Allelochemicals and activities in a replanted Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) tree ecosystem

Chemically Mediated Plant-Plant Interactions: Allelopathy and Allelobiosis

Plant-plant interactions are a central driver for plant coexistence and community assembly. Chemically mediated plant-plant interactions are represented by allelopathy and allelobiosis. Both allelopathy and allelobiosis are achieved through specialized metabolites (allelochemicals or signaling chemicals) produced and released from neighboring plants. Allelopathy exerts mostly negative effects on the establishment and growth of neighboring plants by allelochemicals, while allelobiosis provides plant neighbor detection and identity recognition mediated by signaling chemicals. Therefore, plants can chemically affect the performance of neighboring plants through the allelopathy and allelobiosis that frequently occur in plant-plant intra-specific and inter-specific interactions. Allelopathy and allelobiosis are two probably inseparable processes that occur together in plant-plant chemical interactions. Here, we comprehensively review allelopathy and allelobiosis in plant-plant interactions, including allelopathy and allelochemicals and their application for sustainable agriculture and forestry, allelobiosis and plant identity recognition, chemically mediated root-soil interactions and plant-soil feedback, and biosynthesis and the molecular mechanisms of allelochemicals and signaling chemicals. Altogether, these efforts provide the recent advancements in the wide field of allelopathy and allelobiosis, and new insights into the chemically mediated plant-plant interactions.

Metagenomics insights into responses of rhizobacteria and their alleviation role in licorice allelopathy

BACKGROUND

Allelopathy is closely associated with rhizosphere biological processes, and rhizosphere microbial communities are essential for plant development. However, our understanding of rhizobacterial communities under influence of allelochemicals in licorice remains limited. In the present study, the responses and effects of rhizobacterial communities on licorice allelopathy were investigated using a combination of multi-omics sequencing and pot experiments, under allelochemical addition and rhizobacterial inoculation treatments.

RESULTS

Here, we demonstrated that exogenous glycyrrhizin inhibits licorice development, and reshapes and enriches specific rhizobacteria and corresponding functions related to glycyrrhizin degradation. Moreover, the Novosphingobium genus accounted for a relatively high proportion of the enriched taxa and appeared in metagenomic assembly genomes. We further characterized the different capacities of single and synthetic inoculants to degrade glycyrrhizin and elucidated their distinct potency for alleviating licorice allelopathy. Notably, the single replenished N (Novosphingobium resinovorum) inoculant had the greatest allelopathy alleviation effects in licorice seedlings.

CONCLUSIONS

Altogether, the findings highlight that exogenous glycyrrhizin simulates the allelopathic autotoxicity effects of licorice, and indigenous single rhizobacteria had greater effects than synthetic inoculants in protecting licorice growth from allelopathy. The results of the present study enhance our understanding of rhizobacterial community dynamics during licorice allelopathy, with potential implications for resolving continuous cropping obstacle in medicinal plant agriculture using rhizobacterial biofertilizers. Video Abstract.

Phenolic Acids Released in Maize Rhizosphere During Maize-Soybean Intercropping Inhibit Phytophthora Blight of Soybean

Interspecies interactions play a key role in soil-borne disease suppression in intercropping systems. However, there are limited data on the underlying mechanisms of soil-borne Phytophthora disease suppression. Here, a field experiment confirmed the effects of maize and soybean intercropping on Phytophthora blight of soybean caused by Phytophthora sojae. Experimentally, the roots and root exudates of maize were found to attract P. sojae zoospores and inhibit their motility and the germination of cystospores. Furthermore, five phenolic acids (p-coumaric acid, cinnamic acid, p-hydroxybenzoic acid, vanillic acid, and ferulic acid) that were consistently identified in the root exudates and rhizosphere soil of maize were found to interfere with the infection behavior of P. sojae. Among them, cinnamic acid was associated with significant chemotaxis in zoospores, and p-coumaric acid and cinnamic acid showed strong antimicrobial activity against P. sojae. However, in the rhizosphere soil of soybean, only p-hydroxybenzoic acid, low concentrations of vanillic acid, and ferulic acid were identified. Importantly, the coexistence of five phenolic acids in the maize rhizosphere compared with three phenolic acids in the soybean rhizosphere showed strong synergistic antimicrobial activity against the infection behavior of P. sojae. In summary, the types and concentrations of phenolic acids in maize and soybean rhizosphere soils were found to be crucial factors for Phytophthora disease suppression in this intercropping system.

Rehmannia glutinosa Replant Issues: Root Exudate-Rhizobiome Interactions Clearly Influence Replant Success

Production of medicinal tubers of Rehmannia glutinosa is severely hindered by replanting issues. However, a mechanistic understanding of the plant-soil factors associated with replant problems is currently limited. Thus, we aimed to identify the R. glutinosa root exudates, evaluate their potential phytotoxicity and profile the interactions between the plant and its associated rhizobiome. Stereomicroscopy and liquid chromatography coupled to a quadrupole/time of flight mass spectrometer were used to monitor and identify secreted metabolites, respectively. Seedling bioassays were used to evaluate the phytotoxicity of R. glutinosa root exudates. Two complimentary experiments were performed to investigate allelochemical fate in rhizosphere soil and profile the associated microbiota. Root specific microbes were further isolated from R. glutinosa rhizosphere. Impacts of isolated strains were evaluated by co-cultivation on plate and on seedlings in tissue culture, with a focus on their pathogenicity. Interactions between key R. glutinosa root exudates and isolated rhizobiomes were investigated to understand the potential for plant-soil feedbacks. Quantification and phytotoxic analysis of metabolites released from R. glutinosa indicated catalpol was the most abundant and bioactive metabolite in root exudates. Subsequent microbial profiling in soil containing accumulated and ecologically significant levels of catalpol identified several taxa (e.g., Agromyces, Lysobacter, Pseudomonas, Fusarium) that were specifically shifted. Isolation of R. glutinosa rhizobiomes obtained several root specific strains. A significant antagonistic effect between strain Rh7 (Pseudomonas aeruginosa) and two pathogenic strains Rf1 (Fusarium oxysporum) and Rf2 (Fusarium solani) was observed. Notably, the growth of strain Rh7 and catalpol concentration showed a hormesis-like effect. Field investigation further indicated catalpol was increasingly accumulated in the rhizosphere of replanted R. glutinosa, suggesting that interactions of biocontrol agents and pathogens are likely regulated by the presence of bioactive root exudates and in turn impact the rhizo-ecological process. In summary, this research successfully monitored the release of R. glutinosa root exudates, identified several abundant bioactive R. glutinosa secreted metabolites, profiled associated root specific microbes, and investigated the plant-soil feedbacks potentially regulated by catalpol and associated rhizobiomes. Our findings provide new perspectives toward an enhanced understanding R. glutinosa replant problems.

Allelochemicals and Signaling Chemicals in Plants

Plants abound with active ingredients. Among these natural constituents, allelochemicals and signaling chemicals that are released into the environments play important roles in regulating the interactions between plants and other organisms. Allelochemicals participate in the defense of plants against microbial attack, herbivore predation, and/or competition with other plants, most notably in allelopathy, which affects the establishment of competing plants. Allelochemicals could be leads for new pesticide discovery efforts. Signaling chemicals are involved in plant neighbor detection or pest identification, and they induce the production and release of plant defensive metabolites. Through the signaling chemicals, plants can either detect or identify competitors, herbivores, or pathogens, and respond by increasing defensive metabolites levels, providing an advantage for their own growth. The plant-organism interactions that are mediated by allelochemicals and signaling chemicals take place both aboveground and belowground. In the case of aboveground interactions, mediated air-borne chemicals are well established. Belowground interactions, particularly in the context of soil-borne chemicals driving signaling interactions, are largely unknown, due to the complexity of plant-soil interactions. The lack of effective and reliable methods of identification and clarification their mode of actions is one of the greatest challenges with soil-borne allelochemicals and signaling chemicals. Recent developments in methodological strategies aim at the quality, quantity, and spatiotemporal dynamics of soil-borne chemicals. This review outlines recent research regarding plant-derived allelochemicals and signaling chemicals, as well as their roles in agricultural pest management. The effort represents a mechanistically exhaustive view of plant-organism interactions that are mediated by allelochemicals and signaling chemicals and provides more realistic insights into potential implications and applications in sustainable agriculture.

…Fell Upas Sits, the Hydra-Tree of Death †, or the Phytotoxicity of Trees

The use of natural products that can serve as natural herbicides and insecticides is a promising direction because of their greater safety for humans and environment. Secondary metabolites of plants that are toxic to plants and insects-allelochemicals-can be used as such products. Woody plants can produce allelochemicals, but they are studied much less than herbaceous species. Meanwhile, there is a problem of interaction of woody species with neighboring plants in the process of introduction or invasion, co-cultivation with agricultural crops (agroforestry) or in plantation forestry (multiclonal or multispecies plantations). This review describes woody plants with the greatest allelopathic potential, allelochemicals derived from them, and the prospects for their use as biopesticides. In addition, the achievement of and the prospects for the use of biotechnology methods in relation to the allelopathy of woody plants are presented and discussed.

Functional characterization of an oxidosqualene cyclase (PdFRS) encoding a monofunctional friedelin synthase in Populus davidiana

An oxidosqualene cyclase (PdFRS) from Populus davidiana was characterized as a monofunctional friedelin synthase by its heterologous expression in yeast and overexpression in plants. Triterpenes are one of the largest classes of plant chemical compounds composed of three terpene units, which form the basic skeleton of all sterols and saponins. Friedelin (friedelan-3-one), a pentacyclic triterpene, occurs in many plant families and is particularly present in rich amounts in cork tissues from trees. The biosynthesis of friedelin occurs through the oxidosqualene cyclase (OSC) enzyme that generates friedelin from 2,3-oxidosqualene after the maximum rearrangement of a triterpene skeleton. Populus davidiana is called Korean aspen and grows in northern East Asia. From 57,322 unique sequences generated from the P. davidiana transcriptome database, one complete coding sequence (PdFRS) was obtained from a contig, which showed 74% identity to Betula platyphylla β-amyrin synthase and 73% identity with friedelin synthase from Maytenus ilicifolia. The open reading frame (ORF) region of the PdFRS sequence was 2280 bp long and composed a 759 amino acid protein with a predicted molecular mass of 87.81 kDa. qPCR analysis revealed that methyl jasmonate treatments strongly upregulated PdFRS gene expression and resulted in enhanced friedelin accumulation in leaves. Heterologous expression of the PdFRS gene in yeast resulted in the production of friedelin triterpene as a single product, which was confirmed by comparison with the mass fragmentation pattern from an authentic friedelin standard by GC/MS analysis. Transgenic P. davidiana overexpressing the PdFRS gene was constructed via Agrobacterium-mediated transformation. Overexpression of PdFRS in transgenic P. davidiana lines resulted in enhanced friedelin production.

A broadleaf species enhances an autotoxic conifers growth through belowground chemical interactions

Plants may affect the performance of neighboring plants either positively or negatively through interspecific and intraspecific interactions. Productivity of mixed-species systems is ultimately the net result of positive and negative interactions among the component species. Despite increasing knowledge of positive interactions occurring in mixed-species tree systems, relatively little is known about the mechanisms underlying such interactions. Based on data from 25-year-old experimental stands in situ and a series of controlled experiments, we test the hypothesis that a broadleaf, non-N fixing species, Michelia macclurei, facilitates the performance of an autotoxic conifer Chinese fir (Cunninghamia lanceolata) through belowground chemical interactions. Chinese fir roots released the allelochemical cyclic dipeptide (6-hydroxy-1,3-dimethyl-8-nonadecyl-[1,4]-diazocane- 2,5-diketone) into the soil environment, resulting in self-growth inhibition, and deterioration of soil microorganisms that improve P availability. However, when grown with M. macclurei the growth of Chinese fir was consistently enhanced. In particular, Chinese fir enhanced root growth and distribution in deep soil layers. When compared with monocultures of Chinese fir, the presence of M. macclurei reduced release and increased degradation of cyclic dipeptide in the soil, resulting in a shift from self-inhibition to chemical facilitation. This association also improved the soil microbial community by increasing arbuscular mycorrhizal fungi, and induced the production of Chinese fir roots. We conclude that interspecific interactions are less negative than intraspecific ones between non-N fixing broadleaf and autotoxic conifer species. The impacts are generated by reducing allelochemical levels, enhancing belowground mutualisms, improving soil properties, and changing root distributions as well as the net effects of all the processes within the soil. In particular, allelochemical context alters the consequences of the belowground ecological interactions with a novel mechanism: reduction of self-inhibition through reduced release and increased degradation of an autotoxic compound in the mixed-species plantations. Such a mechanism would be useful in reforestation programs undertaken to rehabilitate forest plantations that suffer from problems associated with autotoxicity.

Allelochemicals from the Rhizosphere Soil of Cultivated Astragalus hoantchy

Astragalus hoantchy, a widely cultivated medicinal plant species in traditional Chinese and Mongolian medicine, has been often hampered by replant failure during cultivation, like many other herbs of the genus Astragalus. Root aqueous extracts of Astragalus herbs were reported to exhibit allelopathic activity against other plants and autotoxic activity on their own seedlings, but the allelochemicals released by Astragalus plants have not been specified so far. Ten compounds were isolated from the rhizosphere soil extract of cultivated A. hoantchy and elucidated by spectroscopic analysis. Compounds 1-6 observably showed allelopathic activity against Lactuca sativa seedlings and autotoxic activity against A. hoantchy seedlings. The isolated compounds were further confirmed and quantified by high-performance liquid chromatography (HPLC) in the rhizosphere soil, with a total concentration of 9.78 μg/g (dry weight). These results specify and verify the allelochemicals released by cultivated A. hoantchy into the soil environment, which may provide new insights into the allelopathic mechanisms of this medicinal plant and probably assist in clarifying the replant problems of Astragalus plants.

Isolation, Identification, and Autotoxicity Effect of Allelochemicals from Rhizosphere Soils of Flue-Cured Tobacco

Autotoxicity, defined as a deleterious allelopathic effect among individuals of the same plant species, is considered as one of the factors that contributes to replant failure. Tobacco, as an important cultured and economic crop over the world, has been often hampered by replant failure. In view of the seriousness of this problem, the allelochemicals of flue-cured tobacco and their allelopathic effects were investigated. The extracts of rhizosphere soil exhibited phytotoxic activities against Lactuca sativa and autotoxic activities against tobacco itself. Bioassay-guided fractionation of the extract led to the isolation of six compounds, the structures of which were elucidated by spectroscopic analysis. Among them, β-cembrenediol (1), di-n-hexyl phthalate (2), and bis(2-propylheptyl) phthalate (3) showed observably phytotoxic activities against L. sativa seedlings and autotoxic activities on tobacco. The allelochemicals were then verified in the root zone soils of flue-cured tobacco by high-performance liquid chromatography (HPLC). These results provide new insights into the allelopathic mechanisms involved in the replant failure of flue-cured tobacco.