Probing the contractile vacuole as Achilles' heel of the biotrophic grapevine pathogen Plasmopara viticola

Contractile vacuoles: a rapidly expanding (and occasionally diminishing?) understanding

Osmoregulation is the homeostatic mechanism essential for the survival of organisms in hypoosmotic and hyperosmotic conditions. In freshwater or soil dwelling protists this is frequently achieved through the action of an osmoregulatory organelle, the contractile vacuole. This endomembrane organelle responds to the osmotic challenges and compensates by collecting and expelling the excess water to maintain the cellular osmolarity. As compared with other endomembrane organelles, this organelle is underappreciated and under-studied. Here we review the reported presence or absence of contractile vacuoles across eukaryotic diversity, as well as the observed variability in the structure, function, and molecular machinery of this organelle. Our findings highlight the challenges and opportunities for constructing cellular and evolutionary models for this intriguing organelle.

Advances in understanding grapevine downy mildew: From pathogen infection to disease management

Plasmopara viticola is geographically widespread in grapevine-growing regions. Grapevine downy mildew disease, caused by this biotrophic pathogen, leads to considerable yield losses in viticulture annually. Because of the great significance of grapevine production and wine quality, research on this disease has been widely performed since its emergence in the 19th century. Here, we review and discuss recent understanding of this pathogen from multiple aspects, including its infection cycle, disease symptoms, genome decoding, effector biology, and management and control strategies. We highlight the identification and characterization of effector proteins with their biological roles in host-pathogen interaction, with a focus on sustainable control methods against P. viticola, especially the use of biocontrol agents and environmentally friendly compounds.

A conserved pressure-driven mechanism for regulating cytosolic osmolarity

Controlling intracellular osmolarity is essential to all cellular life. Cells that live in hypo-osmotic environments, such as freshwater, must constantly battle water influx to avoid swelling until they burst. Many eukaryotic cells use contractile vacuoles to collect excess water from the cytosol and pump it out of the cell. Although contractile vacuoles are essential to many species, including important pathogens, the mechanisms that control their dynamics remain unclear. To identify the basic principles governing contractile vacuole function, we investigate here the molecular mechanisms of two species with distinct vacuolar morphologies from different eukaryotic lineages: the discoban Naegleria gruberi and the amoebozoan slime mold Dictyostelium discoideum. Using quantitative cell biology, we find that although these species respond differently to osmotic challenges, they both use vacuolar-type proton pumps for filling contractile vacuoles and actin for osmoregulation, but not to power water expulsion. We also use analytical modeling to show that cytoplasmic pressure is sufficient to drive water out of contractile vacuoles in these species, similar to findings from the alveolate Paramecium multimicronucleatum. These analyses show that cytoplasmic pressure is sufficient to drive contractile vacuole emptying for a wide range of cellular pressures and vacuolar geometries. Because vacuolar-type proton-pump-dependent contractile vacuole filling and pressure-dependent emptying have now been validated in three eukaryotic lineages that diverged well over a billion years ago, we propose that this represents an ancient eukaryotic mechanism of osmoregulation.

Plant Resistance Inducers Affect Multiple Epidemiological Components of Plasmopara viticola on Grapevine Leaves

Plant resistance inducers (PRIs) harbor promising potential for use in downy mildew (DM) control in viticulture. Here, the effects of six commercial PRIs on some epidemiological components of Plasmopara viticola (Pv) on grapevine leaves were studied over 3 years. Disease severity, mycelial colonization of leaf tissue, sporulation severity, production of sporangia on affected leaves, and per unit of DM lesion were evaluated by inoculating the leaves of PRI-treated plants at 1, 3, 6, 12, and 19 days after treatment (DAT). Laminarin, potassium phosphonate (PHO), and fosetyl-aluminium (FOS) were the most effective in reducing disease severity as well as the Pv DNA concentration of DM lesions on leaves treated and inoculated at 1 and 3 DAT; PHO and FOS also showed long-lasting effects on leaves established after treatment (inoculations at 6 to 19 DAT). PRIs also prevented the sporulation of Pv on lesions; all the PRI-treated leaves produced fewer sporangia than the nontreated control, especially in PHO-, FOS-, and cerevisane-treated leaves (>75% reduction). These results illustrate the broader and longer effect of PRIs on DM epidemics. The findings open up new perspectives for using PRIs in a defense program based on single, timely, and preventative field interventions.

A conserved pressure-driven mechanism for regulating cytosolic osmolarity

Controlling intracellular osmolarity is essential to all cellular life. Cells that live in hypo-osmotic environments like freshwater must constantly battle water influx to avoid swelling until they burst. Many eukaryotic cells use contractile vacuoles to collect excess water from the cytosol and pump it out of the cell. Although contractile vacuoles are essential to many species, including important pathogens, the mechanisms that control their dynamics remain unclear. To identify basic principles governing contractile vacuole function, we here investigate the molecular mechanisms of two species with distinct vacuolar morphologies from different eukaryotic lineagesâ€"the discoban Naegleria gruberi , and the amoebozoan slime mold Dictyostelium discoideum . Using quantitative cell biology we find that, although these species respond differently to osmotic challenges, they both use actin for osmoregulation, as well as vacuolar-type proton pumps for filling contractile vacuoles. We also use analytical modeling to show that cytoplasmic pressure is sufficient to drive water out of contractile vacuoles in these species, similar to findings from the alveolate Paramecium multimicronucleatum . Because these three lineages diverged well over a billion years ago, we propose that this represents an ancient eukaryotic mechanism of osmoregulation.

Biocontrol of Plant Diseases Using Glycyrrhiza glabra Leaf Extract

The growing concern regarding the potential risks of pesticides and their impact on nontarget organisms stimulates the development and application of alternative, environmentally friendly products. It seems necessary to develop alternatives for conventional products and for those already widely used in organic agriculture, e.g., copper. Very importantly, such alternative products should not limit the productivity and profitability of agriculture. In this study, we examined the efficacy of licorice (Glycyrrhiza glabra) leaf extract as such an alternative. We tested its impact on the virulence of Pseudomonas syringae toward the model plant Arabidopsis thaliana and the crop plant tomato (Solanum lycopersicum) as well as of Clavibacter michiganensis, Xanthomonas campestris, and Phytophthora infestans toward tomato, at multiple levels. We demonstrate that licorice leaf extract acts as a direct fungicide and bactericide. Moreover, it acts against a metalaxyl-resistant P. infestans strain. In addition, the extract from licorice leaves influences the plant immune system, modulating the plant responses to the challenge with pathogen(s); this involves both salicylic acid and ethylene-based responses. Our results show that in addition to the well-known use of licorice root extract in medicine, the leaf extract can be an effective alternative in organic and integrated farming, contributing to copper reduction and resistance management.[Formula: see text] Copyright © 2022 The Author(s). This is an open-access article distributed under the CC BY 4.0 International license.

Aluminum can activate grapevine defense through actin remodeling

In the current study, we used a grapevine cell line in which actin filaments are labeled by GFP to show that aluminum causes actin remodeling through activation of NADPH oxidase in the plasma membrane, followed by activation of phytoalexin synthesis genes. Elimination of actin filaments by latrunculin B disrupts gene activation and inhibition of MAPK signaling by the inhibitor PD98059. Interestingly, aluminum also induces the transcription of ISOCHORISMATE SYNTHASE, a key enzyme for the synthesis of salicylic acid, as well as PR1, a gene that is known to be responsive to salicylic acid. However, while salicylic acid responses are usually a hallmark of the hypersensitive response, aluminum-triggered defense is not accompanied by cell death. Both actin remodeling and gene activation in response to aluminum can be suppressed by the natural auxin indole acetic acid, suggesting that the actin response is not caused by nonspecific signaling. Further evidence for the specificity of the aluminum-triggered activation of phytoalexin synthesis genes comes from experiments in which plant peptide elicitors induce significant cellular mortality but do not evoke induction of these transcription. The response in grapevine cells can be recapitulated in grapevine leaf discs from two genotypes contrasting in stilbene inducibility. Here, aluminum can induce accumulation of the central grapevine phytoalexin, the stilbene aglycone trans-resveratrol; this is preceded by a rapid induction of transcription for RESVERATROL SYNTHASE and the regulating transcription factor MYB14. The amplitude of this induction reflects the general stilbene inducibility of these genotypes, indicating that the aluminum effect is not caused by nonspecific toxicity but by activation of specific signaling pathways. The findings are discussed in relation to a model in which actin filaments activate a specific branch of defense signaling, acting in concert with calcium-dependent PAMP-triggered immunity. This pathway links the apoplastic oxidative burst through MAPK signaling with the activation of defense-related transcription.

Glycyrrhizin, the active compound of the TCM drug Gan Cao stimulates actin remodelling and defence in grapevine

Actin remodelling by a membrane-associated oxidative process can sense perturbations of membrane integrity and activate defence. In the current work, we show that glycyrrhizin, a muscle relaxant used in Traditional Chinese Medicine, can activate oxidative burst and actin remodelling in tobacco BY-2 cells, which could be suppressed by diphenylene iodonium, an inhibitor of NADPH oxidases. Glycyrrhizin caused a dose-dependent delay of proliferation, and induced cell death, which was suppressed by addition of indole-acetic acid, a natural auxin that can mitigate RboH dependent actin remodelling. To test, whether the actin remodelling induced by glycyrrhizin was followed by activation of defence, several events of basal immunity were probed. We found that glycyrrhizin induced a transient extracellular alkalinisation, indicative of calcium influx. Furthermore, transcripts of phytoalexins genes, were activated in cells of the grapevine Vitis rupestris, and this induction was followed by accumulation of the glycosylated stilbene α-piceid. We also observed that glycyrrhizin was able to induce actin bundling in leaves of a transgenic grape, especially in guard cells. We discuss these data in frame of a model, where glycyrrhizin, through stimulation of RboH, can cause actin remodelling, followed by defence responses, such as calcium influx, induction of phytoalexins transcripts, and accumulation of stilbene glycosides.

Kaolin Particle Film Protects Grapevine cv. Cabernet Sauvignon Against Downy Mildew by Forming Particle Film at the Leaf Surface, Directly Acting on Sporangia and Inducing the Defense of the Plant

Downy mildew is a major threat to viticulture, leading to severe yield loss. The use of traditional copper-based fungicides is effective, but has adverse effects on the environment and human health, making it urgent to develop an environmentally friendly disease management program. Multi-functional kaolin particle film (KPF) is promising as an effective and safer treatment strategy, since this material lacks chemically active ingredients. In this study, ability of Kaolin particle film (KPF) pretreatment to protect grapevine leaves from Plasmopara viticola was tested and the mode of action of KPF was analyzed. KPF application reduced the disease severity and the development of intercellular hyphae. Additionally, there was reduced accumulation of H₂O₂ and malondialdehyde (MDA) with pretreatment. The observation of ultrastructure on the leaf surface showed KPF deposition and stomatal obstruction, indicating that KPF protected plants against disease by preventing the adhesion of pathogens to the leaf surface and blocking invasion through the stomata. KPF pretreatment also activated host defense responses, as evidenced by increased activities of anti-oxidative enzymes [superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)] and defense-related enzymes [phenylalanine ammonia-lyase (PAL), chitinases, and β-1,3-glucanases], increased phytohormone signals [abscisic acid (ABA), salicylic acid (SA), and jasmonic acid (JA)] and the up-regulation of defense genes related to plant defense. Overall, these results demonstrate that KPF treatment counters grapevine downy mildew by protecting leaves and enhancing plant defense responses.

Activity and Mechanism of Action of the Bioceramic Silicon Nitride as an Environmentally Friendly Alternative for the Control of the Grapevine Downy Mildew Pathogen Plasmopara viticola

Downy mildew of grapevine, caused by Plasmopara viticola (Berk. and Curt.) Berl. and de Toni, is one of the most devastating diseases of grapevine, severely affecting grape and wine production and quality worldwide. Infections are usually controlled by the intensive application of synthetic fungicides or by copper-based products in organic farming, rising problems for soil contamination and adverse impacts on environment and human health. While strict regulations attempt to minimize their harmful consequences, the situation calls for the development of alternative fungicidal strategies. This study presents the unprecedented case of a bioceramic, silicon nitride, with antimicrobial properties against P. viticola, but without adverse effects on human cells and environment, opening the way to the possible extension of silicon nitride applications in agriculture. Raman spectroscopic assessments of treated sporangia in conjunction with microscopic observations mechanistically showed that the nitrogen-chemistry of the bioceramic surface affects pathogen's biochemical components and cell viability, thus presenting a high potential for host protection from P. viticola infections.

A Plant Extract Acts Both as a Resistance Inducer and an Oomycide Against Grapevine Downy Mildew

Protecting vineyards from cryptogamic diseases such as downy mildew, caused by Plasmopara viticola, generally requires a massive use of phytochemicals. However, the issues on unintentional secondary effects on environment and human health, and the occurrence of P. viticola resistant strains, are leading to the development of alternative strategies, such as the use of biocontrol products. In this paper, we evidenced the ability of a plant extract to protect grapevine from P. viticola. Further experiments carried out both on cell suspensions and on plants revealed that plant extract activates typical defense-related responses such as the production of H₂O₂, the up-regulation of genes encoding pathogenesis-related proteins and stilbene synthase, as well as the accumulation of resveratrol or its derivative piceid. We also brought to light a strong direct effect of PE on the release and motility of P. viticola zoospores. Furthermore, we found out that PE application left dried residues on leaf surface, impairing zoospores to reach stomata. Altogether, our results highlight the different modes of action of a new biocontrol product able to protect grapevine against downy mildew.