Lipid exchanges drove the evolution of mutualism during plant terrestrialization

The phosphate starvation response regulator PHR2 antagonizes arbuscule maintenance in Medicago

Phosphate starvation response (PHR) transcription factors play essential roles in regulating phosphate uptake in plants through binding to the P1BS cis-element in the promoter of phosphate starvation response genes. Recently, PHRs were also shown to positively regulate arbuscular mycorrhizal colonization in rice and lotus by controlling the expression of many symbiotic genes. However, their role in arbuscule development has remained unclear. In Medicago, we previously showed that arbuscule degradation is controlled by two SPX proteins that are highly expressed in arbuscule-containing cells. Since SPX proteins bind to PHRs and repress their activity in a phosphate-dependent manner, we investigated whether arbuscule maintenance is also regulated by PHR. Here, we show that PHR2 is a major regulator of the phosphate starvation response in Medicago. Knockout of phr2 showed reduced phosphate starvation response, symbiotic gene expression, and fungal colonization levels. However, the arbuscules that formed showed less degradation, suggesting a negative role for PHR2 in arbuscule maintenance. This was supported by the observation that overexpression of PHR2 led to enhanced degradation of arbuscules. Although many arbuscule-induced genes contain P1BS elements in their promoters, we found that the P1BS cis-elements in the promoter of the symbiotic phosphate transporter PT4 are not required for arbuscule-containing cell expression. Since both PHR2 and SPX1/3 negatively affect arbuscule maintenance, our results indicate that they control arbuscule maintenance partly via different mechanisms. While PHR2 potentiates symbiotic gene expression and colonization, its activity in arbuscule-containing cells needs to be tightly controlled to maintain a successful symbiosis in Medicago.

Phylogenomics reveals the evolutionary origins of lichenization in chlorophyte algae

Mutualistic symbioses have contributed to major transitions in the evolution of life. Here, we investigate the evolutionary history and the molecular innovations at the origin of lichens, which are a symbiosis established between fungi and green algae or cyanobacteria. We de novo sequence the genomes or transcriptomes of 12 lichen algal symbiont (LAS) and closely related non-symbiotic algae (NSA) to improve the genomic coverage of Chlorophyte algae. We then perform ancestral state reconstruction and comparative phylogenomics. We identify at least three independent gains of the ability to engage in the lichen symbiosis, one in Trebouxiophyceae and two in Ulvophyceae, confirming the convergent evolution of the lichen symbioses. A carbohydrate-active enzyme from the glycoside hydrolase 8 (GH8) family was identified as a top candidate for the molecular-mechanism underlying lichen symbiosis in Trebouxiophyceae. This GH8 was acquired in lichenizing Trebouxiophyceae by horizontal gene transfer, concomitantly with the ability to associate with lichens fungal symbionts (LFS) and is able to degrade polysaccharides found in the cell wall of LFS. These findings indicate that a combination of gene family expansion and horizontal gene transfer provided the basis for lichenization to evolve in chlorophyte algae.

Evolution of endosymbiosis-mediated nuclear calcium signaling in land plants

The ability of fungi to establish mycorrhizal associations with plants and enhance the acquisition of mineral nutrients stands out as a key feature of terrestrial life. Evidence indicates that arbuscular mycorrhizal (AM) association is a trait present in the common ancestor of land plants,1,2,3,4 suggesting that AM symbiosis was an important adaptation for plants in terrestrial environments.5 The activation of nuclear calcium signaling in roots is essential for AM within flowering plants.6 Given that the earliest land plants lacked roots, whether nuclear calcium signals are required for AM in non-flowering plants is unknown. To address this question, we explored the functional conservation of symbiont-induced nuclear calcium signals between the liverwort Marchantia paleacea and the legume Medicago truncatula. In M. paleacea, AM fungi penetrate the rhizoids and form arbuscules in the thalli.7 Here, we demonstrate that AM germinating spore exudate (GSE) activates nuclear calcium signals in the rhizoids of M. paleacea and that this activation is dependent on the nuclear-localized ion channel DOES NOT MAKE INFECTIONS 1 (MpaDMI1). However, unlike flowering plants, MpaDMI1-mediated calcium signaling is only required for the thalli colonization but not for the AM penetration within rhizoids. We further demonstrate that the mechanism of regulation of DMI1 has diverged between M. paleacea and M. truncatula, including a key amino acid residue essential to sustain DMI1 in an inactive state. Our study reveals functional evolution of nuclear calcium signaling between liverworts and flowering plants and opens new avenues of research into the mechanism of endosymbiosis signaling.

Epitranscriptome insights into Riccia fluitans L. (Marchantiophyta) aquatic transition using nanopore direct RNA sequencing

BACKGROUND

Riccia fluitans, an amphibious liverwort, exhibits a fascinating adaptation mechanism to transition between terrestrial and aquatic environments. Utilizing nanopore direct RNA sequencing, we try to capture the complex epitranscriptomic changes undergone in response to land-water transition.

RESULTS

A significant finding is the identification of 45 differentially expressed genes (DEGs), with a split of 33 downregulated in terrestrial forms and 12 upregulated in aquatic forms, indicating a robust transcriptional response to environmental changes. Analysis of N6-methyladenosine (m6A) modifications revealed 173 m6A sites in aquatic and only 27 sites in the terrestrial forms, indicating a significant increase in methylation in the former, which could facilitate rapid adaptation to changing environments. The aquatic form showed a global elongation bias in poly(A) tails, which is associated with increased mRNA stability and efficient translation, enhancing the plant's resilience to water stress. Significant differences in polyadenylation signals were observed between the two forms, with nine transcripts showing notable changes in tail length, suggesting an adaptive mechanism to modulate mRNA stability and translational efficiency in response to environmental conditions. This differential methylation and polyadenylation underline a sophisticated layer of post-transcriptional regulation, enabling Riccia fluitans to fine-tune gene expression in response to its living conditions.

CONCLUSIONS

These insights into transcriptome dynamics offer a deeper understanding of plant adaptation strategies at the molecular level, contributing to the broader knowledge of plant biology and evolution. These findings underscore the sophisticated post-transcriptional regulatory strategies Riccia fluitans employs to navigate the challenges of aquatic versus terrestrial living, highlighting the plant's dynamic adaptation to environmental stresses and its utility as a model for studying adaptation mechanisms in amphibious plants.

Complexity of responses to ionizing radiation in plants, and the impact on interacting biotic factors

In nature, plants are simultaneously exposed to different abiotic (e.g., heat, drought, and salinity) and biotic (e.g., bacteria, fungi, and insects) stresses. Climate change and anthropogenic pressure are expected to intensify the frequency of stress factors. Although plants are well equipped with unique and common defense systems protecting against stressors, they may compromise their growth and development for survival in such challenging environments. Ionizing radiation is a peculiar stress factor capable of causing clustered damage. Radionuclides are both naturally present on the planet and produced by human activities. Natural and artificial radioactivity affects plants on molecular, biochemical, cellular, physiological, populational, and transgenerational levels. Moreover, the fitness of pests, pathogens, and symbionts is concomitantly challenged in radiologically contaminated areas. Plant responses to artificial acute ionizing radiation exposure and laboratory-simulated or field chronic exposure are often discordant. Acute or chronic ionizing radiation exposure may occasionally prime the defense system of plants to better tolerate the biotic stress or could often exhaust their metabolic reserves, making plants more susceptible to pests and pathogens. Currently, these alternatives are only marginally explored. Our review summarizes the available literature on the responses of host plants, biotic factors, and their interaction to ionizing radiation exposure. Such systematic analysis contributes to improved risk assessment in radiologically contaminated areas.

Unraveling the potential of the strigolactones-NSP1/NSP2 friendship in crop improvement

Strigolactones (SLs) are fundamental to the ability of plants to cope with phosphate deficiency. A recent study by Yuan et al. indicates that the genetic module PHR2/NSP1/NSP2 is crucial in activating SL biosynthesis and signaling under inorganic phosphate (Pi) deficiency. Furthermore, this genetic module is essential for improving Pi and nitrogen homeostasis in rice.

Genomes of multicellular algal sisters to land plants illuminate signaling network evolution

Zygnematophyceae are the algal sisters of land plants. Here we sequenced four genomes of filamentous Zygnematophyceae, including chromosome-scale assemblies for three strains of Zygnema circumcarinatum. We inferred traits in the ancestor of Zygnematophyceae and land plants that might have ushered in the conquest of land by plants: expanded genes for signaling cascades, environmental response, and multicellular growth. Zygnematophyceae and land plants share all the major enzymes for cell wall synthesis and remodifications, and gene gains shaped this toolkit. Co-expression network analyses uncover gene cohorts that unite environmental signaling with multicellular developmental programs. Our data shed light on a molecular chassis that balances environmental response and growth modulation across more than 600 million years of streptophyte evolution.

Symbiont effector-guided mapping of proteins in plant networks to improve crop climate stress resilience: Symbiont effectors inform highly interconnected plant protein networks and provide an untapped resource for crop climate resilience strategies

There is an urgent need for novel protection strategies to sustainably secure crop production under changing climates. Studying microbial effectors, defined as microbe-derived proteins that alter signalling inside plant cells, has advanced our understanding of plant immunity and microbial plant colonisation strategies. Our understanding of effectors in the establishment and beneficial outcome of plant symbioses is less well known. Combining functional and comparative interaction assays uncovered specific symbiont effector targets in highly interconnected plant signalling networks and revealed the potential of effectors in beneficially modulating plant traits. The diverse functionality of symbiont effectors differs from the paradigmatic immuno-suppressive function of pathogen effectors. These effectors provide solutions for improving crop resilience against climate stress by their evolution-driven specification in host protein targeting and modulation. Symbiont effectors represent stringent tools not only to identify genetic targets for crop breeding, but to serve as applicable agents in crop management strategies under changing environments.

Characterisation of the fig-fig wasp holobiont

Plants and animals have long been considered distinct kingdoms, yet here a 'plant-animal' is described. An extraordinary symbiosis in which neither organism can reproduce without the other, the fig tree (Ficus) provides the habitat for its exclusive pollinator: the fig wasp (Agaonidae). Characterising the 'fig-fig wasp holobiont' acknowledges, for the first time, 'plant-animal symbiogenesis'.

Transcriptional alterations of peanut root during interaction with growth-promoting Tsukamurella tyrosinosolvens strain P9

The plant growth-promoting rhizobacterium Tsukamurella tyrosinosolvens P9 can improve peanut growth. In this study, a co-culture system of strain P9 and peanut was established to analyze the transcriptome of peanut roots interacting with P9 for 24 and 72 h. During the early stage of co-culturing, genes related to mitogen-activated protein kinase (MAPK) and Ca2+ signal transduction, ethylene synthesis, and cell wall pectin degradation were induced, and the up-regulation of phenylpropanoid derivative, flavonoid, and isoflavone synthesis enhanced the defense response of peanut. The enhanced expression of genes associated with photosynthesis and carbon fixation, circadian rhythm regulation, indoleacetic acid (IAA) synthesis, and cytokinin decomposition promoted root growth and development. At the late stage of co-culturing, ethylene synthesis was reduced, whereas Ca2+ signal transduction, isoquinoline alkaloid synthesis, and ascorbate and aldarate metabolism were up-regulated, thereby maintaining root ROS homeostasis. Sugar decomposition and oxidative phosphorylation and nitrogen and fatty acid metabolism were induced, and peanut growth was significantly promoted. Finally, the gene expression of seedlings inoculated with strain P9 exhibited temporal differences. The results of our study, which explored transcriptional alterations of peanut root during interacting with P9, provide a basis for elucidating the growth-promoting mechanism of this bacterial strain in peanut.

Unveiling the complexity of strigolactones: exploring structural diversity, biosynthesis pathways, and signaling mechanisms

Strigolactone is the collective name for compounds containing a butenolide as a part of their structure, first discovered as compounds that induce seed germination of root parasitic plants. They were later found to be rhizosphere signaling molecules that induce hyphal branching of arbuscular mycorrhizal fungi, and, finally, they emerged as a class of plant hormones. Strigolactones are found in root exudates, where they display a great variability in their chemical structure. Their structure varies among plant species, and multiple strigolactones can exist in one species. Over 30 strigolactones have been identified, yet the chemical structure of the strigolactone that functions as an endogenous hormone and is found in the above-ground parts of plants remains unknown. We discuss our current knowledge of the synthetic pathways of diverse strigolactones and their regulation, as well as recent progress in identifying strigolactones as plant hormones. Strigolactone is perceived by the DWARF14 (D14), receptor, an α/β hydrolase which originated by gene duplication of KARRIKIN INSENSITIVE 2 (KAI2). D14 and KAI2 signaling pathways are partially overlapping paralogous pathways. Progress in understanding the signaling mechanisms mediated by two α/β hydrolase receptors as well as remaining challenges in the field of strigolactone research are reviewed.

At the root of plant symbioses: Untangling the genetic mechanisms behind mutualistic associations

Mutualistic interactions between plants and microorganisms shape the continuous evolution and adaptation of plants such as to the terrestrial environment that was a founding event of subsequent life on land. Such interactions also play a central role in the natural and agricultural ecosystems and are of primary importance for a sustainable future. To boost plant's productivity and resistance to biotic and abiotic stresses, new approaches involving associated symbiotic organisms have recently been explored. New discoveries on mutualistic symbioses evolution and the interaction between partners will be key steps to enhance plant potential.

Evolution of immunity networks across embryophytes

Land plants (embryophytes), including vascular (tracheophytes) and non-vascular plants (bryophytes), co-evolved with microorganisms since descendants of an algal ancestor colonized terrestrial habitats around 500 million years ago. To cope with microbial pathogen infections, embryophytes evolved a complex immune system for pathogen perception and activation of defenses. With the growing number of sequenced genomes and transcriptome datasets from algae, bryophytes, tracheophytes, and available plant models, comparative analyses are increasing our understanding of the evolution of molecular mechanisms underpinning immune responses in different plant lineages. In this review, recent progress on plant immunity networks is highlighted with emphasis on the identification of key components that shaped immunity against pathogens in bryophytes compared to angiosperms during plant evolution.

The role of arbuscular mycorrhizal symbiosis in plant abiotic stress

Arbuscular mycorrhizal fungi (AMF) can penetrate plant root cortical cells, establish a symbiosis with most land plant species, and form branched structures (known as arbuscules) for nutrient exchange. Plants have evolved a complete plant-AMF symbiosis system to sustain their growth and development under various types of abiotic stress. Here, we highlight recent studies of AM symbiosis and the regulation of symbiosis process. The roles of mycorrhizal symbiosis and host plant interactions in enhancing drought resistance, increasing mineral nutrient uptake, regulating hormone synthesis, improving salt resistance, and alleviating heavy metal stress were also discussed. Overall, studies of AM symbiosis and a variety of abiotic stresses will aid applications of AMF in sustainable agriculture and can improve plant production and environmental safety.

Immunobiodiversity: Conserved and specific immunity across land plants and beyond

Angiosperms represent most plants that humans cultivate, grow, and eat. However, angiosperms are only one of five major land plant lineages. As a whole lineage, plants also include algal groups. All these clades represent a tremendous genetic diversity that can be investigated to reveal the evolutionary history of any given mechanism. In this review, we describe the current model of the plant immune system, discuss its evolution based on the recent literature, and propose future directions for the field. In angiosperms, plant-microbe interactions have been intensively studied, revealing essential cell surface and intracellular immune receptors, as well as metabolic and hormonal defense pathways. Exploring diversity at the genomic and functional levels demonstrates the conservation of these pathways across land plants, some of which are beyond plants. On basis of the conserved mechanisms, lineage-specific variations have occurred, leading to diversified reservoirs of immune mechanisms. In rare cases, this diversity has been harnessed and successfully transferred to other species by integration of wild immune receptors or engineering of novel forms of receptors for improved resistance to pathogens. We propose that exploring further the diversity of immune mechanisms in the whole plant lineage will reveal completely novel sources of resistance to be deployed in crops.

Arbuscular mycorrhizal fungi heterokaryons have two nuclear populations with distinct roles in host-plant interactions

Arbuscular mycorrhizal fungi (AMF) are prominent root symbionts that can carry thousands of nuclei deriving from two parental strains in a large syncytium. These co-existing genomes can also vary in abundance with changing environmental conditions. Here we assemble the nuclear genomes of all four publicly available AMF heterokaryons using PacBio high-fidelity and Hi-C sequencing. We find that the two co-existing genomes of these strains are phylogenetically related but differ in structure, content and epigenetics. We confirm that AMF heterokaryon genomes vary in relative abundance across conditions and show this can lead to nucleus-specific differences in expression during interactions with plants. Population analyses also reveal signatures of genetic exchange indicative of past events of sexual reproduction in these strains. This work uncovers the origin and contribution of two nuclear genomes in AMF heterokaryons and opens avenues for the improvement and environmental application of these strains.

The worldwide allometric relationship in anatomical structures for plant roots

The cortex (i.e., absorptive tissue) and stele (transportive vascular tissue) are fundamental to the function of plant roots. Unraveling how these anatomical structures are assembled in absorptive roots is essential for our understanding of plant ecology, physiology, and plant responses to global environmental changes. In this review, we first compile a large data set on anatomical traits in absorptive roots, including cortex thickness and stele radius, across 698 observations and 512 species. Using this data set, we reveal a common root allometry in absorptive root structures, i.e., cortex thickness increases much faster than stele radius with increasing root diameter (hereafter, root allometry). Root allometry is further validated within and across plant growth forms (woody, grass, and liana species), mycorrhiza types (arbuscular mycorrhiza, ectomycorrhiza, and orchid mycorrhizas), phylogenetic gradients (from ferns to Orchidaceae), and environmental change scenarios (e.g., elevation of atmospheric CO2 concentration and nitrogen fertilization). These findings indicate that root allometry is common in plants. Importantly, root allometry varies greatly across species. We then summarize recent research on the mechanisms of root allometry and potential issues regarding these mechanisms. We further discuss ecological and evolutionary implications of root allometry. Finally, we propose several important research directions that should be pursued regarding root allometry.

Palmitoleic acid as a coordinating molecule between the invasive pinewood nematode and its newly associated fungi

Symbiotic microorganisms are ubiquitous on the body surface or internal tissues of invertebrates, providing them with benefits. Developing symbiotic relationships requires synchronization of developmental stages and physical proximity of partners. Therefore, the identification of metabolites that coordinate the reproduction of symbiotic partners is essential. This study demonstrates that palmitoleic acid (C16: 1) coordinates bilateral propagation by regulating the synchronization of reproduction between the invasive pinewood nematode (PWN) and its newly associated blue-stain fungus, Sporothrix sp.1. When the PWN fed on Sporothrix sp.1, there was a significant increase in lipid metabolism gene expression and metabolite abundance. Through further investigations, it highlighted a significant enhancement in the reproduction of the PWN through direct acquisition of C16: 1, which was abundantly present in Sporothrix sp.1. Furthermore, the PWN biosynthesized C16: 1 through the involvement of the stearoyl-CoA 9-desaturase gene fat-5 and its hormone nuclear receptor nhr-80, which was clarified to promote the egg-laying capacity of females. Moreover, it is worth noting that the production of C16: 1 was significantly higher by the associated fungus Sporothrix sp.1 to enhance sporulation during the spore formation phase compared to the hypha growth phase. Thus, by coordinating the fecundity and spore production, the key lipid metabolite C16: 1 facilitates the rapid and successful colonization of a mutually beneficial symbiotic relationship between the invasive PWN and the native Sporothrix sp.1 within the host. This finding emphasizes the significant role of metabolite sharing and its function in promoting partner synchronization within symbiotic relationships.

A module centered on the transcription factor Msn2 from arbuscular mycorrhizal fungus Rhizophagus irregularis regulates drought stress tolerance in the host plant

Arbuscular mycorrhizal (AM) fungi can form mutualistic endosymbiosis with > 70% of land plants for obtaining fatty acids and sugars, in return, AM fungi promote plant nutrients and water acquisition to enhance plant fitness. However, how AM fungi orchestrate its own signaling components in response to drought stress remains elusive. Here, we identify a transcription factor containing C2H2 zinc finger domains, RiMsn2 from Rhizophagus irregularis. To characterize the RiMsn2, we combined heterologous expression, subcellular localization in yeasts, and biochemical and molecular studies with reverse genetics approaches during the in planta phase. The results indicate that RiMsn2 is highly conserved across AM fungal species and induced during the early stages of symbiosis. It is significantly upregulated in mycorrhizal roots under severe drought conditions. The nucleus-localized RiMsn2 regulates osmotic homeostasis and trehalose contents of yeasts. Importantly, gene silencing analyses indicate that RiMsn2 is essential for arbuscule formation and enhances plant tolerance to drought stress. Results from yeasts and biochemical experiments suggest that the RiHog1-RiMsn2-STREs module controls the drought stress-responsive genes in AM fungal symbiont. In conclusion, our findings reveal that a module centered on the transcriptional activator RiMsn2 from AM fungus regulates drought stress tolerance in host plant.

Control of arbuscule development by a transcriptional negative feedback loop in Medicago

Most terrestrial plants establish a symbiosis with arbuscular mycorrhizal fungi (AMF), which provide them with lipids and sugars in exchange for phosphorus and nitrogen. Nutrient exchange must be dynamically controlled to maintain a mutually beneficial relationship between the two symbiotic partners. The WRI5a and its homologues play a conserved role in lipid supply to AMF. Here, we demonstrate that the AP2/ERF transcription factor MtERM1 binds directly to AW-box and AW-box-like cis-elements in the promoters of MtSTR2 and MtSTR, which are required for host lipid efflux and arbuscule development. The EAR domain-containing transcription factor MtERF12 is also directly activated by MtERM1/MtWRI5a to negatively regulate arbuscule development, and the TOPLESS co-repressor is further recruited by MtERF12 through EAR motif to oppose MtERM1/MtWRI5a function, thereby suppressing arbuscule development. We therefore reveal an ERM1/WRI5a-ERF12-TOPLESS negative feedback loop that enables plants to flexibly control nutrient exchange and ensure a mutually beneficial symbiosis.

LysM-mediated signaling in Marchantia polymorpha highlights the conservation of pattern-triggered immunity in land plants

Pattern-recognition receptor (PRR)-triggered immunity (PTI) wards off a wide range of pathogenic microbes, playing a pivotal role in angiosperms. The model liverwort Marchantia polymorpha triggers defense-related gene expression upon sensing components of bacterial and fungal extracts, suggesting the existence of PTI in this plant model. However, the molecular components of the putative PTI in M. polymorpha and the significance of PTI in bryophytes have not yet been described. We here show that M. polymorpha has four lysin motif (LysM)-domain-containing receptor homologs, two of which, LysM-receptor-like kinase (LYK) MpLYK1 and LYK-related (LYR) MpLYR, are responsible for sensing chitin and peptidoglycan fragments, triggering a series of characteristic immune responses. Comprehensive phosphoproteomic analysis of M. polymorpha in response to chitin treatment identified regulatory proteins that potentially shape LysM-mediated PTI. The identified proteins included homologs of well-described PTI components in angiosperms as well as proteins whose roles in PTI are not yet determined, including the blue-light receptor phototropin MpPHOT. We revealed that MpPHOT is required for negative feedback of defense-related gene expression during PTI. Taken together, this study outlines the basic framework of LysM-mediated PTI in M. polymorpha and highlights conserved elements and new aspects of pattern-triggered immunity in land plants.

Unravelling molecular mechanisms involved in resistance priming against downy mildew (Plasmopara viticola) in grapevine (Vitis vinifera L.)

Downy mildew (DM; Plasmopara viticola) is amongst the most severe fungal diseases in viticulture and the reason for the majority of fungicide applications. To reduce synthetic and copper-based fungicides, there is an urgent need for natural alternatives, which are being increasingly tested by the industry and the research community. However, their mode of action remains unclear. Therefore, our study aimed to investigate the transcriptomic changes induced by oregano essential oil vapour (OEOV) in DM-infected grapevines. OEOV was applied at different time points before and after DM infection to differentiate between a priming effect and a direct effect. Both pre-DM treatment with OEOV and post-infection treatment resulted in a significant reduction in DM sporulation. RNA-seq, followed by differential gene expression and weighted gene co-expression network analysis, identified co-expressed gene modules associated with secondary metabolism, pathogen recognition and response. Surprisingly, the molecular mechanisms underlying the efficiency of OEOV against DM appear to be independent of stilbene synthesis, and instead involve genes from a putative signalling pathway that has yet to be characterized. This study enhances our understanding of the molecular regulation of innate plant immunity and provides new insights into the mode of action of alternative natural antifungal agents.

TETRASPANIN 8-1 from Phaseolus vulgaris plays a key role during mutualistic interactions

Arbuscular mycorrhizal (AM) fungi and rhizobia form two of the most important plant-microbe associations for the assimilation of phosphorus (P) and nitrogen (N). Symbiont-derived signals are able to coordinate the infection process by triggering multiple responses in the plant root, such as calcium influxes and oscillations, increased reactive oxygen species (ROS), cytoskeletal rearrangements and altered gene expression. An examination was made of the role of tetraspanins, which are transmembrane proteins that self-organize into tetraspanin web regions, where they recruit specific proteins into platforms required for signal transduction, membrane fusion, cell trafficking, and ROS generation. In plant cells, tetraspanins are scaffolding proteins associated with root radial patterning, biotic and abiotic stress responses, cell fate determination, plasmodesmata and hormonal regulation. Some plant tetraspanins, such as Arabidopsis thaliana TETRASPANIN 8 and TETRASPANIN 9 (AtTET8 and AtTET9) are associated with exosomes during inter-kingdom communication. In this study, a homolog of AtTET8, PvTET8-1, in common bean (Phaseolus vulgaris L. var. Negro Jamapa) was examined in roots during interactions with Rhizobium tropici and Rhizophagus irregularis. The promoter of PvTET8-1 contained several cis-acting regulatory DNA elements potentially related to mutualistic interactions, and PvTET8-1 was transcriptionally activated during AM fungal and rhizobial associations. Silencing it decreased the size and number of nodules, nitrogen fixation, and mycorrhizal arbuscule formation, whereas overexpressing it increased the size and number of nodules, and mycorrhizal arbuscule formation but decreased nitrogen fixation. PvTET8-1 appears to be an important element in both of these mutualistic interactions, perhaps through its interaction with NADPH oxidase and the generation of ROS during the infection processes.

Characterization of the arbuscular mycorrhizal fungal community associated with rosewood in threatened Miombo forests

Understanding the dynamics of arbuscular mycorrhizal fungi (AMF) in response to land use change is important for the restoration of degraded forests. Here, we investigated the AMF community composition in the roots of Pterocarpus tinctorius sampled from agricultural and forest fallow soils rich in aluminum and iron. By sequencing the large subunit region of the rRNA gene, we identified a total of 30 operational taxonomic units (OTUs) in 33 root samples. These OTUs belonged to the genera Rhizophagus, Dominikia, Glomus, Sclerocystis, and Scutellospora. The majority of these OTUs did not closely match any known AMF species. We found that AMF species richness was significantly influenced by soil properties and overall tree density. Acidic soils with high levels of aluminum and iron had a low mean AMF species richness of 3.2. Indicator species analyses revealed several AMF OTUs associated with base saturation (4 OTUs), high aluminum (3 OTUs), and iron (2 OTUs). OTUs positively correlated with acidity (1 OTU), iron, and available phosphorus (2 OTUs) were assigned to the genus Rhizophagus, suggesting their tolerance to aluminum and iron. The results highlight the potential of leguminous trees in tropical dry forests as a reservoir of unknown AMF species. The baseline data obtained in this study opens new avenues for future studies, including the use of indigenous AMF-based biofertilizers to implement ecological revegetation strategies and improve land use.

Comparative phylotranscriptomics reveals ancestral and derived root nodule symbiosis programmes

Symbiotic interactions such as the nitrogen-fixing root nodule symbiosis (RNS) have structured ecosystems during the evolution of life. Here we aimed at reconstructing ancestral and intermediate steps that shaped RNS observed in extant flowering plants. We compared the symbiotic transcriptomic responses of nine host plants, including the mimosoid legume Mimosa pudica for which we assembled a chromosome-level genome. We reconstructed the ancestral RNS transcriptome composed of most known symbiotic genes together with hundreds of novel candidates. Cross-referencing with transcriptomic data in response to experimentally evolved bacterial strains with gradual symbiotic proficiencies, we found the response to bacterial signals, nodule infection, nodule organogenesis and nitrogen fixation to be ancestral. By contrast, the release of symbiosomes was associated with recently evolved genes encoding small proteins in each lineage. We demonstrate that the symbiotic response was mostly in place in the most recent common ancestor of the RNS-forming species more than 90 million years ago.

Integrated miRNA-mRNA analysis reveals candidate miRNA family regulating arbuscular mycorrhizal symbiosis of Poncirus trifoliata

Over 70% land plants live in mutualistic symbiosis with arbuscular mycorrhizal (AM) fungi, and maintenance of symbiosis requires transcriptional and post-transcriptional regulation. The former has been widely studied, whereas the latter mediated by symbiotic microRNAs (miRNAs) remains obscure, especially in woody plants. Here, we performed high-throughput sequencing of the perennial woody citrus plant Poncirus trifoliata and identified 3750 differentially expressed genes (DEGs) and 42 miRNAs (DEmiRs) upon AM fungal colonization. By analyzing cis-regulatory elements in the promoters of the DEGs, we predicted 329 key AM transcription factors (TFs). A miRNA-mRNA regulatory network was then constructed by integrating these data. Several candidate miRNA families of P. trifoliata were identified whose members target known symbiotic genes, such as miR167h-AMT2;3 and miR156e-EXO70I, or key TFs, such as miR164d-NAC and miR477a-GRAS, thus are involved in AM symbiotic processes of fungal colonization, arbuscule development, nutrient exchange and phytohormone signaling. Finally, analysis of selected miRNA family revealed that a miR159b conserved in mycorrhizal plant species and a Poncirus-specific miR477a regulate AM symbiosis. The role of miR477a was likely to target GRAS family gene RAD1 in citrus plants. Our results not only revealed that miRNA-mRNA network analysis, especially miRNA-TF analysis, is effective in identifying miRNA family regulating AM symbiosis, but also shed light on miRNA-mediated post-transcriptional regulation of AM symbiosis in woody citrus plants.

ARBUSCULAR MYCORRHIZA-INDUCED KINASES AMK8 and AMK24 associate with the receptor-like kinase KINASE3 to regulate arbuscular mycorrhizal symbiosis in Lotus japonicus

Arbuscular mycorrhizal (AM) symbiosis is a widespread, ancient mutualistic association between plants and fungi, and facilitates nutrient uptake into plants. Cell surface receptor-like kinases (RLKs) and receptor-like cytoplasmic kinases (RLCKs) play pivotal roles in transmembrane signaling, while few RLCKs are known to function in AM symbiosis. Here, we show that 27 out of 40 AM-induced kinases (AMKs) are transcriptionally upregulated by key AM transcription factors in Lotus japonicus. Nine AMKs are only conserved in AM-host lineages, among which the SPARK-RLK-encoding gene KINASE3 (KIN3) and the RLCK paralogues AMK8 and AMK24 are required for AM symbiosis. KIN3 expression is directly regulated by the AP2 transcription factor CTTC MOTIF-BINDING TRANSCRIPTION FACTOR1 (CBX1), which regulates the reciprocal exchange of nutrients in AM symbiosis, via the AW-box motif in the KIN3 promoter. Loss of function mutations in KIN3, AMK8, or AMK24 result in reduced mycorrhizal colonization in L. japonicus. AMK8 and AMK24 physically interact with KIN3. KIN3 and AMK24 are active kinases and AMK24 directly phosphorylates KIN3 in vitro. Moreover, CRISPR-Cas9-mediated mutagenesis of OsRLCK171, the sole homolog of AMK8 and AMK24 in rice (Oryza sativa), leads to diminished mycorrhization with stunted arbuscules. Overall, our results reveal a crucial role of the CBX1-driven RLK/RLCK complex in the evolutionarily conserved signaling pathway enabling arbuscule formation.

A glycan receptor kinase facilitates intracellular accommodation of arbuscular mycorrhiza and symbiotic rhizobia in the legume Lotus japonicus

Receptors that distinguish the multitude of microbes surrounding plants in the environment enable dynamic responses to the biotic and abiotic conditions encountered. In this study, we identify and characterise a glycan receptor kinase, EPR3a, closely related to the exopolysaccharide receptor EPR3. Epr3a is up-regulated in roots colonised by arbuscular mycorrhizal (AM) fungi and is able to bind glucans with a branching pattern characteristic of surface-exposed fungal glucans. Expression studies with cellular resolution show localised activation of the Epr3a promoter in cortical root cells containing arbuscules. Fungal infection and intracellular arbuscule formation are reduced in epr3a mutants. In vitro, the EPR3a ectodomain binds cell wall glucans in affinity gel electrophoresis assays. In microscale thermophoresis (MST) assays, rhizobial exopolysaccharide binding is detected with affinities comparable to those observed for EPR3, and both EPR3a and EPR3 bind a well-defined β-1,3/β-1,6 decasaccharide derived from exopolysaccharides of endophytic and pathogenic fungi. Both EPR3a and EPR3 function in the intracellular accommodation of microbes. However, contrasting expression patterns and divergent ligand affinities result in distinct functions in AM colonisation and rhizobial infection in Lotus japonicus. The presence of Epr3a and Epr3 genes in both eudicot and monocot plant genomes suggest a conserved function of these receptor kinases in glycan perception.

Temporal variation of mycorrhization rates in a tree diversity experiment

While mycorrhization rates have been studied in different contexts, not much is known about their temporal patterns across seasons. Here, we asked how mycorrhization rates of 10 deciduous trees assessed by microscopy changed from winter to spring to early summer. We made use of a tree diversity experiment on nutrient-rich soil (former farmland) in Central Germany. In the experiment, saplings of host species with a preference for either arbuscular mycorrhiza (AM) or ectomycorrhiza (EM) were planted in monocultures, two-species, and four-species mixtures. In addition, mixtures were composed of tree species of only one mycorrhizal type or by AM/EM trees. For almost all species, with the exception of Aesculus hippocastanum and Acer pseudoplatanus (only AM), dual mycorrhization with both types (AM and EM) was found at every sampling time (December, March, and May), although the expected preferences for certain mycorrhizal types were confirmed. The sampling date had a significant influence on mycorrhization rates of both EM and AM tree species. Frequencies of EM and AM were lowest in May, but there were no differences between December and March. The causes of this seasonal variation may be associated with climate-induced differences in carbon allocation to mycorrhizal tree roots in the temperate climate. Within individual trees, mycorrhization rates by AM and EM fungi were not correlated over time, pointing to asynchronous variation between both types and to independent drivers for AM and EM mycorrhization. At the community level, mycorrhiza frequency of either of the two types became more asynchronous from two-species to four-species mixtures. Thus, increased community asynchrony in mycorrhization could be another important mechanism in biodiversity-ecosystem functioning relationships.

What can hornworts teach us?

The hornworts are a small group of land plants, consisting of only 11 families and approximately 220 species. Despite their small size as a group, their phylogenetic position and unique biology are of great importance. Hornworts, together with mosses and liverworts, form the monophyletic group of bryophytes that is sister to all other land plants (Tracheophytes). It is only recently that hornworts became amenable to experimental investigation with the establishment of Anthoceros agrestis as a model system. In this perspective, we summarize the recent advances in the development of A. agrestis as an experimental system and compare it with other plant model systems. We also discuss how A. agrestis can help to further research in comparative developmental studies across land plants and to solve key questions of plant biology associated with the colonization of the terrestrial environment. Finally, we explore the significance of A. agrestis in crop improvement and synthetic biology applications in general.

Phylogenomics reveal that plants colonized land together with their fungal symbiotic partners

Most extant land plants establish a mutually beneficial relationship with soil fungi called mycorrhizal symbiosis. From their partners, plants get access to mineral nutrient and water resources transported via a fungal network that acts like an extension of their root systems. Using genetic and molecular tools, we showed that distant plant species use similar molecular mechanisms during the symbiosis. This similarity suggests that those mechanisms were inherited from their last common ancestor, a lineage that emerged from an aquatic environment 450 million years ago. Thus, this plant fungal interaction could have helped the first land plants without structures adapted to soil exploration to survive and colonize this new environment.

A transcriptional activator from Rhizophagus irregularis regulates phosphate uptake and homeostasis in AM symbiosis during phosphorous starvation

Introduction

Phosphorus (P) is one of the most important nutrient elements for plant growth and development. Under P starvation, arbuscular mycorrhizal (AM) fungi can promote phosphate (Pi) uptake and homeostasis within host plants. However, the underlying mechanisms by which AM fungal symbiont regulates the AM symbiotic Pi acquisition from soil under P starvation are largely unknown. Here, we identify a HLH domain containing transcription factor RiPho4 from Rhizophagus irregularis.

Methods

To investigate the biological functions of the RiPho4, we combined the subcellular localization and Yeast One-Hybrid (Y1H) experiments in yeasts with gene expression and virus-induced gene silencing approach during AM symbiosis.

Results

The approach during AM symbiosis. The results indicated that RiPho4 encodes a conserved transcription factor among different fungi and is induced during the in planta phase. The transcription of RiPho4 is significantly up-regulated by P starvation. The subcellular localization analysis revealed that RiPho4 is located in the nuclei of yeast cells during P starvation. Moreover, knock-down of RiPho4 inhibits the arbuscule development and mycorrhizal Pi uptake under low Pi conditions. Importantly, RiPho4 can positively regulate the downstream components of the phosphate (PHO) pathway in R. irregularis.

Discussion

In summary, these new findings reveal that RiPho4 acts as a transcriptional activator in AM fungus to maintain arbuscule development and regulate Pi uptake and homeostasis in the AM symbiosis during Pi starvation.

The RNAome landscape of tomato during arbuscular mycorrhizal symbiosis reveals an evolving RNA layer symbiotic regulatory network

Arbuscular mycorrhizal symbiosis (AMS) is an ancient plant-fungus relationship that is widely distributed in terrestrial plants. The formation of symbiotic structures and bidirectional nutrient exchange requires the regulation of numerous genes. However, the landscape of RNAome during plant AMS involving different types of regulatory RNA is poorly understood. In this study, a combinatorial strategy utilizing multiple sequencing approaches was used to decipher the landscape of RNAome in tomato, an emerging AMS model. The annotation of the tomato genome was improved by a multiple-platform sequencing strategy. A total of 3,174 protein-coding genes were upregulated during AMS, 42% of which were alternatively spliced. Comparative-transcriptome analysis revealed that genes from 24 orthogroups were consistently induced by AMS in eight phylogenetically distant angiosperms. Seven additional orthogroups were specifically induced by AMS in all surveyed dicot AMS host plants. However, these orthogroups were absent or not induced in monocots and/or non-AMS hosts, suggesting a continuously evolving AMS-responsive network in addition to a conserved core regulatory module. Additionally, we detected 587 lncRNAs, ten miRNAs, and 146 circRNAs that responded to AMS, which were incorporated to establish a tomato AMS-responsive, competing RNA-responsive endogenous RNA (ceRNA) network. Finally, a tomato symbiotic transcriptome database (TSTD, https://efg.nju.edu.cn/TSTD) was constructed to serve as a resource for deep deciphering of the AMS regulatory network. These results help elucidate the reconfiguration of the tomato RNAome during AMS and suggest a sophisticated and evolving RNA layer responsive network during AMS processes.

Functional Endophytes Regulating Plant Secondary Metabolism: Current Status, Prospects and Applications

Endophytes, which are widely found in host plants and have no harmful effects, are a vital biological resource. Plant endophytes promote plant growth and enhance plants' resistance to diseases, pests, and environmental stresses. In addition, they enhance the synthesis of important secondary metabolites in plants and improve the potential applicability of plants in agriculture, medicine, food, and horticulture. In this review, we summarize the recent progress in understanding the interaction between endophytes and plants and summarize the construction of synthetic microbial communities (SynComs) and metaomics analysis of the interaction between endophytes and plants. The application and development prospects of endophytes in agriculture, medicine, and other industries are also discussed to provide a reference for further study of the interaction between endophytes and plants and further development and utilization of endophytes.

Exploring the role of plant lysin motif receptor-like kinases in regulating plant-microbe interactions in the bioenergy crop Populus

For plants, distinguishing between mutualistic and pathogenic microbes is a matter of survival. All microbes contain microbe-associated molecular patterns (MAMPs) that are perceived by plant pattern recognition receptors (PRRs). Lysin motif receptor-like kinases (LysM-RLKs) are PRRs attuned for binding and triggering a response to specific MAMPs, including chitin oligomers (COs) in fungi, lipo-chitooligosaccharides (LCOs), which are produced by mycorrhizal fungi and nitrogen-fixing rhizobial bacteria, and peptidoglycan in bacteria. The identification and characterization of LysM-RLKs in candidate bioenergy crops including Populus are limited compared to other model plant species, thus inhibiting our ability to both understand and engineer microbe-mediated gains in plant productivity. As such, we performed a sequence analysis of LysM-RLKs in the Populus genome and predicted their function based on phylogenetic analysis with known LysM-RLKs. Then, using predictive models, molecular dynamics simulations, and comparative structural analysis with previously characterized CO and LCO plant receptors, we identified probable ligand-binding sites in Populus LysM-RLKs. Using several machine learning models, we predicted remarkably consistent binding affinity rankings of Populus proteins to CO. In addition, we used a modified Random Walk with Restart network-topology based approach to identify a subset of Populus LysM-RLKs that are functionally related and propose a corresponding signal transduction cascade. Our findings provide the first look into the role of LysM-RLKs in Populus-microbe interactions and establish a crucial jumping-off point for future research efforts to understand specificity and redundancy in microbial perception mechanisms.

Endofungal bacteria boost anthelminthic host protection with the biosurfactant symbiosin

Effective protection of soil fungi from predators is crucial for their survival in the niche. Thus, fungi have developed efficient defence strategies. We discovered that soil beneficial Mortierella fungi employ a potent cytotoxin (necroxime) against fungivorous nematodes. Interestingly, this anthelminthic agent is produced by bacterial endosymbionts (Candidatus Mycoavidus necroximicus) residing within the fungus. Analysis of the symbiont's genome indicated a rich biosynthetic potential, yet nothing has been known about additional metabolites and their potential synergistic functions. Here we report that two distinct Mortierella endosymbionts produce a novel cyclic lipodepsipeptide (symbiosin), that is clearly of bacterial origin, but has striking similarities to various fungal specialized metabolites. The structure and absolute configuration of symbiosin were fully elucidated. By comparative genomics of symbiosin-positive strains and in silico analyses of the deduced non-ribosomal synthetases, we assigned the (sym) biosynthetic gene cluster and proposed an assembly line model. Bioassays revealed that symbiosin is not only an antibiotic, in particular against mycobacteria, but also exhibits marked synergistic effects with necroxime in anti-nematode tests. By functional analyses and substitution experiments we found that symbiosin is a potent biosurfactant and that this particular property confers a boost in the anthelmintic action, similar to formulations of therapeutics in human medicine. Our findings illustrate that "combination therapies" against parasites already exist in ecological contexts, which may inspire the development of biocontrol agents and therapeutics.

Plant-microbe symbiosis widens the habitability range of the Daisyworld

Plant-microbe symbiosis is pervasive in the Earth's ecosystems and dates back to the early land colonisation by plants. Mutualistic partnership with rhizobia bacteria and mycorrhizal fungi promotes plant nutrition, growth and diversity, impacting important ecosystem functions. However, how the global behaviour and dynamical properties of an ecosystem are modified by plant-microbe symbiosis is still unclear. To tackle this theoretical question, we resorted to the Daisyworld as a toy model of the global ecosystem. We redesigned the original model to allow accounting for seed production, spreading, germination, and seedling development to mature seed-producing plants to describe how symbiotic and non-symbiotic daisy species differ in these key processes. Using the steady-state and bifurcation analysis of this model, we demonstrate that symbiosis with microbes broadens the habitability range of the Daisyworld by enhancing plant growth and/or facilitating plant access to otherwise uninhabitable nutrient-poor regions.

Lipid transfer proteins involved in plant-pathogen interactions and their molecular mechanisms

Nonspecific lipid transfer proteins (LTPs) are small, cysteine-rich proteins that play numerous functional roles in plant growth and development, including cutin wax formation, pollen tube adhesion, cell expansion, seed development, germination, and adaptation to changing environmental conditions. LTPs contain eight conserved cysteine residues and a hydrophobic cavity that provides a wide variety of lipid-binding specificities. As members of the pathogenesis-related protein 14 family (PR14), many LTPs inhibit fungal or bacterial growth, and act as positive regulators in plant disease resistance. Over the past decade, these essential immunity-related roles of LTPs in plant immune processes have been documented in a growing body of literature. In this review, we summarize the roles of LTPs in plant-pathogen interactions, emphasizing the underlying molecular mechanisms in plant immune responses and specific LTP functions.

A phosphate starvation response-regulated receptor-like kinase, OsADK1, is required for mycorrhizal symbiosis and phosphate starvation responses

Most land plants associate with arbuscular mycorrhizal (AM) fungi to secure mineral nutrient acquisition, especially that of phosphorus. A phosphate starvation response (PHR)-centered network regulates AM symbiosis. Here, we identified 520 direct target genes for the rice transcription factor OsPHR1/2/3 during AM symbiosis using transcriptome deep sequencing and DNA affinity purification sequencing. These genes were involved in strigolactone biosynthesis, transcriptional reprogramming, and bidirectional nutrient exchange. Moreover, we identified the receptor-like kinase, Arbuscule Development Kinase 1 (OsADK1), as a new target of OsPHR1/2/3. Electrophoretic mobility shift assays and transactivation assays showed that OsPHR2 can bind directly to the P1BS elements within the OsADK1 promoter to activate its transcription. OsADK1 appeared to be required for mycorrhizal colonization and arbuscule development. In addition, hydroponic experiments suggested that OsADK1 may be involved in plant Pi starvation responses. Our findings validate a role for OsPHR1/2/3 as master regulators of mycorrhizal-related genes involved in various stages of symbiosis, and uncover a new RLK involved in AM symbiosis and plant Pi starvation responses.

Functions of Lipids in Development and Reproduction of Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhizal fungi (AMF) form mutualistic associations with most land plants. The symbiosis is based on the exchange of nutrients: AMF receive photosynthetically fixed carbon from the plants and deliver mineral nutrients in return. Lipids are important players in the symbiosis. They act as components of the plant-derived membrane surrounding arbuscules, as carbon sources transferred from plants to AMF, as a major form of carbon storage in AMF and as triggers of developmental responses in AMF. In this review, we describe the role of lipids in arbuscular mycorrhizal symbiosis and AMF development.

A simple and versatile fluorochrome-based procedure for imaging of lipids in arbuscule-containing cells

The arbuscular mycorrhizal (AM) symbiosis is characterized by the reciprocal exchange of nutrients. AM fungi are oleaginous microorganisms that obtain essential fatty acids from host plants. A lipid biosynthesis and delivery pathway has been proposed to operate in inner root cortex cells hosting arbuscules, a cell type challenging to access microscopically. Despite the central role lipids play in the association, lipid distribution patterns during arbuscule development are currently unknown. We developed a simple co-staining method employing fluorophore-conjugated Wheat Germ Agglutinin (WGA) and a lipophilic blue fluorochrome, Ac-201, for the simultaneous imaging of arbuscules and lipids distributed within arbuscule-containing cells in high resolution. We observed lipid distribution patterns in wild-type root infection zones in a variety of plant species. In addition, we applied this methodology to mutants of the Lotus japonicus GRAS transcription factor RAM1 and the Oryza sativa half-size ABC transporter STR1, both proposed to be impaired in the symbiotic lipid biosynthesis-delivery pathway. We found that lipids accumulated in cortical cells hosting stunted arbuscules in Ljram1 and Osstr1, and observed lipids in the arbuscule body of Osstr1, suggesting that in the corresponding plant species, RAM1 and STR1 may not be essential for symbiotic lipid biosynthesis and transfer from arbuscule-containing cells, respectively. The versatility of this methodology has the potential to help elucidate key questions on the complex lipid dynamics fostering AM symbioses.

The renaissance and enlightenment of Marchantia as a model system

The liverwort Marchantia polymorpha has been utilized as a model for biological studies since the 18th century. In the past few decades, there has been a Renaissance in its utilization in genomic and genetic approaches to investigating physiological, developmental, and evolutionary aspects of land plant biology. The reasons for its adoption are similar to those of other genetic models, e.g. simple cultivation, ready access via its worldwide distribution, ease of crossing, facile genetics, and more recently, efficient transformation, genome editing, and genomic resources. The haploid gametophyte dominant life cycle of M. polymorpha is conducive to forward genetic approaches. The lack of ancient whole-genome duplications within liverworts facilitates reverse genetic approaches, and possibly related to this genomic stability, liverworts possess sex chromosomes that evolved in the ancestral liverwort. As a representative of one of the three bryophyte lineages, its phylogenetic position allows comparative approaches to provide insights into ancestral land plants. Given the karyotype and genome stability within liverworts, the resources developed for M. polymorpha have facilitated the development of related species as models for biological processes lacking in M. polymorpha.

Transcriptional regulation of oil biosynthesis in seed plants: Current understanding, applications, and perspectives

Plants produce and accumulate triacylglycerol (TAG) in their seeds as an energy reservoir to support the processes of seed germination and seedling development. Plant seed oils are vital not only for the human diet but also as renewable feedstocks for industrial use. TAG biosynthesis consists of two major steps: de novo fatty acid biosynthesis in the plastids and TAG assembly in the endoplasmic reticulum. The latest advances in unraveling transcriptional regulation have shed light on the molecular mechanisms of plant oil biosynthesis. We summarize recent progress in understanding the regulatory mechanisms of well-characterized and newly discovered transcription factors and other types of regulators that control plant fatty acid biosynthesis. The emerging picture shows that plant oil biosynthesis responds to developmental and environmental cues that stimulate a network of interacting transcriptional activators and repressors, which in turn fine-tune the spatiotemporal regulation of the pathway genes.

Parametarhizium hingganense, a Novel Ectomycorrhizal Fungal Species, Promotes the Growth of Mung Beans and Enhances Resistance to Disease Induced by Rhizoctonia solani

The mutualistic interactions between mycorrhizae and plants first occurred along with the terrestrialization of plants. The majority of vascular plants are in symbiosis with mycorrhizal fungi. Due to their importance to the economy and ecology, arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi emerge as the most popular ones. However, the mechanism underlying the beneficial function of ECM fungi is not as clear as AM fungi. Here, the interaction between Parametarhizium hingganense, a novel fungal species isolated from forest litter, and mung bean (Vigna radiata) was studied. P. hingganense demonstrated P solubilization ability in vitro. Treatment of P. hingganense on the seeds resulted in promoted growth with enhanced P content. The hyphae of green fluorescence protein (GFP)-tagged P. hingganense were found to surround the roots and develop between cells, suggesting the establishment of an ectomycorrhizal symbiosis. Upon symbiosis with P. hingganense, the levels of jasmonic acid (JA) and gibberellin (GA₁) and total phenolic and flavonoid content elevated. Meanwhile, damping off caused by Rhizoctonia solani in mycorrhizal plants was alleviated. Taken together, the above findings suggested that symbiosis with P. hingganense conferred growth promotion and priming of defense responses to host plants which should be associated with facilitated P uptake and increased JA and GA₁ levels.

The greening ashore

More than half a billion years ago a streptophyte algal lineage began terraforming the terrestrial habitat and the Earth's atmosphere. This pioneering step enabled the subsequent evolution of all complex life on land, and the past decade has uncovered that many traits, both morphological and genetic, once thought to be unique to land plants, are conserved across some streptophyte algae. They provided the common ancestor of land plants with a repertoire of genes, of which many were adapted to overcome the new biotic and abiotic challenges. Exploring these molecular adaptations in non-tracheophyte species may help us to better prepare all green life, including our crops, for the challenges precipitated by the climate change of the Anthropocene because the challenges mostly differ by the speed with which they are now being met.

Plant-microbe interactions that have impacted plant terrestrializations

Plants display a tremendous diversity of developmental and physiological features, resulting from gains and losses of functional innovations across the plant phylogeny. Among those, the most impactful have been undoubtedly the ones that allowed plant terrestrializations, the transitions from an aquatic to a terrestrial environment. Although the embryophyte terrestrialization has been particularly scrutinized, others occurred across the plant phylogeny with the involvement of mutualistic symbioses as a common theme. Here, we review the current pieces of evidence supporting that the repeated colonization of land by plants has been facilitated by interactions with mutualistic symbionts. In that context, we detail two of these mutualistic symbioses: the arbuscular mycorrhizal symbiosis in embryophytes and the lichen symbiosis in chlorophyte algae. We suggest that associations with bacteria should be revisited in that context, and we propose that overlooked symbioses might have facilitated the emergence of other land plant clades.

Noncanonical Strigolactone Analogues Highlight Selectivity for Stimulating Germination in Two Phelipanche ramosa Populations

Strigolactones (SLs) are plant hormones exuded in the rhizosphere with a signaling role for the development of arbuscular mycorrhizal (AM) fungi and as stimulants of seed germination of the parasitic weeds Orobanche, Phelipanche, and Striga, the most threatening weeds of major crops worldwide. Phelipanche ramosa is present mainly on rape, hemp, and tobacco in France. P. ramosa 2a preferentially attacks hemp, while P. ramosa 1 attacks rapeseed. The recently isolated cannalactone (14) from hemp root exudates has been characterized as a noncanonical SL that selectively stimulates the germination of P. ramosa 2a seeds in comparison with P. ramosa 1. In the present work, (-)-solanacol (5), a canonical orobanchol-type SL exuded by tobacco and tomato, was established to possess a remarkable selective germination stimulant activity for P. ramosa 2a seeds. Two cannalactone analogues, named (±)-SdL19 and (±)-SdL118, have been synthesized. They have an unsaturated acyclic carbon chain with a tertiary hydroxy group and a methyl or a cyclopropyl group instead of a cyclohexane A-ring, respectively. (±)-SdL analogues are able to selectively stimulate P. ramosa 2a, revealing that these minimal structural elements are key for this selective bioactivity. In addition, (±)-SdL19 is able to inhibit shoot branching in Pisum sativum and Arabidopsis thaliana and induces hyphal branching in the AM fungus Rhizophagus irregularis, like SLs.

An expanded role for the transcription factor WRINKLED1 in the biosynthesis of triacylglycerols during seed development

The transcription factor WRINKLED1 (WRI1) is known as a master regulator of fatty acid synthesis in developing oilseeds of Arabidopsis thaliana and other species. WRI1 is known to directly stimulate the expression of many fatty acid biosynthetic enzymes and a few targets in the lower part of the glycolytic pathway. However, it remains unclear to what extent and how the conversion of sugars into fatty acid biosynthetic precursors is controlled by WRI1. To shortlist possible gene targets for future in-planta experimental validation, here we present a strategy that combines phylogenetic foot printing of cis-regulatory elements with additional layers of evidence. Upstream regions of protein-encoding genes in A. thaliana were searched for the previously described DNA-binding consensus for WRI1, the ASML1/WRI1 (AW)-box. For about 900 genes, AW-box sites were found to be conserved across orthologous upstream regions in 11 related species of the crucifer family. For 145 select potential target genes identified this way, affinity of upstream AW-box sequences to WRI1 was assayed by Microscale Thermophoresis. This allowed definition of a refined WRI1 DNA-binding consensus. We find that known WRI1 gene targets are predictable with good confidence when upstream AW-sites are phylogenetically conserved, specifically binding WRI1 in the in vitro assay, positioned in proximity to the transcriptional start site, and if the gene is co-expressed with WRI1 during seed development. When targets predicted in this way are mapped to central metabolism, a conserved regulatory blueprint emerges that infers concerted control of contiguous pathway sections in glycolysis and fatty acid biosynthesis by WRI1. Several of the newly predicted targets are in the upper glycolysis pathway and the pentose phosphate pathway. Of these, plastidic isoforms of fructokinase (FRK3) and of phosphoglucose isomerase (PGI1) are particularly corroborated by previously reported seed phenotypes of respective null mutations.

Mycorrhizal symbiosis reprograms ion fluxes and fatty acid metabolism in wild jujube during salt stress

Chinese jujube (Ziziphus jujuba) is an important fruit tree in China, and soil salinity is the main constraint affecting jujube production. It is unclear how arbuscular mycorrhizal (AM) symbiosis supports jujube adaptation to salt stress. Herein, we performed comparative physiological, ion flux, fatty acid (FA) metabolomic, and transcriptomic analyses to examine the mechanism of AM jujube responding to salt stress. AM seedlings showed better performance during salt stress. AM symbiosis altered phytohormonal levels: indole-3-acetic acid and abscisic acid contents were significantly increased in AM roots and reduced by salt stress. Mycorrhizal colonization enhanced root H+ efflux and K+ influx, while inducing expression of plasma membrane-type ATPase 7 (ZjAHA7) and high-affinity K+ transporter 2 (ZjHAK2) in roots. High K+/Na+ homeostasis was maintained throughout salt exposure. FA content was elevated in AM leaves as well as roots, especially for palmitic acid, oleic acid, trans oleic acid, and linoleic acid, and similar effects were also observed in AM poplar (Populus. alba × Populus. glandulosa cv. 84K) and Medicago truncatula, indicating AM symbiosis elevating FA levels could be a conserved physiological effect. Gene co-expression network analyses uncovered a core gene set including 267 genes in roots associated with AM symbiosis and conserved transcriptional responses, for example, FA metabolism, phytohormone signal transduction, SNARE interaction in vesicular transport, and biotin metabolism. In contrast to widely up-regulated genes related to FA metabolism in AM roots, limited genes were affected in leaves. We propose a model of AM symbiosis-linked reprogramming of FA metabolism and provide a comprehensive insight into AM symbiosis with a woody species adaptation to salt stress.