Hormonal Responses to Susceptible, Intermediate, and Resistant Interactions in the Brassica napus-Leptosphaeria maculans Pathosystem

From Recognition to Response: Resistance-Effector Gene Interactions in the Brassica napus and Leptosphaeria maculans Patho-System

Blackleg disease, caused by the hemibiotrophic fungal pathogen Leptosphaeria maculans, poses a serious threat to Brassica crops and requires a broad understanding of the plant defence mechanisms. The Brassica. napus-L. maculans pathosystem provides a useful model to understand plant resistance response to hemibiotrophs. This review aims to explain the mechanisms underlying R-Avr interaction, signalling cascades, and the hypersensitive response (HR) produced by B. napus towards L. maculans, causing local cell death that restricts the pathogen to the site of infection. The role of transcription factors is pivotal to the process of HR, coordinating the regulation of genes involved in pathogen recognition and the activation of SA responsive genes and production of secondary metabolites. The R-Avr interaction signalling cascade involves production of reactive oxygen species (ROS), calcium ion influx, Salicylic acid (SA) hormonal signalling and mitogen activated protein kinases (MAPKs), which are critical in the HR in B. napus. The in-depth understanding of molecular signalling pathway of the R-Avr interaction between B. napus-L. maculans pathosystem provides valuable information for future research endeavours regarding enhancing disease resistance in Brassica crops.

Glycerophosphodiester phosphodiesterase 1 mediates G3P accumulation for Eureka lemon resistance to citrus yellow vein clearing virus

Glycerophosphodiester phosphodiesterase 1 (GDPD1) plays an important function in the abiotic stress responses and participates in the accumulation of sn-glycerol-3-phosphate (G3P) in plants, which is key to plant systemic acquired resistance (SAR). However, the role of GDPD1 in plant responses to biotic stress remains poorly understood. This study characterized the antivirus function of the GDPD1 gene (designated as ClGDPD1) from Eureka lemon. ClGDPD1 is located in the membrane and endoplasmic reticulum, where it interacts with the citrus yellow vein clearing virus (CYVCV) coat protein (CP). Compared to individually expressed ClGDPD1 or coexpressed ClGDPD1 + CP140-326, transiently coexpressed ClGDPD1 + CP or ClGDPD1 + CP1-139 significantly upregulated the key substance contents and genes expression involved in glycerophospholipid metabolism. Over-expression of ClGDPD1 significantly facilitated the accumulation of G3P, upregulated the expression of SAR-related genes, and increased the resistance of transgenic Eureka lemon to CYVCV infection. Furthermore, exogenous glycerol treatment and over-expression of ClGPDH increased the G3P content and reduced CYVCV titers in plants or hairy roots. These results indicated that the enhanced resistance of ClGDPD1 transgenic Eureka lemon to CYVCV may be due to facilitating G3P accumulation through the interaction of ClGDPD1 with CP. Our findings provide novel insights into the role of ClGDPD1 as an important regulatory center in mediating the citrus defense response to viral infections.

Studying temperature's impact on Brassica napus resistance to identify key regulatory mechanisms using comparative metabolomics

To investigate the effects of temperature on Brassica napus (canola) resistance to Leptosphaeria maculans (LM), the causal agent of blackleg disease, metabolic profiles of LM infected resistant (R) and susceptible (S) canola cultivars at 21 °C and 28 °C were analyzed. Metabolites were detected in cotyledons of R and S plants at 48- and 120-h post-inoculation with LM using UPLC-QTOF/MS. The mock-inoculated plants were used as controls. Some of the resistance-related specific pathways, including lipid metabolism, amino acid metabolism, carbohydrate metabolism, and aminoacyl-tRNA biosynthesis, were down-regulated in S plants but up-regulated in R plants at 21 °C. However, some of these pathways were down-regulated in R plants at 28 °C. Amino acid metabolism, lipid metabolism, alkaloid biosynthesis, phenylpropanoid biosynthesis, and flavonoid biosynthesis were the pathways linked to combined heat and pathogen stresses. By using network analysis and enrichment analysis, these pathways were identified as important. The pathways of carotenoid biosynthesis, pyrimidine metabolism, and lysine biosynthesis were identified as unique mechanisms related to heat stress and may be associated with the breakdown of resistance against the pathogen. The increased susceptibility of R plants at 28 °C resulted in the down-regulation of signal transduction pathway components and compromised signaling, particularly during the later stages of infection. Deactivating LM-specific signaling networks in R plants under heat stress may result in compatible responses and deduction in signaling metabolites, highlighting global warming challenges in crop disease control.

Integrative Analysis of Expression Profiles of mRNA and MicroRNA Provides Insights of Cotton Response to Verticillium dahliae

Cotton Verticillium wilt, caused by the notorious fungal phytopathogen Verticillium dahliae (V. dahliae), is a destructive soil-borne vascular disease and severely decreases cotton yield and quality worldwide. Transcriptional and post-transcriptional regulation of genes responsive to V. dahliae are crucial for V. dahliae tolerance in plants. However, the specific microRNAs (miRNAs) and the miRNA/target gene crosstalk involved in cotton resistance to Verticillium wilt remain largely limited. To investigate the roles of regulatory RNAs under V. dahliae induction in upland cotton, mRNA and small RNA libraries were constructed from mocked and infected roots of two upland cotton cultivars with the V. dahliae-sensitive cultivar Jimian 11 (J11) and the V. dahliae-tolerant cultivar Zhongzhimian 2 (Z2). A comparative transcriptome analysis revealed 8330 transcripts were differentially expressed under V. dahliae stress and associated with several specific biological processes. Moreover, small RNA sequencing identified a total of 383 miRNAs, including 330 unique conserved miRNAs and 53 novel miRNAs. Analysis of the regulatory network involved in the response to V. dahliae stress revealed 31 differentially expressed miRNA−mRNA pairs, and the up-regulation of GhmiR395 and down-regulation of GhmiR165 were possibly involved in the response to V. dahliae by regulating sulfur assimilation through the GhmiR395-APS1/3 module and the establishment of the vascular pattern and secondary cell wall formation through GhmiR165-REV module, respectively. The integrative analysis of mRNA and miRNA expression profiles from upland cotton lays the foundation for further investigation of regulatory mechanisms of resistance to Verticillium wilt in cotton and other crops.

Less Virulent Leptosphaeria biglobosa Immunizes the Canola Plant to Resist Highly Virulent L. maculans, the Blackleg Pathogen

Leptosphaeria biglobosa is a less virulent Leptosphaeria spp. that causes blackleg disease in canola. Previous studies from our lab have shown that inoculation with the less virulent L. biglobosa can boost the resistance of canola plants against the highly virulent L. maculans. The objective of this study was to confirm the effectiveness of L. biglobosa as a biocontrol agent against L. maculans utilizing morphology, fluorescence microscopy, gene quantification, and transcriptomic analysis. The in planta development of two Leptosphaeria species inoculated at different time points was assessed using fluorescent protein-tagged isolates which are GFP-tagged L. maculans and DsRed-tagged L. biglobosa. The growth inhibition of L. maculans by pre-and co-inoculated L. biglobosa was supported by no lesion development on cotyledons and no or weak fluorescence protein-tagged mycelia under the confocal microscope. The host defense-related genes, WRKY33, PR1, APX6, and CHI, were upregulated in L. biglobosa inoculated Westar cotyledons compared to L. maculans inoculated cotyledons. The quantification of each pathogen through qPCR assay and gene expressions analysis on host defense-related genes by RT-qPCR confirmed the potential of L. biglobosa “brassicae’ in the management of the blackleg disease pathogen, L. maculans ‘brassicae’, in canola.

Leptosphaeria maculans-Brassica napus Battle: A Comparison of Incompatible vs. Compatible Interactions Using Dual RNASeq

Leptosphaeria maculans causes blackleg disease, which is one of the most destructive diseases of canola (Brassica napus L.). Due to the erosion of the current resistance in B. napus, it is pivotal to introduce new resistant genotypes to the growers. This study evaluated the potential of Rlm7 gene as resistance to its corresponding avirulence AvrLm7 gene is abundant. The Rlm7 line was inoculated with L. maculans isolate with AvrLm7; UMAvr7; and the CRISPR/Cas9 knockout AvrLm7 mutant, umavr7, of the same isolate to cause incompatible and compatible interactions, respectively. Dual RNA-seq showed differential gene expressions in both interactions. High expressions of virulence-related pathogen genes-CAZymes, merops, and effector proteins after 7-dpi in compatible interactions but not in incompatible interaction—confirmed that the pathogen was actively virulent only in compatible interactions. Salicyclic and jasmonic acid biosynthesis and signaling-related genes, defense-related PR1 gene (GSBRNA2T00150001001), and GSBRNA2T00068522001 in the NLR gene family were upregulated starting as early as 1- and 3-dpi in the incompatible interaction and the high upregulation of those genes after 7-dpi in compatible interactions confirmed the early recognition of the pathogen by the host and control it by early activation of host defense mechanisms in the incompatible interaction.

Comparison of the Distinct, Host-Specific Response of Three Solanaceae Hosts Induced by Phytophthora infestans

Three Solanaceae hosts (TSHs), S. tuberosum, N. benthamiana and S. lycopersicum, represent the three major phylogenetic clades of Solanaceae plants infected by Phytophthora infestans, which causes late blight, one of the most devastating diseases seriously affecting crop production. However, details regarding how different Solanaceae hosts respond to P. infestans are lacking. Here, we conducted RNA-seq to analyze the transcriptomic data from the TSHs at 12 and 24 h post P. infestans inoculation to capture early expression effects. Macroscopic and microscopic observations showed faster infection processes in S. tuberosum than in N. benthamiana and S. lycopersicum under the same conditions. Analysis of the number of genes and their level of expression indicated that distinct response models were adopted by the TSHs in response to P. infestans. The host-specific infection process led to overlapping but distinct in GO terms and KEGG pathways enriched for differentially expressed genes; many were tightly linked to the immune response in the TSHs. S. tuberosum showed the fastest response and strongest accumulation of reactive oxygen species compared with N. benthamiana and S. lycopersicum, which also had similarities and differences in hormone regulation. Collectively, our study provides an important reference for a better understanding of late blight response mechanisms of different Solanaceae host interactions.

Leaf Transcriptome Analysis of Broomcorn Millet Uncovers Key Genes and Pathways in Response to Sporisorium destruens

Broomcorn millet (Panicum miliaceum L.) affected by smut (caused by the pathogen Sporisorium destruens) has reduced production yields and quality. Determining the tolerance of broomcorn millet varieties is essential for smut control. This study focuses on the differences in the phenotypes, physiological characteristics, and transcriptomes of resistant and susceptible broomcorn millet varieties under Sporisorium destruens stress. In diseased broomcorn millet, the plant height and stem diameter were reduced, while the number of nodes increased. After infection, the activities of superoxide dismutase and peroxidase decreased, and malondialdehyde and relative chlorophyll content (SPAD) decreased. Transcriptome analysis showed 514 and 5452 differentially expressed genes (DEGs) in the resistant and susceptible varieties, respectively. The Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis of DEGs showed that pathways related to plant disease resistance, such as phenylpropanoid biosynthesis, plant–pathogen interaction, and plant hormone signal transduction, were significantly enriched. In addition, the transcriptome changes of cluster leaves and normal leaves in diseased broomcorn millet were analysed. Gene ontology and KEGG enrichment analyses indicated that photosynthesis played an important role in both varieties. These findings lay a foundation for future research on the molecular mechanism of the interaction between broomcorn millet and Sporisorium destruens.