3-D imaging of temporal and spatial development of Puccinia striiformis haustoria in wheat

Beneficial Microorganisms as Bioprotectants against Foliar Diseases of Cereals: A Review

Cereal production plays a major role in both animal and human diets throughout the world. However, cereal crops are vulnerable to attacks by fungal pathogens on the foliage, disrupting their biological cycle and photosynthesis, which can reduce yields by 15-20% or even 60%. Consumers are concerned about the excessive use of synthetic pesticides given their harmful effects on human health and the environment. As a result, the search for alternative solutions to protect crops has attracted the interest of scientists around the world. Among these solutions, biological control using beneficial microorganisms has taken on considerable importance, and several biological control agents (BCAs) have been studied, including species belonging to the genera Bacillus, Pseudomonas, Streptomyces, Trichoderma, Cladosporium, and Epicoccum, most of which include plants of growth-promoting rhizobacteria (PGPRs). Bacillus has proved to be a broad-spectrum agent against these leaf cereal diseases. Interaction between plant and beneficial agents occurs as direct mycoparasitism or hyperparasitism by a mixed pathway via the secretion of lytic enzymes, growth enzymes, and antibiotics, or by an indirect interaction involving competition for nutrients or space and the induction of host resistance (systemic acquired resistance (SAR) or induced systemic resistance (ISR) pathway). We mainly demonstrate the role of BCAs in the defense against fungal diseases of cereal leaves. To enhance a solution-based crop protection approach, it is also important to understand the mechanism of action of BCAs/molecules/plants. Research in the field of preventing cereal diseases is still ongoing.

Double-stranded RNA prevents and cures infection by rust fungi

Fungal pathogens that impact perennial plants or natural ecosystems require management strategies beyond fungicides and breeding for resistance. Rust fungi, some of the most economically and environmentally important plant pathogens, have shown amenability to double-stranded RNA (dsRNA) mediated control. To date, dsRNA treatments have been applied prior to infection or together with the inoculum. Here we show that a dsRNA spray can effectively prevent and cure infection by Austropuccinia psidii (cause of myrtle rust) at different stages of the disease cycle. Significant reductions in disease coverage were observed in plants treated with dsRNA targeting essential fungal genes 48 h pre-infection through to 14 days post-infection. For curative treatments, improvements in plant health and photosynthetic capacity were seen 2-6 weeks post-infection. Two-photon microscopy suggests inhibitory activity of dsRNA on intercellular hyphae or haustoria. Our results show that dsRNA acts both preventively and curatively against myrtle rust disease, with treated plants recovering from severe infection. These findings have immediate potential in the management of the more than 10-year epidemic of myrtle rust in Australia.

Pathogenesis of Plasmopara viticola Depending on Resistance Mediated by Rpv3_1, and Rpv10 and Rpv3_3, and by the Vitality of Leaf Tissue

Grapevine cultivars vary in their resistance to Plasmopara viticola, causal agent of downy mildew. Genes from various Vitis species confer pathogen resistance (Rpv), resulting in reduced compatibility of the host-pathogen interaction and partial disease resistance that may become apparent at different stages of pathogenesis. This study describes the pathogenesis of P. viticola on the partially resistant cultivars Regent (Rpv3-1) and Solaris (Rpv3-3, Rpv10) as compared with the susceptible cultivar Mueller-Thurgau using various microscopic techniques, visual disease rating as well as qPCR. Host plant resistance had no effect on the initial steps of pathogenesis outside the host plant cells (zoospore attachment, formation of substomatal vesicle) and became detectable only after the formation of primary haustoria. The restricted compatibility resulted in reductions in haustorium size and in the number of secondary haustoria and was associated with callose depositions around haustoria and stomatal guard cells, collapsed mesophyll cells (hypersensitive reaction), and additional production of an amorphous substance in the intercellular space of cultivar Solaris. Resistance mechanisms reduced the efficiency of P. viticola haustoria and largely restricted tissue colonization to the spongy parenchyma; resistance of cultivar Solaris having thicker leaves was more effective than that of cultivar Regent. Despite of the effects of resistance genes, P. viticola was able to complete its life cycle by forming sporangiophores with sporangia through the stomata on both resistant cultivars indicating partial resistance. Differences in the pathogenesis on detached and attached grapevine leaves highlighted the impact of host tissue vitality on both resistance and susceptibility to the biotrophic pathogen.

Use of red, far-red, and near-infrared light in imaging of yeasts and filamentous fungi

Abstract

While phototoxicity can be a useful therapeutic modality not only for eliminating malignant cells but also in treating fungal infections, mycologists aiming to observe morphological changes or molecular events in fungi, especially when long observation periods or high light fluxes are warranted, encounter problems owed to altered regulatory pathways or even cell death caused by various photosensing mechanisms. Consequently, the ever expanding repertoire of visible fluorescent protein toolboxes and high-resolution microscopy methods designed to investigate fungi in vitro and in vivo need to comply with an additional requirement: to decrease the unwanted side effects of illumination. In addition to optimizing exposure, an obvious solution is red-shifted illumination, which, however, does not come without compromises. This review summarizes the interactions of fungi with light and the various molecular biology and technology approaches developed for exploring their functions on the molecular, cellular, and in vivo microscopic levels, and outlines the progress towards reducing phototoxicity through applying far-red and near-infrared light.

Key points

• Fungal biological processes alter upon illumination, also under the microscope

• Red shifted fluorescent protein toolboxes decrease interference by illumination

• Innovations like two-photon, lightsheet, and near IR microscopy reduce phototoxicity

Temporal and Spatial Variability of Fungal Structures and Host Responses in an Incompatible Rust-Wheat Interaction

Information about temporal and spatial variability of fungal structures and host responses is scarce in comparison to the vast amount of genetic, biochemical, and physiological studies of host-pathogen interactions. In this study, we used avirulent wild type and virulent mutant isolates of Puccinia striiformis to characterize the interactions in wheat carrying yellow rust Yr2 resistance. Both conventional and advanced microscopic techniques were used for a detailed study of morphology and growth of fungal colonies and associated host cell responses. The growth of the wild type isolates was highly restricted due to hypersensitive response (HR, plant cell death) indicated by autofluorescence and change in the shape of the affected plant cells. The host response appeared post-haustorial, but large variation in the time and stage of arrest was observed for individual fungal colonies, probably due to a delay between detection and response. Some colonies were stopped right after the formation of the primary infection hyphae whereas others formed highly branched mycelia. HR was first observed in host cells in direct contact with fungal structures, after which the defense responses spread to adjacent host cells, and eventually led to encasement of the fungal colony. Several cells with HR contained haustoria, which were small and underdeveloped, but some cells contained normal sized haustoria without signs of hypersensitivity. The growth of the virulent mutants in the resistant plants was similar to the growth in plants without Yr2 resistance, which is a strong indication that the incompatible phenotype was associated with Yr2. The interaction between P. striiformis and wheat with Yr2 resistance was highly variable in time and space, which demonstrate that histological studies are important for a deeper understanding of host-pathogen interactions and plant defense mechanisms in general.

Semiautomated confocal imaging of fungal pathogenesis on plants: Microscopic analysis of macroscopic specimens

The study of phenotypic variation in plant pathogenesis provides fundamental information about the nature of disease resistance. Cellular mechanisms that alter pathogenesis can be elucidated with confocal microscopy; however, systematic phenotyping platforms-from sample processing to image analysis-to investigate this do not exist. We have developed a platform for 3D phenotyping of cellular features underlying variation in disease development by fluorescence-specific resolution of host and pathogen interactions across time (4D). A confocal microscopy phenotyping platform compatible with different maize-fungal pathosystems (fungi: Setosphaeria turcica, Cochliobolus heterostrophus, and Cercospora zeae-maydis) was developed. Protocols and techniques were standardized for sample fixation, optical clearing, species-specific combinatorial fluorescence staining, multisample imaging, and image processing for investigation at the macroscale. The sample preparation methods presented here overcome challenges to fluorescence imaging such as specimen thickness and topography as well as physiological characteristics of the samples such as tissue autofluorescence and presence of cuticle. The resulting imaging techniques provide interesting qualitative and quantitative information not possible with conventional light or electron 2D imaging. Microsc. Res. Tech., 81:141-152, 2018. © 2016 Wiley Periodicals, Inc.

Small RNAs from the wheat stripe rust fungus (Puccinia striiformis f.sp. tritici)

BACKGROUND

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici, is a costly global disease that burdens farmers with yield loss and high fungicide expenses. This sophisticated biotrophic parasite infiltrates wheat leaves and develops infection structures inside host cells, appropriating nutrients while suppressing the plant defense response. Development in most eukaryotes is regulated by small RNA molecules, and the success of host-induced gene silencing technology in Puccinia spp. implies the existence of a functional RNAi system. However, some fungi lack this capability, and small RNAs have not yet been reported in rust fungi. The objective of this study was to determine whether P. striiformis carries an endogenous small RNA repertoire.

RESULTS

We extracted small RNA from rust-infected wheat flag leaves and performed high-throughput sequencing. Two wheat cultivars were analyzed: one is susceptible; the other displays partial high-temperature adult plant resistance. Fungal-specific reads were identified by mapping to the P. striiformis draft genome and removing reads present in uninfected control libraries. Sequencing and bioinformatics results were verified by RT-PCR. Like other RNAi-equipped fungi, P. striiformis produces large numbers of 20-22 nt sequences with a preference for uracil at the 5' position. Precise post-transcriptional processing and high accumulation of specific sRNA sequences were observed. Some predicted sRNA precursors possess a microRNA-like stem-loop secondary structure; others originate from much longer inverted repeats containing gene sequences. Finally, sRNA-target prediction algorithms were used to obtain a list of putative gene targets in both organisms. Predicted fungal target genes were enriched for kinases and small secreted proteins, while the list of wheat targets included homologs of known plant resistance genes.

CONCLUSIONS

This work provides an inventory of small RNAs endogenous to an important plant pathogen, enabling further exploration of gene regulation on both sides of the host/parasite interaction. We conclude that small RNAs are likely to play a role in regulating the complex developmental processes involved in stripe rust pathogenicity.

Combining microtomy and confocal laser scanning microscopy for structural analyses of plant-fungus associations

The serious problem of extended tissue thickness in the analysis of plant-fungus associations was overcome using a new method that combines physical and optical sectioning of the resin-embedded sample by microtomy and confocal microscopy. Improved tissue infiltration of the fungal-specific, high molecular weight fluorescent probe wheat germ agglutinin conjugated to Alexa Fluor® 633 resulted in high fungus-specific fluorescence even in deeper tissue sections. If autofluorescence was insufficient, additional counterstaining with Calcofluor White M2R or propidium iodide was applied in order to visualise the host plant tissues. Alternatively, the non-specific fluorochrome acid fuchsine was used for rapid staining of both, the plant and the fungal cells. The intricate spatial arrangements of the plant and fungal cells were preserved by immobilization in the hydrophilic resin Unicryl™. Microtomy was used to section the resin-embedded roots or leaves until the desired plane was reached. The data sets generated by confocal laser scanning microscopy of the remaining resin stubs allowed the precise spatial reconstruction of complex structures in the plant-fungus associations of interest. This approach was successfully tested on tissues from ectomycorrhiza (Betula pendula), arbuscular mycorrhiza (Galium aparine; Polygala paniculata, Polygala rupestris), ericoid mycorrhiza (Calluna vulgaris), orchid mycorrhiza (Limodorum abortivum, Serapias parviflora) and on one leaf-fungus association (Zymoseptoria tritici on Triticum aestivum). The method provides an efficient visualisation protocol applicable with a wide range of plant-fungus symbioses.

Filamentous plant pathogen effectors in action

Live-cell imaging assisted by fluorescent markers has been fundamental to understanding the focused secretory 'warfare' that occurs between plants and biotrophic pathogens that feed on living plant cells. Pathogens succeed through the spatiotemporal deployment of a remarkably diverse range of effector proteins to control plant defences and cellular processes. Some effectors can be secreted by appressoria even before host penetration, many enter living plant cells where they target diverse subcellular compartments and others move into neighbouring cells to prepare them before invasion. This Review summarizes the latest advances in our understanding of the cell biology of biotrophic interactions between plants and their eukaryotic filamentous pathogens based on in planta analyses of effectors.