Mycorrhiza-induced protection against pathogens is both genotype-specific and graft-transmissible

Plant Immunity Modulation in Arbuscular Mycorrhizal Symbiosis and Its Impact on Pathogens and Pests

Arbuscular mycorrhizal (AM) symbiosis is the oldest and most widespread mutualistic association on Earth and involves plants and soil fungi belonging to Glomeromycotina. A complex molecular, cellular, and genetic developmental program enables partner recognition, fungal accommodation in plant tissues, and activation of symbiotic functions such as transfer of phosphorus in exchange for carbohydrates and lipids. AM fungi, as ancient obligate biotrophs, have evolved strategies to circumvent plant defense responses to guarantee an intimate and long-lasting mutualism. They are among those root-associated microorganisms able to boost plants' ability to cope with biotic stresses leading to mycorrhiza-induced resistance (MIR), which can be effective across diverse hosts and against different attackers. Here, we examine the molecular mechanisms underlying the modulation of plant immunity during colonization by AM fungi and at the onset and display of MIR against belowground and aboveground pests and pathogens. Understanding the MIR efficiency spectrum and its regulation is of great importance to optimizing the biotechnological application of these beneficial microbes for sustainable crop protection.

Arbuscular Mycorrhizal Fungi as Biostimulant and Biocontrol Agents: A Review

Arbuscular mycorrhizal fungi (AMF) are soil microorganisms living in symbiosis with most terrestrial plants. They are known to improve plant tolerance to numerous abiotic and biotic stresses through the systemic induction of resistance mechanisms. With the aim of developing more sustainable agriculture, reducing the use of chemical inputs is becoming a major concern. After providing an overview on AMF history, phylogeny, development cycle and symbiosis benefits, the current review aims to explore the potential of AMF as biostimulants and/or biocontrol agents. Nowadays, AMF inoculums are already increasingly used as biostimulants, improving mineral nutrient plant acquisition. However, their role as a promising tool in the biocontrol market, as an alternative to chemical phytosanitary products, is underexplored and underdiscussed. Thus, in the current review, we will address the mechanisms of mycorrhized plant resistance to biotic stresses induced by AMF, and highlight the various factors in favor of inoculum application, but also the challenges that remain to be overcome.

Decrypting the multi-functional biological activators and inducers of defense responses against biotic stresses in plants

Plant diseases are still the main problem for the reduction in crop yield and a threat to global food security. Additionally, excessive usage of chemical inputs such as pesticides and fungicides to control plant diseases have created another serious problem for human and environmental health. In view of this, the application of plant growth-promoting rhizobacteria (PGPR) for controlling plant disease incidences has been identified as an eco-friendly approach for coping with the food security issue. In this review, we have identified different ways by which PGPRs are capable of reducing phytopathogenic infestations and enhancing crop yield. PGPR suppresses plant diseases, both directly and indirectly, mediated by microbial metabolites and signaling components. Microbial synthesized anti-pathogenic metabolites such as siderophores, antibiotics, lytic enzymes, hydrogen cyanide, and several others act directly on phytopathogens. The indirect mechanisms of reducing plant disease infestation are caused by the stimulation of plant immune responses known as initiation of systemic resistance (ISR) which is mediated by triggering plant immune responses elicited through pathogen-associated molecular patterns (PAMPs). The ISR triggered in the infected region of the plant leads to the development of systemic acquired resistance (SAR) throughout the plant making the plant resistant to a wide range of pathogens. A number of PGPRs including Pseudomonas and Bacillus genera have proven their ability to stimulate ISR. However, there are still some challenges in the large-scale application and acceptance of PGPR for pest and disease management. Further, we discuss the newly formulated PGPR inoculants possessing both plant growth-promoting activities and plant disease suppression ability for a holistic approach to sustaining plant health and enhancing crop productivity.

Boosting Sustainable Agriculture by Arbuscular Mycorrhiza under Stress Condition: Mechanism and Future Prospective

Global agriculture is frequently subjected to stresses from increased salt content, drought, heavy metals, and other factors, which limit plant growth and production, deteriorate soil health, and constitute a severe danger to global food security. Development of environmentally acceptable mitigation techniques against stresses and restrictions on the use of chemical fertilizers in agricultural fields is essential. Therefore, eco-friendly practises must be kept to prevent the detrimental impacts of stress on agricultural regions. The advanced metabolic machinery needed to handle this issue is not now existent in plants to deal against the stresses. Research has shown that the key role and mechanisms of arbuscular mycorrhiza fungi (AMF) to enhance plant nutrient uptake, immobilisation and translocation of heavy metals, and plant growth-promoting attributes may be suitable agents for plant growth under diversed stressed condition. The successful symbiosis and the functional relationship between the plant and AMF may build the protective regulatory mechansm against the key challenge in particular stress. AMF's compatibility with hyperaccumulator plants has also been supported by studies on gene regulation and theoretical arguments. In order to address this account, the present review included reducing the impacts of biotic and abiotic stress through AMF, the mechanisms of AMF to improve the host plant's capacity to endure stress, and the strategies employed by AM fungus to support plant survival in stressful conditions.

Arbuscular Mycorrhizal Symbiosis Leads to Differential Regulation of Genes and miRNAs Associated with the Cell Wall in Tomato Leaves

Arbuscular mycorrhizal symbiosis is an association that provides nutritional benefits to plants. Importantly, it induces a physiological state allowing plants to respond to a subsequent pathogen attack in a more rapid and intense manner. Consequently, mycorrhiza-colonized plants become less susceptible to root and shoot pathogens. This study aimed to identify some of the molecular players and potential mechanisms related to the onset of defense priming by mycorrhiza colonization, as well as miRNAs that may act as regulators of priming genes. The upregulation of cellulose synthases, pectinesterase inhibitors, and xyloglucan endotransglucosylase/hydrolase, as well as the downregulation of a pectinesterase, suggest that the modification and reinforcement of the cell wall may prime the leaves of mycorrhizal plants to react faster and stronger to subsequent pathogen attack. This was confirmed by the findings of miR164a-3p, miR164a-5p, miR171e-5p, and miR397, which target genes and are also related to the biosynthesis or modification of cell wall components. Our findings support the hypothesis that the reinforcement or remodeling of the cell wall and cuticle could participate in the priming mechanism triggered by mycorrhiza colonization, by strengthening the first physical barriers upstream of the pathogen encounter.

Arbuscular Mycorrhizal Fungi Induced Plant Resistance against Fusarium Wilt in Jasmonate Biosynthesis Defective Mutant and Wild Type of Tomato

Arbuscular mycorrhizal (AM) fungi can form mutual symbiotic associations with most terrestrial plants and improve the resistance of host plants against pathogens. However, the bioprotection provided by AM fungi can depend on the host–fungus combinations. In this study, we unraveled the effects of pre-inoculation with AM fungus Rhizophagus irregularis on plant resistance against the hemibiotrophic fungal pathogen Fusarium oxysporum in jasmonate (JA) biosynthesis mutant tomato, suppressor of prosystemin-mediated responses8 (spr8) and the wild type Castlemart (CM). Results showed that R. irregularis colonization in CM plants significantly decreased the disease index, which was not observed in spr8 plants, suggesting that the disease protection of AM fungi was a plant-genotype-specific trait. Inoculation with R. irregularis significantly increased the shoot dry weight of CM plants when infected with F. oxysporum, with increased plant P content and net photosynthetic rate. Induced expression of the JA synthesis genes, including allene oxide cyclase gene (AOC) and lipoxygenase D gene (LOXD), and increased activities of polyphenol oxidase (PPO) and phenylalanine ammonia lyase (PAL) were recorded in mycorrhizal CM plants infected with F. oxysporum, but not in spr8 plants. Thus, mycorrhiza-induced resistance (MIR) to fungal pathogen in tomato was highly relevant to the JA signaling pathway.

ARBUSCULAR MYCORRHIZA SYMBIOSIS REDUCES THE RHIZOCTONIA ROOT ROT AND ALTERS THE PHENOLIC PROFILE IN COMMON BEAN

Arbuscular mycorrhizal fungi (AMF) have the potential to reduce the deleterious effect of soil-borne pathogens, but their ability for pathogen biocontrol may vary depending on the genotype of the plant, the pathogen, and the AMF interaction. Thus, the aim of this work was to evaluate the effect of the Mexican biofertilizer Rizofermic-UV based on a mix of AMF formulation against the common bean root rot caused by the pathogenic fungus Rhizoctonia solani Kühn 1858 (Teleomorph: Thanatephorus cucumeris). Additionally, the total phenolic content, individual phenolic acid (caffeic, ferulic, o-cumaric, p-cumaric, sinapic, and vanillic), and the flavonoid (catechin, kaempferol, quercetin and, rutin) profiles were analyzed. Our results show that the AMF biofertilization reduces the disease severity up to 68 %, and this was accompanied by a boost in total phenolic content in dual inoculation. Furthermore, a variation in the individual phenolic profiles caused by both AMF interaction and pathogen treatment alone were observed. In dual inoculations, vanillic acid was significantly different among treatments, suggesting it may contribute to the enhanced resistance of mycorrhizal roots to soil-borne pathogens. Further work is required to elucidate the exact role of these compounds in the bioprotection of arbuscular mycorrhizal to plant pathogens.

Influence of mycorrhiza and fly ash on the survival, growth and heavy metal accumulation in three Acacia species grown in Cu-Ni mine soil

Acacia albida, Acacia luederitzii, and Acacia tortilis are dominant acacia species in Botswana and have the potential to rehabilitate the heavy metal degraded environment. To establish this claim, experiments to assess the influence of mycorrhizal inoculation and fly ash amendments on the survival, growth and heavy metal accumulation of these species in mine tailings were conducted. A two-factor (AM inoculation × fly ash) in CRD was done on each of the three Acacia species consisting of four treatments: control (no mycorrhizal, no fly ash coded as - AM/- FA), with mycorrhizal but no fly ash (+ AM/- FA), no mycorrhizal but with fly ash (- AM/+ FA), and with mycorrhizal and with fly ash (+ AM/+ FA). After 24 weeks, results showed that the survival and dry matter yield of all Acacia species were enhanced by 10% with fly ash amendments. However, mycorrhiza inoculation alone improved the survival of A. albida and A. luederitzii but reduced that of the A. tortilis in mine tailings. Fly ash amendments increased the pH of the mine tailings, reduced the availability of Cu, Ni, Pb, Mn and Zn and consequently reduced the concentration of these metals in shoots. On the other hand, it increased the availability of As in the mine tailings. In addition, mycorrhizal inoculation reduced the concentration of these metals in shoots regardless of fly ash amendments. Overall, combined mycorrhizal inoculation and fly ash amendment enhanced the establishment of A. luederitzii in heavy metal-contaminated soils by reducing the heavy metal availability and metal uptake, thus increasing the survival and dry matter yield of plants.

Root-to-shoot signalling in mycorrhizal tomato plants upon Botrytis cinerea infection

Arbuscular mycorrhizal symbiosis is restricted in roots, but it also improves shoot responses against leaf challenges, a phenomenon known as Mycorrhiza-Induced Resistance (MIR). This study focuses on mycorrhizal root signals that may orchestrate shoot defence responses. Metabolomic analysis of non-mycorrhizal and mycorrhizal plants upon Botrytis cinerea infection showed that roots rearrange their metabolome mostly in response to the symbiosis, whereas in shoots a stronger impact of the infection is observed. Specific clusters of compounds in shoots and roots display a priming profile suggesting an implication in the enhanced resistance observed in mycorrhizal plants. Among the primed pathways in roots, lignans showed the highest number of hits followed by oxocarboxylic acids, compounds of the amino acid metabolism, and phytohormones. The lignan yatein was present at higher concentrations in roots, root efflux and leaves of mycorrhizal plants This lignan displayed in vitro antimicrobial activity against B. cinerea and it was also functional protecting tomato plants. Besides, several JA defence-related genes were upregulated in mycorrhizal roots regardless of the pathogen infection, whereas PIN-II was primed in roots of mycorrhizal infected plants. These observations suggest that the enhanced resistance in shoots during MIR may be coordinated by lignans and oxylipins with the participation of roots.

Reduction of Sudden Death Syndrome Foliar Symptoms and Fusarium virguliforme DNA in Roots Inoculated With Rhizophagus intraradices

There is increasing interest in incorporating arbuscular mycorrhizal fungi (AMF) into agricultural production because of the benefits they provide, including protection against pathogens and pests. Sudden death syndrome (SDS) of soybean is a devastating disease caused by the soilborne pathogen Fusarium virguliforme. Multiple management methods are needed to control SDS. The relationship between F. virguliforme and AMF is not well documented. The goal of this study was to determine whether soybean plants co-inoculated with F. virguliforme and the AMF species Rhizophagus intraradices showed reduced SDS foliar symptom severity and reduced relative F. virguliforme DNA quantities in soybean roots. Six soybean genotypes were inoculated with F. virguliforme alone or with R. intraradices in a greenhouse experiment. Averaged over the six soybean genotypes, area under the disease progress curve values and relative F. virguliforme DNA quantities were 45 and 28% lower (P < 0.05), respectively, in roots co-inoculated with R. intraradices compared with roots of control plants inoculated with F. virguliforme only. Weight of roots co-inoculated with R. intraradices were 58% higher (P < 0.05) compared with roots of plants not inoculated with R. intraradices. Nutrient analysis showed higher boron, phosphorus, potassium, sodium, and sulfur concentrations in root tissues of plants co-inoculated with R. intraradices compared with plants inoculated with F. virguliforme (P < 0.05). Overall, this study showed that R. intraradices reduced SDS severity and relative F. virguliforme DNA quantities while simultaneously increasing growth and nutrient uptake of plants. Further testing of AMF inoculants in the field will indicate whether incorporating them into soybean SDS management practices will reduce the impact of SDS on soybean production.

Phosphorus source driving the soil microbial interactions and improving sugarcane development

The world demand for phosphate has gradually increased over the last decades, currently achieving alarming levels considering available rock reserves. The use of soil microorganisms, such as arbuscular mycorrhizal fungi (AMF), has been suggested as a promising alternative to improve phosphorus-use efficiency. However, the effect of the source of phosphorus on the interactions within the soil microbial community remains unclear. Here, we evaluated the links between the total dry matter content of sugarcane and the interactions within the soil microbial community under different phosphate sources, with/without AMF inoculation. The phosphate sources were Simple Superphosphate (SS, 18% of P2O5), Catalão rock phosphate (CA, 2.93% of P2O5) and Bayovar rock phosphate (BA, 14% of P2O5). The results indicated that the BA source led to the largest total dry matter content. The phosphate source affected total dry matter and the structure of the soil microbial communities. The bacterial interactions increased across sources with high percentage of P2O5, while the fungal interactions decreased. The interactions between bacterial and fungal microorganisms allowed to identify the percentage of P2O5 resulting in the highest total sugarcane dry matter. Our findings suggested the soil microbial interactions as a potential microbial indicator helping to improve the agricultural management.

Effects of inoculation by arbuscular mycorrhizal fungi on the composition of the essential oil, plant growth, and lipoxygenase activity of Piper aduncum L

The aim of this study was to evaluate the changes in the production of secondary metabolites Piper aduncum seedlings were inoculated by spores of the arbuscular mycorrhizal fungi (AMF) Rhizophagus clarus and Claroideoglomus etunicatum. P. aduncum seedlings were inoculated by spores of R. clarus and C. etunicatum and then, development parameters, root colonization, lipoxygenase (LOX) activity, and essential oil (OE) chemical composition were monitored at 30, 60 and 90 days' post-inoculation (dpi). The inoculation had influenced the plant height and root length at 30 and 90 dpi and microscopic analysis of roots showed the presence of hyphae, arbuscules and vesicles in the inoculated plants. Phenylpropanoids and sesquiterpene hydrocarbons were the main compounds in the EO. In the leaves, the concentration of phenylpropanoids showed a decrease, mainly at 60 dpi, with increased sesquiterpene hydrocarbon production. The main compounds were dillapiole, myristicin, and germacrene D; the dillapiole concentration decreased in all treatments. LOX activity had an increase in the leaves and roots at 90 dpi. These results suggest that alterations in the secondary metabolites of P. aduncum can be induced by its mechanisms of resistance during AMF interaction.

Identification of tomato introgression lines with enhanced susceptibility or resistance to infection by parasitic giant dodder (Cuscuta reflexa)

The parasitic flowering plant genus Cuscuta (dodder) is a parasitic weed that infects many important crops. Once it winds around the shoots of potential host plants and initiates the development of penetration organs, called haustoria, only a few plant species have been shown to deploy effective defense mechanisms to ward off Cuscuta parasitization. However, a notable exception is Solanum lycopersicum (tomato), which exhibits a local hypersensitive reaction when attacked by giant dodder (Cuscuta reflexa). Interestingly, the closely related wild desert tomato, Solanum pennellii, is unable to stop the penetration of its tissue by the C. reflexa haustoria. In this study, we observed that grafting a S. pennellii scion onto the rootstock of the resistant S. lycopersicum did not change the susceptibility phenotype of S. pennellii. This suggests that hormones, or other mobile substances, produced by S. lycopersicum do not induce a defense reaction in the susceptible tissue. Screening of a population of introgression lines harboring chromosome fragments from S. pennellii in the genome of the recurrent parent S. lycopersicum, revealed that most lines exhibit the same defense reaction as shown by the S. lycopersicum parental line. However, several lines showed different responses and exhibited either susceptibility, or cell death that extended considerably beyond the infection site. These lines will be valuable for the future identification of key loci involved in the perception of, and resistance to, C. reflexa and for developing strategies to enhance resistance to infection in crop species.