Functional study of Csrbohs in defence response against Xanthomonas citri ssp. citri

Elucidation of mechanisms underlying active oxygen burst in Citrus sinensis after Diaporthe citri infection using transcriptome analysis

Introduction

Reactive oxygen species (ROS) generation is a common disease defense mechanism in plants. However, it is unclear whether Citrus host activates defense response against Diaporthe citri causing citrus melanose disease by producing ROS, and the underlying molecular mechanisms are unknown.

Methods

DAB staining and RNA-Seq technology were used to compare the active oxygen burst and differential gene expression, respectively, in uninfected and infected Citrus sinensis leaves at different time points during D. citri infection in vivo. The functions of CsRBOH (a significant DEG) were confirmed in N. benthamiana through the Agrobacterium-mediated transient expression system.

Results

DAB staining indicated that C. sinensis initiated defense against D. citri infection within 24 h by generating ROS. Illumina sequencing revealed 25,557 expressed genes of C. sinensis. The most upregulated DEGs (n = 1,570) were identified 72 h after fungal inoculation (sample denoted as CD72). In the CD72 vs. Cs (samples at 0 h after fungal inoculation) comparison, the KEGG pathway category with the highest number of genes (n = 62) and most significant enrichment was Protein processing in endoplasmic reticulum, followed by Glutathione metabolism and MAPK signaling pathway-plant. GO analysis revealed that the DEGs of CD72 vs. Cs related to active oxygen burst and chitin recognition were significantly grouped into the regulation of biological processes and molecular functions, with GO terms including response to ROS, response to fungus, and oxidoreductase activity. Remarkably, CsRBOH was significantly enriched in the GO and KEGG analyses, and its expression pattern in qRT-PCR and DAB staining results were consistent. Among the 63 ROS-related DEGs, HSP genes and genes associated with the peroxidase family were highly significant as revealed by protein-protein interaction networks. Furthermore, ROS accumulation, cell death, and upregulation of defense-related genes were observed in N. benthamiana leaves with CsRBOH expressed through the Agrobacterium-mediated transient expression system.

Conclusion

Our findings suggested that C. sinensis activates CsRBOH and ROS-related genes, leading to ROS accumulation to resist the invasion by D. citri. This study laid the foundation for future research on molecular mechanisms and breeding of C. sinensis cultivars resistant to citrus melanose.

The CsAP2-09-CsWRKY25-CsRBOH2 cascade confers resistance against citrus bacterial canker by regulating ROS homeostasis

Citrus bacterial canker (CBC) is a serious bacterial disease caused by Xanthomonas citri subsp. citri (Xcc) that adversely impacts the global citrus industry. In a previous study, we demonstrated that overexpression of an Xcc-inducible apetala 2/ethylene response factor encoded by Citrus sinensis, CsAP2-09, enhances CBC resistance. The mechanism responsible for this effect, however, is not known. In the present study, we showed that CsAP2-09 targeted the promoter of the Xcc-inducible WRKY transcription factor coding gene CsWRKY25 directly, activating its transcription. CsWRKY25 was found to localize to the nucleus and to activate transcriptional activity. Plants overexpressing CsWRKY25 were more resistant to CBC and showed higher expression of the respiratory burst oxidase homolog (RBOH) CsRBOH2, in addition to exhibiting increased RBOH activity. Transient overexpression assays in citrus confirmed that CsWRKY25 and CsRBOH2 participated in the generation of reactive oxygen species (ROS) bursts, which were able to restore the ROS degradation caused by CsAP2-09 knockdown. Moreover, CsWRKY25 was found to bind directly to W-box elements within the CsRBOH2 promoter. Notably, CsRBOH2 knockdown had been reported previously to reduce the CBC resistance, while demonstrated in this study, CsRBOH2 transient overexpression can enhance the CBC resistance. Overall, our results outline a pathway through which CsAP2-09-CsWRKY25 transcriptionally reprograms CsRBOH2-mediated ROS homeostasis in a manner conducive to CBC resistance. These data offer new insight into the mechanisms and regulatory pathways through which CsAP2-09 regulates CBC resistance, highlighting its potential utility as a target for the breeding of CBC-resistant citrus varieties.

Molecular basis for host responses to Xanthomonas infection

This review highlights the most relevant and recent updated information available on the defense responses of selected hosts against Xanthomonas spp. Xanthomonas is one of the most important genera of Gram-negative phytopathogenic bacteria, severely affecting the productivity of economically important crops worldwide, colonizing either the vascular system or the mesophyll tissue of the host. Due to its rapid propagation, Xanthomonas poses an enormous challenge to farmers, because it is usually controlled using huge quantities of copper-based chemicals, adversely impacting the environment. Thus, developing new ways of preventing colonization by these bacteria has become essential. Advances in genomic and transcriptomic technologies have significantly elucidated at molecular level interactions between various crops and Xanthomonas species. Understanding how these hosts respond to the infection is crucial if we are to exploit potential approaches for improving crop breeding and cutting productivity losses. This review focuses on our current knowledge of the defense response mechanisms in agricultural crops after Xanthomonas infection. We describe the molecular basis of host-bacterium interactions over a broad spectrum with the aim of improving our fundamental understanding of which genes are involved and how they work in this interaction, providing information that can help to speed up plant breeding programs, namely using gene editing approaches.