The role of strigolactones during plant interactions with the pathogenic fungus Fusarium oxysporum

Signals and Machinery for Mycorrhizae and Cereal and Oilseed Interactions towards Improved Tolerance to Environmental Stresses

In the quest for sustainable agricultural practices, there arises an urgent need for alternative solutions to mineral fertilizers and pesticides, aiming to diminish the environmental footprint of farming. Arbuscular mycorrhizal fungi (AMF) emerge as a promising avenue, bestowing plants with heightened nutrient absorption capabilities while alleviating plant stress. Cereal and oilseed crops benefit from this association in a number of ways, including improved growth fitness, nutrient uptake, and tolerance to environmental stresses. Understanding the molecular mechanisms shaping the impact of AMF on these crops offers encouraging prospects for a more efficient use of these beneficial microorganisms to mitigate climate change-related stressors on plant functioning and productivity. An increased number of studies highlighted the boosting effect of AMF on grain and oil crops' tolerance to (a)biotic stresses while limited ones investigated the molecular aspects orchestrating the different involved mechanisms. This review gives an extensive overview of the different strategies initiated by mycorrhizal cereal and oilseed plants to manage the deleterious effects of environmental stress. We also discuss the molecular drivers and mechanistic concepts to unveil the molecular machinery triggered by AMF to alleviate the tolerance of these crops to stressors.

Strigolactones as promising biomolecule for oxidative stress management: A comprehensive review

Strigolactones, which are a group of plant hormones, have emerged as promising biomolecules for effectively managing oxidative stress in plants. Oxidative stress occurs when the production of reactive oxygen species (ROS) exceeds the plant's ability to detoxify or scavenge these harmful molecules. An elevation in reactive oxygen species (ROS) levels often occurs in response to a range of stressors in plants. These stressors encompass both biotic factors, such as fungal, viral, or nematode attacks, as well as abiotic challenges like intense light exposure, drought, salinity, and pathogenic assaults. This ROS surge can ultimately lead to cellular harm and damage. One of the key ways in which strigolactones help mitigate oxidative stress is by stimulating the synthesis and accumulation of antioxidants. These antioxidants act as scavengers of ROS, neutralizing their harmful effects. Additionally, strigolactones also regulate stomatal closure, which reduces water loss and helps alleviate oxidative stress during conditions of drought stress or water deficiencies. By understanding and harnessing the capabilities of strigolactones, it becomes possible to enhance crop productivity and enable plants to withstand environmental stresses in the face of a changing climate. This comprehensive review provides an in-depth exploration of the various roles of strigolactones in plant growth, development, and response to various stresses, with a specific emphasis on their involvement in managing oxidative stress. Strigolactones also play a critical role in detoxifying ROS while regulating the expression of genes related to antioxidant defense pathways, striking a balance between ROS detoxification and production.

Strigolactones GR-24 and Nijmegen Applications Result in Reduced Susceptibility of Tobacco and Grapevine Plantlets to Botrytis cinerea Infection

Priming agents are plant defence-inducing compounds which can prompt a state of protection but may also aid in plant growth and interactions with beneficial microbes. The synthetic strigolactones (±)-GR24 and Nijmegen-1 were evaluated as potential priming agents for induced resistance against Botrytis cinerea in tobacco and grapevine plants. The growth and stress response profiles of B. cinerea to strigolactones were also investigated. Soil drench treatment with strigolactones induced resistance in greenhouse-grown tobacco plants and restricted lesion development. The mode of action appeared to function by priming redox-associated compounds to produce an anti-oxidant protective response for limiting the infection. The results obtained in the in vitro assays mirrored that of the greenhouse-grown plants. Exposure of B. cinerea to the strigolactones resulted in increased hyphal branching, with (±)-GR24 stimulating a stronger effect than Nijmegen-1 by affecting colony diameter and radial growth. An oxidative stress response was observed, with B. cinerea exhibiting increased ROS and SOD levels when grown with strigolactones. This study identified the application of strigolactones as potential priming agents to induce disease resistance in both tobacco and grapevine plants. In addition, strigolactones may alter the ROS homeostasis of B. cinerea, resulting in both morphological and physiological changes, thereby reducing virulence.

Phenotypic and Genetic Characterization of the Lentil Single Plant-Derived Core Collection for Resistance to Root Rot Caused by Fusarium avenaceum

Lentil (Lens culinaris) is a pulse crop grown for its amino acid profile, moderate drought tolerance, and ability to fix nitrogen. As the global demand for lentils expands and new production regions emerge so too have the complement of diseases that reduce yield, including the root rot complex. Although the predominant causal pathogen varies based on growing region, Fusarium avenaceum is often found to be an important contributor to disease. This study screened part of the lentil single plant-derived core collection for resistance to F. avenaceum in a greenhouse. Plants were phenotyped for disease severity using three scoring scales and the differences in biomass traits due to pathogen presence were measured. Lentil accessions varied in disease severity and differences in biomass traits were found to be correlated with each visual severity estimate (r = -0.37 to -0.63, P < 0.001), however, heritability estimates were low to moderate among traits (H² = 0.12 to 0.43). Results of a genome-wide association study (GWAS) using single nucleotide polymorphism (SNP) markers derived from genotyping-by-sequencing revealed 11 quantitative trait loci (QTL) across four chromosomes. Two pairs of QTL colocated for two traits and were found near putative orthologs that have been previously associated with plant disease resistance. The identification of lentil accessions that did not exhibit a difference in biomass traits may serve as parental material in breeding or in the development of biparental mapping populations to further validate and dissect the genetic control of resistance to root rot caused by F. avenaceum.

Chemotropic Assay for Testing Fungal Response to Strigolactones and Strigolactone-Like Compounds

Current knowledge on the mechanism of strigolactones (SLs) as signaling molecules during specific interactions in the rhizosphere is mainly related to the control of germination of parasitic weed seeds and hyphal branching of arbuscular mycorrhizal fungi. Thus, the role of plant secreted SLs in regulating the growth and development of root-colonizing fungi still remains controversial. Fusarium oxysporum can sense and respond to extracellular signals through oriented germ tube emergence and redirectioning of hyphal growth toward gradients of nutrients, sex pheromones, or plant root exudates. However, chemoattractant activity of SLs against microorganisms living in the soil has not been tested so far. Here we propose a quantitative chemotropic assay to understand if and how soil fungi could sense gradients of SLs and SLs-like sources. In the example case of F. oxysporum, hyphae of fungal representative mutants preferentially grow toward the synthetic SL analog GR24; and this chemotropic response requires conserved elements of the fungal invasive growth mitogen-activated protein kinase (MAPK) cascade.

Overexpression of a Cytochrome P450 Monooxygenase Involved in Orobanchol Biosynthesis Increases Susceptibility to Fusarium Head Blight

Fusarium Head Blight (FHB) is a cereal disease caused primarily by the ascomycete fungus Fusarium graminearum with public health issues due to the production of mycotoxins including deoxynivalenol (DON). Genetic resistance is an efficient protection means and numerous quantitative trait loci have been identified, some of them related to the production of resistance metabolites. In this study, we have functionally characterized the Brachypodium distachyon BdCYP711A29 gene encoding a cytochrome P450 monooxygenase (CYP). We showed that BdCYP711A29 belongs to an oligogenic family of five members. However, following infection by F. graminearum, BdCYP711A29 is the only copy strongly transcriptionally induced in a DON-dependent manner. The BdCYP711A29 protein is homologous to the Arabidopsis thaliana MAX1 and Oryza sativa MAX1-like CYPs representing key components of the strigolactone biosynthesis. We show that BdCYP711A29 is likely involved in orobanchol biosynthesis. Alteration of the BdCYP711A29 sequence or expression alone does not modify plant architecture, most likely because of functional redundancy with the other copies. B. distachyon lines overexpressing BdCYP711A29 exhibit an increased susceptibility to F. graminearum, although no significant changes in defense gene expression were detected. We demonstrate that both orobanchol and exudates of Bd711A29 overexpressing lines stimulate the germination of F. graminearum macroconidia. We therefore hypothesize that orobanchol is a susceptibility factor to FHB.

Science and application of strigolactones

Strigolactones (SLs) represent a class of plant hormones that regulate developmental processes and play a role in the response of plants to various biotic and abiotic stresses. Both in planta hormonal roles and ex planta signalling effects of SLs are potentially interesting agricultural targets. In this review, we explore various aspects of SL function and highlight distinct areas of agriculture that may benefit from the use of synthetic SL analogues, and we identify possible bottlenecks. Our objective is to identify where the contributions of science and stakeholders are still needed to achieve harnessing the benefits of SLs for a sustainable agriculture of the near future.

Recent progress in the chemistry and biochemistry of strigolactones

Strigolactones (SLs) are plant secondary metabolites derived from carotenoids. SLs play important roles in the regulation of plant growth and development in planta and coordinate interactions between plants and other organisms including root parasitic plants, and symbiotic and pathogenic microbes in the rhizosphere. In the 50 years since the discovery of the first SL, strigol, our knowledge about the chemistry and biochemistry of SLs has advanced explosively, especially over the last two decades. In this review, recent advances in the chemistry and biology of SLs are summarized and possible future outcomes are discussed.

Directing Trophic Divergence in Plant-Pathogen Interactions: Antagonistic Phytohormones With NO Doubt?

A fundamental process culminating in the mechanisms of plant-pathogen interactions is the regulation of trophic divergence into biotrophic, hemibiotrophic, and necrotrophic interactions. Plant hormones, of almost all types, play significant roles in this regulatory apparatus. In plant-pathogen interactions, two classical mechanisms underlying hormone-dependent trophic divergence are long recognized. While salicylic acid dominates in the execution of host defense response against biotrophic and early-stage hemibiotrophic pathogens, jasmonic acid, and ethylene are key players facilitating host defense response against necrotrophic and later-stage hemibiotrophic pathogens. Evidence increasingly suggests that trophic divergence appears to be modulated by more complex signaling networks. Acting antagonistically or agonistically, other hormones such as auxins, cytokinins, abscisic acid, gibberellins, brassinosteroids, and strigolactones, as well as nitric oxide, are emerging candidates in the regulation of trophic divergence. In this review, the latest advances in the dynamic regulation of trophic divergence are summarized, emphasizing common and contrasting hormonal and nitric oxide signaling strategies deployed in plant-pathogen interactions.

Strigolactones positively regulate defense against Magnaporthe oryzae in rice (Oryza sativa)

This study presents evidence that strigolactones (SLs) promote defense against devastating rice blast fungal pathogen Magnaporthe oryzae. Impairment in either SL-biosynthetic dwarf17 (d17) or -signaling (d14) led to increased susceptibility towards M. oryzae. Comparative transcriptome profiling of the SL-signaling d14 mutant and WT plants revealed that a large number of defense-associated genes including hydrogen peroxide (H₂O₂)-, ethylene- and cell wall-synthesis-related genes were remarkably suppressed in d14 with respect to that of WT plants, during M. oryzae infection. In addition, various KEGG metabolic pathways related to sugar metabolism were significantly suppressed in the d14 plants compared to WT, during M. oryzae infection. Accordingly, WT plants accumulated increased levels of H₂O₂ and soluble sugar content compared to that of d17 and d14 in response to M. oryzae infection. Altogether, these results propose that SLs positively regulated rice defense against M. oryzae through involvement in the induction of various defense associated genes/pathways.

Induction of ethylene inhibits development of soybean sudden death syndrome by inducing defense-related genes and reducing Fusarium virguliforme growth

Ethylene is a gaseous hormone that regulates plant responses to biotic and abiotic stresses. To investigate the importance of ethylene in soybean resistance to Fusarium virguliforme (Fv), the causal agent of sudden death syndrome (SDS), soybean cultivars Williams 82 (SDS-susceptible) and MN1606 (SDS-resistant) were treated 24 h before and 24h after Fv inoculation with either ethephon (ethylene inducer), cobalt chloride (ethylene biosynthesis inhibitor), or 1-MCP (ethylene perception inhibitor). Inoculated plants were grown for 21 days at 24°C in the greenhouse and then evaluated for SDS severity and expression of soybean defense genes. In both cultivars, plants treated with ethephon showed lower SDS foliar severity compared to the other treatments, whereas those treated with cobalt chloride or 1-MCP showed the same or higher SDS foliar severity compared to the water-treated control. Ethephon application resulted in activation of genes involved in ethylene biosynthesis, such as ethylene synthase (ACS) and ethylene oxidase (ACO), and genes involved in soybean defense response, such as pathogenesis-related protein (PR), basic peroxidase (IPER), chalcone synthase (CHS), and defense-associated transcription factors. Cobalt chloride and 1-MCP treatments had little or no effect on the expression of these genes. In addition, ethephon had a direct inhibitory effect on in-vitro growth of Fv on PDA media. Our results suggest that ethephon application inhibits SDS development directly by slowing Fv growth and/or by inducing soybean ethylene signaling and the expression of defense related genes.

Strigolactones positively regulate defense against root-knot nematodes in tomato

Strigolactones (SLs) are carotenoid-derived phytohormones that are known to influence various aspects of plant growth and development. As root-derived signals, SLs can enhance symbiosis between plants and arbuscular mycorrhizal fungi (AMF). However, little is known about the roles of SLs in plant defense against soil-borne pathogens. Here, we determined that infection with root-knot nematodes (RKNs; Meloidogyne incognita) induced SL biosynthesis in roots of tomato (Solanum lycopersicum). Silencing of SL biosynthesis genes compromised plant defense against RKNs, whilst application of the SL analog racGR24 enhanced it. Accumulation of endogenous jasmonic acid (JA) and abscisic acid (ABA) in the roots in response to RKN infection was enhanced by silencing of SL biosynthetic genes and was suppressed by application of racGR24. Genetic evidence showed that JA was a positive regulator of defense against RKNs while ABA was a negative regulator. In addition, racGR24 enhanced the defense against nematode in a JA-deficient mutant but not in an ABA-deficient mutant. Silencing of SL biosynthetic genes resulted in up-regulation of MYC2, which negatively regulated defense against RKNs. Our results demonstrate that SLs play a positive role in nematode defense in tomato and that MYC2 negatively regulates this defense, potentially by mediating hormone crosstalk among SLs, ABA and JA.

Symbiosis genes for immunity and vice versa

Basic molecular knowledge on plant-pathogen interactions has largely been gained from reverse and forward genetics in Arabidopsis thaliana. However, as this model plant is unable to establish endosymbiosis with mycorrhizal fungi or rhizobia, plant responses to mutualistic symbionts have been studied in parallel in other plant species, mainly legumes. The resulting analyses led to the identification of gene networks involved in various functions, from microbe recognition to signalling and plant responses, thereafter assigned to either mutualistic symbiosis or immunity, according to the nature of the initially inoculated microbe. The increasing development of new pathosystems and genetic resources in model legumes and the implementation of reverse genetics in plants such as rice and tomato that interact with both mycorrhizal fungi and pathogens, have highlighted the dual role of plant genes previously thought to be specific to mutualistic or pathogenic interactions. The next challenges will be to determine whether such genes have similar functions in both types of interaction and if not, whether the perception of microbial compounds or the involvement of specific plant signalling components is responsible for the appropriate plant responses to the encountered microorganisms.

Strigolactones cross the kingdoms: plants, fungi, and bacteria in the arbuscular mycorrhizal symbiosis

Strigolactones (SLs) first evolved as regulators of simple developmental processes in very ancient plant lineages, and then assumed new roles to sustain the increasing biological complexity of land plants. Their versatility is also shown by the fact that during evolution they have been exploited, once released in the rhizosphere, as a communication system towards plant-interacting organisms even belonging to different kingdoms. Here, we reviewed the impact of SLs on soil microbes, paying particular attention to arbuscular mycorrhizal fungi (AMF). SLs induce several responses in AMF, including spore germination, hyphal branching, mitochondrial metabolism, transcriptional reprogramming, and production of chitin oligosaccharides which, in turn, stimulate early symbiotic responses in the host plant. In the specific case study of the AMF Gigaspora margarita, SLs are also perceived, directly or indirectly, by the well-characterized population of endobacteria, with an increase of bacterial divisions and the activation of specific transcriptional responses. The dynamics of SLs during AM root colonization were also surveyed. Although not essential for the establishment of this mutualistic association, SLs act as positive regulators as they are relevant to achieve the full extent of colonization. This possibly occurs through a complex crosstalk with other hormones such as auxin, abscisic acid, and gibberellins.

Chemical signaling involved in plant-microbe interactions

Microorganisms are found everywhere, and they are closely associated with plants. Because the establishment of any plant-microbe association involves chemical communication, understanding crosstalk processes is fundamental to defining the type of relationship. Although several metabolites from plants and microbes have been fully characterized, their roles in the chemical interplay between these partners are not well understood in most cases, and they require further investigation. In this review, we describe different plant-microbe associations from colonization to microbial establishment processes in plants along with future prospects, including agricultural benefits.

The Role of Strigolactone in the Cross-Talk Between Arabidopsis thaliana and the Endophytic Fungus Mucor sp

Over the last years the role of fungal endophytes in plant biology has been extensively studied. A number of species were shown to positively affect plant growth and fitness, thus attempts have been made to utilize these microorganisms in agriculture and phytoremediation. Plant-fungi symbiosis requires multiple metabolic adjustments of both of the interacting organisms. The mechanisms of these adaptations are mostly unknown, however, plant hormones seem to play a central role in this process. The plant hormone strigolactone (SL) was previously shown to activate hyphae branching of mycorrhizal fungi and to negatively affect pathogenic fungi growth. Its role in the plant-endophytic fungi interaction is unknown. The effect of the synthetic SL analog GR24 on the endophytic fungi Mucor sp. growth, respiration, H2O2 production and the activity of antioxidant enzymes was evaluated. We found fungi colony growth rate was decreased in a GR24 concentration dependent manner. Additionally, the fungi accumulated more H2O2 what was accompanied by an altered activity of antioxidant enzymes. Symbiosis with Mucor sp. positively affected Arabidopsis thaliana growth, but SL was necessary for the establishment of the beneficial interaction. A. thaliana biosynthesis mutants max1 and max4, but not the SL signaling mutant max2 did not develop the beneficial phenotype. The negative growth response was correlated with alterations in SA homeostasis and a significant upregulation of genes encoding selected plant defensins. The fungi were also shown to be able to decompose SL in planta and to downregulate the expression of SL biosynthesis genes. Additionally, we have shown that GR24 treatment with a dose of 1 μM activates the production of SA in A. thaliana. The results presented here provide evidence for a role of SL in the plant-endophyte cross-talk during the mutualistic interaction between Arabidopsis thaliana and Mucor sp.

Strigolactones in the Rhizosphere: Friend or Foe?

Strigolactones are well-known endogenous plant hormones that play a major role in planta by influencing different physiological processes. Moreover, ex planta, strigolactones are important signaling molecules in root exudates and function as host detection cues to launch mutualistic interactions with arbuscular mycorrhizal fungi in the rhizosphere. However, parasitic plants belonging to the Orobanchaceae family hijacked this communication system to stimulate their seed germination when in close proximity to the roots of a suitable host. As a result, the secretion of strigolactones by the plant can have both favorable and detrimental outcomes. Here, we discuss these dual positive and negative effects of strigolactones and we provide a detailed overview on the role of these molecules in the complex dialogs between plants and different organisms in the rhizosphere.

New Strigolactone Mimics as Exogenous Signals for Rhizosphere Organisms

The importance of strigolactones in plant biology prompted us to synthesize simplified strigolactone mimics effective as exogenous signals for rhizosphere organisms. New strigolactone mimics easily derived from simple and available starting materials in significant amounts were prepared and fully characterized. These compounds contain an aromatic or heterocyclic ring, usually present in various bioactive molecules, connected by an ether link to a furan-2-one moiety. The new synthesized strigolactone mimics were confirmed to be active on plant pathogenic fungi and parasitic weed seeds.

Strigolactones in Plant Interactions with Beneficial and Detrimental Organisms: The Yin and Yang

Strigolactones (SLs) are plant hormones that have important roles as modulators of plant development. They were originally described as ex planta signaling molecules in the rhizosphere that induce the germination of parasitic plants, a role that was later linked to encouraging the beneficial symbiosis with arbuscular mycorrhizal (AM) fungi. Recently, the focus has shifted to examining the role of SLs in plant-microbe interactions, and has revealed roles for SLs in the association of legumes with nitrogen-fixing rhizobacteria and in interactions with disease-causing pathogens. Here, we examine the role of SLs in plant interactions with beneficial and detrimental organisms, and propose possible future biotechnological applications.

The Role of Strigolactones and Their Potential Cross-talk under Hostile Ecological Conditions in Plants

The changing environment always questions the survival mechanism of life on earth. The plant being special in the sense of their sessile habit need to face many of these environmental fluctuations as they have a lesser escape option. To counter these adverse conditions, plants have developed efficient sensing, signaling, and response mechanism. Among them the role of phytohormones in the management of hostile ecological situations is remarkable. The strigolactone, a newly emerged plant hormone has been identified with many functions such as growth stimulant of parasitic plants, plant architecture determinant, arbuscular mycorrhiza symbiosis promoter, and also in many other developmental and environmental cues. Despite of their immense developmental potential, the strigolactone research in the last few years has also established their significance in adverse environmental condition. In the current review, its significance under drought, salinity, nutrient starvation, temperature, and pathogenic assail has been discussed. This review also opens the research prospects of strigolactone to better manage the crop loss under hostile ecological conditions.

Strigolactones as Part of the Plant Defence System

Strigolactones (SLs) are plant hormones, described as regulators of plant growth and development. Recently, it was proposed that these hormones might also be involved in the biotic stress response. However, SLs do not have a universal role in plant protection, instead only playing a part in resistance to specific pathogens.