Two genes conferring resistance to Pythium stalk rot in maize inbred line Qi319

A Large-Scale Hydroponic Evaluation of Rice Mutants for Pythium Resistance

Rice is a major staple crop worldwide. However, the occurrence of rice diseases during cultivation poses a significant challenge to achieving optimal yields. Among the major pathogens, Pythium species, which cause seedling blight, are of particular concern. Pythium infects rice roots through zoospores, mycelia, or oospores, leading to root rot, stunting, yellowing, and ultimately seedling damping-off. While many disease resistance-related genes have been reported in rice, only very limited research has been associated with resistance to Pythium infection. In this study, we aimed to establish a rapid screening system to identify rice lines that are resistant or susceptible to the Pythium pathogen in rice nurseries. We conducted evaluations on important factors, including virulence, inoculation method, seed-soaking period, and the measurement of disease severity. As a result, we successfully developed a screening system that allows for high-throughput and rapid screening of the Taiwan Rice Insertional Mutant (TRIM) library for mutant lines exhibiting resistance to P. arrhenomanes. Furthermore, we identified a slightly resistant TRIM line and explored potential genes encoding endoglucanase-1 precursor and malonyl-CoA decarboxylase that may be involved in conferring resistance to P. arrhenomanes.

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.

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.

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.

Knockout of Diguanylate Cyclase Genes in Lysobacter enzymogenes to Improve Production of Antifungal Factor and Increase Its Application in Seed Coating

Heat-stable antifungal factor (HSAF) is a broad-spectrum antifungal antibiotic produced by the biological control agent, Lysobacter enzymogenes. In our earlier works, we have applied HSAF to effectively control wheat and pear fungal disease. However, a major bottleneck in its practical application is the low HSAF production level; therefore, boosting its production is essential for its wide application. In the past, we find that c-di-GMP, a universal bacterial second messenger, is inhibitory to HSAF production. In this work, we further identified eight active diguanylate cyclases (DGCs) responsible for c-di-GMP synthesis in Lysobacter enzymogenes via both bioinformatics and genetic analyses. We generated a strain lacking seven active DGC genes and found that this DGC-modified strain, OH11LC, produced a higher HSAF amount in a c-di-GMP concentration-dependent manner. Subsequently, by employing OH11LC as the host fermentation strain, we could even produce a much higher HSAF amount (> 200-fold). After improving the HSAF production, we further developed a technique of seed coating method with HSAF, which turned out to be effective in fighting against the maize seed-borne filamentous pathogen, Pythium gramineacola. Overall, via combining strain modification and fermentation optimization, we demonstrated a good example of translating fundamental knowledge of bacterial c-di-GMP signaling into biological control application in which we relieved the inhibitory effect of c-di-GMP on HSAF biosynthesis by deleting a bunch of potentially active L. enzymogenes DGC genes to improve HSAF yield and to expand its usage in antifungal seed coating.

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

Pythium stalk rot caused by Pythium inflatum is becoming a more and more serious disease in maize, and it has caused severe yield loss in China in recent years. Deployment of resistant maize varieties is the most effective way to control this disease. Searching for the resistant maize germplasm and identifying the resistance genes are the vital processes in the breeding program. The maize inbred line X178 previously showed high resistance to Pythium stalk rot. Thus, this study aimed to reveal the gene(s) resistance to Pythium stalk rot in X178 by resistance inheritance analysis using the derived F₂ and F_2:3 genetic populations. The results showed that two independently inherited dominant genes, designated RpiX178-1 and RpiX178-2, carried by X178 are responsible for its resistance relative to the susceptible parent Ye107; they are located on regions of maize chromosome (chr.) 1 bin 1.09 and chr. 4 bin 4.08, respectively, and flanked by markers umc2047 and bnlg1671 as well as bnlg1444 and umc1313, respectively, by linkage analysis. Subsequently, RpiX178-1 was finely mapped between markers SSRZ8 and IDP2347, with genetic distances of 0.6 and 1.1 cM, respectively, and the physical distance of the target region was about 700 kb. RpiX178-2 was also further located between markers bnlg1444 and umc2041, with a genetic distance of 2.4 cM. Moreover, we confirmed that the two genes RpiX178-1 and RpiX178-2 were newly identified and different from those genes known on chrs. 1 and 4 according to results of allelism testing. Herein, we newly identified two genes resistant to P. inflatum, which provided important genetic information for resistance to Pythium stalk rot in maize.

qRfg3, a novel quantitative resistance locus against Gibberella stalk rot in maize

A quantitative trait locus  qRfg3 imparts recessive resistance to maize Gibberella stalk rot. qRfg3 has been mapped into a 350-kb interval and could reduce the disease severity index by ~26.6%. Gibberella stalk rot, caused by the fungal pathogen Fusarium graminearum, severely affects maize yield and grain quality worldwide. To identify more resistance quantitative trait loci (QTLs) against this disease, we analyzed a recombinant inbred line (RIL) population derived from a cross between resistant H127R and susceptible C7-2 inbred lines. Within this population, maize resistance to Gibberella stalk rot had high broad-sense heritability. A major QTL, qRfg3, on chromosome 3 was consistently detected across three field trials, accounting for 10.7-19.4% of the total phenotypic variation. Using a progeny-based sequential fine-mapping strategy, we narrowed qRfg3 down to an interval of ~350 kb. We further demonstrated that qRfg3 is a recessive resistance locus to Gibberella stalk rot that reduced the disease severity index by ~26.6%. Both the gene location and recessive genetic mode distinguish qRfg3 from other stalk rot resistance loci. Hence, qRfg3 is valuable as a complement to existing resistance QTLs to improve maize resistance to Gibberella stalk rot.

Transcriptome analysis of maize resistance to Fusarium graminearum

BACKGROUND

Gibberella stalk rot caused by Fusarium graminearum is one of the most destructive soil-borne diseases of maize (Zea mays L.). Chemical means of controlling Gibberella stalk rot are not very effective; development of highly resistant hybrids is the best choice for disease control. Hence, understanding of the molecular basis underlying maize resistance against Gibberella stalk rot would undoubtedly facilitate the resistance breeding for stalk rot.

RESULTS

Two quantitative trait loci (QTL), qRfg1 and qRfg2, conferring resistance to Gibberella stalk rot were detected in our previous study. Three near-isogenic lines (NILs) of maize with either qRfg1 (NIL1) or qRfg2 (NIL2), or neither (NIL3) were generated and subjected to RNA sequencing to study the transcriptional changes after F. graminearum inoculation at 0 (control), 6, and 18 h post-inoculation (hpi). In total, 536,184,652 clean reads were generated, and gene expression levels were calculated using FPKM (fragments per kilobase of exon model per million mapped reads). A total of 7252 differentially expressed genes (DEGs) were found in the three NILs after F. graminearum inoculation. As many as 2499 DEGs were detected between NIL1 and NIL3 at 0 hpi, of which 884 DEGs were more abundant in NIL1 and enriched in defense responses. After F. graminearum inoculation, 1070 and 751 genes were exclusively up- and downregulated, respectively, in NIL1 as compared to NIL3. The 1070 upregulated DEGs were enriched in growth/development, photosynthesis/biogenesis, and defense-related responses. Genes encoding putative auxin-induced proteins and GH3 family proteins in auxin signaling pathway were highly induced and lasted longer in NIL3. Genes involved in polar auxin transport (PAT) were more abundant in NIL3 as compared with NIL2.

CONCLUSIONS

The qRfg1 confers its resistance to Gibberella stalk rot through both constitutive and induced high expression of defense-related genes; while qRfg2 enhances maize resistance to the disease via relatively lower induction of auxin signaling and repression of PAT. The defense-related transcriptional changes underlying each QTL will undoubtedly facilitate our understanding of the resistance mechanism and resistance breeding for maize stalk rot.