminearum (23.2%), F. subglutinans (18.9%), F. cerealis (18.9%), Bipolaris zeicola (13.0%), F. brachygibbosum (13.0%), F. temperatum (7.2%), and F. proliferatum (5.8%). Among them, F. graminearum (>20%) was the predominant species among the isolates causing maize stalk rot. B. zeicola had not previously been reported causing maize stalk rot in China. Resistance of 67 maize inbred lines to maize stalk rot was assessed, and 24 lines (35.8% of them) were highly resistant or resistant, indicating that approximately 65% of these lines were susceptible to maize stalk rot. Maize inbred lines were analyzed using simple sequence repeat markers and divided into five genetic groups with 12 pairs of primers. Additionally, analysis of molecular variance indicated that 44.2% of the genetic variation in disease resistance was distributed among populations. This study provides insight into the genetic diversity of inbred maize and may contribute useful information for breeding stalk rot disease-resistant hybrids, and facilitates development of effective strategies for managing this destructive disease complex.

ZmGLP1, a Germin-like Protein from Maize, Plays an Important Role in the Regulation of Pathogen Resistance

A gene encoding a protein similar to germin-like proteins (GLPs) was obtained from maize (Zea mays) and designated as ZmGLP1. Based on the ZmGLP1 conserved domain and phylogenetic status, ZmGLP1 was grouped into GLP subfamily b and has high similarity to OsGLP8-14 from Oryza sativa. ZmGLP1 is expressed in different maize tissues during different growth stages and is mainly expressed in the stems and leaves. The induced expression patterns confirmed that ZmGLP1 is differentially expressed under abiotic and hormone stress; it had an early response to jasmonic acid (JA) and ethephon (ET) but a late response to salicylic acid (SA) and was significantly upregulated under Bipolaris maydis infection. The overexpression of ZmGLP1 in Arabidopsis improved the resistance to biotrophic Pseudomonas syringae pv. tomato DC3000 (PstDC3000) and necrotrophic Sclerotinia sclerotiorum by inducing the expression of JA signaling-related genes. Moreover, the hydrogen peroxide (H₂O₂) content increased due to the overexpression of ZmGLP1 in Arabidopsis after pathogen infection. Compared to the wild-type control, the H₂O₂ content of ZmGLP1-overexpressing Arabidopsis infected by PstDC3000 increased significantly but was lower in transgenic plants infected with S. sclerotiorum. Furthermore, high-performance liquid chromatography-tandem mass (HPLC-MS/MS) spectrometry showed that the JA contents of ZmGLP1-overexpressing Arabidopsis markedly increased after pathogen infection. However, the improved resistance of ZmGLP1-overexpressing Arabidopsis pretreated with the JA biosynthetic inhibitor, sodium diethyldithiocarbamate trihydrate (DIECA), was suppressed. Based on these findings, we speculate that ZmGLP1 plays an important role in the regulation of Arabidopsis resistance to biotrophic PstDC3000 and necrotrophic S. sclerotiorum; the regulatory effects are achieved by inducing plant oxidative burst activity and activation of the JA signaling pathway.

Genetic dissection of maize disease resistance and its applications in molecular breeding

Disease resistance is essential for reliable maize production. In a long-term tug-of-war between maize and its pathogenic microbes, naturally occurring resistance genes gradually accumulate and play a key role in protecting maize from various destructive diseases. Recently, significant progress has been made in deciphering the genetic basis of disease resistance in maize. Enhancing disease resistance can now be explored at the molecular level, from marker-assisted selection to genomic selection, transgenesis technique, and genome editing. In view of the continuing accumulation of cloned resistance genes and in-depth understanding of their resistance mechanisms, coupled with rapid progress of biotechnology, it is expected that the large-scale commercial application of molecular breeding of resistant maize varieties will soon become a reality.