The LYSIN MOTIF-CONTAINING RECEPTOR-LIKE KINASE 1 protein of banana is required for perception of pathogenic and symbiotic signals

A gap in the recognition of two mycorrhizal factors: new insights into two LysM-type mycorrhizal receptors

Arbuscular mycorrhizal (AM) fungi are crucial components of the plant microbiota and can form symbioses with 72% of land plants. Researchers have long known that AM symbioses have dramatic effects on plant performance and also provide multiple ecological services in terrestrial environments. The successful establishment of AM symbioses relies on the host plant recognition of the diffusible mycorrhizal (Myc) factors, lipo-chitooligosaccharides (LCOs) and chitooligosaccharides (COs). Among them, the short-chain COs such as CO4/5 secreted by AM fungi are the major Myc factors in COs. In this review, we summarize current advances, develop the concept of mycorrhizal biceptor complex (double receptor complexes for Myc-LCOs and CO4/5 in the same plant), and provide a perspective on the future development of mycorrhizal receptors. First, we focus on the distinct perception of two Myc factors by different host plant species, highlighting the essential role of Lysin-Motif (LysM)-type mycorrhizal receptors in perceiving them. Second, we propose the underlying molecular mechanisms by which LysM-type mycorrhizal receptors in various plants recognize both the Myc-LCOs and -COs. Finally, we explore future prospects for studies on the biceptor complex (Myc-LCO and -CO receptors) in dicots to facilitate the utilization of them in cereal crops (particularly in modern cultivated rice). In conclusion, our understanding of the precise perception processes during host plant interacting with AM fungi, where LysM-type mycorrhizal receptors act as recruiters, provides the tools to design biotechnological applications addressing agricultural challenges.

Carbohydrate elicitor-induced plant immunity: Advances and prospects

The perceived negative impacts of synthetic agrochemicals gave way to alternative, biological plant protection strategies. The deployment of induced resistance, comprising boosting the natural defense responses of plants, is one of those. Plants developed multi-component defense mechanisms to defend themselves against biotic and abiotic stresses. These are activated upon recognition of stress signatures via membrane-localized receptors. The induced immune responses enable plants to tolerate and limit the impact of stresses. A systemic cascade of signals enables plants to prime un-damaged tissues, which is crucial during secondary encounters with stress. Comparable stress tolerance mechanisms can be induced in plants by the application of carbohydrate elicitors such as chitin/chitosan, β-1,3-glucans, oligogalacturonides, cellodextrins, xyloglucans, alginates, ulvans, and carrageenans. Treating plants with carbohydrate-derived elicitors enable the plants to develop resistance appliances against diverse stresses. Some carbohydrates are also known to have been involved in promoting symbiotic signaling. Here, we review recent progresses on plant resistance elicitation effect of various carbohydrate elicitors and the molecular mechanisms of plant cell perception, cascade signals, and responses to cascaded cues. Besides, the molecular mechanisms used by plants to distinguish carbohydrate-induced immunity signals from symbiotic signals are discussed. The structure-activity relationships of the carbohydrate elicitors are also described. Furthermore, we forwarded future research outlooks that might increase the utilization of carbohydrate elicitors in agriculture in order to improve the efficacy of plant protection strategies.

A receptor required for chitin perception facilitates arbuscular mycorrhizal associations and distinguishes root symbiosis from immunity

Plants establish symbiotic associations with arbuscular mycorrhizal fungi (AMF) to facilitate nutrient uptake, particularly in nutrient-limited conditions. This partnership is rooted in the plant's ability to recognize fungal signaling molecules, such as chitooligosaccharides (chitin) and lipo-chitooligosaccharides. In the legume Medicago truncatula, chitooligosaccharides trigger both symbiotic and immune responses via the same lysin-motif-receptor-like kinases (LysM-RLKs), notably CERK1 and LYR4. The nature of plant-fungal engagement is opposite according to the outcomes of immunity or symbiosis signaling, and as such, discrimination is necessary, which is challenged by the dual roles of CERK1/LYR4 in both processes. Here, we describe a LysM-RLK, LYK8, that is functionally redundant with CERK1 for mycorrhizal colonization but is not involved in chitooligosaccharides-induced immunity. Genetic mutation of both LYK8 and CERK1 blocks chitooligosaccharides-triggered symbiosis signaling, as well as mycorrhizal colonization, but shows no further impact on immunity signaling triggered by chitooligosaccharides, compared with the mutation of CERK1 alone. LYK8 interacts with CERK1 and forms a receptor complex that appears essential for chitooligosaccharides activation of symbiosis signaling, with the lyk8/cerk1 double mutant recapitulating the impact of mutations in the symbiosis signaling pathway. We conclude that this novel receptor complex allows chitooligosaccharides activation specifically of symbiosis signaling and helps the plant to differentiate between activation of these opposing signaling processes.

Cadmium-tolerant Bacillus cereus 2-7 alleviates the phytotoxicity of cadmium exposure in banana plantlets

Bananas are the world's important fruit and staple crop in the developing countries. Cadmium (Cd) contamination in soils results in the decrease of crop yield and food safety. Bioremediation is an environmental-friendly and effective measure using Cd-tolerant plant growth promoting rhizobacteria (PGPR). In our study, a Cd-resistant PGPR Bacillus cereus 2-7 was isolated and identified from a discarded gold mine. It could produce multiple plant growth promoting biomolecules such as siderophores, indole-3-acetic acid (IAA), 1-aminocyclopropane-1-carboxylate (ACC)-deaminase and phosphatase. The extracellular accumulation was a main manner of Cd removal. Surplus Cd induced the expression of Cd resistance/transport genes of B. cereus 2-7 to maintain the intracellular Cd homeostasis. The pot experiment showed that Cd contents decreased by 50.31 % in soil, 45.43 % in roots, 56.42 % in stems and 79.69 % in leaves after the strain 2-7 inoculation for 40 d. Bacterial inoculation alleviated the Cd-induced oxidative stress to banana plantlets, supporting by the increase of chlorophyll contents, plant height and total protein contents. The Cd remediation mechanism revealed that B. cereus 2-7 could remodel the rhizosphere bacterial community structure and improve soil enzyme activities to enhance the immobilization of Cd. Our study provides a Cd-bioremediation strategy using Cd-resistant PGPR in tropical and subtropical area.

Microbe-associated molecular pattern recognition receptors have little effect on endophytic Arabidopsis thaliana microbiome assembly in the field

Plant microbiome structure affects plant health and productivity. A limited subset of environmental microbes successfully establishes within plant tissues, but the forces underlying this selectivity remain poorly characterized. Transmembrane pattern recognition receptors (PRRs), used by plants to detect microbe-associated molecular patterns (MAMPs), are strong candidates for achieving this selectivity because PRRs can potentially interact with many members of the microbiome. Indeed, MAMPs found in many microbial taxa, including beneficials and commensals, can instigate a robust immune response that affects microbial growth. Surprisingly, we found that MAMP-detecting PRRs have little effect on endophytic bacterial and fungal microbiome structure in the field. We compared the microbiomes of four PRR knockout lines of Arabidopsis thaliana to wild-type plants in multiple tissue types over several developmental stages and detected only subtle shifts in fungal, but not bacterial, β-diversity in one of the four PRR mutants. In one developmental stage, lore mutants had slightly altered fungal β-diversity, indicating that LORE may be involved in plant-fungal interactions in addition to its known role in detecting certain bacterial lipids. No other effects of PRRs on α-diversity, microbiome variability, within-individual homogeneity, or microbial load were found. The general lack of effect suggests that individual MAMP-detecting PRRs are not critical in shaping the endophytic plant microbiome. Rather, we suggest that MAMP-detecting PRRs must either act in concert and/or are individually maintained through pleiotropic effects or interactions with coevolved mutualists or pathogens. Although unexpected, these results offer insights into the role of MAMP-detecting PRRs in plant-microbe interactions and help direct future efforts to uncover host genetic elements that control plant microbiome assembly.

The grapevine LysM receptor-like kinase VvLYK5-1 recognizes chitin oligomers through its association with VvLYK1-1

The establishment of defense reactions to protect plants against pathogens requires the recognition of invasion patterns (IPs), mainly detected by plasma membrane-bound pattern recognition receptors (PRRs). Some IPs, also termed elicitors, are used in several biocontrol products that are gradually being developed to reduce the use of chemicals in agriculture. Chitin, the major component of fungal cell walls, as well as its deacetylated derivative, chitosan, are two elicitors known to activate plant defense responses. However, recognition of chitooligosaccharides (COS) in Vitis vinifera is still poorly understood, hampering the improvement and generalization of protection tools for this important crop. In contrast, COS perception in the model plant Arabidopsis thaliana is well described and mainly relies on a tripartite complex formed by the cell surface lysin motif receptor-like kinases (LysM-RLKs) AtLYK1/CERK1, AtLYK4 and AtLYK5, the latter having the strongest affinity for COS. In grapevine, COS perception has for the moment only been demonstrated to rely on two PRRs VvLYK1-1 and VvLYK1-2. Here, we investigated additional players by overexpressing in Arabidopsis the two putative AtLYK5 orthologs from grapevine, VvLYK5-1 and VvLYK5-2. Expression of VvLYK5-1 in the atlyk4/5 double mutant background restored COS sensitivity, such as chitin-induced MAPK activation, defense gene expression, callose deposition and conferred non-host resistance to grapevine downy mildew (Erysiphe necator). Protein-protein interaction studies conducted in planta revealed a chitin oligomer-triggered interaction between VvLYK5-1 and VvLYK1-1. Interestingly, our results also indicate that VvLYK5-1 mediates the perception of chitin but not chitosan oligomers showing a part of its specificity.

Arbuscular mycorrhizal fungi induce lateral root development in angiosperms via a conserved set of MAMP receptors

Root systems regulate their branching patterns in response to environmental stimuli. Lateral root development in both monocotyledons and dicotyledons is enhanced in response to inoculation with arbuscular mycorrhizal (AM) fungi, which has been interpreted as a developmental response to specific, symbiosis-activating chitinaceous signals. Here, we report that generic instead of symbiosis-specific, chitin-derived molecules trigger lateral root formation. We demonstrate that this developmental response requires the well-known microbe-associated molecular pattern (MAMP) receptor, ChitinElicitorReceptorKinase 1 (CERK1), in rice, Medicago truncatula, and Lotus japonicus, as well as the non-host of AM fungi, Arabidopsis thaliana, lending further support for a broadly conserved signal transduction mechanism across angiosperms. Using rice mutants impaired in strigolactone biosynthesis and signaling, we show that strigolactone signaling is necessary to regulate this developmental response. Rice CERK1 operates together with either Chitin Elicitor Binding Protein (CEBiP) or Nod Factor Receptor 5 (NFR5) in immunity and symbiosis signaling, respectively; for the lateral root response, however, all three LysM receptors are required. Our work, therefore, reveals an overlooked but a conserved role of LysM receptors integrating MAMP perception with developmental responses in plants, an ability that might influence the interaction between roots and the rhizosphere biota.

Potential Biological Control of Endophytic Streptomyces sp. 5-4 Against Fusarium Wilt of Banana Caused by Fusarium oxysporum f. sp. cubense Tropical Race 4

Fusarium wilt of banana caused by Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4) is one of the most disastrous fungal diseases. Biological control is a promising strategy for controlling Fusarium wilt of banana. To explore endophytic actinomycetes as biocontrol resources against Foc TR4, antagonistic strains were isolated from different tissues of medicinal plants. Here, a total of 144 actinomycetes were isolated and belonged to Nonomuraea, Kitasatospora, and Streptomyces. Forty-three isolates exhibited antifungal activities against Foc TR4. The strain labeled with 5-4 isolated from roots of Piper austrosinense had a broad-spectrum antifungal activity by the production of chitinase and β-1,3-glucanase and was identified as Streptomyces hygroscopicus subsp. hygroscopicus 5-4. Furthermore, disease index of banana wilt was significantly reduced by application of strain 5-4 in comparison with application of Foc TR4 alone. Exogenous application of strain 5-4 increased the expression levels of defense genes such as (PAL), peroxidase (POD), pathogenesis-related protein 1 (PR-1), hydrolytic enzymes (β-1,3-glucanase), lysin motif receptor kinase 1 (LYK-1), and mitogen-activated protein kinase 1 (MPK-1). The antifungal mechanism assay demonstrated that extracts of strain 5-4 inhibited spore gemination and hyphal growth of Foc TR4, and caused abnormally swollen, deformity, and rupture of Foc TR4 hypha. Thus, S. hygroscopicus subsp. hygroscopicus 5-4 could be used as a potential biological agent for controlling Fusarium wilt of banana.

Molecular Regulation of Arbuscular Mycorrhizal Symbiosis

Plant-microorganism interactions at the rhizosphere level have a major impact on plant growth and plant tolerance and/or resistance to biotic and abiotic stresses. Of particular importance for forestry and agricultural systems is the cooperative and mutualistic interaction between plant roots and arbuscular mycorrhizal (AM) fungi from the phylum Glomeromycotina, since about 80% of terrestrial plant species can form AM symbiosis. The interaction is tightly regulated by both partners at the cellular, molecular and genetic levels, and it is highly dependent on environmental and biological variables. Recent studies have shown how fungal signals and their corresponding host plant receptor-mediated signalling regulate AM symbiosis. Host-generated symbiotic responses have been characterized and the molecular mechanisms enabling the regulation of fungal colonization and symbiosis functionality have been investigated. This review summarizes these and other recent relevant findings focusing on the molecular players and the signalling that regulate AM symbiosis. Future progress and knowledge about the underlying mechanisms for AM symbiosis regulation will be useful to facilitate agro-biotechnological procedures to improve AM colonization and/or efficiency.

Innovation and appropriation in mycorrhizal and rhizobial Symbioses

Most land plants benefit from endosymbiotic interactions with mycorrhizal fungi, including legumes and some nonlegumes that also interact with endosymbiotic nitrogen (N)-fixing bacteria to form nodules. In addition to these helpful interactions, plants are continuously exposed to would-be pathogenic microbes: discriminating between friends and foes is a major determinant of plant survival. Recent breakthroughs have revealed how some key signals from pathogens and symbionts are distinguished. Once this checkpoint has been passed and a compatible symbiont is recognized, the plant coordinates the sequential development of two types of specialized structures in the host. The first serves to mediate infection, and the second, which appears later, serves as sophisticated intracellular nutrient exchange interfaces. The overlap in both the signaling pathways and downstream infection components of these symbioses reflects their evolutionary relatedness and the common requirements of these two interactions. However, the different outputs of the symbioses, phosphate uptake versus N fixation, require fundamentally different components and physical environments and necessitated the recruitment of different master regulators, NODULE INCEPTION-LIKE PROTEINS, and PHOSPHATE STARVATION RESPONSES, for nodulation and mycorrhization, respectively.

Biocontrol Potential of Endophytic Streptomyces malaysiensis 8ZJF-21 From Medicinal Plant Against Banana Fusarium Wilt Caused by Fusarium oxysporum f. sp. cubense Tropical Race 4

Banana (Musa spp.) is an important fruit crop cultivated in most tropical countries. Banana Fusarium wilt caused by Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4) is the most destructive fungal disease. Biocontrol using endophytic microorganisms is considered as a safety and sustainable strategy. Actinomycetes have a potential for the production of diverse metabolites. Isolation of endophytic actinomycetes with high efficiency and broad-spectrum antagonism is key for exploring biocontrol agents. Our previous study showed that a total of 144 endophytic actinomycetes were isolated from different tissues of medicinal plants in Hainan, China. Especially, strain 8ZJF-21 exhibited a broad-spectrum antifungal activity. Its morphological, physiological, and biochemical characteristics were consistent with the genus Streptomyces. The phylogenetic tree demonstrated that strain 8ZJF-21 formed a distinct clade with Streptomyces malaysiensis. Average nucleotide identity (ANI) was 98.49% above the threshold of novel species. The pot experiment revealed that endophytic Streptomyces malaysiensis 8ZJF-21 could improve the plant resistance to Foc TR4 by enhancing the expression levels of defense-related and antioxidant enzyme genes. It also promoted the plant growth by producing several extracellular enzymes and metabolites. Antifungal mechanism assays showed that S. malaysiensis 8ZJF-21 extract inhibited mycelial growth and spore germination of Foc TR4 in vitro. Pathogenic cells occurred cytoplasmic heterogeneity, disappeared organelles, and ruptured ultrastructure. Sequencing and annotation of genome suggested that S. malaysiensis 8ZJF-21 had a potential of producing novel metabolites. Nineteen volatile organic compounds were obtained from the extract by Gas Chromatography-Mass Spectrometry (GC-MS). Hence, endophytic Streptomyces strains will become essential biocontrol agents of modern agricultural practice.

Toward Understanding the Molecular Recognition of Fungal Chitin and Activation of the Plant Defense Mechanism in Horticultural Crops

Large volumes of fruit and vegetable production are lost during postharvest handling due to attacks by necrotrophic fungi. One of the promising alternatives proposed for the control of postharvest diseases is the induction of natural defense responses, which can be activated by recognizing molecules present in pathogens, such as chitin. Chitin is one of the most important components of the fungal cell wall and is recognized through plant membrane receptors. These receptors belong to the receptor-like kinase (RLK) family, which possesses a transmembrane domain and/or receptor-like protein (RLP) that requires binding to another RLK receptor to recognize chitin. In addition, these receptors have extracellular LysM motifs that participate in the perception of chitin oligosaccharides. These receptors have been widely studied in Arabidopsis thaliana (A. thaliana) and Oryza sativa (O. sativa); however, it is not clear how the molecular recognition and plant defense mechanisms of chitin oligosaccharides occur in other plant species or fruits. This review includes recent findings on the molecular recognition of chitin oligosaccharides and how they activate defense mechanisms in plants. In addition, we highlight some of the current advances in chitin perception in horticultural crops.

Isolation and Evaluation of Rhizosphere Actinomycetes With Potential Application for Biocontrolling Fusarium Wilt of Banana Caused by Fusarium oxysporum f. sp. cubense Tropical Race 4

Fusarium wilt of banana caused by Fusarium oxysporum f. sp. cubense tropical race 4 (TR4) is globally one of the most destructive soil-borne fungal diseases. Biological control using environmental microorganisms is considered as an alternative and sustainable strategy. Actinomycetes have the potential to explore biocontrol agents due to their production of diverse metabolites. The isolation and identification of high-efficiency and broad-spectrum antagonistic actinomycetes are the key for the application of biocontrol agents. In the present study, 60 actinomycetes were obtained from the rhizosphere soil of Machilus pingii in the primitive ecological natural reserve of Hainan province, China. Seventeen isolates and their extracts exhibited significant antifungal activity against F. oxysporum TR4. Particularly, strain BITDG-11 with the strongest inhibition ability had a broad-spectrum antifungal activity. The assay of its physiological and biochemical profiles showed that strain BITDG-11 had the ability to produce IAA and siderophores and had a positive response to gelatin liquefaction and nitrate reduction. Enzyme activities of chitinase, β-1,3-glucanase, lipase, and urease were also detected. Average nucleotide identity calculated by comparison with the standard strain genome of Streptomyces albospinus JCM3399 was 86.55% below the novel species threshold, suggesting that the strain could be a novel species. In addition, Streptomyces BITDG-11 obviously reduced the disease index of banana plantlets and promoted plant growth at 45 days post inoculation. The higher and lasting expression levels of defense genes and activities of antioxidant enzymes were induced in the roots of banana. Genome sequencing revealed that the Streptomyces BITDG-11 chromosome contained large numbers of conserved biosynthesis gene clusters encoding terpenes, non-ribosomal peptides, polyketides, siderophores, and ectoines. Fifteen bioactive secondary metabolites were further identified from Streptomyces BITDG-11 extract by gas chromatography-mass spectrometry. Dibutyl phthalate demonstrating a strong antifungal activity was the major compound with the highest peak area. Hence, Streptomyces sp. BITDG-11 has a great potential to become an essential constituent of modern agricultural practice as biofertilizers and biocontrol agents.

Lysin Motif (LysM) Proteins: Interlinking Manipulation of Plant Immunity and Fungi

The proteins with lysin motif (LysM) are carbohydrate-binding protein modules that play a critical role in the host-pathogen interactions. The plant LysM proteins mostly function as pattern recognition receptors (PRRs) that sense chitin to induce the plant's immunity. In contrast, fungal LysM blocks chitin sensing or signaling to inhibit chitin-induced host immunity. In this review, we provide historical perspectives on plant and fungal LysMs to demonstrate how these proteins are involved in the regulation of plant's immune response by microbes. Plants employ LysM proteins to recognize fungal chitins that are then degraded by plant chitinases to induce immunity. In contrast, fungal pathogens recruit LysM proteins to protect their cell wall from hydrolysis by plant chitinase to prevent activation of chitin-induced immunity. Uncovering this coevolutionary arms race in which LysM plays a pivotal role in manipulating facilitates a greater understanding of the mechanisms governing plant-fungus interactions.

Using Genetic Engineering Techniques to Develop Banana Cultivars With Fusarium Wilt Resistance and Ideal Plant Architecture

Bananas (Musa spp.) are an important fruit crop worldwide. The fungus Fusarium oxysporum f. sp. cubense (Foc), which causes Fusarium wilt, is widely regarded as one of the most damaging plant diseases. Fusarium wilt has previously devastated global banana production and continues to do so today. In addition, due to the current use of high-density banana plantations, desirable banana varieties with ideal plant architecture (IPA) possess high lodging resistance, optimum photosynthesis, and efficient water absorption. These properties may help to increase banana production. Genetic engineering is useful for the development of banana varieties with Foc resistance and ideal plant architecture due to the sterility of most cultivars. However, the sustained immune response brought about by genetic engineering is always accompanied by yield reductions. To resolve this problem, we should perform functional genetic studies of the Musa genome, in conjunction with genome editing experiments, to unravel the molecular mechanisms underlying the immune response and the formation of plant architecture in the banana. Further explorations of the genes associated with Foc resistance and ideal architecture might lead to the development of banana varieties with both ideal architecture and pathogen super-resistance. Such varieties will help the banana to remain a staple food worldwide.

LysM Receptor-Like Kinase LYK9 of Pisum Sativum L. May Regulate Plant Responses to Chitooligosaccharides Differing in Structure

This study focused on the interactions of pea (Pisum sativum L.) plants with phytopathogenic and beneficial fungi. Here, we examined whether the lysin-motif (LysM) receptor-like kinase PsLYK9 is directly involved in the perception of long- and short-chain chitooligosaccharides (COs) released after hydrolysis of the cell walls of phytopathogenic fungi and identified in arbuscular mycorrhizal (AM) fungal exudates. The identification and analysis of pea mutants impaired in the lyk9 gene confirmed the involvement of PsLYK9 in symbiosis development with AM fungi. Additionally, PsLYK9 regulated the immune response and resistance to phytopathogenic fungi, suggesting its bifunctional role. The existence of co-receptors may provide explanations for the potential dual role of PsLYK9 in the regulation of interactions with pathogenic and AM fungi. Co-immunoprecipitation assay revealed that PsLYK9 and two proposed co-receptors, PsLYR4 and PsLYR3, can form complexes. Analysis of binding capacity showed that PsLYK9 and PsLYR4, synthesized as extracellular domains in insect cells, were able to bind the deacetylated (DA) oligomers CO5-DA-CO8-DA. Our results suggest that the receptor complex consisting of PsLYK9 and PsLYR4 can trigger a signal pathway that stimulates the immune response in peas. However, PsLYR3 seems not to be involved in the perception of CO4-5, as a possible co-receptor of PsLYK9.

A combination of chitooligosaccharide and lipochitooligosaccharide recognition promotes arbuscular mycorrhizal associations in Medicago truncatula

Plants associate with beneficial arbuscular mycorrhizal fungi facilitating nutrient acquisition. Arbuscular mycorrhizal fungi produce chitooligosaccharides (COs) and lipo-chitooligosaccharides (LCOs), that promote symbiosis signalling with resultant oscillations in nuclear-associated calcium. The activation of symbiosis signalling must be balanced with activation of immunity signalling, which in fungal interactions is promoted by COs resulting from the chitinaceous fungal cell wall. Here we demonstrate that COs ranging from CO4-CO8 can induce symbiosis signalling in Medicago truncatula. CO perception is a function of the receptor-like kinases MtCERK1 and LYR4, that activate both immunity and symbiosis signalling. A combination of LCOs and COs act synergistically to enhance symbiosis signalling and suppress immunity signalling and receptors involved in both CO and LCO perception are necessary for mycorrhizal establishment. We conclude that LCOs, when present in a mix with COs, drive a symbiotic outcome and this mix of signals is essential for arbuscular mycorrhizal establishment.