Christine Vos

What lies beneath: belowground defense strategies in plants

Diseases caused by soil-borne pathogens result worldwide in significant yield losses in economically important crops. In contrast to foliar diseases, relatively little is known about the nature of root defenses against these pathogens. This review summarizes the current knowledge on root infection strategies, root-specific preformed barriers, pathogen recognition, and defense signaling. Studies reviewed here suggest that many commonalities as well as differences exist in defense strategies employed by roots and foliar tissues during pathogen attack. Importantly, in addition to pathogens, plant roots interact with a plethora of non-pathogenic and symbiotic microorganisms. Therefore, a good understanding of how plant roots interact with the microbiome would be particularly important to engineer resistance to root pathogens without negatively altering root-beneficial microbe interactions.

RNAseq‐based transcriptome analysis of Lactuca sativa infected by the fungal necrotroph Botrytis cinerea

The fungal pathogen Botrytis cinerea establishes a necrotrophic interaction with its host plants, including lettuce (Lactuca sativa), causing it to wilt, collapse and eventually dry up and die, which results in serious economic losses. Global expression profiling using RNAseq and the newly sequenced lettuce genome identified a complex network of genes involved in the lettuce-B. cinerea interaction. The observed high number of differentially expressed genes allowed us to classify them according to the biological pathways in which they are implicated, generating a holistic picture. Most pronounced were the induction of the phenylpropanoid pathway and terpenoid biosynthesis, whereas photosynthesis was globally down-regulated at 48 h post-inoculation. Large-scale comparison with data available on the interaction of B. cinerea with the model plant Arabidopsis thaliana revealed both general and species-specific responses to infection with this pathogen. Surprisingly, expression analysis of selected genes could not detect significant systemic transcriptional alterations in lettuce leaves distant from the inoculation site. Additionally, we assessed the response of these lettuce genes to a biotrophic pathogen, Bremia lactucae, revealing that similar pathways are induced during compatible interactions of lettuce with necrotrophic and biotrophic pathogens.

Arbuscular mycorrhizal fungi reduce root-knot nematode penetration through altered root exudation of their host

AimsArbuscular mycorrhizal fungi (AMF) can control root-knot nematode infection, but the mode of action is still unknown. We investigated the effects of AMF and mycorrhizal root exudates on the initial steps of Meloidogyne incognita infection, namely movement towards and penetration of tomato roots.MethodsM. incognita soil migration and root penetration were evaluated in a twin-chamber set-up consisting of a control and mycorrhizal (Glomus mosseae) plant compartment (Solanum lycopersicum cv. Marmande) connected by a bridge. Penetration into control and mycorrhizal roots was also assessed when non-mycorrhizal or mycorrhizal root exudates were applied and nematode motility in the presence of the root exudates was tested in vitro.ResultsM. incognita penetration was significantly reduced in mycorrhizal roots compared to control roots. In the twin-chamber set-up, equal numbers of nematodes moved to both compartments, but the majority accumulated in the soil of the mycorrhizal plant compartment, while for the control plants the majority penetrated the roots. Application of mycorrhizal root exudates further reduced nematode penetration in mycorrhizal plants and temporarily paralyzed nematodes, compared with application of water or non-mycorrhizal root exudates.ConclusionsNematode penetration was reduced in mycorrhizal tomato roots and mycorrhizal root exudates probably contributed at least partially by affecting nematode motility.

Arbuscular Mycorrhizal Fungi for the Biocontrol of Plant-Parasitic Nematodes: A Review of the Mechanisms Involved

Arbuscular mycorrhizal fungi (AMF) are obligate root symbionts that can protect their host plant against biotic stress factors such as plant-parasitic nematode (PPN) infection. PPN consist of a wide range of species with different life styles that can cause major damage in many important crops worldwide. Various mechanisms have been proposed to play a role in the biocontrol effect of AMF against PPN. This review presents an overview of the different mechanisms that have been proposed, and discusses into more detail the plausibility of their involvement in the biocontrol against PPN specifically. The proposed mechanisms include enhanced plant tolerance, direct competition for nutrients and space, induced systemic resistance (ISR) and altered rhizosphere interactions. Recent studies have emphasized the importance of ISR in biocontrol and are increasingly placing rhizosphere effects on the foreground as well, both of which will be the focal point of this review. Though AMF are not yet widely used in conventional agriculture, recent data help to develop a better insight into the modes of action, which will eventually lead toward future field applications of AMF against PPN. The scientific community has entered an exciting era that provides the tools to actually unravel the underlying molecular mechanisms, making this a timely opportunity for a review of our current knowledge and the challenges ahead.

Mining the genome of Arabidopsis thaliana as a basis for the identification of novel bioactive peptides involved in oxidative stress tolerance

Although evidence has accumulated on the role of plant peptides in the response to external conditions, the number of peptide-encoding genes in the genome is still underestimated. Using tiling arrays, we identified 176 unannotated transcriptionally active regions (TARs) in Arabidopsis thaliana that were induced upon oxidative stress generated by the herbicide paraquat (PQ). These 176 TARs could be translated into 575 putative oxidative stress-induced peptides (OSIPs). A high-throughput functional assay was used in the eukaryotic model organism Saccharomyces cerevisiae allowing us to test for bioactive peptides that increase oxidative stress tolerance. In this way, we identified three OSIPs that, upon overexpression in yeast, resulted in a significant rise in tolerance to hydrogen peroxide (H2O2). For one of these peptides, the decapeptide OSIP108, exogenous application to H2O2-treated yeast also resulted in significantly increased survival. OSIP108 is contained within a pseudogene and is induced in A. thaliana leaves by both the reactive oxygen species-inducer PQ and the necrotrophic fungal pathogen Botrytis cinerea. Moreover, infiltration and overexpression of OSIP108 in A. thaliana leaves resulted in increased tolerance to treatment with PQ. In conclusion, the identification and characterization of OSIP108 confirms the validity of our high-throughput approach, based on tiling array analysis in A. thaliana and functional screening in yeast, to identify bioactive peptides.

The toolbox of Trichoderma spp. in the biocontrol of Botrytis cinerea disease

Botrytis cinerea is a necrotrophic fungal pathogen causing disease in many plant species, leading to economically important crop losses. So far, fungicides have been widely used to control this pathogen. However, in addition to their detrimental effects on the environment and potential risks for human health, increasing fungicide resistance has been observed in the B. cinerea population. Biological control, that is the application of microbial organisms to reduce disease, has gained importance as an alternative or complementary approach to fungicides. In this respect, the genus Trichoderma constitutes a promising pool of organisms with potential for B. cinerea control. In the first part of this article, we review the specific mechanisms involved in the direct interaction between the two fungi, including mycoparasitism, the production of antimicrobial compounds and enzymes (collectively called antagonism), and competition for nutrients and space. In addition, biocontrol has also been observed when Trichoderma is physically separated from the pathogen, thus implying an indirect systemic plant defence response. Therefore, in the second part, we describe the consecutive steps leading to induced systemic resistance (ISR), starting with the initial Trichoderma-plant interaction and followed by the activation of downstream signal transduction pathways and, ultimately, the defence response resulting in ISR (ISR-prime phase). Finally, we discuss the ISR-boost phase, representing the effect of ISR priming by Trichoderma spp. on plant responses after additional challenge with B. cinerea.

Belowground Defence Strategies in Plants

Plant roots have long been literally and figuratively hidden from sight, despite their unmistakable importance in a plant’s life. Interactions between plant roots and soil microbes indeed seem to take place in a black box, but science is starting to shed some light into this box. This book aims to bring together our current knowledge on the belowground interactions of plant roots with both detrimental and beneficial microbes. This knowledge can form the basis for more environmentally friendly plant disease management of soil-borne pathogens and pests, and the book will be of interest to both plant scientists and students eager to discover the hidden part of a plant’s daily life and survival. Plants are multicellular photosynthetic organisms that have evolved from unicellular fresh water green algae. During their evolution, plants have acquired diverse capabilities that enabled them not only to survive but also to adapt and successfully colonize diverse land environments. In particular, the acquisition of roots or rootlike structures that facilitate extracting water from soil rather than relying on limited amounts of moisture available on the soil surface has no doubt played an important role in plant’s adaptation to life on land. Obviously, roots are also essential for physical attachment of plants to the soil, as well as for nutrient uptake and interaction with soil biota. Plant roots continuously C.M.F. Vos (*) Centre of Microbial and Plant Genetics, KU Leuven, Kasteelpark Arenberg 20, 3001 Leuven, Belgium Department of Plant Systems Biology, Vlaams Instituut voor Biotechnologie (VIB), Technologie park 927, 9052 Ghent, Belgium Commonwealth Scientific and Industrial Research Organisation (CSIRO) Agriculture, 306 Carmody Road, St Lucia, Brisbane, QLD 4067, Australia Scientia Terrae Research Institute, Fortsesteenweg 30A, Sint-Katelijne-Waver, Belgium e-mail: cvo@scientiaterrae.org K. Kazan CSIRO Agriculture St Lucia, 306 Carmody Road, St Lucia, QLD 4067, Australia The Queensland Alliance for Agriculture & Food Innovation (QAAFI), Queensland Bioscience Precinct, University of Queensland, Brisbane, QLD 4072, Australia

Belowground Defence Strategies in Plants: The Plant– Trichoderma Dialogue

Trichoderma spp. are cosmopolitan soil fungi that hold great promise as biocontrol organisms. Their biocontrol capacity was initially thought to be based on their direct suppressive effects on plant pathogens, with most strains showing mycoparasitic potential and producing a large variety of enzymes and secondary metabolites. More recently however Trichoderma was also recognized as an opportunistic plant root colonizer that can trigger induced systemic resistance (ISR) in the plant, typically leading to a more rapid and robust systemic activation of defences after pathogen attack. As our understanding of the Trichoderma–plant interaction advances, it is becoming increasingly clear that Trichoderma is initially also perceived by the host plant as a potential invader. Trichoderma thus needs to find a way to deal with the plant defence response, either by avoiding or suppressing it, in order to establish a durable interaction with their host. In this chapter, we cover our current knowledge on the initial dialogue between Trichoderma and its host, including the defence responses mounted by the host plant and how Trichoderma attempts to circumvent it. Next, we describe how the host plant can benefit from this interaction. Trichoderma colonization can indeed prime the host defence, enabling it to react faster and stronger to subsequent pathogen attack. We then conclude with examples of Trichoderma-induced resistance and direct antagonism against different types of soil pathogens and pests.

Rhizophagus irregularis MUCL 41833 decreases the reproduction ratio of Radopholus similis in the banana cultivar Yangambi km5

Bananas are susceptible to various plant-parasitic nematodes, including Radopholus similis. The control of this migratory endoparasite relies on the application of nematicides as well as on measures such as fallow, paring and hot water treatment of the corms, the use of resistant cultivars and the large-scale micropropagation of plantlets (Quénéhervé, 2009). The application of biocontrol organisms is another option considered nowadays as a potential alternative to decrease the damage caused by nematodes (FAO, 1997). Within the rhizosphere, arbuscular mycorrhizal fungi (AMF) are key microorganisms that form symbiotic associations with nearly 80% of plant species, including bananas. They improve plant nutrition and have also been reported to decrease nematode infestation. Elsen et al. (2001) demonstrated that Rhizophagus irregularis MUCL 41833 was able to decrease the reproduction capacity of R. similis in excised root organs of carrot (i.e., in root organ culture). More recently, Koffi et al. (2009) developed an in vitro culture system associating autotrophic micropropagated banana plantlets with R. irregularis MUCL 41833. With a closely related in vitro cultivation system, Koffi et al. (2012) were the first to investigate the impact of R. irregularis MUCL 41833 on the resistance of cv. Grande Naine, a banana cultivar particularly sensitive to R. similis. They observed a decrease of 60 and 56% in the nematode population and surface of

Fungal Glucosylceramide-Specific Camelid Single Domain Antibodies Are Characterized by Broad Spectrum Antifungal Activity

Chemical crop protection is widely used to control plant diseases. However, the adverse effects of pesticide use on human health and environment, resistance development and the impact of regulatory requirements on the crop protection market urges the agrochemical industry to explore innovative and alternative approaches. In that context, we demonstrate here the potential of camelid single domain antibodies (VHHs) generated against fungal glucosylceramides (fGlcCer), important pathogenicity factors. To this end, llamas were immunized with purified fGlcCer and a mixture of mycelium and spores of the fungus Botrytis cinerea, one of the most important plant pathogenic fungi. The llama immune repertoire was subsequently cloned in a phage display vector to generate a library with a diversity of at least 108 different clones. This library was incubated with fGlcCer to identify phages that bind to fGlcCer, and VHHs that specifically bound fGlcCer but not mammalian or plant-derived GlcCer were selected. They were shown to inhibit the growth of B. cinerea in vitro, with VHH 41D01 having the highest antifungal activity. Moreover, VHH 41D01 could reduce disease symptoms induced by B. cinerea when sprayed on tomato leaves. Based on all these data, anti-fGlcCer VHHs show the potential to be used as an alternative approach to combat fungal plant diseases.