Contribution of a phytotoxic compound to the allelopathy of Ginkgo biloba

Isolation and identification of allelochemicals and their activities and functions

Allelopathy is the interaction between donor plants and receiver plants through allelochemicals. According to a great number of publications, allelopathy may be involved in several ecological aspects such as the formation of monospecific stands and sparse understory vegetation for certain plant species. Allelopathy also contributes to the naturalization of invasive plant species in introduced ranges. Autotoxicity is a particular type of allelopathy involving certain compounds. Many medicinal plants have been reported to show relatively high allelopathic activity. We selected plant species that show high allelopathic activity and isolated allelochemicals through the bioassay-guided purification process. More than 100 allelochemicals, including novel compounds have been identified in some medicinal and invasive plants, plants forming monospecific stands, plants with sparse understory vegetation, and plants showing autotoxicity. The allelopathic activity of benzoxazinones and related compounds was also determined.

Allelochemicals: A source for developing economically and environmentally friendly plant growth regulators

Allelochemicals are specific secondary metabolites that can exhibit autotoxicity by inhibiting the growth of the same plant species that produced them. These metabolites have been found to affect various physical processes during plant growth and development, including inhibition of seed germination, photosynthesis, respiration, root growth, and nutrient uptake, with diverse mechanisms involving cell destruction, oxidative homeostasis and photoinhibition. In some cases, allelochemicals can also have positive effects on plant growth and development. In addition to their ecological significance, allelochemicals also possess potential as plant growth regulators (PGRs) due to their extensive physiological effects. However, a comprehensive summary of the development and applications of allelochemicals as PGRs is currently lacking. In this review, we present an overview of the sources and categories of allelochemicals, discuss their effects and the underlying mechanisms on plant growth and development. We showcase numerous instances of key phytohormonal allelochemicals and non-phytohormonal allelochemicals, highlighting their potential as candidates for the development of PGRs. This review aims to provide a theoretical basis for the development of economical, safe and effective PGRs utilizing allelochemicals, and emphasizes the need for further research in this area.

Regulation of the regulators: Transcription factors controlling biosynthesis of plant secondary metabolites during biotic stresses and their regulation by miRNAs

Biotic stresses threaten to destabilize global food security and cause major losses to crop yield worldwide. In response to pest and pathogen attacks, plants trigger many adaptive cellular, morphological, physiological, and metabolic changes. One of the crucial stress-induced adaptive responses is the synthesis and accumulation of plant secondary metabolites (PSMs). PSMs mitigate the adverse effects of stress by maintaining the normal physiological and metabolic functioning of the plants, thereby providing stress tolerance. This differential production of PSMs is tightly orchestrated by master regulatory elements, Transcription factors (TFs) express differentially or undergo transcriptional and translational modifications during stress conditions and influence the production of PSMs. Amongst others, microRNAs, a class of small, non-coding RNA molecules that regulate gene expression post-transcriptionally, also play a vital role in controlling the expression of many such TFs. The present review summarizes the role of stress-inducible TFs in synthesizing and accumulating secondary metabolites and also highlights how miRNAs fine-tune the differential expression of various stress-responsive transcription factors during biotic stress.

Effects of Ginkgo biloba extract on growth, photosynthesis, and photosynthesis-related gene expression in Microcystis flos-aquae

The inhibitory effect of plants on algae offers a new and promising alternative method for controlling harmful algal blooms. Previous studies showed that anti-algal effects might be obvious from extracts of fallen leaves from terrestrial plants, which had great potential for cyanobacterial control in field tests. To investigate the anti-algal activities and main algicidal mechanisms of Ginkgo biloba fallen leaves extracts (GBE) on Microcystis flos-aquae, the cell density, photosynthetic fluorescence, and gene expression under different concentrations of GBE treatments were tested. GBE (3.00 g L^-1) showed a strong inhibitory effect against M. flos-aquae with an IC₅₀ (96h) of 0.79 g L^-1. All the inhibition rates of maximal quantum yield (F_v/F_m), effective quantum yield (F_q'/F_m'), and maximal relative electron transfer rate (rETR_max) were more than 70% at 96 h at 3.00 g L^-1 and more than 90% at 6.00 g L^-1. Further results of gene expression of the core proteins of PSII (psbD), limiting enzyme in carbon assimilation (rbcL), and phycobilisome degradation protein (nblA) were downregulated after exposure. These findings emphasized that photosynthetic damage is one of the main toxic mechanisms of GBE on M. flos-aquae. When exposed to 12.00 g L^-1 GBE, no significant influence on the death rate of zebrafish or photosynthetic activity of the three submerged plants was found. Therefore, appropriate use of GBE could control the expansion of M. flos-aquae colonies without potential risks to the ecological safety of aquatic environments, which means that GBE could actually be used to regulate cyanobacterial blooms in natural waters.

Allelopathy of Knotweeds as Invasive Plants

Perennial herbaceous Fallopia is native to East Asia, and was introduced to Europe and North America in the 19th century as an ornamental plant. Fallopia has been spreading quickly and has naturalized in many countries. It is listed in the world’s 100 worst alien species. Fallopia often forms dense monospecies stands through the interruption of the regeneration process of indigenous plant species. Allelopathy of Japanese knotweed (Fallopia japonica), giant knotweed (Fallopia sachalinensis), and Bohemian knotweed (Fallopia x bohemica) has been reported to play an essential role in its invasion. The exudate from their roots and/or rhizomes, and their plant residues inhibited the germination and growth of some other plant species. These knotweeds, which are non-mycorrhizal plants, also suppressed the abundance and species richness of arbuscular mycorrhizal fungi (AMF) in the rhizosphere soil. Such suppression was critical for most territorial plants to form the mutualism with AMF, which enhances the nutrient and water uptake, and the tolerance against pathogens and stress conditions. Several allelochemicals such as flavanols, stilbenes, and quinones were identified in the extracts, residues, and rhizosphere soil of the knotweeds. The accumulated evidence suggests that some of those allelochemicals in knotweeds may be released into the rhizosphere soil through the decomposition process of their plant parts, and the exudation from their rhizomes and roots. Those allelochemicals may inhibit the germination and growth of native plants, and suppress the mycorrhizal colonization of native plants, which provides the knotweeds with a competitive advantage, and interrupts the regeneration processes of native plants. Therefore, allelopathy of knotweeds may contribute to establishing their new habitats in the introduced ranges as invasive plant species. It is the first review article focusing on the allelopathy of knotweeds.

Allelopathy of Lantana camara as an Invasive Plant

Lantana camara L. (Verbenaceae) is native to tropical America and has been introduced into many other countries as an ornamental and hedge plant. The species has been spreading quickly and has naturalized in more than 60 countries as an invasive noxious weed. It is considered to be one of the world's 100 worst alien species. L. camara often forms dense monospecies stands through the interruption of the regeneration process of indigenous plant species. Allelopathy of L. camara has been reported to play a crucial role in its invasiveness. The extracts, essential oil, leachates, residues, and rhizosphere soil of L. camara suppressed the germination and growth of other plant species. Several allelochemicals, such as phenolic compounds, sesquiterpenes, triterpenes, and a flavonoid, were identified in the extracts, essential oil, residues, and rhizosphere soil of L. camara. The evidence also suggests that some of those allelochemicals in L. camara are probably released into the rhizosphere soil under the canopy and neighboring environments during the decomposition process of the residues and as leachates and volatile compounds from living plant parts of L. camara. The released allelochemicals may suppress the regeneration process of indigenous plant species by decreasing their germination and seedling growth and increasing their mortality. Therefore, the allelopathic property of L. camara may support its invasive potential and formation of dense monospecies stands.

Procyanidin B2 and rutin in Ginkgo biloba extracts protect human retinal pigment epithelial (RPE) cells from oxidative stress by modulating Nrf2 and Erk1/2 signalling

Oxidative stress plays an important role in the pathogenesis of human retinal diseases. Ginkgo biloba products are widely consumed herbal supplements that contain ingredients with anti-oxidant potentials. However, the active agents in ginkgo biloba extracts (GBE) are unclear. This study assessed the anti-oxidant effects of 19 natural compounds isolated from GBE to provide a rational basis for their use in preventing retinal diseases. The compounds were tested in retinal pigment epithelial (RPE) cells subjected to tert-butyl hydroperoxide (t-BHP)-induced oxidative stress. Cell viability and intracellular reactive oxygen species (ROS) were assessed and flow cytometry was used to delineate the cell death profile. The expression of nuclear factor erythroid 2-related factor-2 (Nrf2) was activated in RPE cells by t-BHP accompanied with an activation of Erk1/2 signaling. GBE-derived rutin and procyanidin B2 ameliorated t-BHP-induced cell death and promoted cell viability by suppressing intracellular ROS generation. These agents also enhanced Nrf2 expression with activating Erk1/2 signaling in RPE cells. In contrast, the other compounds tested were minimally active and did not prevent the loss of cell viability elicited by t-BHP. The present findings suggest that rutin and procyanidin B2 may have potential therapeutic values in the prevention of retinal diseases induced by oxidative damage.