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Biodiversity is adversely affected by the growing levels of synthetic chemicals released into the environment due to agricultural activities. This has been the driving force for embracing sustainable agriculture. Plant secondary metabolites offer promising alternatives for protecting plants against microbes, feeding herbivores, and weeds. Terpenes are the largest among PSMs and have been extensively studied for their potential as antimicrobial, insecticidal, and weed control agents. They also attract natural enemies of pests and beneficial insects, such as pollinators and dispersers. However, most of these research findings are shelved and fail to pass beyond the laboratory and greenhouse stages. This review provides an overview of terpenes, types, biosynthesis, and their roles in protecting plants against microbial pathogens, insect pests, and weeds to rekindle the debate on using terpenes for the development of environmentally friendly biopesticides and herbicides.

Just as plants attack heterospecific competitors with allelopathic phytotoxins, they also attack conspecifics with phytotoxins to inhibit seedling germination and growth (autotoxicity). As a result, for many plant species, autotoxicity limits offspring germination and growth proximate to parental plants—consequently reducing deleterious density dependent effects. Autotoxicity appears to vary across species, but it also may vary within species. We tested autotoxicity and variability in six ecotypes of the model plant, Arabidopsis thaliana, using allelopathy bioassays. We found that autotoxic impacts varied across the Eurasian and African ecotypes, and the negative effects on conspecific root growth were greater from above-than belowground exudate. In half the ecotypes, root growth decreased 71% in seedlings treated with exudate from the same ecotype than when treated with exudate from other ecotypes. That the ecotypes limited themselves more than they did other ecotypes is consistent with coexistence theory, which assumes species limit themselves more than others. Moreover, it is consistent with negative density dependent theories that suggest seedling mortality is highest near conspecific adults. Finally, the variation in autotoxicity across ecotypes suggests that intraspecific genetic variability and/or local habitat influence autotoxic intensity. It is well recognized that phytotoxic effect (allelopathy and autotoxicity) varies interspecifically but ecotype-level effects suggests that plants may exhibit greater intraspecific variation in autotoxicity than currently recognized.

An organism with a single recessive loss-of-function allele will typically have a wild-type phenotype, whereas individuals homozygous for two copies of the allele will display a mutant phenotype. We have developed a method called the mutagenic chain reaction (MCR), which is based on the CRISPR/Cas9 genome-editing system for generating autocatalytic mutations, to produce homozygous loss-of-function mutations. In Drosophila, we found that MCR mutations efficiently spread from their chromosome of origin to the homologous chromosome, thereby converting heterozygous mutations to homozygosity in the vast majority of somatic and germline cells. MCR technology should have broad applications in diverse organisms.

The frequency and intensity of cyanobacterial blooms are increasing worldwide with major societal and economic costs. Interactions between toxic cyanobacteria and eukaryotic algal competitors can affect toxic bloom formation, but the exact mechanisms of interspecies interactions remain unknown. Using metabolomic and proteomic profiling of co-cultures of the toxic cyanobacterium Microcystis aeruginosa with a green alga as well as of microorganisms collected in a Microcystis spp. bloom in Lake Taihu (China), we disentangle novel interspecies allelopathic interactions. We describe an interspecies molecular network in which M. aeruginosa inhibits growth of Chlorella vulgaris , a model green algal competitor, via the release of linoleic acid. In addition, we demonstrate how M. aeruginosa takes advantage of the cell signaling compound nitric oxide produced by C. vulgaris , which stimulates a positive feedback mechanism of linoleic acid release by M. aeruginosa and its toxicity. Our high-throughput system-biology approach highlights the importance of previously unrecognized allelopathic interactions between a broadly distributed toxic cyanobacterial bloom former and one of its algal competitors.

Plant–soil feedback (PSF) occurs when plants alter soil properties that influence the performance of seedlings, with consequent effects on plant populations and communities. Many processes influence PSF, including changes in nutrient availability and the accumulation of natural enemies, mutualists or secondary chemicals. Typically, these mechanisms are investigated in isolation, yet no single mechanism is likely to be completely responsible for PSF as these processes can interact. Further, the outcome depends on which resources are limiting and the other plants and soil biota in the surrounding environment. As such, understanding the mechanisms of PSF and their role within plant communities requires quantification of the interactions among the processes influencing PSF and the associated abiotic and biotic contexts.

The successful invasion of Ambrosia artemisiifolia is largely due to allelopathy. As an invasive alien plant, A. artemisiifolia has spread rapidly in Asia and Europe. Studies have shown that sesquiterpenoids play an important role in plant allelopathy. However, it is unclear whether the inflorescence of A. artemisiifolia also contains allelopathic components. In this paper, our phytochemical research focuses on the inflorescence of A. artemisiifolia. Twenty sesquiterpenoids, including four new ones (1–4) were isolated through successive chromatographic columns and identified by spectroscopic methods. At a concentration of 200 μg/mL, all the compounds tested were evaluated for their allelopathic activities on seedling growth of wheat. Our results indicate that nine compounds inhibited both the root and shoot growth of seedlings. Compounds 14, 15, 17, and 20 significantly inhibited root length, which was more than 50% shorter than the control. This study identified the chemical profile of the sesquiterpenoids occurring in the inflorescence of A. artemisiifolia. The bioactivity screening results provide further understanding of the chemical basis of allelopathy in A. artemisiifolia.

Allelopathy is a process by which a plant releases chemicals that inhibit competitors. Recent phytochemical studies have reported the presence of phenolic molecules in some species of Pilosocereus, which could be indicative of allelopathic effect. The aim of this study was to assess the allelopathic potential of the fruit pulp extract of Pilosocereus gounellei on the germination of seeds of Lactuca sativa and P. gounellei. We tested the effects of pulp extract at four different concentrations. We measured the osmotic potential of the extract at the different concentrations, in order to assess its effect on seed germination. The extract negatively affected the germination parameters of both species and effect was more significant at higher extract concentrations. The influence of the osmotic potential on the germination parameters was not significant for most of the studied parameters. We believe that our results are indicative that the fruit pulp extract of P. gounellei contains allelochemicals that delayed or inhibited seed germination of not only L. sativa, but also of seeds of their own species (autoallelopathy). Further studies are necessary to clarify the inhibition mechanisms, and how widespread the allelopathy is across other cactus species.

In conventional tea plantations, a large amount of pruned material returns to the soil surface, putting a high quantity of polyphenols into the soil. The accumulation of active allelochemicals in the tea rhizosphere and subsequent shift in beneficial microbes may be the cause of acidification, soil sickness, and regeneration problem, which may be attributed to hindrance of plant growth, development, and low yield in long-term monoculture tea plantation. However, the role of pruning leaf litter in soil sickness under consecutive tea monoculture is unclear. Here, we investigated soil samples taken from conventional tea gardens of different ages (2, 15, and 30 years) and under the effect of regular pruning. Different approaches including liquid chromatography–mass spectrometry (LC-MS) analysis of the leaf litter, metagenomic study of root-associated bacterial communities, and in vitro interaction of polyphenols with selected bacteria were applied to understand the effect of leaf litter-derived polyphenols on the composition and structure of the tea rhizosphere microbial community. Our results indicated that each pruning practice returns a large amount of leaf litter to each tea garden. LC-MS results showed that leaf litter leads to the accumulation of various allelochemicals in the tea rhizosphere, including epigallocatechin gallate , epigallocatechin , epicatechin gallate , catechin , and epicatechin with increasing age of the tea plantation. Meanwhile, in the tea garden grown consecutively for 30 years (30-Y), the phenol oxidase and peroxidase activities increased significantly. Pyrosequencing identified Burkholderia and Pseudomonas as the dominant genera, while plant growth-promoting bacteria, especially Bacillus , Prevotella , and Sphingomonas , were significantly reduced in the long-term tea plantation. The qPCR results of 30-Y soil confirmed that the copy numbers of bacterial genes per gram of the rhizosphere soil were significantly reduced, while that of Pseudomonas increased significantly. In vitro study showed that the growth of catechin-degrading bacteria (e.g., Pseudomonas ) increased and plant-promoting bacteria (e.g., Bacillus ) decreased significantly with increasing concentration of these allelochemicals. Furthermore, in vitro interaction showed a 0.36-fold decrease in the pH of the broth after 72 h with the catechin degradation. In summary, the increase of Pseudomonas and Burkholderia in the 30-Y garden was found to be associated with the accumulation of catechin substrates. In response to the long-term monoculture of tea, the variable soil pH along with the litter distribution negatively affect the population of plant growth-promoting bacteria (e.g., Sphingomonas , Bacillus , and Prevotella ). Current research suggests that the removal of pruned branches from tea gardens can prevent soil sickness and may lead to sustainable tea production.

An allelopathic rice PI312777 and a non-allelopathic rice Lemont were hydroponically cultured and treated with extracts of seeds and tissues and root exudates of barnyard grass (Echinochloa crus-galli, BYG) to induce an allelopathic response. The results of bioassays showed that the % inhibition of two rice leaf extracts was significantly (p < 0.05) increased by treatment with BYG root exudates. This induced trait was dosage- and time-dependent. The highest % inhibition for both PI312777 and Lemont was obtained following treatment with 15 mL of BYG root exudates for 2 days. Under these conditions, total allelopathy (TA) values of PI312777 on root length, plant height, and plant dry weight of BYG were 73.39, 68.01, and 70.42%, respectively, and induced allelopathy (IA) values were 21.53, 17.04, and 16.62%, respectively, accounting for 24–29% of TA. Correspondingly, TA values of Lemont were 28.38, 21.38, and 23.38%, respectively, and IA values were 14.49, 11.37, and 12.11%, respectively, accounting for 51–53% of TA. The % inhibition of two rice culture solutions was in agreement with the results of their leaf extracts. The total contents of 7 phenolic acids in the culture solutions of PI312777 and Lemont were 2.18 and 1.47 times, respectively, as much as those in the control solutions. The phenylalanine ammonia-lyase (PAL) gene in PI312777 leaves was significantly up-regulated after induction treatments. The results indicated that rice allelopathy is a chemical induction mechanism, and confirmed that chemical induction to raise rice allelopathy is a practical and feasible method.

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 selfinhibition 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.