Transcriptome analysis of maize leaf systemic symptom infected by Bipolaris zeicola

Comparative transcriptome analysis highlights resistance regulatory networks of maize in response to Exserohilum turcicum infection at the early stage

Northern corn leaf blight, caused by Exserohilum turcicum (E. turcicum), is one of the most destructive diseases in maize, leading to serious yield losses. However, the underlying molecular mechanisms of E. turcicum infection response in maize remain unclear. In this study, we performed comparative transcriptome analysis in resistant maize inbred line J9D207 (R) and susceptible maize inbred line PH4CV (S) after infecting with E. turcicum at 0 h, 24 h and 72 h, respectively. Compared with 0 h, 9656 (24 h) and 8748 (72 h) differentially expressed genes (DEGs) were identified in J9D207, and 7915 (24 h) and 7865 (72 h) DEGs were identified in PH4CV. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that alpha-linolenic acid metabolism, benzoxazinoid biosynthesis, flavonoid biosynthesis and phenylpropanoid biosynthesis might be involved in maize defense reactions. Some DEGs coded for transcription factors, such as MYB-related, ERF, NAC, bZIP, bHLH and WRKY families, which indicated that they may participate in resistance against E. turcicum. In addition, DEGs involved in SA, JA, ABA and ET signaling pathways were revealed. Moreover, 75 SOD activity-related genes and 421 POD activity-related genes were identified through weighted gene co-expression network analysis (WGCNA), respectively. These results provide a novel insight into the resistance mechanism of maize in response to E. turcicum inoculation.

The complete mitochondrial genomes of five critical phytopathogenic Bipolaris species: features, evolution, and phylogeny

In the present study, three mitogenomes from the Bipolaris genus (Bipolaris maydis, B. zeicola, and B. oryzae) were assembled and compared with the other two reported Bipolaris mitogenomes (B. oryzae and B. sorokiniana). The five mitogenomes were all circular DNA molecules, with lengths ranging from 106,403 bp to 135,790 bp. The mitogenomes of the five Bipolaris species mainly comprised the same set of 13 core protein-coding genes (PCGs), two rRNAs, and a certain number of tRNAs and unidentified open reading frames (ORFs). The PCG length, AT skew and GC skew showed large variability among the 13 PCGs in the five mitogenomes. Across the 13 core PCGs tested, nad6 had the least genetic distance among the 16 Pleosporales species we investigated, indicating that this gene was highly conserved. In addition, the Ka/Ks values for all 12 core PCGs (excluding rps3) were < 1, suggesting that these genes were subject to purifying selection. Comparative mitogenomic analyses indicate that introns were the main factor contributing to the size variation of Bipolaris mitogenomes. The introns of the cox1 gene experienced frequent gain/loss events in Pleosporales species. The gene arrangement and collinearity in the mitogenomes of the five Bipolaris species were almost highly conserved within the genus. Phylogenetic analysis based on combined mitochondrial gene datasets showed that the five Bipolaris species formed well-supported topologies. This study is the first report on the mitogenomes of B. maydis and B. zeicola, as well as the first comparison of mitogenomes among Bipolaris species. The findings of this study will further advance investigations into the population genetics, evolution, and genomics of Bipolaris species.

Interactive transcriptome analyses of Northern Wild Rice (Zizania palustris L.) and Bipolaris oryzae show convoluted communications during the early stages of fungal brown spot development

Fungal diseases, caused mainly by Bipolaris spp., are past and current threats to Northern Wild Rice (NWR) grain production and germplasm preservation in both natural and cultivated settings. Genetic resistance against the pathogen is scarce. Toward expanding our understanding of the global gene communications of NWR and Bipolaris oryzae interaction, we designed an RNA sequencing study encompassing the first 12 h and 48 h of their encounter. NWR activated numerous plant recognition receptors after pathogen infection, followed by active transcriptional reprogramming of signaling mechanisms driven by Ca2+ and its sensors, mitogen-activated protein kinase cascades, activation of an oxidative burst, and phytohormone signaling-bound mechanisms. Several transcription factors associated with plant defense were found to be expressed. Importantly, evidence of diterpenoid phytoalexins, especially phytocassane biosynthesis, among expression of other defense genes was found. In B. oryzae, predicted genes associated with pathogenicity including secreted effectors that could target plant defense mechanisms were expressed. This study uncovered the early molecular communication between the NWR-B. oryzae pathosystem, which could guide selection for allele-specific genes to boost NWR defenses, and overall aid in the development of more efficient selection methods in NWR breeding through the use of the most virulent fungal isolates.

Microbial and enzymatic changes in cigar tobacco leaves during air-curing and fermentation

Metabolic enzyme activity and microbial composition of the air-curing and fermentation processes determine the quality of cigar tobacco leaves (CTLs). In this study, we reveal the evolution of the dominant microorganisms and microbial community structure at different stages of the air-curing and fermentation processes of CTLs. The results showed that the changes in metabolic enzymes occurred mainly during the air-curing phase, with polyphenol oxidase (PPO) being the most active at the browning phase. Pseudomonas, Bacteroides, Vibrio, Monographella, Bipolaris, and Aspergillus were the key microorganisms in the air-curing and fermentation processes. Principal coordinate analysis revealed significant separation of microbial communities between the air-curing and fermentation phases. Redundancy analysis showed that bacteria such as Proteobacteria, Firmicutes, Bacteroidota, and Acidobacteriota and fungi such as Ascomycota and Basidiomycota were correlated with enzyme activity and temperature and humidity. Bacteria mainly act in sugar metabolism, lipid metabolism, and amino acid metabolism, while fungi mainly degrade lignin, cellulose, and pectin through saprophytic action. Spearman correlation network analysis showed that Firmicutes, Proteobacteria, and Actinobacteria were the key bacterial taxa, while Dothideomycetes, Sordariomycetes, and Eurotiomycetes were the key fungal taxa. This research provides the basis for improving the quality of cigars by improving the air-curing and fermentation processes. KEY POINTS: • Changes in POD and PPO activity control the color change of CTLs at the air-curing stage. • Monographella, Aspergillus, Pseudomonas, and Vibrio play an important role in air-curing and fermentation. • Environmental temperature and humidity mainly affect the fermentation process, whereas bacteria such as Proteobacteria, Firmicutes, Bacteroidota, and Acidobacteriota and fungi such as Ascomycota and Basidiomycota are associated with enzyme activity and temperature and humidity.

Maize FERONIA-like receptor genes are involved in the response of multiple disease resistance in maize

Receptor-like kinases (RLKs) are key modulators of diverse cellular processes such as development and sensing the extracellular environment. FERONIA, a member of the CrRLK1L subfamily, acts as a pleiotropic regulator of plant immune responses, but little is known about how maize FERONIA-like receptors (FLRs) function in responding to the major foliar diseases of maize such as northern corn leaf blight (NLB), northern corn leaf spot (NLS), anthracnose stalk rot (ASR), and southern corn leaf blight (SLB). Here, we identified three ZmFLR homologous proteins that showed cell membrane localization. Transient expression in Nicotiana benthamiana proved that ZmFLRs were capable of inducing cell death. To investigate the role of ZmFLRs in maize, we used virus-induced gene silencing to knock down expression of ZmFLR1/2 and ZmFLR3 resulting in reduced reactive oxygen species production induced by flg22 and chitin. The resistance of maize to NLB, NLS, ASR, and SLB was also reduced in the ZmFLRs knockdown maize plants. These results indicate that ZmFLRs are positively involved in broad-spectrum disease resistance in maize.

Comparative Transcriptome Analysis of Fungal Pathogen Bipolaris maydis to Understand Pathogenicity Behavior on Resistant and Susceptible Non-CMS Maize Genotypes

Bipolaris maydis is pathogen of maize which causes maydis leaf blight disease. In India major losses occur due to the B. maydis race “O” pathogen, whereas in other parts of the world, major losses are due to the race “T” pathogen. In the present study, we conducted an in planta transcriptomics study of the B. maydis race “O” pathogen after infection on non-CMS maize resistant and susceptible genotypes by mRNA sequencing to understand the molecular basis of pathogenicity for better management of the pathogen. Approximately 23.4 GB of mRNA-seq data of B. maydis were obtained from both resistant and susceptible maize backgrounds for fungus. Differentially expressed genes (DEGs) analysis of B. maydis in two different genetic backgrounds suggested that the majority of highly DEGs were associated with mitochondrial, cell wall and chitin synthesis, sugar metabolism, peroxidase activity, mitogen-activated protein kinase (MAPK) activity, and shikimate dehydrogenase. KEGG analysis showed that the biosynthetic pathways for secondary metabolism, antibiotics, and carbon metabolism of fungus were highly enriched, respectively, in susceptible backgrounds during infection. Previous studies in other host pathogen systems suggest that these genes play a vital role in causing disease in their host plants. Our study is probably the first transcriptome study of the B. maydis race “O” pathogen and provides in-depth insight of pathogenicity on the host.

Crosstalk of DNA Methylation Triggered by Pathogen in Poplars With Different Resistances

DNA methylation plays crucial roles in responses to environmental stimuli. Modification of DNA methylation during development and abiotic stress responses has been confirmed in increasing numbers of plants, mainly annual plants. However, the epigenetic regulation mechanism underlying the immune response to pathogens remains largely unknown in plants, especially trees. To investigate whether DNA methylation is involved in the response to infection process or is related to the resistance differences among poplars, we performed comprehensive whole-genome bisulfite sequencing of the infected stem of the susceptible type Populus × euramerican ‘74/76’ and resistant type Populus tomentosa ‘henan’ upon Lonsdalea populi infection. The results revealed that DNA methylation changed dynamically in poplars during the infection process with a remarkable decrease seen in the DNA methylation ratio. Intriguingly, the resistant P. tomentosa ‘henan’ had a much lower basal DNA methylation ratio than the susceptible P. × euramerican ‘74/76’. Compared to mock-inoculation, both poplar types underwent post-inoculation CHH hypomethylation; however, significant decreases in mC and mCHH proportions were found in resistant poplar. In addition, most differentially CHH-hypomethylated regions were distributed in repeat and promoter regions. Based on comparison of DNA methylation modification with the expression profiles of genes, DNA methylation occurred in resistance genes, pathogenesis-related genes, and phytohormone genes in poplars during pathogen infection. Additionally, transcript levels of genes encoding methylation-related enzymes changed during pathogen infection. Interestingly, small-regulator miRNAs were subject to DNA methylation in poplars experiencing pathogen infection. This investigation highlights the critical role of DNA methylation in the poplar immune response to pathogen infection and provides new insights into epigenetic regulation in perennial plants in response to biotic stress.

First Report of Root Rot Caused by Bipolaris zeicola on Maize in Hebei Province

Maize (Zea mays L.) is one of the most important cereal crops in China, and the planting area reached 41.3 million hectares in 2019. Root rot is a widespread disease that occurs at the seedling stage of maize, resulting in leaf wilting, root rot and even plant death, and consequently yield and quality losses. During an investigation of spring maize in 2020, seedlings with wilted leaves and dark brown necrotic spots on root were observed in the fields in Kuancheng Manchu Autonomous County, Hebei Province, China. Symptomatic plants were collected for pathogen isolation and identification. The soil on roots was washed off with running water. Then, 2-3 mm necrotic root segments were sampled and surface sterilized with 75% ethanol for 2 min, rinsed three times with sterile distilled water, air-dried on sterile filter paper, and plated on potato dextrose agar (PDA). Plates were incubated at 28℃ in darkness for 3 days. A nonsporulating, dematiaceous fungus growing from root segments was transferred to fresh PDA plates. The colonies were round or irregular round, black, villiform with dense grayish white mycelia. Water agar amended with wheat straw was used for sporulation. Conidiophores were single, light brown, multiseptate, geniculate. Conidia were 38.68 x 10.69 to 71.98 x 20.57 μm, brown, oval, slightly curved, with 2 to 8 septa, and an obviously flattened hilum on the basal cell. Conidia germinated from both poles. The causal agent was identified as Bipolaris zeicola (G.L. Stout) Shoemaker (teleomorph = Cochliobolus carbonum R. R. Nelson) based on its morphological features. For molecular identification, genomic DNA was extracted from fresh mycelia cultured on PDA plates. Partial sequences of ITS-rDNA region and Brn1 reductase melanin biosynthesis gene were amplified using primers ITS1/ ITS4 (TCCGTAGGTGAACCTGCGG/ TCCTCCGCTTATTGATATGC) (White et al. 1990) and Brn01/ Brn02 (GCCAACATCGAGCAAACATGG/ GCAAGCAGCACCGTCAATACCAAT) (Shimizu et al. 1998), respectively. A DNA fragment of 532 bp was obtained from ITS-rDNA region and the sequence (GenBank Accession No. MW407046) was 100% identical to sequence of B. zeicola (GenBank Accession MH864760). The sequence of Brn1 gene was 816 bp (GenBank Accession No. MW415899) and was 99.75% identical to sequence of C. carbonum (GenBank Accession No. AB011658). The morphological and molecular evidence proved that the causal agent isolated from maize roots in Hebei province was B. zeicola. Pathogenicity assays were conducted with one week old (V1 stage) maize seedlings grown from the surface-sterilized seed of cv. Zhengdan 958. The mesocotyl and radicle of each plant (N=3) were inoculated with a 5 mm fungal disk of B. zeicola. Mock-inoculated plants (N=3) with sterile PDA disk served as the negative control. After 7 days, plants inoculated with B. zeicola were wilted with dark brown necrotic spots on mesocotyl and radicle. Meanwhile, the negative controls did not present any symptoms. Koch's postulate was proved with successful re-isolation of the same fungus from the inoculated maize plants. These results confirmed the pathogenicity of B. zeicola on maize root. B. zeicola mainly causes an important foliar disease in many regions of the world, known as Northern corn leaf spot, in addition, it can also cause ear rot and stalk rot of maize (Liu et al. 2015). To our knowledge, this is the first report of root rot caused by B. zeicola on maize in China, which extends the known agents of maize root rot. Therefore, it is necessary to explore effective seed-applied fungicides for disease control. Also, more attention should be paid to develop hybrids with resistance to this disease.

Genome-Wide Association and Gene Co-expression Network Analyses Reveal Complex Genetics of Resistance to Goss's Wilt of Maize

Goss’s bacterial wilt and leaf blight is a disease of maize caused by the gram positive bacterium Clavibacter michiganensis subsp. nebraskensis (Cmn). First discovered in Nebraska, Goss’s wilt has now spread to major maize growing states in the United States and three provinces in Canada. Previous studies conducted using elite maize inbred lines and their hybrids have shown that resistance to Goss’s wilt is a quantitative trait. The objective of this study was to further our understanding of the genetic basis of resistance to Goss’s wilt by using a combined approach of genome-wide association mapping and gene co-expression network analysis. Genome-wide association analysis was accomplished using a diversity panel consisting of 555 maize inbred lines and a set of 450 recombinant inbred lines (RILs) from three bi-parental mapping populations, providing the most comprehensive screening of Goss’s wilt resistance to date. Three SNPs in the diversity panel and 10 SNPs in the combined dataset, including the diversity panel and RILs, were found to be significantly associated with Goss’s wilt resistance. Each significant SNP explained 1–5% of the phenotypic variation for Goss’s wilt (total of 8–11%). To augment the results of genome-wide association mapping and help identify candidate genes, a time course RNA sequencing experiment was conducted using resistant (N551) and susceptible (B14A) maize inbred lines. Gene co-expression network analysis of this time course experiment identified one module of 141 correlated genes that showed differential regulation in response to Cmn inoculations in both resistant and susceptible lines. SNPs inside and flanking these genes explained 13.3% of the phenotypic variation. Among 1,000 random samples of genes, only 8% of samples explained more phenotypic variance for Goss’s wilt resistance than those implicated by the co-expression network analysis. While a statistically significant enrichment was not observed (P < 0.05), these results suggest a possible role for these genes in quantitative resistance at the field level and warrant more research on combining gene co-expression network analysis with quantitative genetic analyses to dissect complex disease resistance traits. The results of the GWAS and co-expression analysis both support the complex nature of resistance to this important disease of maize.

Changes in maize transcriptome in response to maize Iranian mosaic virus infection

Background

Maize Iranian mosaic virus (MIMV, genus Nucleorhabdovirus, family Rhabdoviridae) causes an economically important disease in maize and other gramineous crops in Iran. MIMV negative-sense RNA genome sequence of 12,426 nucleotides has recently been completed. Maize Genetics and Genomics database shows that 39,498 coding genes and 4,976 non-coding genes of maize have been determined, but still some transcripts could not be annotated. The molecular host cell responses of maize to MIMV infection including differential gene expression have so far not been elucidated.

Methodology/Principal findings

Complementary DNA libraries were prepared from total RNA of MIMV-infected and mock-inoculated maize leaves and sequenced using Illumina HiSeq 2500. Cleaned raw transcript reads from MIMV-infected maize were mapped to reads from uninfected maize and to a maize reference genome. Differentially expressed transcripts were characterized by gene ontology and biochemical pathway analyses. Transcriptome data for selected genes were validated by real-time quantitative PCR.

Conclusion/Significance

Approximately 42 million clean reads for each treatment were obtained. In MIMV-infected maize compared to uninfected plants, 1689 transcripts were up-regulated and 213 transcripts were down-regulated. In response to MIMV infection, several pathways were activated in maize including immune receptor signaling, metabolic pathways, RNA silencing, hormone-mediated pathways, protein degradation, protein kinase and ATP binding activity, and fatty acid metabolism. Also, several transcripts including those encoding hydrophobic protein RCI2B, adenosylmethionine decarboxylase NAC transcription factor and nucleic acid binding, leucine-rich repeat, heat shock protein, 26S proteasome, oxidoreductases and endonuclease activity protein were up-regulated. These data will contribute to the identification of genes and pathways involved in plant-virus interactions that may serve as future targets for improved disease control.