Molecular complexity of quantitative immunity in plants: from QTL mapping to functional and systems biology

In nature, plants defend themselves against pathogen attack by activating an arsenal of defense mechanisms. During the last decades, work mainly focused on the understanding of qualitative disease resistance mediated by a few genes conferring an almost complete resistance, while quantitative disease resistance (QDR) remains poorly understood despite the fact that it represents the predominant and more durable form of resistance in natural populations and crops. Here, we review our past and present work on the dissection of the complex mechanisms underlying QDR in Arabidopsis thaliana. The strategies, main steps and challenges of our studies related to one atypical QDR gene, RKS1 (Resistance related KinaSe 1), are presented. First, from genetic analyses by QTL (Quantitative Trait Locus) mapping and GWAs (Genome Wide Association studies), the identification, cloning and functional analysis of this gene have been used as a starting point for the exploration of the multiple and coordinated pathways acting together to mount the QDR response dependent on RKS1. Identification of RKS1 protein interactors and complexes was a first step, systems biology and reconstruction of protein networks were then used to decipher the molecular roadmap to the immune responses controlled by RKS1. Finally, exploration of the potential impact of key components of the RKS1-dependent gene network on leaf microbiota offers interesting and challenging perspectives to decipher how the plant immune systems interact with the microbial communities' systems.

Predicting rice diseases using advanced technologies at different scales: present status and future perspectives

The past few years have witnessed significant progress in emerging disease detection techniques for accurately and rapidly tracking rice diseases and predicting potential solutions. In this review we focus on image processing techniques using machine learning (ML) and deep learning (DL) models related to multi-scale rice diseases. Furthermore, we summarize applications of different detection techniques, including genomic, physiological, and biochemical approaches. In addition, we also present the state-of-the-art in contemporary optical sensing applications of pathogen-plant interaction phenotypes. This review serves as a valuable resource for researchers seeking effective solutions to address the challenges of high-throughput data and model recognition for early detection of issues affecting rice crops through ML and DL models.

Dynamic Regulatory Event Mining by iDREM in Large-Scale Multi-omics Datasets During Biotic and Abiotic Stress in Plants

The system-wide complexity of genome regulation encoding the organism phenotypic diversity is well understood. However, a major challenge persists about the appropriate method to describe the systematic dynamic genome regulation event utilizing enormous multi-omics datasets. Here, we describe Interactive Dynamic Regulatory Events Miner (iDREM) which reconstructs gene-regulatory networks from temporal transcriptome, proteome, and epigenome datasets during stress to envisage "master" regulators by simulating cascades of temporal transcription-regulatory and interactome events. The iDREM is a Java-based software that integrates static and time-series transcriptomics and proteomics datasets, transcription factor (TF)-target interactions, microRNA (miRNA)-target interaction, and protein-protein interactions to reconstruct temporal regulatory network and identify significant regulators in an unsupervised manner. The hidden Markov model detects specialized manipulated pathways as well as genes to recognize statistically significant regulators (TFs/miRNAs) that diverge in temporal activity. This method can be translated to any biotic or abiotic stress in plants and animals to predict the master regulators from condition-specific multi-omics datasets including host-pathogen interactions for comprehensive understanding of manipulated biological pathways.

Plant Primary Metabolism Regulated by Nitrogen Contributes to Plant-Pathogen Interactions

Nitrogen contributes to plant defense responses by the regulation of plant primary metabolism during plant-pathogen interactions. Based on biochemical, physiological, bioinformatic and transcriptome approaches, we investigated how different nitrogen forms (ammonium vs. nitrate) regulate the physiological response of cucumber (Cucumis sativus) to Fusarium oxysporum f. sp. cucumerinum (FOC) infection. The metabolic profile revealed that nitrate-grown plants accumulated more organic acids, while ammonium-grown plants accumulated more amino acids; FOC infection significantly increased levels of both amino acids and organic acids in the roots of ammonium-grown plants. Transcriptome analysis showed that genes related to carbon metabolism were mostly up-regulated in plants grown with nitrate, whereas in ammonium-grown plants the up-regulated genes were mostly those that were related to primary nitrogen metabolism. Root FOC colonization and disease incidence were positively correlated with levels of root amino acids and negatively correlated with levels of root organic acids. In conclusion, organic acid metabolism and expression of related genes increased under nitrate, whereas ammonium increased the level of amino acids and expression of related genes; these altered levels of organic acids and amino acids resulted in different tolerances to FOC infection depending on the nitrogen forms supplied.

ROS and Cell Death in Tomato Roots Infected by Meloidogyne Incognita

Phytoparasitic nematodes are plant pests causing serious problems to a broad range of hosts, and Meloidogyne species are widely recognized as the most damaging among the root knot nematode groups. During the incompatible interaction between avirulent pathogens and resistant tomato cultivars, juvenile nematode invasions provoke a defense cascade, culminating in hypersensitive responses. Methods to detect the key molecules involved in oxidative metabolism of the infected tomato roots are described here.

Visualizing the Movement of Magnaporthe oryzae Effector Proteins in Rice Cells During Infection

During biotrophy, filamentous pathogens such as the rice blast fungus Magnaporthe oryzae deliver effector proteins into live host cells to facilitate colonization. We describe three complementary assays for visualizing M. oryzae effector translocation into the rice cytoplasm and cell-to-cell movement during infection. Our assays make use of live-cell confocal imaging of optically clear rice sheath cells infected with transgenic strains of M. oryzae that express the fluorescent protein-tagged effector known as PWL2. We highlight several important considerations for the analysis of effector translocation and movement dynamics during infection of host plants.

Vacuole Dynamics in Rice Cells Invaded by the Blast Fungus Magnaporthe oryzae

We describe a fluorescence imaging method to visualize the dynamics of the central vacuole in rice cells during invasion by the blast fungus Magnaporthe oryzae. This method utilizes the combination of confocal microscopy, rice sheath cells (optically transparent), fluorescently tagged M. oryzae (red fluorescence), and fluorescein diacetate staining (green fluorescence; visualizing vacuole dynamics). Using this method, we demonstrate that the vacuole undergoes progressive shrinkage and collapse during M. oryzae infection.

Plant actin depolymerizing factor: actin microfilament disassembly and more

ACTIN DEPOLYMERIZING FACTOR (ADF) is a conserved protein among eukaryotes. The main function of ADF is the severing and depolymerizing filamentous actin (F-actin), thus regulating F-actin organization and dynamics and contributing to growth and development of the organisms. Mammalian genomes contain only a few ADF genes, whereas angiosperm plants have acquired an expanding number of ADFs, resulting in the differentiation of physiological functions. Recent studies have revealed functions of ADFs in plant growth and development, and various abiotic and biotic stress responses. In biotic stress responses, ADFs are involved in both susceptibility and resistance, depending on the pathogens. Furthermore, recent studies have highlighted a new role of ADF in the nucleus, possibly in the regulation of gene expression. In this review, I will summarize the current status of plant ADF research and discuss future research directions.

Time-resolved pathogenic gene expression analysis of the plant pathogen Xanthomonas oryzae pv. oryzae

Background

Plant-pathogen interactions at early stages of infection are important to the fate of interaction. Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight, which is a devastating disease in rice. Although in vivo and in vitro systems have been developed to study rice-Xoo interactions, both systems have limitations. The resistance mechanisms in rice can be better studied by the in vivo approach, whereas the in vitro systems are suitable for pathogenicity studies on Xoo. The current in vitro system uses minimal medium to activate the pathogenic signal (expression of pathogenicity-related genes) of Xoo, but lacks rice-derived factors needed for Xoo activation. This fact emphasizes the need of developing a new in vitro system that allow for an easy control of both pathogenic activation and for the experiment itself.

Results

We employed an in vitro system that can activate pathogenicity-related genes in Xoo using rice leaf extract (RLX) and combined the in vitro assay with RNA-Seq to analyze the time-resolved genome-wide gene expression of Xoo. RNA-Seq was performed with samples from seven different time points within 1 h post-RLX treatment and the expression of up- or downregulated genes in RNA-Seq was validated by qRT-PCR. Global analysis of gene expression and regulation revealed the most dramatic changes in functional categories of genes related to inorganic ion transport and metabolism, and cell motility. Expression of many pathogenicity-related genes was induced within 15 min upon contact with RLX. hrpG and hrpX expression reached the maximum level within 10 and 15 min, respectively. Chemotaxis and flagella biosynthesis-related genes and cyclic-di-GMP controlling genes were downregulated for 10 min and were then upregulated. Genes related to inorganic ion uptake were upregulated within 5 min. We introduced a non-linear regression fit to generate continuous time-resolved gene expression levels and tested the essentiality of the transcriptionally upregulated genes by a pathogenicity assay of lesion length using single-gene knock-out Xoo strains.

Conclusions

The in vitro system combined with RNA-Seq generated a genome-wide time-resolved pathogenic gene expression profile within 1 h of initial rice-Xoo interactions, demonstrating the expression order and interaction dependency of pathogenic genes. This combined system can be used as a novel tool to study the initial interactions between rice and Xoo during bacterial blight progression.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2657-7) contains supplementary material, which is available to authorized users.

Laser Microdissection of Grapevine Leaves Reveals Site-Specific Regulation of Transcriptional Response to Plasmopara viticola

Grapevine is one of the most important fruit crops in the world, and it is highly susceptible to downy mildew caused by the biotrophic oomycete Plasmopara viticola. Gene expression profiling has been used extensively to investigate the regulation processes of grapevine-P. viticola interaction, but all studies to date have involved the use of whole leaves. However, only a small fraction of host cells is in contact with the pathogen, so highly localized transcriptional changes of infected cells may be masked by the large portion of non-infected cells when analyzing the whole leaf. In order to understand the transcriptional regulation of the plant reaction at the sites of pathogen infection, we optimized a laser microdissection protocol and analyzed the transcriptional changes in stomata cells and surrounding areas of grapevine leaves at early stages of P. viticola infection. The results indicate that the expression levels of seven P. viticola-responsive genes were greater in microdissected cells than in whole leaves, highlighting the site-specific transcriptional regulation of the host response. The gene modulation was restricted to the stomata cells and to the surrounding areas of infected tissues, indicating that the host response is mainly located at the infection sites and that short-distance signals are implicated. In addition, due to the high sensitivity of the laser microdissection technique, significant modulations of three genes that were completely masked in the whole tissue analysis were detected. The protocol validated in this study could greatly increase the sensitivity of further transcriptomic studies of the grapevine-P. viticola interaction.

Salicylic acid is an indispensable component of the Ny-1 resistance-gene-mediated response against Potato virus Y infection in potato

The purpose of the study was to investigate the role of salicylic acid (SA) signalling in Ny-1-mediated hypersensitive resistance (HR) of potato (Solanum tuberosum L.) to Potato virus Y (PVY). The responses of the Ny-1 allele in the Rywal potato cultivar and transgenic NahG-Rywal potato plants that do not accumulate SA were characterized at the cytological, biochemical, transcriptome, and proteome levels. Analysis of noninoculated and inoculated leaves revealed that HR lesions started to develop from 3 d post inoculation and completely restricted the virus spread. At the cytological level, features of programmed cell death in combination with reactive oxygen species burst were observed. In response to PVY infection, SA was synthesized de novo. The lack of SA accumulation in the NahG plants led to the disease phenotype due to unrestricted viral spreading. Grafting experiments show that SA has a critical role in the inhibition of PVY spreading in parenchymal tissue, but not in vascular veins. The whole transcriptome analysis confirmed the central role of SA in orchestrating Ny-1-mediated responses and showed that the absence of SA leads to significant changes at the transcriptome level, including a delay in activation of expression of genes known to participate in defence responses. Moreover, perturbations in the expression of hormonal signalling genes were detected, shown as a switch from SA to jasmonic acid/ethylene signalling. Viral multiplication in the NahG plants was accompanied by downregulation of photosynthesis genes and activation of multiple energy-producing pathways.