Plant neighbor detection and allelochemical response are driven by root-secreted signaling chemicals

Interspecific plant interaction via root exudates structures the disease suppressiveness of rhizosphere microbiomes

Terrestrial plants can affect the growth and health of adjacent plants via interspecific interaction. Here, we studied the mechanism by which plant root exudates affect the recruitment of the rhizosphere microbiome in adjacent plants-with implications for plant protection-using a tomato (Solanum lycopersicum)-potatoonion (Allium cepa var. agrogatum) intercropping system. First, we showed that the intercropping system results in a disease-suppressive rhizosphere microbiome that protects tomato plants against Verticillium wilt disease caused by the soilborne pathogen Verticillium dahliae. Second, 16S rRNA gene sequencing revealed that intercropping with potatoonion altered the composition of the tomato rhizosphere microbiome by promoting the colonization of specific Bacillus sp. This taxon was isolated and shown to inhibit V. dahliae growth and induce systemic resistance in tomato plants. Third, a belowground segregation experiment found that root exudates mediated the interspecific interaction between potatoonion and tomato. Moreover, experiments using split-root tomato plants found that root exudates from potatoonion, especially taxifolin-a flavonoid compound-stimulate tomato plants to recruit plant-beneficial bacteria, such as Bacillus sp. Lastly, ultra-high-pressure liquid chromatography-mass spectrometry analysis found that taxifolin alters tomato root exudate chemistry; thus, this compound acts indirectly in modulating root colonization by Bacillus sp. Our results revealed that this intercropping system can improve tomato plant fitness by changing rhizosphere microbiome recruitment via the use of signaling chemicals released by root exudates of potatoonion. This study revealed a novel mechanism by which interspecific plant interaction modulates the establishment of a disease-suppressive microbiome, thus opening up new avenues of research for precision plant microbiome manipulations.

Weed-induced crop yield loss: a new paradigm and new challenges

Direct competition for resources is generally considered the primary mechanism for weed-induced yield loss. A re-evaluation of physiological evidence suggests weeds initially impact crop growth and development through resource-independent interference. We suggest weed perception by crops induce a shift in crop development, before resources become limited, which ultimately reduce crop yield, even if weeds are subsequently removed. We present the mechanisms by which crops perceive and respond to weeds and discuss the technologies used to identify these mechanisms. These data lead to a fundamental paradigm shift in our understanding of how weeds reduce crop yield and suggest new research directions and opportunities to manipulate or engineer crops and cropping systems to reduce weed-induced yield losses.

Linkages of microbial community structure and root exudates: Evidence from microbial nitrogen limitation in soils of crop families

Rhizosphere microorganisms are critical for crop nutrient cycling and soil ecological functions in agroecosystem soils; however, there is limited information regarding the role of root exudates in determining soil microbial communities and functions in plant-soil systems, especially for microbial nutrient limitations. In the present study, rhizosphere soil samples were collected from the main food crop families, including maize, soybean, potato, and buckwheat, representing the cereals, Leguminosae, Solanaceae, and Polygonaceae families, in the northern Loess Plateau, China, to investigate soil microbial co-occurrences and assembly processes and the relationship between soil microbes and root exudates. The results showed that the crop families greatly regulated the soil microbial community composition and assembly, and all microorganisms of the four species were subjected to N limitation via the vector analysis. The topological properties of the soil microbial networks varied with the crop family, demonstrating that the ecological relationships of bacterial taxa are more complex than those of fungi. Stochastic processes were more important in stimulating assembly across the four crop families; the non-dominated process governed >60 % of the critical ecological turnover in community assembly, whereas dispersal limitation was the key factor influencing fungal community assembly. Furthermore, the metabolic profiles of root exudates in response to microbial N limitation varied by family. Microbial function and metabolic limitations were strongly associated with variations in root exudates, especially amino acids and organic acids, which were directly facilitated by crop families. Our results highlight the key roles of root exudates in stimulating microbial community structure and ecological functions from the perspective of microbial nutrient limitation and improve our understanding of plant-microbe interactions in agricultural ecosystems.

Integrated phylogenomic analyses unveil reticulate evolution in Parthenocissus (Vitaceae), highlighting speciation dynamics in the Himalayan-Hengduan Mountains

Hybridization caused by frequent environmental changes can lead both to species diversification (speciation) and to speciation reversal (despeciation), but the latter has rarely been demonstrated. Parthenocissus, a genus with its trifoliolate lineage in the Himalayan-Hengduan Mountains (HHM) region showing perplexing phylogenetic relationships, provides an opportunity for investigating speciation dynamics based on integrated evidence. We investigated phylogenetic discordance and reticulate evolution in Parthenocissus based on rigorous analyses of plastome and transcriptome data. We focused on reticulations in the trifoliolate lineage in the HHM region using a population-level genome resequencing dataset, incorporating evidence from morphology, distribution, and elevation. Comprehensive analyses confirmed multiple introgressions within Parthenocissus in a robust temporal-spatial framework. Around the HHM region, at least three hybridization hot spots were identified, one of which showed evidence of ongoing speciation reversal. We present a solid case study using an integrative methodological approach to investigate reticulate evolutionary history and its underlying mechanisms in plants. It demonstrates an example of speciation reversal through frequent hybridizations in the HHM region, which provides new perspectives on speciation dynamics in mountainous areas with strong topographic and environmental heterogeneity.

Rhizosphere engineering for sustainable crop production: entropy-based insights

There is a growing interest in exploring interactions at root-soil interface in natural and agricultural ecosystems, but an entropy-based understanding of these dynamic rhizosphere processes is lacking. We have developed a new conceptual model of rhizosphere regulation by localized nutrient supply using thermodynamic entropy. Increased nutrient-use efficiency is achieved by rhizosphere management based on self-organization and minimized entropy via equilibrium attractors comprising (i) optimized root strategies for nutrient acquisition and (ii) improved information exchange related to root-soil-microbe interactions. The cascading effects through different hierarchical levels amplify the underlying processes in plant-soil system. We propose a strategy for manipulating rhizosphere dynamics and improving nutrient-use efficiency by localized nutrient supply with minimization of entropy to underpin sustainable food/feed/fiber production.

Kin Recognition in an Herbicide-Resistant Barnyardgrass (Echinochloa crus-galli L.) Biotype

Despite increasing evidence of kin recognition in natural and crop plants, there is a lack of knowledge of kin recognition in herbicide-resistant weeds that are escalating in cropping systems. Here, we identified a penoxsulam-resistant barnyardgrass biotype with the ability for kin recognition from two biotypes of penoxsulam-susceptible barnyardgrass and normal barnyardgrass at different levels of relatedness. When grown with closely related penoxsulam-susceptible barnyardgrass, penoxsulam-resistant barnyardgrass reduced root growth and distribution, lowering belowground competition, and advanced flowering and increased seed production, enhancing reproductive effectiveness. However, such kin recognition responses were not occurred in the presence of distantly related normal barnyardgrass. Root segregation, soil activated carbon amendment, and root exudates incubation indicated chemically-mediated kin recognition among barnyardgrass biotypes. Interestingly, penoxsulam-resistant barnyardgrass significantly reduced a putative signaling (-)-loliolide production in the presence of closely related biotype but increased production when growing with distantly related biotype and more distantly related interspecific allelopathic rice cultivar. Importantly, genetically identical penoxsulam-resistant and -susceptible barnyardgrass biotypes synergistically interact to influence the action of allelopathic rice cultivar. Therefore, kin recognition in plants could also occur at the herbicide-resistant barnyardgrass biotype level, and intraspecific kin recognition may facilitate cooperation between genetically related biotypes to compete with interspecific rice, offering many potential implications and applications in paddy systems.

Penoxsulam-resistant barnyardgrass-mediated rhizosphere microbial communities affect the growth of rice

BACKGROUND

The incidence of herbicide-resistant barnyardgrass is escalating in paddy fields, yet the interactions between resistant weeds and rice are largely unknown. The microbiota of herbicide-resistant barnyardgrass rhizosphere soil is critical for both barnyardgrass and rice fitness.

RESULTS

Rice has different biomass allocation and root traits in the presence of penoxsulam-resistant versus penoxsulam-susceptible barnyardgrass or in their conditioned soil. Compared to susceptible barnyardgrass, resistant barnyardgrass led to an allelopathic increase in rice root, shoot, and whole-plant biomasses. Resistant barnyardgrass recruited distinct core and unique microbes in rhizosphere soil compared to susceptible barnyardgrass. In particular, resistant barnyardgrass assembled more Proteobacteria and Ascomycota to enhance plant stress tolerance. Furthermore, the root exudates from resistant and susceptible barnyardgrass were responsible for the assembly and establishment of the root microbial structure. Importantly, (-)-loliolide and jasmonic acid in root exudates were correlated with the core microbes in the rhizosphere soil.

CONCLUSION

The interference of barnyardgrass with rice can be mediated by rhizosphere microbial communities. Biotype-specific variation in the ability to generate soil microbial communities appears to ameliorate the negative consequences for rice growth, providing an intriguing possibility for modulation of the rhizosphere microbiota to increase crop productivity and sustainability. © 2023 Society of Chemical Industry.

L-phenylalanine in potato onion (Allium cepa var. aggregatum G. Don) root exudates mediates neighbor detection and trigger physio-morphological root responses of tomato

INTERACTION

Despite numerous recent insights into neighbor detection and belowground plant communication mediated by root exudates, less is known about the specificity and nature of substances within root exudates and the mechanism by which they may act belowground in root-root interactions.

METHODS

Here, we used a coculture experiment to study the root length density (RLD) of tomato (Solanum lycopersicum L.) grown with potato onion (Allium cepa var. aggregatum G. Don) cultivars with growth-promoting (S-potato onion) or no growth-promoting (N-potato onion) effects.

RESULTS AND DISCUSSION

Tomato plants grown with growth-promoting potato onion or its root exudates increased root distribution and length density oppositely and grew their roots away as compared to when grown with potato onion of no growth-promoting potential, its root exudates, and control (tomato monoculture/distilled water treatment). Root exudates profiling of two potato onion cultivars by UPLC-Q-TOF/MS showed that L-phenylalanine was only found in root exudates of S-potato onion. The role of L-phenylalanine was further confirmed in a box experiment in which it altered tomato root distribution and forced the roots grow away. In vitro trial revealed that tomato seedlings root exposed to L-phenylalanine changed the auxin distribution, decreased the concentration of amyloplasts in columella cells of roots, and changed the root deviation angle to grow away from the addition side. These results suggest that L-phenylalanine in S-potato onion root exudates may act as an "active compound" and trigger physio-morphological changes in neighboring tomato roots.

Host root exudates initiate a foraging preference by the root parasite Santalum album

Haustoria of root-parasitic plants draw nutrients from the roots of host species. While recent studies have assessed host preferences of parasitic plants, how root-exuded chemicals can mediate host tropism and selection by root-parasitic plants is poorly understood. Under greenhouse conditions, we performed two pot experiments to determine whether the root parasite Santalum album selectively forages for superior hosts (N2-fixing Acacia confusa Merr. or Dalbergia odorifera T. Chen) rather than for inferior hosts (non-N2-fixing Bischofia polycarpa (levl.) Airy Shaw or Dracontomelon duperreranum Pierre), and whether S. album uses host root exudates and/or specific chemicals in these root exudates to locate and trigger haustorium formation. Lateral roots and haustoria of S. album seedlings exhibited greater growth in the direction of D. odorifera roots than toward roots from the other three hosts. Comparative metabolic analysis revealed that D. odorifera root exudates were enriched in isoflavonoid, flavonoid and flavone/flavonol biosynthesis pathways, and that the relative contents of flavonoids were significantly greater in the root exudates of D. odorifera than in those of the other three hosts. Root exudates from D. odorifera significantly promoted S. album root growth, haustorium formation and reactive oxygen species accumulation in haustoria. Our results demonstrate that the key step in plant parasitism by S. album is based on root exudation by a host plant; the exudates function as a metabolite signal that activate lateral root growth and haustorium formation. Our results also indicate that flavonoids in the root exudates could play an important role in S. album foraging activity. Information on the responses of root parasites to host root exudates and/or haustorium-inducing chemicals may be useful for selecting superior host species to plant with valuable species of root parasites.

Root placement patterns in allelopathic plant-plant interactions

Plants actively respond to their neighbors by altering root placement patterns. Neighbor-modulated root responses involve root detection and interactions mediated by root-secreted functional metabolites. However, chemically mediated root placement patterns and their underlying mechanisms remain elusive. We used an allelopathic wheat model system challenged with 60 target species to identify root placement responses in window rhizobox experiments. We then tested root responses and their biochemical mechanisms in incubation experiments involving the addition of activated carbon and functional metabolites with amyloplast staining and auxin localization in roots. Wheat and each target species demonstrated intrusive, avoidant or unresponsive root placement, resulting in a total of nine combined patterns. Root placement patterns were mediated by wheat allelochemicals and (-)-loliolide signaling of neighbor species. In particular, (-)-loliolide triggered wheat allelochemical production that altered root growth and placement, degraded starch grains in the root cap and induced uneven distribution of auxin in target species roots. Root placement patterns in wheat-neighbor interactions were perception dependent and species dependent. Signaling (-)-loliolide induced the production and release of wheat allelochemicals that modulated root placement patterns. Therefore, root placement patterns are generated by both signaling chemicals and allelochemicals in allelopathic plant-plant interactions.

The ecologically relevant genetics of plant-plant interactions

Interactions among plants have been long recognized as a major force driving plant community dynamics and crop yield. Surprisingly, our knowledge of the ecological genetics associated with variation of plant-plant interactions remains limited. In this opinion article by scientists from complementary disciplines, the international PLANTCOM network identified four timely questions to foster a better understanding of the mechanisms mediating plant assemblages. We propose that by identifying the key relationships among phenotypic traits involved in plant-plant interactions and the underlying adaptive genetic and molecular pathways, while considering environmental fluctuations at diverse spatial and time scales, we can improve predictions of genotype-by-genotype-by-environment interactions and modeling of productive and stable plant assemblages in wild habitats and crop fields.

Distinct metabolite classes in root exudates are indicative for field- or hydroponically-grown cover crops

Introduction

Plants release a large variety of metabolites via their roots to shape physico-chemical soil properties and biological processes in the rhizosphere. While hydroponic growth conditions facilitate accessibility of the root system and recovery of root exudates, the natural soil environment can alter root metabolism and exudate secretion, raising the question to what extent the quantity and composition of root exudates released in hydroponic growth systems reflect those recovered from soil-grown roots.

Methods

Using a root washing method, we sampled root exudates from four field-grown cover crop species with wide taxonomic distance, namely white mustard, lacy phacelia, bristle oat, and Egyptian clover. A set of primary metabolites and secondary metabolites were analysed in a targeted and untargeted LC-MS-based approach, respectively, for comparison with exudates obtained from hydroponically cultured plants.

Results and discussion

We found that hydroponically cultivated plants released a larger amount of total carbon, but that the recovery of total carbon was not indicative for the diversity of metabolites in root exudates. In the field, root exudates from phacelia and clover contained 2.4 to 3.8 times more secondary metabolites, whereas carbon exudation in hydroponics was 5- to 4-fold higher. The composition of the set of metabolites identified using the untargeted approach was much more distinct among all species and growth conditions than that of quantified primary metabolites. Among secondary metabolite classes, the presence of lipids and lipid-like molecules was highly indicative for field samples, while the release of a large amount of phenylpropanoids, organoheterocyclic compounds or benzenoids was characteristic for clover, mustard or oat, respectively, irrespective of the cultivation condition. However, at the compound level the bulk of released metabolites was specific for cultivation conditions in every species, which implies that hydroponically sampled root exudates poorly reflect the metabolic complexity of root exudates recovered from field-grown plants.

Effects of indirect plant-plant interaction via root exudate on growth and leaf chemical contents in Rumex obtusifolius

Belowground plant-plant interactions can affect the concentrations of leaf chemicals, but the mechanism is not clear. Here, we investigated the effects of intra- and interspecific root exudates on the growth and leaf chemical content of Rumex obtusifolius. Seedlings of R. obtusifolius were grown with exposure to root exudates collected from other R. obtusifolius plants or from Trifolium repens, Festuca ovina, or Plantago asiatica plants, and the total phenolic, condensed tannin, dry biomass, and chlorophyll contents of the leaves were examined. The root exudates from conspecific plants had no effect on the total phenolic, condensed tannin, and chlorophyll contents of the leaves but did significantly reduce the dry leaf biomass. Root exudates from heterospecific plants had different effects depending on the species. These results were different from the results of a previous study that examined the effects of direct plant-plant interaction in R. obtusifolius. Thus, indirect interaction via root exudates induces different effects in leaves from direct interaction.

Quick and In Situ Detection of Different Polar Allelochemicals in Taxus Soil by Microdialysis Combined with UPLC-MS/MS

The action of allelopathy need that allelochemicals exist in the soil and reach a certain concentration. Also, the detection of allelochemicals in the soil is one of the most important research topics in the process of exploring allelopathy. To solve the problem of the simultaneous detection of allelochemicals with low concentrations and different polarities, a novel strategy for the quick detection of the allelochemicals in Taxus soil by microdialysis combined with UPLC-MS/MS on the basis of in situ detection without destroying the original structure of soil was developed for the first time in the work. The dialysis conditions were optimized by the Box-Behnken design (BBD): 70% methanol, 3 μL/min flow rate, and 3 cm long membrane tube. A reliable UPLC-MS/MS program was systematically optimized for the simultaneous detection of nine allelochemicals with different polarities. The results proved the differences in the contents and distributions of nine allelochemicals in three different Taxus soils.

Response of Cucumis sativus to Neighbors in a Species-Specific Manner

Plants exhibit various behaviors of growth and allocation that play an important role in plant performance and social interaction as they grow together. However, it is unclear how Cucumis sativus plants respond to different neighbors. Here, we performed 5 neighbor combinations with C. sativus as the focal species. The selected materials of C. sativus responded to neighbors and exhibited different behavior strategies in a species-specific manner. All competition treatments reduced the growth of C. sativus seedlings to a certain extent, but only the Eruca sativa neighbor treatment reached a significant level in total root length and shoot biomass. Compared with growing under solitary conditions, focal plants avoided, tended to and did not change their allocation to their nearby plants. The larger the biomass of their neighbors, the stronger the inhibition of the focal plants. In addition, no significant correlations between growth and allocation variables were found, suggesting that growth and allocation are two important aspects of C. sativus behavioral strategies. Our findings provide reference and support for agricultural production of C. sativus, but further research and practice are still needed.

Root exudates enhanced rhizobacteria complexity and microbial carbon metabolism of toxic plants

Root exudates and rhizosphere microorganisms play key roles in the colonization of toxic plants under climate change and land degradation. However, how root exudates affect the rhizosphere microorganisms and soil nutrients of toxic plants in degraded grasslands remains unknown. We compared the interaction of soil microbial communities, root exudates, microbial carbon metabolism, and environmental factors in the rhizosphere of toxic and non-toxic plants. Deterministic processes had a greater effect on toxic than non-toxic plants, as root exudates affected rhizosphere microorganisms directly. The 328 up-regulated compounds in root exudates of toxic plants affected the diversity of rhizosphere microorganisms. Rhizosphere bacteria-enriched enzymes were involved in the phenylpropanoid biosynthesis pathway. Root exudates of toxic plants form complex networks of rhizosphere microorganisms, provide high rhizosphere nutrients, and increase microbial carbon metabolism. The interaction between root exudates and rhizosphere microorganisms is the key mechanism that enables toxic plants to spread in degraded grassland habitats.

Tapping into Plant-Microbiome Interactions through the Lens of Multi-Omics Techniques

This review highlights the pivotal role of root exudates in the rhizosphere, especially the interactions between plants and microbes and between plants and plants. Root exudates determine soil nutrient mobilization, plant nutritional status, and the communication of plant roots with microbes. Root exudates contain diverse specialized signaling metabolites (primary and secondary). The spatial behavior of these metabolites around the root zone strongly influences rhizosphere microorganisms through an intimate compatible interaction, thereby regulating complex biological and ecological mechanisms. In this context, we reviewed the current understanding of the biological phenomenon of allelopathy, which is mediated by phytotoxic compounds (called allelochemicals) released by plants into the soil that affect the growth, survival, development, ecological infestation, and intensification of other plant species and microbes in natural communities or agricultural systems. Advances in next-generation sequencing (NGS), such as metagenomics and metatranscriptomics, have opened the possibility of better understanding the effects of secreted metabolites on the composition and activity of root-associated microbial communities. Nevertheless, understanding the role of secretory metabolites in microbiome manipulation can assist in designing next-generation microbial inoculants for targeted disease mitigation and improved plant growth using the synthetic microbial communities (SynComs) tool. Besides a discussion on different approaches, we highlighted the advantages of conjugation of metabolomic approaches with genetic design (metabolite-based genome-wide association studies) in dissecting metabolome diversity and understanding the genetic components of metabolite accumulation. Recent advances in the field of metabolomics have expedited comprehensive and rapid profiling and discovery of novel bioactive compounds in root exudates. In this context, we discussed the expanding array of metabolomics platforms for metabolome profiling and their integration with multivariate data analysis, which is crucial to explore the biosynthesis pathway, as well as the regulation of associated pathways at the gene, transcript, and protein levels, and finally their role in determining and shaping the rhizomicrobiome.

Negative Interactions Balance Growth and Defense in Plants Confronted with Herbivores or Pathogens

Plants have evolved a series of defensive mechanisms against pathogens and herbivores, but the defense response always leads to decreases in growth or reproduction, which has serious implications for agricultural production. Growth and defense are negatively regulated not only through metabolic consumption but also through the antagonism of different phytohormones, such as jasmonic acid (JA) and salicylic acid (SA). Meanwhile, plants can limit the expression of defensive metabolites to reduce the costs of defense by producing constitutive defenses such as glandular trichomes or latex and accumulating specific metabolites, determining the activation of plant defense or the maintenance of plant growth. Interestingly, plant defense pathways might be prepared in advance which may be transmitted to descendants. Plants can also use external organisms to protect themselves, thus minimizing the costs of defense. In addition, plant relatives exhibit cooperation to deal with pathogens and herbivores, which is also a way to regulate growth and defense.

Root exudates and rhizosphere soil bacterial relationships of Nitraria tangutorum are linked to k-strategists bacterial community under salt stress

When plants are subjected to various biotic and abiotic stresses, the root system responds actively by secreting different types and amounts of bioactive compounds, while affects the structure of rhizosphere soil bacterial community. Therefore, understanding plant-soil-microbial interactions, especially the strength of microbial interactions, mediated by root exudates is essential. A short-term experiment was conducted under drought and salt stress to investigate the interaction between root exudates and Nitraria tangutorum rhizosphere bacterial communities. We found that drought and salt stress increased rhizosphere soil pH (9.32 and 20.6%) and electrical conductivity (1.38 and 11 times), respectively, while decreased organic matter (27.48 and 31.38%), total carbon (34.55 and 29.95%), and total phosphorus (20 and 28.57%) content of N. tangutorum rhizosphere soil. Organic acids, growth hormones, and sugars were the main differential metabolites of N. tangutorum under drought and salt stress. Salt stress further changed the N. tangutorum rhizosphere soil bacterial community structure, markedly decreasing the relative abundance of Bacteroidota as r-strategist while increasing that of Alphaproteobacteria as k-strategists. The co-occurrence network analysis showed that drought and salt stress reduced the connectivity and complexity of the rhizosphere bacterial network. Soil physicochemical properties and root exudates in combination with salt stress affect bacterial strategies and interactions. Our study revealed the mechanism of plant-soil-microbial interactions under the influence of root exudates and provided new insights into the responses of bacterial communities to stressful environments.

Environmental strigolactone drives early growth responses to neighboring plants and soil volume in pea

There has been a dramatic recent increase in the understanding of the mechanisms by which plants detect their neighbors,1 including by touch,2 reflected light,3 volatile organic chemicals, and root exudates.4,5 The importance of root exudates remains ill-defined because of confounding experimental variables6,7 and difficulties disentangling neighbor detection in shoot and roots.8-10 There is evidence that root exudates allow distinction between kin and non-kin neighbors,11-13 but identification of specific exudates that function in neighbor detection and/or kin recognition remain elusive.1 Strigolactones (SLs), which are exuded into the soil in significant quantities in flowering plants to promote recruitment of arbuscular mycorrhizal fungi (AMF),14 seem intuitive candidates to act as plant-plant signals, since they also act as hormones in plants,15-17 with dramatic effects on shoot growth18,19 and milder effects on root development.20 Here, using pea, we test whether SLs act as either cues or signals for neighbor detection. We show that peas detect neighbors early in the life cycle through their root systems, resulting in strong changes in shoot biomass and branching, and that this requires SL biosynthesis. We demonstrate that uptake and detection of SLs exuded by neighboring plants are needed for this early neighbor detection, and that plants that cannot exude SLs are outcompeted by neighboring plants and fail to adjust growth to their soil volume. We conclude that plants both exude SLs as signals to modulate neighbor growth and detect environmental SLs as a cue for neighbor presence; collectively, this allows plants to proactively adjust their shoot growth according to neighbor density.

Root tragedy of the commons: Revisiting the mechanisms of a misunderstood theory

Fine root density in the soil is a plant functional trait of paramount importance for plant ecology and agriculture. Fine root proliferation by plants involves complex plant strategies that may depend on various abiotic and biotic factors. Concretely, the root tragedy of the commons (RToC) is a behavioral strategy predicted by game theory models in which interacting plants forage for soil resources inefficiently. Generally, researchers assume that the RToC is a proactive competition strategy directly induced by the non-self roots. In this opinion, I recall Hardin's original definition of the tragedy of the commons to challenge this notion. I argue that the RToC is a suboptimal phenotypically plastic response of the plants based on the soil resource information exclusively, and I discuss how this alternative perspective carries important implications for the design of experiments investigating the physiological mechanisms underlying observable plant root responses.

Supra-organismal regulation of strigolactone exudation and plant development in response to rhizospheric cues in rice

Plants have evolved elaborate mechanisms to detect neighboring plants, which typically involve the perception of "cues" inadvertently produced by the neighbor.¹ Strigolactones are hormonal signaling molecules^2,3 that are also exuded into the rhizosphere by most flowering plant species to promote arbuscular mycorrhizal symbioses.⁴ Since flowering plants have an endogenous perception system for strigolactones,⁵ strigolactones are obvious candidates to act as a cue for neighbor presence, but have not been shown to act as such. To test this hypothesis in rice plants, we quantified two major strigolactones of rice plants, orobanchol and 4-deoxyorobanchol, in root exudates by using LC-MS/MS (MRM) and examined feedback regulation of strigolactone biosynthesis and changes in shoot branching phenotypes in rice plants grown at different densities in hydroponics and soil culture. We show that the presence of neighboring plants, or greater root volume, results in rapidly induced changes in strigolactone biosynthesis, sensitivity, and exudation and the subsequent longer-term changes in shoot architecture. These changes require intact strigolactone biosynthesis in neighboring plants and intact strigolactone signaling in focal plants. These results suggest that strigolactone biosynthesis and exudation in rice plants are driven by supra-organismal environmental strigolactone levels. Strigolactones thus act as a cue for neighbor presence in rice plants, but also seem to act as a more general root density-sensing mechanism in flowering plants that integrates soil volume and neighbor density and allows plants to adapt to the limitations of the rhizosphere.

Harnessing belowground processes for sustainable intensification of agricultural systems

Increasing food demand coupled with climate change pose a great challenge to agricultural systems. In this review we summarize recent advances in our knowledge of how plants, together with their associated microbiota, shape rhizosphere processes. We address (molecular) mechanisms operating at the plant-microbe-soil interface and aim to link this knowledge with actual and potential avenues for intensifying agricultural systems, while at the same time reducing irrigation water, fertilizer inputs and pesticide use. Combining in-depth knowledge about above and belowground plant traits will not only significantly advance our mechanistic understanding of involved processes but also allow for more informed decisions regarding agricultural practices and plant breeding. Including belowground plant-soil-microbe interactions in our breeding efforts will help to select crops resilient to abiotic and biotic environmental stresses and ultimately enable us to produce sufficient food in a more sustainable agriculture in the upcoming decades.

Endophytic Fungal and Bacterial Microbiota Shift in Rice and Barnyardgrass Grown under Co-Culture Condition

Although barnyardgrass (Echinochloa crus-galli L.) is more competitive than rice (Oryza sativa L.) in the aboveground part, little is known about whether barnyardgrass is still competitive in recruiting endophytes and the root microbiota composition variation of rice under the barnyardgrass stress. Here, by detailed temporal characterization of root-associated microbiomes of rice plants during co-planted barnyardgrass stress and a comparison with the microbiomes of unplanted soil, we found that the bacterial community diversity of rice was dramatically higher while the fungal community richness was significantly lower than that of barnyardgrass at BBCH 45 and 57. More importantly, rice recruited more endophytic bacteria at BBCH 45 and 57, and more endophytic fungi at BBCH 17, 24, 37 to aginst the biotic stress from barnyardgrass. Principal coordinates analysis (PCoA) showed that rice and barnyardgrass had different community compositions of endophytic bacteria and fungi in roots. The PICRUSt predictive analysis indicated that majority of metabolic pathways of bacteria were overrepresented in barnyardgrass. However, eleven pathways were significantly presented in rice. In addition, rice and barnyardgrass harbored different fungal trophic modes using FUNGuild analysis. A negative correlation between bacteria and fungi in rice and barnyardgrass roots was found via network analysis. Actinobacteria was the vital bacteria in rice, while Proteobacteria dominated in barnyardgrass, and Ascomycota was the vital fungi in each species. These findings provided data and a theoretical basis for the in-depth understanding of the competition of barnyardgrass and endophytes and have implications relevant to weed prevention and control strategies using root microbiota.

Chemo-Ecological Insights into the Use of the Non-Host Plant Vegetable Black-Jack to Protect Two Susceptible Solanaceous Crops from Root-Knot Nematode Parasitism

Plant parasitic nematodes (PPNs) develop through three major stages in their life cycle: hatching, infection, and reproduction. Interruption of any of these stages can affect their growth and survival. We used screenhouse pot experiments, laboratory in vitro hatching and mortality assays, and chemical analysis to test the hypothesis that the non-host Asteraceae plant vegetable black-jack (Bidens pilosa) suppresses infection of the PPN Meloidogyne incognita in two susceptible Solanaceae host plants, tomato (Solanum lycopersicum) and black nightshade (S. nigrum). In intercrop and drip pot experiments, B. pilosa significantly reduced the number of galls and egg masses in root-knot nematode (RKN)-susceptible host plants by 3-9-fold compared to controls. Chemical analysis of the most bioactive fraction from the root exudates of B. pilosa identified several classes of compounds, including vitamins, a dicarboxylic acid, amino acids, aromatic acids, and a flavonoid. In in vitro assays, the vitamins and aromatic acids elicited the highest inhibition in egg hatching, whereas ascorbic acid (vitamin) and 2-hydroxybenzoic acid (aromatic acid) elicited strong nematicidal activity against M. incognita, with LC_50/48 h values of 12 and 300 ng/μL, respectively. Our results provide insights into how certain non-host plants can be used as companion crops to disrupt PPN infestation.

Enantioselective Response of Wheat Seedlings to Imazethapyr: From the Perspective of Fe and the Secondary Metabolite DIMBOA

The responses of trace elements and secondary metabolites to stress can reflect plant adaptation to the environment. If and how the imperative trace element Fe and the defensive secondary metabolite 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazine-3(4H)-one (DIMBOA) mediate the toxicity of chiral herbicides to nontarget plants remains inconclusive. We found that the herbicidal-active imazethapyr enantiomer [(R)-IM] stimulated heme oxygenase-1 activity, triggered the release of the catalytic product Fe²⁺, increased reactive oxygen species production, decreased the DIMBOA content, and increased the DIMBOA-Fe content. XAFS analyses and in vitro Fenton assays demonstrated that DIMBOA could relieve phytotoxicity by chelating excessive Fe³⁺ to restore Fe homeostasis. The free radical scavenging ability of the chelate of DIMBOA and Fe was also involved. This work refines the dual role of DIMBOA and Fe in mediating the enantioselective phytotoxicity of chiral herbicides, which provides a new direction for improving the herbicide resistance of crops.

Weed pressure determines the chemical profile of wheat (Triticum aestivum L.) and its allelochemicals potential

BACKGROUND

Common purslane (Portulaca oleracea) and annual ryegrass (Lolium rigidum) are important infesting weeds of field crops. Herbicides are mostly used for weed suppression, while their environmental toxicity and resistance in weeds against them demand considering alternative options, such as the use of allelopathic crops for weed management. Wheat is an important allelopathic crop and present research focused on the identification and quantification of benzoxazinoids (BXZs) and polyphenols (phenolic acids and flavonoids) of the wheat accession 'Ursita' and to screen its allelopathic impact on P. oleracea and Lolium rigidum through equal-compartment-agar (ECA) method.

RESULTS

Weed germination, radicle length, biomass and photosynthetic pigments were altered following co-growth of weeds with Ursita for 10-day. Root exudates from Ursita reduced (29-60%) the seedling growth and photosynthetic pigments of Lolium rigidum depending on co-culture conditions of planting density. Weed pressure caused significant increase in the production of phenolic acids (vanillic, ferulic, syringic and p-coumaric acids) and root exudation of BXZs, in particular benzoxazolin-2-one (BOA), 2-hydroxy-7-methoxy-1,4-benzoxazin-3-one (HMBOA), 2-hydroxy-1,4-benzoxazin3-one (HBOA) and 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) in wheat tissues (shoots, roots) and exudate in root rhizosphere agar medium in response to co-cultivation with Lolium rigidum and P. oleracea, depending on weed/crop density.

CONCLUSION

The work revealed that Ursita is allelopathic in nature and can be used in breeding programs to enhance its allelopathic activity. Meanwhile, there are opportunities to explore allelopathic effect of wheat cultivars to control P. oleracea and Lolium rigidum under field conditions. © 2022 Society of Chemical Industry.

Genetic and molecular mechanisms underlying root architecture and function under heat stress-A hidden story

Heat stress events are resulting in a significant negative impact on global food production. The dynamics of cellular, molecular and physiological homoeostasis in aboveground parts under heat stress are extensively deciphered. However, root responses to higher soil/air temperature or stress signalling from shoot to root are limited. Therefore, this review presents a holistic view of root physio-morphological and molecular responses to adapt under hotter environments. Heat stress reprogrammes root cellular machinery, including crosstalk between genes, phytohormones, reactive oxygen species (ROS) and antioxidants. Spatio-temporal regulation and long-distance transport of phytohormones, such as auxin, cytokinin and abscisic acid (ABA) determine the root growth and development under heat stress. ABA cardinally integrates a signalling pathway involving heat shock factors, heat shock proteins and ROS to govern heat stress responses. Additionally, epigenetic modifications by transposable elements, DNA methylation and acetylation also regulate root growth under heat stress. Exogenous application of chemical compounds or biological agents such as ascorbic acid, metal ion chelators, fungi and bacteria can alleviate heat stress-induced reduction in root biomass. Future research should focus on the systemic effect of heat stress from shoot to root with more detailed investigations to decipher the molecular cues underlying the roots architecture and function.

The wheat secreted root proteome: Implications for phosphorus mobilisation and biotic interactions

Root secreted acid phosphatases and organic anions are widely perceived as major players of plant phosphorus (P) mobilisation from the rhizosphere under P limiting growth conditions. Previous research indicated that other mechanisms play a role, especially in species with fine roots, such as wheat. In this study we characterised the plant-derived extracellular proteome of wheat roots by profiling root tip mucilage, soluble root secreted and root tip proteomes. Extracellular acid phosphatases and enzymes of the central carbon metabolism were targeted using selected reaction monitoring. More than 140 proteins with extracellular localisation prediction were identified in mucilage. P starvation induced proteins predicted to be localised to the apoplast which are related to cell wall modification and defence in both, root tip and soluble root-secreted proteomes. Glycolytic enzymes were strongly increased in abundance by P limitation in root tips, as were PEPC and plastidial MDH. Soluble acid phosphatases were not identified in extracellular protein samples. Our results indicate that root tip mucilage contains proteins with the functional potential to actively shape their immediate environment by modification of plant structural components and biotic interactions. Wheat acid phosphatases appear to play a minor role in P mobilisation beyond the immediate root surface. SIGNIFICANCE: Phosphorus (P) is a plant growth limiting nutrient in many agricultural situations and the development of phosphorus efficient crops is of paramount importance for future agricultural management practices. As P is relatively immobile in soils, processes occurring at the root-soil interface, the rhizosphere, are suspected to play a key role in plant-induced P mobilisation. According to the current view, the secretion of extracellular acid phosphatases and organic anions enhances P mobilisation within several millimetres beyond the root surface, either directly or indirectly through the selection and appropriate soil microbes. However, the mechanisms of P mobilisation in species with fine roots, such as wheat, and the role of other secreted root proteins are poorly understood. Here, we carried out the profiling of wheat root tip mucilage, soluble root secreted and root tip proteomes. We analysed proteome changes in response to P starvation. We found that proteins with a predicted localisation to the apoplast made up a major proportion of stress-responsive proteins. Acid phosphatases were not identified within extracellular protein samples, which were enriched in proteins with predicted extracellular localisation. The absence of extracellular APases was further validated by multiple reaction monitoring. Our data indicates that wheat acid phosphatases play a minor role in P mobilisation beyond the immediate root surface and provides a resource for breeding strategies and further investigations of the functional roles of root tip-released proteins in the rhizosphere under P limitation.

Kin Recognition in Plants: Did We Learn Anything From Roots?

Kin recognition, manifesting through various traits such as changes in root or shoot growth, has been documented in several species of plants. Identifying this phenomenon in plants has intrinsic value itself, understanding why plants recognize kin and how it might benefit them evolutionarily has been of recent interest. Here we explore studies regarding nutrient and resource allocation in regard to kin recognition as well as discuss how kin recognition is involved in multispecies interactions with an emphasis on how plant roots are involved in these processes. Future directions of this research are also discussed.

Sesquiterpenes from Ambrosia artemisiifolia and their allelopathy

Ambrosia artemisiifolia, an invasive plant, has seriously harmed the agricultural production, native ecosystems and human health. Allelopathy is an important reason for the successful invasion of this alien plant. However, the chemical basis, action effects, action mechanism and release pathway of its allelopathy remain unclear. To address these problems, four sesquiterpenes (1-4), consisting of three new sesquiterpenes (1-2, 4), were isolated from the whole plant of A. artemisiifolia using a variety of column chromatography techniques, and identified using HR-ESIMS, 1D-NMR, 2D-NMR, and ECD. All the compounds exhibited different levels of inhibitory effects on three native plants (Setaria viridis, Digitaria sanguinalis, Chenopodium album) and one model plant (Arabidopsis thaliana), especially compound 1. In addition, the preliminary action mechanism of active compound 1 was revealed by FDA/PI staining assay. Furthermore, the allelopathic substances 1-3 were released into environment through the root secretion pathway by UPLC-MS/MS analyses.

Exploring the Diversity and Regulation of Apocarotenoid Metabolic Pathways in Plants

In plants, carotenoids are subjected to enzyme-catalyzed oxidative cleavage reactions as well as to non-enzymatic degradation processes, which produce various carbonyl products called apocarotenoids. These conversions control carotenoid content in different tissues and give rise to apocarotenoid hormones and signaling molecules, which play important roles in plant growth and development, response to environmental stimuli, and in interactions with surrounding organisms. In addition, carotenoid cleavage gives rise to apocarotenoid pigments and volatiles that contribute to the color and flavor of many flowers and several fruits. Some apocarotenoid pigments, such as crocins and bixin, are widely utilized as colorants and additives in food and cosmetic industry and also have health-promoting properties. Considering the importance of this class of metabolites, investigation of apocarotenoid diversity and regulation has increasingly attracted the attention of plant biologists. Here, we provide an update on the plant apocarotenoid biosynthetic pathway, especially highlighting the diversity of the enzyme carotenoid cleavage dioxygenase 4 (CCD4) from different plant species with respect to substrate specificity and regioselectivity, which contribute to the formation of diverse apocarotenoid volatiles and pigments. In addition, we summarize the regulation of apocarotenoid metabolic pathway at transcriptional, post-translational, and epigenetic levels. Finally, we describe inter- and intraspecies variation in apocarotenoid production observed in many important horticulture crops and depict recent progress in elucidating the genetic basis of the natural variation in the composition and amount of apocarotenoids. We propose that the illustration of biochemical, genetic, and evolutionary background of apocarotenoid diversity would not only accelerate the discovery of unknown biosynthetic and regulatory genes of bioactive apocarotenoids but also enable the identification of genetic variation of causal genes for marker-assisted improvement of aroma and color of fruits and vegetables and CRISPR-based next-generation metabolic engineering of high-value apocarotenoids.

Belowground facilitation and trait matching: two or three to tango?

High biodiversity increases ecosystem functions; however, belowground facilitation remains poorly understood in this context. Here, we explore mechanisms that operate via 'giving-receiving feedbacks' for belowground facilitation. These include direct effects via root exudates, signals, and root trait plasticity, and indirect biotic facilitation via the effects of root exudates on soil biota and feedback from biota to plants. We then highlight that these two- or three-way mechanisms must affect biodiversity-ecosystem function relationships via specific combinations of matching traits. To tango requires a powerful affinity and harmony between well-matched partners, and such matches link belowground facilitation to the effect of biodiversity on function. Such matching underpins applications in intercropping, forestry, and pasture systems, in which diversity contributes to greater productivity and sustainability.

Intra-specific kin recognition contributes to inter-specific allelopathy: A case study of allelopathic rice interference with paddy weeds

Species interactions and mechanisms affect plant coexistence and community assembly. Despite increasing knowledge of kin recognition and allelopathy in regulating inter-specific and intra-specific interactions among plants, little is known about whether kin recognition mediates allelopathic interference. We used allelopathic rice cultivars with the ability for kin recognition grown in kin versus non-kin mixtures to determine their impacts on paddy weeds in field trials and a series of controlled experiments. We experimentally tested potential mechanisms of the interaction via altered root behaviour, allelochemical production and resource partitioning in the dominant weed competitor, as well as soil microbial communities. We consistently found that the establishment and growth of paddy weeds were more inhibited by kin mixtures compared to non-kin mixtures. The effect was driven by kin recognition that induced changes in root placement, altered weed carbon and nitrogen partitioning, but was associated with similar soil microbial communities. Importantly, genetic relatedness enhanced the production of intrusive roots towards weeds and reduced the production of rice allelochemicals. These findings suggest that relatedness allows allelopathic plants to discriminate their neighbouring collaborators (kin) or competitors and adjust their growth, competitiveness and chemical defense accordingly.

Nanoselenium Enhanced Wheat Resistance to Aphids by Regulating Biosynthesis of DIMBOA and Volatile Components

The mechanism of nanoselenium (nano-Se) improving the resistance induced by plant components to aphids is unclear. In this study, foliar sprayed nano-Se (5.0 mg/L) could significantly reduce the Sitobion avenae number (36%) compared with that in the control. Foliar application of nano-Se enhanced the antioxidant capacity by reducing malondialdehyde (MDA) and increasing GSH-Px, CAT, GSH, Pro, and VE concentrations in wheat seedlings. The phenylpropane pathway was activated by nano-Se biofortification, which increased apigenin and caffeic acid concentrations. The high-level expression of the related genes (TaBx1A, TaBx3A, TaBx4A, TaASMT2, and TaCOMT) induced the promotion of melatonin (88.6%) and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) (64.3%). Different ratios of the secondary metabolites to nano-Se were taken to examine the effects on resistance of wheat to S. avenae. The results revealed that the combination of nano-Se and melatonin could achieve the best overall performance by reducing the S. avenae number by 52.2%. The study suggests that the coordinated applications of nano-Se and melatonin could more effectively improve the wheat resistance to aphids via the promotion of volatile organic compound synthesis and modulation in phenylpropane and indole metabolism pathways.

Plant Communication

Communication occurs when a sender emits a cue perceived by a receiver that changes the receiver's behavior. Plants perceive information regarding light, water, other nutrients, touch, herbivores, pathogens, mycorrhizae, and nitrogen-fixing bacteria. Plants also emit cues perceived by other plants, beneficial microbes, herbivores, enemies of herbivores, pollinators, and seed dispersers. Individuals responding to light cues experienced increased fitness. Evidence for benefits of responding to cues involving herbivores and pathogens is more limited. The benefits of emitting cues are also less clear, particularly for plant–plant communication. Reliance on multiple or dosage-dependent cues can reduce inappropriate responses, and plants often remember past cues. Plants have multiple needs and prioritize conflicting cues such that the risk of abiotic stress is treated as greater than that of shading, which is in turn treated as greater than that of consumption. Plants can distinguish self from nonself and kin from strangers. They can identify the species of competitor or consumer and respond appropriately. Cues involving mutualists often contain highly specific information.

Plant neighbour-modulated susceptibility to pathogens in intraspecific mixtures

As part of a trend towards diversifying cultivated areas, varietal mixtures are subject to renewed interest as a means to manage diseases. Besides the epidemiological effects of varietal mixtures on pathogen propagation, little is known about the effect of intraspecific plant-plant interactions and their impact on responses to disease. In this study, genotypes of rice (Oryza sativa) or durum wheat (Triticum turgidum) were grown with different conspecific neighbours and manually inoculated under conditions preventing pathogen propagation. Disease susceptibility was measured together with the expression of basal immunity genes as part of the response to intra-specific neighbours. The results showed that in many cases for both rice and wheat susceptibility to pathogens and immunity was modified by the presence of intraspecific neighbours. This phenomenon, which we term 'neighbour-modulated susceptibility' (NMS), could be caused by the production of below-ground signals and does not require the neighbours to be infected. Our results suggest that the mechanisms responsible for reducing disease in varietal mixtures in the field need to be re-examined.

Determination of the Effect of Co-cultivation on the Production and Root Exudation of Flavonoids in Four Legume Species Using LC-MS/MS Analysis

Flavonoids play a key role in the regulation of plant-plant and plant-microbe interactions, and factors determining their release have been investigated in most of the common forage legumes. However, little is known about the response of flavonoid production and release to co-cultivation with other crop species. This study investigated alterations in the concentration of flavonoids in plant tissues and root exudates in four legumes [alfalfa (Medicago sativa L.), black medic (Medicago polymorpha L.), crimson clover (Trifolium incarnatum L.), and subterranean clover (Trifolium subterraneum L.)] co-cultivated with durum wheat [Triticum turgidum subsp. durum (Desf.) Husn.]. For this purpose, we carried out two experiments in a greenhouse, one with glass beads as growth media for root exudate extraction and one with soil as growth media for flavonoid detection in shoot and root biomass, using LC-MS/MS analysis. This study revealed that interspecific competition with wheat negatively affected legume growth and led to a significant reduction in shoot and root biomass compared with the same legume species grown in monoculture. In contrast, the concentration of flavonoids significantly increased both in legume biomass and in root exudates. Changes in flavonoid concentration involved daidzein, genistein, medicarpin, and formononetin, which have been found to be involved in legume nodulation and regulation of plant-plant interaction. We hypothesize that legumes responded to the co-cultivation with wheat by promoting nodulation and increasing exudation of allelopathic compounds, respectively, to compensate for the lack of nutrients caused by the presence of wheat in the cultivation system and to reduce the competitiveness of neighboring plants. Future studies should elucidate the bioactivity of flavonoid compounds in cereal-legume co-cultivation systems and their specific role in the nodulation process and inter-specific plant interactions such as potential effects on weeds.

Plant immunity: Good fences make good neighbors?

Plant immunity is modulated by several abiotic factors, and microbiome has emerged as a major biotic driver of plant resistance. Recently, a few studies showed that plants also modify resistance to pests and pathogens in their neighborhood. Several types of neighborhood could be identified depending on the biological processes at play: intraspecific and interspecific competition, kin and stranger recognition, plant-soil feedbacks, and danger signaling. This review highlights that molecules exchanged aboveground and belowground between plants can modulate plant immunity, either constitutively or after damage or attack. An intriguing relationship between allelopathy and immunity has been evidenced and should merit further investigation. Interestingly, most reported cases of modulation of immunity by the neighbors are positive, opening new perspectives for the understanding of natural plant communities as well as for the design of more diverse cultivated systems.

The effects of plant-soil feedback on invasion resistance are soil context dependent

There is growing interest in understanding the role that plant-soil feedbacks (PSFs) may play in invasion resistance. However, recent studies have shown that there is great uncertainty in explaining community patterns by PSF studies regarding invasions. This uncertainty may be partly because soils used for PSF studies are usually collected from open areas rather than natural communities, thus ignoring the effects of community contexts that may specifically influence the soil feedbacks of community residents to invaders. We performed a two-phase pot experiment to study the soil feedback initiated by ten co-occurring native and exotic species to a forest invader, Phytolacca americana, and the experiments were performed in forest soil and open area soil. The context-dependent mechanisms were further explored by studying different components of PSF. The results showed that natives and exotics had positive and negative effects on P. americana in the open area soil, respectively, but both had negative effects in the forest soil. Nutrient limitation was more important for the PSF in open area soil, whereas biotic factors were likely the primary mechanisms explaining the PSF in forest soil. Additionally, the litter-mediated allelopathy of dominant Quercus acutissima caused the strongest inhibition of the invader. These results suggest that native species can effectively resist invasion by producing negative PSF depending on the community context. Evidence that exotic species promote invasion through positive PSFs was not obtained. This study provided preliminary insights into the possibility of bridging PSF studies and community patterns.

Density Dependence Influences the Efficacy of Wastewater Remediation by Lemna minor

As part of a circular economy (CE) approach to food production systems, Lemnaceae, i.e., duckweed species, can be used to remediate wastewater due to rapid nutrient assimilation and tolerance of non-optimal growing conditions. Further, given rapid growth rates and high protein content, duckweed species are a valuable biomass. An important consideration for duckweed-mediated remediation is the density at which the plants grow on the surface of the wastewater, i.e., how much of the surface of the medium they cover. Higher duckweed density is known to have a negative effect on duckweed growth, which has implications for the development of duckweed-based remediation systems. In the present study, the effects of density (10-80% plant surface coverage) on Lemna minor growth, chlorophyll fluorescence and nutrient remediation of synthetic dairy processing wastewater were assessed in stationary (100 mL) and re-circulating non-axenic (11.7 L) remediation systems. Overall, L. minor growth, and TN and TP removal rates decreased as density increased. However, in the stationary system, absolute TN and TP removal were greater at higher densities (50-80% coverage). The exact cause of density related growth reduction in duckweed is unclear, especially at densities well below 100% surface coverage. A further experiment comparing duckweed grown at 'low' and 'high' density conditions with the same biomass and media volume conditions, showed that photosynthetic yield, Y(II), is reduced at high density despite the same nutrient availability at both densities, and arguably similar shading. The results demonstrate a negative effect of high density on duckweed growth and nutrient uptake, and point towards signals from neighbouring duckweed colonies as the possible cause.

Allelopathy in rice: a story of momilactones, kin recognition, and weed management

In the struggle to secure nutrient access and to outperform competitors, some plant species have evolved a biochemical arsenal with which they inhibit the growth or development of neighbouring plants. This process, known as allelopathy, exists in many of today's major crops, including rice. Rice synthesizes momilactones, diterpenoids that are released into the rhizosphere and inhibit the growth of numerous plant species. While the allelopathic potential of rice was recognized decades ago, many questions remain unresolved regarding the biosynthesis, exudation, and biological activity of momilactones. Here, we review current knowledge on momilactones, their role in allelopathy, and their potential to serve as a basis for sustainable weed management. We emphasize the gaps in our current understanding of when and how momilactones are produced and of how they act in plant cells, and outline what we consider the next steps in momilactone and rice allelopathy research.

Preparative separation and purification of loliolide and epiloliolide from Ascophyllum nodosum using amine-based microporous organic polymer for solid phase extraction coupled with macroporous resin and prep-HPLC

Herein, we reported a novel approach for the preparative separation and purification of loliolide and epiloliolide from Ascophyllum nodosum. An amine-based microporous organic polymer (MOP) was used for the pretreatment of the nodosum extract via solid-phase extraction (SPE). The obtained extract was further purified using macroporous resin chromatography and preparative high-performance liquid chromatography (prep-HPLC). The loading and elution parameters of the MOP were evaluated using standard loliolide, and the optimized conditions were used during the SPE of the nodosum extract (37.5 g). After the pretreatment with MOP, the extract (2.79 g) was obtained and further purified using a D101 resin column followed by prep-HPLC. A pair of epimers were isolated and identified as loliolide (5.83 mg) and epiloliolide (2.50 mg) using high-resolution electrospray ionization tandem mass spectrometry (HRESI-MS), 1D- and 2D-nuclear magnetic resonance (NMR) spectroscopy. This study demonstrates the potential of MOPs in the separation and purification of monoterpenoids from complex plant samples.

Friends, neighbours and enemies: an overview of the communal and social biology of plants

Plants were traditionally seen as rather passive actors in their environment, interacting with each other only in so far as they competed for the same resources. In the last 30 years, this view has been spectacularly overturned, with a wealth of evidence showing that plants actively detect and respond to their neighbours. Moreover, there is evidence that these responses depend on the identity of the neighbour, and that plants may cooperate with their kin, displaying social behaviour as complex as that observed in animals. These plant-plant interactions play a vital role in shaping natural ecosystems, and are also very important in determining agricultural productivity. However, in terms of mechanistic understanding, we have only just begun to scratch the surface, and many aspects of plant-plant interactions remain poorly understood. In this review, we aim to provide an overview of the field of plant-plant interactions, covering the communal interactions of plants with their neighbours as well as the social behaviour of plants towards their kin, and the consequences of these interactions. We particularly focus on the mechanisms that underpin neighbour detection and response, highlighting both progress and gaps in our understanding of these fascinating but previously overlooked interactions.

Social networking in crop plants: Wired and wireless cross-plant communications

The plant-associated microbial community (microbiome) has an important role in plant-plant communications. Plants decipher their complex habitat situations by sensing the environmental stimuli and molecular patterns and associated with microbes, herbivores and dangers. Perception of these cues generates inter/intracellular signals that induce modifications of plant metabolism and physiology. Signals can also be transferred between plants via different mechanisms, which we classify as wired- and wireless communications. Wired communications involve direct signal transfers between plants mediated by mycorrhizal hyphae and parasitic plant stems. Wireless communications involve plant volatile emissions and root exudates elicited by microbes/insects, which enable inter-plant signalling without physical contact. These producer-plant signals induce microbiome adaptation in receiver plants via facilitative or competitive mechanisms. Receiver plants eavesdrop to anticipate responses to improve fitness against stresses. An emerging body of information in plant-plant communication can be leveraged to improve integrated crop management under field conditions.

Soil chemical legacies trigger species-specific and context-dependent root responses in later arriving plants

Soil legacies play an important role for the creation of priority effects. However, we still poorly understand to what extent the metabolome found in the soil solution of a plant community is conditioned by its species composition and whether soil chemical legacies affect subsequent species during assembly. To test these hypotheses, we collected soil solutions from forb or grass communities and evaluated how the metabolome of these soil solutions affected the growth, biomass allocation and functional traits of a forb (Dianthus deltoides) and a grass species (Festuca rubra). Results showed that the metabolomes found in the soil solutions of forb and grass communities differed in composition and chemical diversity. While soil chemical legacies did not have any effect on F. rubra, root foraging by D. deltoides decreased when plants received the soil solution from a grass or a forb community. Structural equation modelling showed that reduced soil exploration by D. deltoides arose via either a root growth-dependent pathway (forb metabolome) or a root trait-dependent pathway (grass metabolome). Reduced root foraging was not connected to a decrease in total N uptake. Our findings reveal that soil chemical legacies can create belowground priority effects by affecting root foraging in later arriving plants.

Root exudate signals in plant-plant interactions

Plant-to-plant signalling is a key mediator of interactions among plant species. Plants can perceive and respond to chemical cues emitted from their neighbours, altering survival and performance, impacting plant coexistence and community assembly. An increasing number of studies indicate root exudates as key players in plant-to-plant signalling. Root exudates mediate root detection and behaviour, kin recognition, flowering and production, driving inter- and intra-specific facilitation in cropping systems and mixed-species plantations. Altered interactions may be attributed to the signalling components within root exudates. Root ethylene, strigolactones, jasmonic acid, (-)-loliolide and allantoin are signalling chemicals that convey information on local conditions in plant-plant interactions. These root-secreted signalling chemicals appear ubiquitous in plants and trigger a series of belowground responses inter- and intra-specifically, involving molecular events in biosynthesis, secretion and action. The secretion of root signals, mainly mediated by ATP-binding cassette transporters, is critical. Root-secreted signalling chemicals and their molecular mechanisms are rapidly revealing a multitude of fascinating plant-plant interactions. However, many root signals, particularly species-specific signals and their underlying mechanisms, remain to be uncovered due to methodological limitations and root-soil interactions. A thorough understanding of root-secreted chemical signals and their mechanisms will offer many ecological implications and potential applications for sustainable agriculture.

Plant volatiles as cues and signals in plant communication

Volatile organic compounds are important mediators of mutualistic interactions between plants and their physical and biological surroundings. Volatiles rapidly indicate competition or potential threat before these can take place, and they regulate and coordinate adaptation responses in neighbouring plants, fine-tuning them to match the exact stress encountered. Ecological specificity and context-dependency of plant-plant communication mediated by volatiles represent important factors that determine plant performance in specific environments. In this review, we synthesise the recent progress made in understanding the role of plant volatiles as mediators of plant interactions at the individual and community levels, highlighting the complexity of the plant receiver response to diverse volatile cues and signals and addressing how specific responses shape plant growth and survival. Finally, we outline the knowledge gaps and provide directions for future research. The complex dialogue between the emitter and receiver based on either volatile cues or signals determines the outcome of information exchange, which shapes the communication pattern between individuals at the community level and determines their ecological implications at other trophic levels.