Systematic dissection of disease resistance to southern corn rust by bulked-segregant and transcriptome analysis

Southern corn rust(SCR) is a destructive maize disease caused by Puccinia polysora Underw. To investigate the mechanism of SCR resistance in maize, a highly resistant inbred line, L119 A, and a highly susceptible line, Lx9801, were subjected to gene mapping and transcriptome analysis. Bulked-segregant analysis coupled with whole-genome sequencing revealed several quantitative trait loci(QTL) on chromosomes 1, 6, 8, and 10. A set of 25 genes, including two coiled-coil nucleotide-binding site leucine-rich repeat(CC-NBS-LRR) genes, were identified as candidate genes for a major-effect QTL on chromosome 10. To investigate the mechanism of SCR resistance in L119 A, RNA-seq of P. polysorainoculated and non-inoculated plants of L119 A and Lx9801 was performed. Unexpectedly, the number of differentially expressed genes in inoculated versus non-inoculated L119 A plants was about 10 times that of Lx9801, with only 29 common genes identified in both lines, suggesting extensive gene expression changes in the highly resistant but not in the susceptible line. Based on the transcriptome analysis, one of the CC-NBS-LRR candidate genes was confirmed to be upregulated in L119 A relative to Lx9801 independently of P. polysora inoculation. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses indicated that transcription factors, as well as genes involved in defense responses and metabolic processes, were dominantly enriched, with the phenylpropanoid biosynthesis pathway most specifically activated. Consistently, accumulation of phenylpropanoid-derived lignin, especially S lignin, was drastically increased in L119 A after P. polysora inoculation, but remained unchanged in Lx9801, suggesting a critical role of lignin in SCR resistance. A regulatory network of defense activation and metabolic change in SCR-resistant maize upon P. polysora infection is described.

The antioxidant protein ZmPrx5 contributes resistance to maize stalk rot

Maize(Zea mays L.)stalk rot is a devastating disease worldwide,causing severe yield losses.Although previous studies have focused on the genetic dissection of maize resistance to stalk rot,the mechanisms of resistance remain largely unknown.We used a comparative proteomics approach to identify candidate proteins associated with stalk rot resistance.Statistical analyses revealed 763 proteins differentially accumulated between Fusarium graminearum and mock-inoculated plants.Among them,the antioxidant protein ZmPrx5,which was up-accumulated in diseased plants,was selected for further study.ZmPrx5 transcripts were present in root,stalk,leaf,ear,and reproductive tissues.The expression of ZmPrx5 in three inbred lines increased significantly upon F.graminearum infection.ZmPrx5 was localized in the cytoplasm.Compared to control plants,maize plants overexpressing ZmPrx5 showed increased resistance to F.graminearum infection,and ZmPrx5 mutant plants were more susceptible than wild-type plants.Defense-associated pathways including plant–pathogen interactions,phenylalanine metabolism,and benzoxazinoid and flavonoid biosynthesis were suppressed in ZmPrx5 homozygous mutant plants compared with wild-type plants.We suggest that ZmPrx5 positively regulates resistance against stalk rot in maize,likely through defense-oriented transcriptome reprogramming.These results lay a foundation for further research on the roles of Prx5 subfamily proteins in resistance to plant fungal diseases,and provide a potential genetic resource for breeding disease-resistance maize lines.

Quantitative disease resistance:Multifaceted players in plant defense

In contrast to large-effect qualitative disease resistance,quantitative disease resistance(QDR)exhibits partial and generally durable resistance and has been extensively utilized in crop breeding.The molecular mechanisms underlying QDR remain largely unknown but considerable progress has been made in this area in recent years.In this review,we summarize the genes that have been associated with plant QDR and their biological functions.Many QDR genes belong to the canonical resistance gene categories with predicted functions in pathogen perception,signal transduction,phytohormone homeostasis,metabolite transport and biosynthesis,and epigenetic regulation.However,other"atypical"QDR genes are predicted to be involved in processes that are not commonly associated with disease resistance,such as vesicle trafficking,molecular chaperones,and others.This diversity of function for QDR genes contrasts with qualitative resistance,which is often based on the actions of nucleotidebinding leucine-rich repeat(NLR)resistance proteins.An understanding of the diversity of QDR mechanisms and of which mechanisms are effective against which classes of pathogens will enable the more effective deployment of QDR to produce more durably resistant,resilient crops.

Regulation of maize growth and immunity by ZmSKI3-mediated RNA decay and post-transcriptional gene silencing

Disease resistance is often associated with compromised plant growth and yield due to defense-growth tradeoffs.However,key components and mechanisms underlying the defensegrowth tradeoffs are rarely explored in maize.In this study,we find that ZmSKI3,a putative subunit of the SUPERKILLER(SKI)complex that mediates the 3′-5′degradation of RNA,regulates both plant development and disease resistance in maize.The Zmski3 mutants showed retarded plant growth and constitutively activated defense responses,while the ZmSKI3 overexpression lines are more susceptible to Curvularia lunata and Bipolaris maydis.Consistently,the expression of defense-related genes was generally up-regulated,while expressions of growth-related genes were mostly down-regulated in leaves of the Zmski3-1 mutant compared to that of wild type.In addition,223 differentially expressed genes that are up-regulated in Zmski3-1 mutant but down-regulated in the ZmSKI3 overexpression line are identified as potential target genes of ZmSKI3.Moreover,small interfering RNAs targeting the transcripts of the defense-and growth-related genes are differentially accumulated,likely to combat the increase of defense-related transcripts but decrease of growthrelated transcripts in Zmski3-1 mutant.Taken together,our study indicates that plant growth and immunity could be regulated by both ZmSKI3-mediated RNA decay and post-transcriptional gene silencing in maize.