Extracellular vesicles as key mediators of plant-microbe interactions

The Trojan Horse of the Plant Kingdom

The advent of host-induced gene silencing (HIGS) technology for the development of pathogen-resistant cultivars led to the discovery of cross-kingdom RNA interference. In a recent Science paper, Cai et al. (2018) discovered that plant extracellular vesicles act as Trojan horses to deliver small RNAs into fungi to fight infection.

Biological properties of plant-derived extracellular vesicles

Identification of active constituents of our diet is crucial to understand the impact of food on health, and disease development, and for the formulation of functional food and nutraceuticals. Until now research into the pharmacological properties of the components of our diet has focused on vitamins, sterols, polyphenols, fiber, etc. But very recently, it has been found that plants contain various types of vesicles which are in contact with the intestinal tract throughout our lives. They participate in intestinal tissue renewal processes and modulate gut microbiota in healthy subjects and have important biological functions against inflammatory diseases (e.g.; colitis injury, liver steatosis) or cancers associated with their specific lipid and miRNA content. In addition, recent data have suggested that plant-derived nanovesicles would be excellent candidates for the delivery of therapeutic agents (e.g.; anti-cancerous drugs, siRNAs) or poorly soluble natural compounds (e.g.; curcumin), as they are able to cross mammalian barriers without inducing either an inflammatory response or necrosis, conversely to conventional liposomes. It is thus important to consider these plant-derived vesicles as new components of our food in order to evaluate their potential for health benefit and food-derived technology.

Cross-Kingdom Small RNAs Among Animals, Plants and Microbes

Small RNAs (sRNAs), a class of regulatory non-coding RNAs around 20~30-nt long, including small interfering RNAs (siRNAs) and microRNAs (miRNAs), are critical regulators of gene expression. Recently, accumulating evidence indicates that sRNAs can be transferred not only within cells and tissues of individual organisms, but also across different eukaryotic species, serving as a bond connecting the animal, plant, and microbial worlds. In this review, we summarize the results from recent studies on cross-kingdom sRNA communication. We not only review the horizontal transfer of sRNAs among animals, plants and microbes, but also discuss the mechanism of RNA interference (RNAi) signal transmission via cross-kingdom sRNAs. We also compare the advantages of host-induced gene silencing (HIGS) and spray-induced gene silencing (SIGS) technology and look forward to their applicable prospects in controlling fungal diseases.

Extracellular Vesicles From the Cotton Pathogen Fusarium oxysporum f. sp. vasinfectum Induce a Phytotoxic Response in Plants

Extracellular vesicles (EVs) represent a system for the coordinated secretion of a variety of molecular cargo including proteins, lipids, nucleic acids, and metabolites. They have an essential role in intercellular communication in multicellular organisms and have more recently been implicated in host-pathogen interactions. Study of the role for EVs in fungal biology has focused on pathogenic yeasts that are major pathogens in humans. In this study we have expanded the investigation of fungal EVs to plant pathogens, specifically the major cotton pathogen Fusarium oxysporum f. sp. vasinfectum. EVs isolated from F. oxysporum f. sp. vasinfectum culture medium have a morphology and size distribution similar to EVs from yeasts such as Candida albicans and Cryptococcus neoformans. A unique feature of the EVs from F. oxysporum f. sp. vasinfectum is their purple color, which is predicted to arise from a napthoquinone pigment being packaged into the EVs. Proteomic analysis of F. oxysporum f. sp. vasinfectum EVs revealed that they are enriched in proteins that function in synthesis of polyketides as well as proteases and proteins that function in basic cellular processes. Infiltration of F. oxysporum f. sp. vasinfectum EVs into the leaves of cotton or N. benthamiana plants led to a phytotoxic response. These observations lead to the hypothesis that F. oxysporum f. sp. vasinfectum EVs are likely to play a crucial role in the infection process.

Tethering of Multi-Vesicular Bodies and the Tonoplast to the Plasma Membrane in Plants

Tethering of the plasma membrane (PM) and many organelles to the endoplasmic reticulum (ER) for communication and lipid exchange has been widely reported. However, despite growing interest in multi-vesicular bodies (MVBs) as possible sources of exosomes, tethering of MVBs to the PM has not been reported. Here we show that MVBs and the vacuolar membrane (tonoplast) could be tethered to the PM (PM-MVB/TP tethering) by artificial protein fusions or bimolecular fluorescence complementation (BiFC) complexes that contain a peripheral membrane protein that binds the PM and also a protein that binds MVBs or the tonoplast. PM-binding proteins capable of participating in PM-MVB/TP tethering included StRem1.3, BIK1, PBS1, CPK21, and the PtdIns(4)-binding proteins FAPP1 and Osh2. MVB/TP-binding proteins capable of participating in tethering included ARA6, ARA7, RHA1, RABG3f, and the PtdIns(3)P-binding proteins Vam7p and Hrs-2xFYVE. BiFC complexes or protein fusions capable of producing PM-MVB/TP tethering were visualized as large well-defined patches of fluorescence on the PM that could displace PM proteins such as AtFlotillin1 and also could displace cytoplasmic proteins such as soluble GFP. Furthermore, we identified paralogous ubiquitin E3 ligase proteins, SAUL1 (AtPUB44), and AtPUB43 that could produce PM-MVB/TP tethering. SAUL1 and AtPUB43 could produce tethering in uninfected tissue when paired with MVB-binding proteins or when their E3 ligase domain was deleted. When Nicotiana benthamiana leaf tissue was infected with Phytophthora capsici, full length SAUL1 and AtPUB43 localized in membrane patches consistent with PM-MVB/TP tethering. Our findings define new tools for studying PM-MVB/TP tethering and its possible role in plant defense.

SIGNIFICANCE STATEMENT

Although not previously observed, the tethering of multi-vesicular bodies to the plasma membrane is of interest due to the potential role of this process in producing exosomes in plants. Here we describe tools for observing and manipulating the tethering of multi-vesicular bodies and the tonoplast to the plant plasma membrane, and describe two plant proteins that may naturally regulate this process during infection.

Planting the Microbiome

Plant-derived microRNAs stabilized by species-specific lipid nanoparticles mediate interkingdom communication through bacterial intermediates and impact consumer health. Ingested by distinct gut bacteria, these microRNA-containing particles alter bacterial gene expression to affect host immunity. This three-kingdom interplay provides compelling approaches for health-directed dietary interventions for consumers.

Role of microRNA (miRNA) and Viroids in Lethal Diseases of Plants and Animals. Potential Contribution to Human Neurodegenerative Disorders

Both plants and animals have adopted a common strategy of using ~18-25-nucleotide small non-coding RNAs (sncRNAs), known as microRNAs (miRNAs), to transmit DNA-based epigenetic information. miRNAs (i) shape the total transcriptional output of individual cells; (ii) regulate and fine-tune gene expression profiles of cell clusters, and (iii) modulate cell phenotype in response to environmental stimuli and stressors. These miRNAs, the smallest known carriers of gene-encoded post-transcriptional regulatory information, not only regulate cellular function in healthy cells but also act as important mediators in the development of plant and animal diseases. Plants possess their own specific miRNAs; at least 32 plant species have been found to carry infectious sncRNAs called viroids, whose mechanisms of generation and functions are strikingly similar to those of miRNAs. This review highlights recent remarkable and sometimes controversial findings in miRNA signaling in plants and animals. Special attention is given to the intriguing possibility that dietary miRNAs and/or sncRNAs can function as mobile epigenetic and/or evolutionary linkers between different species and contribute to both intra- and interkingdom signaling. Wherever possible, emphasis has been placed on the relevance of these miRNAs to the development of human neurodegenerative diseases, such as Alzheimer's disease. Based on the current available data, we suggest that such xeno-miRNAs may (i) contribute to the beneficial properties of medicinal plants, (ii) contribute to the negative properties of disease-causing or poisonous plants, and (iii) provide cross-species communication between kingdoms of living organisms involving multiple epigenetic and/or potentially pathogenic mechanisms associated with the onset and pathogenesis of various diseases.

Plant lipids: Key players of plasma membrane organization and function

The plasma membrane (PM) is the biological membrane that separates the interior of all cells from the outside. The PM is constituted of a huge diversity of proteins and lipids. In this review, we will update the diversity of molecular species of lipids found in plant PM. We will further discuss how lipids govern global properties of the plant PM, explaining that plant lipids are unevenly distributed and are able to organize PM in domains. From that observation, it emerges a complex picture showing a spatial and multiscale segregation of PM components. Finally, we will discuss how lipids are key players in the function of PM in plants, with a particular focus on plant-microbe interaction, transport and hormone signaling, abiotic stress responses, plasmodesmata function. The last chapter is dedicated to the methods that the plant membrane biology community needs to develop to get a comprehensive understanding of membrane organization in plants.

Small RNA trafficking at the forefront of plant-pathogen interactions

Plants and pathogenic microbes are engaged in constant attacks and counterattacks at the interface of the interacting organisms. Much of the molecular warfare involves cross-kingdom trafficking of proteins, nucleic acids, lipids, and metabolites that act as toxins, inhibitors, lytic enzymes, and signaling molecules. How various molecules are transported across the boundaries of plants and pathogens has remained largely unknown until now. Extracellular vesicles have emerged as likely carriers of molecular ammunition for both plants and pathogens. Recent advances are beginning to show how extracellular vesicles serve as powerful vehicles that transfer small RNAs from plants to fungal cells to diminish pathogen virulence and from fungi to plant cells to dampen host immunity.

In Search of Darwin's Imaginary Gemmules

Darwin's gemmules were supposed to be "thrown off" by cells and were "inconceivably minute and numerous as the stars in heaven." They were capable of self-propagation and diffusion from cell to cell, and circulation through the system. The word "gene" coined by Wilhelm Johannsen, was derived from de Vries's term "pangen," itself a substitute for "gemmule" in Darwin's Pangenesis. Johannsen resisted the "morphological" conception of genes as particles with a certain structure. Morgan's genes were considered to be stable entities arranged in an orderly linear pattern on chromosomes, like beads on a string. In the late 1940s, McClintock challenged the concept of the stability of the gene when she discovered that some genes could move within a chromosome and between chromosomes. In 1948, Mandel and Metais reported the presence of cell-free nucleic acids in human blood for the first time. Over the past several decades, it has been universally accepted that almost all types of cells not only shed molecules such as cell-free DNA (including genomic DNA, tumor DNA and fetal DNA), RNAs (including mRNA and small RNAs) and prions, but also release into the extracellular environment diverse types of membrane vesicles (known as extracellular vesicles) containing DNA, RNA and proteins. Thus Darwin's speculative gemmules of the 19th century have become the experimentally demonstrated circulating cell-free DNA, mobile RNAs, prions and extracellular vesicles.

Biogenesis and Function of Multivesicular Bodies in Plant Immunity

Multivesicular bodies (MVBs) are specialized endosomes that contain intraluminal vesicles generated from invagination and budding of the limiting membrane. In the endocytic pathway, MVBs are late endosomes whose content can be degraded through fusion with lysosomes/vacuoles or released into the extracellular space after fusion with the plasma membrane (PM). The proteins retained on the limiting membrane of MVBs are translocated to the membrane of lysosomes/vacuoles or delivered back to the PM. It has been long suspected that MVBs might fuse with the PM to form paramural bodies in plant cells, possibly leading to release of building blocks for deposition of papillae and antimicrobial molecules against invading pathogens. Over the past decade or so, major progress has been made in establishing the critical roles of MVBs and associated membrane trafficking in pathogen recognition, defense signaling, and deployment of defense-related molecules during plant immune responses. Regulatory proteins and signaling pathways associated with induced biogenesis and trafficking of MVBs during plant immune responses have also been identified and characterized. Recent successful isolation of plant extracellular vesicles and proteomic profiling of their content have provided additional support for the roles of MVBs in plant-pathogen interactions. In this review, we summarize the important progress and discuss how MVBs, particularly through routing of cellular components to different destinations, contribute to the complex network of plant immune system.