Sorgoleone from root exudate inhibits mitochondrial functions

Selective Biotic Stressors' Action on Seed Germination: A Review

In the realm of plant biology and agriculture, seed germination serves as a fundamental process with far-reaching implications for crop production and environmental health. This comprehensive review seeks to unravel the intricate web of interactions between some biotic stressors and seed germination, addressing the pertinent issue of how these stressors influence seed germination. Different chemicals produced by interacting plants (different parts), fungi, bacteria, or insects can either promote or inhibit seed germination. Releasing chemicals that modulate signaling pathways and cellular processes significantly disrupt essential cellular functions. This disruption leads to diverse germination outcomes, introducing additional layers of complexity to this regulatory landscape. The chemicals perturb enzyme activity and membrane integrity, imposing unique challenges on the germination process. Understanding the mechanisms- how allelochemicals, mycotoxins, or bacterial toxins affect seed germination or the modes of action holds promise for more sustainable agricultural practices, enhanced pest control, and improved environmental outcomes. In sum, this review contributes to a fundamental exposition of the pivotal role of biotic stressors in shaping the germination of seeds.

Physiological and molecular insights into the allelopathic effects on agroecosystems under changing environmental conditions

Allelopathy is a natural phenomenon of competing and interfering with other plants or microbial growth by synthesizing and releasing the bioactive compounds of plant or microbial origin known as allelochemicals. This is a sub-discipline of chemical ecology concerned with the effects of bioactive compounds produced by plants or microorganisms on the growth, development and distribution of other plants and microorganisms in natural communities or agricultural systems. Allelochemicals have a direct or indirect harmful effect on one plant by others, especially on the development, survivability, growth, and reproduction of species through the production of chemical inhibitors released into the environment. Cultivation systems that take advantage of allelopathic plants' stimulatory/inhibitory effects on plant growth and development while avoiding allelopathic autotoxicity is critical for long-term agricultural development. Allelopathy is one element that defines plant relationships and is involved in weed management, crop protection, and microbial contact. Besides, the allelopathic phenomenon has also been reported in the forest ecosystem; however, its presence depends on the forest type and the surrounding environment. In the present article, major aspects addressed are (1) literature review on the impacts of allelopathy in agroecosystems and underpinning the research gaps, (2) chemical, physiological, and ecological mechanisms of allelopathy, (3) genetic manipulations, plant defense, economic benefits, fate, prospects and challenges of allelopathy. The literature search and consolidation efforts in this article shall pave the way for future research on the potential application of allelopathic interactions across various ecosystems.

Passive permeability controls synthesis for the allelochemical sorgoleone in sorghum root exudate

Competition for soil nutrients and water with other plants foster competition within the biosphere for access to these limited resources. The roots for the common grain sorghum produce multiple small molecules that are released via root exudates into the soil to compete with other plants. Sorgoleone is one such compound, which suppresses weed growth near sorghum by acting as a quinone analog and interferes with photosynthesis. Since sorghum also grows photosynthetically, and may be susceptible to sorgoleone action if present in tissues above ground, it is essential to exude sorgoleone efficiently. However, since the P450 enzymes that synthesize sorgoleone are intracellular, the release mechanism for sorgoleone remain unclear. In this study, we conducted an in silico assessment for sorgoleone and its precursors to passively permeate biological membranes. To facilitate accurate simulation, CHARMM parameters were newly optimized for sorgoleone and its precursors. These parameters were used to conduct 1 μs of unbiased molecular dynamics simulations to compare the permeability of sorgoleone with its precursors molecules. We find that interleaflet transfer is maximized for sorgoleone, suggesting that the precursor molecules may remain in the same leaflet for access by biosynthetic P450 enzymes. Since no sorgoleone was extracted during unbiased simulations, we compute a permeability coefficient using the inhomogeneous solubility diffusion model. The requisite free energy and diffusivity profiles for sorgoleone through a sorghum membrane model were determined through Replica Exchange Umbrella Sampling (REUS) simulations. The REUS calculations highlight that any soluble sorgoleone would quickly insert into a lipid bilayer, and would readily transit. When sorgoleone forms aggregates in root exudate as indicated by our equilibrium simulations, aggregate formation would lower the effective concentration in aqueous solution, creating a concentration gradient that would facilitate passive transport. This suggests that sorgoleone synthesis occurs within sorghum root cells and that sorgoleone is exuded by permeating through the cell membrane without the need for a transport protein once the extracellular sorgoleone aggregate is formed.

Persistence of Root Exudates of Sorghum bicolor and Solidago canadensis: Impacts on Invasive and Native Species

Root exudates of the invasive Solidago canadensis and the cereal crop Sorghum bicolor (L.) Moench cv. 'Hybridsorgo' were tested for allelopathic interactions against native and invasive plant species in a controlled environment. After the surface was sterilized, the seeds of two invasive species (Bromus sterilis and Veronica persica) and two native species (Youngia japonica and Rumex acetosa) were germinated and transplanted into the soil (1:1 mixture of coco peat and sand) that had been conditioned for one month by the cultivation of Solidago canadensis and Sorghum bicolor, both in combination or as unplanted controls. After an additional eight weeks of growth, morphometric measurements of the shoot and root, including foliar characteristics and above- and below-ground biomass accumulation, were performed. The results revealed significant inhibitory effects of root exudates released by Sorghum bicolor and Solidago canadensis on native species' productivity and physiology. The invasive species exhibited variable growth responses, with Veronica persica showing reduced shoot and root expansion, but Bromus sterilis revealed increased shoot and root biomass allocation and nutrition under the exudate treatments. Exudates from Solidago canadensis and Sorghum bicolor together showed synergistic negative effects on native species, while they promoted growth and nutrition in Veronica persica. Taken together, the differential species responses indicate that the tested native species were more sensitive to the allelopathic compounds than the invasive species, which is in line with the theory of novel weapons. The legacy effects of root exudates of both Sorghum bicolor and Solidago canadensis could promote invasive establishment through imposing allelochemical interference competition against native plant species. Understanding the specific allelopathic mechanisms may help with the development of integrated strategies for managing invasive species.

In vivo assembly of the sorgoleone biosynthetic pathway and its impact on agroinfiltrated leaves of Nicotiana benthamiana

Sorgoleone, a hydrophobic compound exuded from root hair cells of Sorghum spp., accounts for much of the allelopathic activity of the genus. The enzymes involved in the biosynthesis of this compound have been identified and functionally characterized. Here, we report the successful assembly of the biosynthetic pathway and the significant impact of in vivo synthesized sorgoleone on the heterologous host Nicotiana benthamiana. A multigene DNA construct was prepared for the expression of genes required for sorgoleone biosynthesis in planta and deployed in N. benthamiana leaf tissues via Agrobacterium-mediated transient expression. RNA-sequencing was conducted to investigate the effects of sorgoleone, via expression of its biosynthesis pathway, on host gene expression. The production of sorgoleone in agroinfiltrated leaves as detected by gas chromatography/mass spectrometry (GC/MS) resulted in the formation of necrotic lesions, indicating that the compound caused severe phytotoxicity to these tissues. RNA-sequencing profiling revealed significant changes in gene expression in the leaf tissues expressing the pathway during the formation of sorgoleone-induced necrotic lesions. Transcriptome analysis suggested that the compound produced in vivo impaired the photosynthetic system as a result of downregulated gene expression for the photosynthesis apparatus and elevated expression of proteasomal genes which may play a major role in the phytotoxicity of sorgoleone.

Research Progress on the use of Plant Allelopathy in Agriculture and the Physiological and Ecological Mechanisms of Allelopathy

Allelopathy is a common biological phenomenon by which one organism produces biochemicals that influence the growth, survival, development, and reproduction of other organisms. These biochemicals are known as allelochemicals and have beneficial or detrimental effects on target organisms. Plant allelopathy is one of the modes of interaction between receptor and donor plants and may exert either positive effects (e.g., for agricultural management, such as weed control, crop protection, or crop re-establishment) or negative effects (e.g., autotoxicity, soil sickness, or biological invasion). To ensure sustainable agricultural development, it is important to exploit cultivation systems that take advantage of the stimulatory/inhibitory influence of allelopathic plants to regulate plant growth and development and to avoid allelopathic autotoxicity. Allelochemicals can potentially be used as growth regulators, herbicides, insecticides, and antimicrobial crop protection products. Here, we reviewed the plant allelopathy management practices applied in agriculture and the underlying allelopathic mechanisms described in the literature. The major points addressed are as follows: (1) Description of management practices related to allelopathy and allelochemicals in agriculture. (2) Discussion of the progress regarding the mode of action of allelochemicals and the physiological mechanisms of allelopathy, consisting of the influence on cell micro- and ultra-structure, cell division and elongation, membrane permeability, oxidative and antioxidant systems, growth regulation systems, respiration, enzyme synthesis and metabolism, photosynthesis, mineral ion uptake, protein and nucleic acid synthesis. (3) Evaluation of the effect of ecological mechanisms exerted by allelopathy on microorganisms and the ecological environment. (4) Discussion of existing problems and proposal for future research directions in this field to provide a useful reference for future studies on plant allelopathy.

Sorghum allelopathy--from ecosystem to molecule

Sorghum allelopathy has been reported in a series of field experiments following sorghum establishment. In recent years, sorghum phytotoxicity and allelopathic interference also have been well-described in greenhouse and laboratory settings. Observations of allelopathy have occurred in diverse locations and with various sorghum plant parts. Phytotoxicity has been reported when sorghum was incorporated into the soil as a green manure, when residues remained on the soil surface in reduced tillage settings, or when sorghum was cultivated as a crop in managed fields. Allelochemicals present in sorghum tissues have varied with plant part, age, and cultivar evaluated. A diverse group of sorghum allelochemicals, including numerous phenolics, a cyanogenic glycoside (dhurrin), and a hydrophobic p-benzoquinone (sorgoleone) have been isolated and identified in recent years from sorghum shoots, roots, and root exudates, as our capacity to analyze and identify complex secondary products in trace quantities in the plant and in the soil rhizosphere has improved. These allelochemicals, particularly sorgoleone, have been widely investigated in terms of their mode(s) of action, specific activity and selectivity, release into the rhizosphere, and uptake and translocation into sensitive indicator species. Both genetics and environment have been shown to influence sorgoleone production and expression of genes involved in sorgoleone biosynthesis. In the soil rhizosphere, sorgoleone is released continuously by living root hairs where it accumulates in significant concentrations around its roots. Further experimentation designed to study the regulation of sorgoleone production by living sorghum root hairs may result in increased capacity to utilize sorghum cover crops more effectively for suppression of germinating weed seedlings, in a manner similar to that of soil-applied preemergent herbicides like trifluralin.

Synthesis and biological evaluation of 2,5-bis(alkylamino)-1,4-benzoquinones

A series of twelve 2,5-bis(alkylamino)-1,4-benzoquinones were prepared in yields ranging from 9–58% via the reaction between p-benzoquinone and various amines. The structures of the synthesized compounds were confirmed by IR, ¹H- and ¹³C-NMR and MS analyses. The phytotoxicity of the 2,5-bis(alkylamino)-1,4-benzoquinones was evaluated against two crop species, Cucumis sativus and Sorgum bicolor, at 1.0 × 10^-3 mol/L. In general, the quinones displayed inhibitory effects on the dicotyledonous species C. sativus (7–74%). On the other hand stimulatory effects were observed on S. bicolor (monocotyledonous). Similar results were observed in the biological assays carried out with the weed species Ipomoea grandifolia (dicotyledonous) and Brachiaria decumbens (monocotyledonous). In addition, the cytotoxicity of the 2,5-bis(alkylamino)-1,4-benzoquinones was assayed against HL-60 (leukemia), MDA-MB-435 (melanoma), SF-295 (brain) and HCT-8 (colon) human cancer cell lines and human peripheral blood mononuclear cells (PBMC), as representatives of healthy cells, using a MTT and an Alamar Blue assay. Compound 12 was the most active, displaying cytotoxicity against all cancer cell lines tested.

Sorgoleone

Sorgoleone, a major component of the hydrophobic root exudate of sorghum [Sorghum bicolor (L.) Moench], is one of the most studied allelochemicals. The exudate also contains an equivalent amount of a lipid resorcinol analog as well as a number of minor sorgoleone congeners. Synthesis of sorgoleone is constitutive and compartmentalized within root hairs, which can accumulate up to 20 microg of exudate/mg root dry weight. The biosynthesis pathway involves unique fatty acid desaturases which produce an atypical 16:3 fatty acyl-CoA starter unit for an alkylresorcinol synthase that catalyzes the formation of a pentadecatrienylresorcinol intermediate. This intermediate is then methylated by SAM-dependent O-methyltransferases and dihydroxylated by cytochrome P450 monooxygenases. An EST data set derived from a S. bicolor root hair-specific cDNA library contained all the candidate sequences potentially encoding enzymes involved in the sorgoleone biosynthetic pathway. Sorgoleone interferes with several molecular target sites, including inhibition of photosynthesis in germinating seedlings. Sorgoleone is not translocated acropetally in older plants, but can be absorbed through the hypocotyl and cotyledonary tissues. Therefore, the mode of action of sorgoleone may be the result of inhibition of photosynthesis in young seedlings in concert with inhibition of its other molecular target sites in older plants. Due to its hydrophobic nature, sorgoleone is strongly sorbed in soil which increases its persistence, but experiments show that it is mineralized by microorganisms over time.

Natural products in crop protection

The tremendous increase in crop yields associated with the 'green' revolution has been possible in part by the discovery and utilization of chemicals for pest control. However, concerns over the potential impact of pesticides on human health and the environment has led to the introduction of new pesticide registration procedures, such as the Food Quality Protection Act in the United States. These new regulations have reduced the number of synthetic pesticides available in agriculture. Therefore, the current paradigm of relying almost exclusively on chemicals for pest control may need to be reconsidered. New pesticides, including natural product-based pesticides are being discovered and developed to replace the compounds lost due to the new registration requirements. This review covers the historical use of natural products in agricultural practices, the impact of natural products on the development of new pesticides, and the future prospects for natural products-based pest management.

Dynamic root exudation of sorgoleone and its in planta mechanism of action

The oily droplets exuded from the root hairs of sorghum are composed of a 1:1 ratio of sorgoleone and its lipid resorcinol analogue. The production of these droplets appears to be suppressed when c. 20 microg of exudate mg(-1) root dry weight accumulates at the tip of the root hairs. However, more exudate is produced following gentle washing of the roots with water, suggesting that the biosynthesis of lipid benzoquinones and resorcinols is a dynamic process. Sorgoleone interferes with several molecular target sites, including photosynthetic electron transport, in in vitro assays. However, the in planta mechanism of action of sorgoleone remains controversial because it is not clear whether this lipid benzoquinone exuding from the roots of sorghum is taken up by roots of the receiving plants and translocated to their foliage where it must enter the chloroplast and inhibit PSII in the thylakoid membrane. Experiments designed to test the in planta mode of action of sorgoleone demonstrated that it has no effect on the photosynthesis of older plants, but inhibits photosynthesis in germinating seedlings. Sorgoleone is not translocated acropetally in older plants, but can be absorbed through the hypocotyl and cotyledonary tissues. Therefore, the mode of action of sorgoleone may be the result of inhibition of photosynthesis in young seedlings in concert with inhibition of its other molecular target sites in older plants.

A functional genomics investigation of allelochemical biosynthesis in Sorghum bicolor root hairs

Sorghum is considered to be one of the more allelopathic crop species, producing phytotoxins such as the potent benzoquinone sorgoleone (2-hydroxy-5-methoxy-3-[(Z,Z)-8',11',14'-pentadecatriene]-p-benzoquinone) and its analogs. Sorgoleone likely accounts for much of the allelopathy of Sorghum spp., typically representing the predominant constituent of Sorghum bicolor root exudates. Previous and ongoing studies suggest that the biosynthetic pathway for this plant growth inhibitor occurs in root hair cells, involving a polyketide synthase activity that utilizes an atypical 16:3 fatty acyl-CoA starter unit, resulting in the formation of a pentadecatrienyl resorcinol intermediate. Subsequent modifications of this resorcinolic intermediate are likely to be mediated by S-adenosylmethionine-dependent O-methyltransferases and dihydroxylation by cytochrome P450 monooxygenases, although the precise sequence of reactions has not been determined previously. Analyses performed by gas chromatography-mass spectrometry with sorghum root extracts identified a 3-methyl ether derivative of the likely pentadecatrienyl resorcinol intermediate, indicating that dihydroxylation of the resorcinol ring is preceded by O-methylation at the 3'-position by a novel 5-n-alk(en)ylresorcinol-utilizing O-methyltransferase activity. An expressed sequence tag data set consisting of 5,468 sequences selected at random from an S. bicolor root hair-specific cDNA library was generated to identify candidate sequences potentially encoding enzymes involved in the sorgoleone biosynthetic pathway. Quantitative real time reverse transcription-PCR and recombinant enzyme studies with putative O-methyltransferase sequences obtained from the expressed sequence tag data set have led to the identification of a novel O-methyltransferase highly and predominantly expressed in root hairs (designated SbOMT3), which preferentially utilizes alk(en)ylresorcinols among a panel of benzene-derivative substrates tested. SbOMT3 is therefore proposed to be involved in the biosynthesis of the allelochemical sorgoleone.

Synthesis of new phytogrowth-inhibitory substituted aryl-p-benzoquinones

Reaction of [(2-alkyloxy)methyl]-1,4-dimethoxybenzene 10 (alkyl=butyl, hexyl, decyl, tridecyl, tetradecyl, hexadecyl, and octadecyl) with ceric ammonium nitrate in order to produce p-benzoquinones (=cyclohexa-2,5-diene-1,4-diones) afforded 5-[(alkyloxy)methyl]-2-(4-formyl-2,5-dimethoxyphenyl)benzo-1,4-quinones 12a-12g in yields that varied from 46 to 97%, accompanied by 2-[(alkyloxy)methyl]benzo-1,4-quinones 11a-11g in only small quantities (< or =5%). These quinones resemble the natural phytotoxic compound sorgoleone, found in Sorghum bicolor. This reaction exemplifies a general procedure for the synthesis of novel aryl-substituted p-benzoquinones. The selective effects of compounds 12a-12g, at the concentration of 5.5 ppm, on the growth of Cucumis sativus, Sorghum bicolor, Euphorbia heterophylla, and Ipomoea grandifolia were evaluated. All compounds caused some inhibition upon the aerial parts and root growth of the tested plants. The most active compound, 2-(4-formyl-2,5-dimethoxyphenyl)-5-[(tridecyloxy)methyl]-benzo-1,4-quinone (12d), caused between 3 and 18%, and 12 and 29% inhibition on the roots and aerial parts development of Cucumis sativus and Sorghum bicolor, respectively, and between 77 and 85%, and 34 and 52% inhibition on the roots and aerial parts growth of Euphorbia heterophylla and Ipomoea grandifolia, respectively.

Functional characterization of desaturases involved in the formation of the terminal double bond of an unusual 16:3Delta(9,12,150) fatty acid isolated from Sorghum bicolor root hairs

Sorgoleone, produced in root hair cells of sorghum (Sorghum bicolor), is likely responsible for much of the allelopathic properties of sorghum root exudates against broadleaf and grass weeds. Previous studies suggest that the biosynthetic pathway of this compound initiates with the synthesis of an unusual 16:3 fatty acid possessing a terminal double bond. The corresponding fatty acyl-CoA serves as a starter unit for polyketide synthases, resulting in the formation of 5-pentadecatrienyl resorcinol. This resorcinolic intermediate is then methylated by an S-adenosylmethionine-dependent O-methyltransferase and subsequently dihydroxylated, yielding the reduced (hydroquinone) form of sorgoleone. To characterize the corresponding enzymes responsible for the biosynthesis of the 16:3 fatty acyl-CoA precursor, we identified and cloned three putative fatty acid desaturases, designated SbDES1, SbDES2, and SbDES3, from an expressed sequence tag (EST) data base prepared from isolated root hairs. Quantitative real-time RT-PCR analyses revealed that these three genes were preferentially expressed in sorghum root hairs where the 16:2 and 16:3 fatty acids were exclusively localized. Heterologous expression of the cDNAs in Saccharomyces cerevisiae revealed that recombinant SbDES2 converted palmitoleic acid (16:1Delta(9)) to hexadecadienoic acid (16:2Delta(9,12)), and that recombinant SbDES3 was capable of converting hexadecadienoic acid into hexadecatrienoic acid (16:3Delta(9,12,15)). Unlike other desaturases reported to date, the double bond introduced by SbDES3 occurred between carbons 15 and 16 resulting in a terminal double bond aliphatic chain. Collectively, the present results strongly suggest that these fatty acid desaturases represent key enzymes involved in the biosynthesis of the allelochemical sorgoleone.

Factors modulating the levels of the allelochemical sorgoleone in Sorghum bicolor

Sorgoleone is the major component of the hydrophobic root exudate of sorghum [Sorghum bicolor (L.) Moench]. The presence of this allelochemical is intrinsically linked to root growth and the development of mature root hairs. However, factors modulating root formation and the biosynthesis of sorgoleone are not well known. Sorgoleone production was independent of early stages of plant development. The optimum temperature for root growth and sorgoleone production was 30 degrees C. Seedling development and sorgoleone levels were greatly reduced at temperatures below 25 degrees C and above 35 degrees C. The level of sorgoleone was also sensitive to light, being reduced by nearly 50% upon exposure to blue light (470 nm) and by 23% with red light (670 nm). Applying mechanical pressure over developing seedlings stimulated root formation but did not affect the biosynthesis of this lipid benzoquinone. Sorgoleone production did not change in seedlings exposed to plant defense elicitors. On the other hand, sorgoleone levels increased in plants treated with a crude extract of velvetleaf (Abutilon theophrasti Medik.) root. This stimulation was not associated with increased osmotic stress, since decreases in water potential (Psi(w)) by increasing solute concentrations with sorbitol reduces sorgoleone production. Sorgoleone production appears to be constitutively expressed in young developing sorghum plants. Other than with temperature, changes in the environmental factors had either no effect or caused a reduction in sorgoleone levels. However, the stimulation observed with velvetleaf root crude extract suggests that sorghum seedlings may respond to the presence of other plants by releasing more of this allelochemical.

The allelochemical sorgoleone inhibits root H+-ATPase and water uptake

Sorghum plants inhibit the growth of some adjacent species. Root exudates from grain sorghum (Sorghum bicolor), consisting primarily of the quinone sorgoleone, are phytotoxic to several plant species, yet the mechanisms of growth inhibition remain to be fully explained. Disruption of electron transport functions in isolated mitochondria and chloroplasts has been reported as one explanation for growth inhibition. In the studies reported here, however, soybean seedlings grown in nutrient solution with 10, 50, or 100 microM sorgoleone showed no disruption of photosynthesis, as measured by leaf fluorescence and oxygen evolution, yet their mean leaf surface area was less when grown in 100 microM sorgoleone. Furthermore, in the presence of these same concentrations of sorgoleone, decreased nutrient solution use by soybean seedlings and decreased H+-ATPase activity in corn root microsomal membranes were observed. This suggests that impairment of essential plant processes, such as solute and water uptake, driven by proton-pumping across the root cell plasmalemma should also be considered as a mechanism contributing to observed plant growth inhibition by sorgoleone.

Chromatographic separation and in vitro activity of sorgoleone congeners from the roots of sorghum bicolor

Sorgoleone, 2-hydroxy-5-methoxy-3-[(8'Z,11'Z)-8',11',14'-pentadecatriene]-p-benzoquinone (1), and its corresponding hydroquinone are the major components of the root exudate of Sorghum bicolor. The name sorgoleone includes minor analogues differing in the length or degree of unsaturation of the 3-alkyl side chain. These compounds are known to be phytotoxic, probably through inhibition of photosystem II (PSII) driven oxygen evolution, as previously demonstrated for 1. Isolation of these sorgoleone congeners was achieved by C(8) column chromatography and argentation thin-layer chromatography, and the purified compounds were structurally characterized. The abilities of the minor sorgoleones to inhibit PSII were similar to that of the major compound, suggesting that all of these sorgoleone congeners contribute to the overall allelopathy of sorghum.

Elucidation of the biosynthetic pathway of the allelochemical sorgoleone using retrobiosynthetic NMR analysis

NMR analyses of the labeling pattern obtained using various 13C-labeled precursors indicated that both the lipid tail and the quinone head of sorgoleone, the main allelopathic component of the oily root exudate of Sorghum bicolor, were derived from acetate units, but that the two moieties were synthesized in different subcellular compartments. The 16:3 fatty acid precursor of the tail is synthesized by the combined action of fatty-acid synthase and desaturases most likely in the plastids. It is then exported out of the plastids and converted to 5-pentadecatriene resorcinol by a polyketide synthase. This resorcinol intermediate was identified in root hair extracts. The lipid resorcinol intermediate is then methylated by a S-adenosylmethionine-dependent O-methyltransferase and subsequently dihydroxylated by a P450 monooxygenase to yield the reduced form of sorgoleone.

Herbicidal action of 2-hydroxy-3-alkyl-1,4-naphthoquinones

The main mode of herbicidal activity of 2-hydroxy-3-alkyl-1,4-naphthoquinones is shown to be inhibition of photosystem II (PSII). The herbicidal and in vitro activities have been measured and correlated with their (Log)octanol/water partition coefficients (Log Ko/w). The length of the 3-n-alkyl substituent for optimal activity differed between herbicidal and in vitro activity. The maximum in vitro activity was given by the nonyl to dodecyl homologues (Log Ko/w between 6.54 and 8.12), whereas herbicidal activity peaked with the n-hexyl compound (Log Ko/w = 4.95). The effect of chain branching was also investigated using isomeric pentyl analogues substituted at position 3. All exhibited similar levels of in vitro activities but herbicidal activities differed, albeit moderately, with the exception of one analogue that was much less phytotoxic. Other modes of action were also investigated using two representative compounds. They did not show any activity on photosystem I or mitochondrial complex I, or generate toxic oxygen radicals by redox cycling reactions. Only moderate activity was found against mitochondrial complex III from plants, in contrast to much higher corresponding activity using an insect enzyme.

Aerotaxis and other energy-sensing behavior in bacteria

Energy taxis is widespread in motile bacteria and in some species is the only known behavioral response. The bacteria monitor their cellular energy levels and respond to a decrease in energy by swimming to a microenvironment that reenergizes the cells. This is in contrast to classical Escherichia coli chemotaxis in which sensing of stimuli is independent of cellular metabolism. Energy taxis encompasses aerotaxis (taxis to oxygen), phototaxis, redox taxis, taxis to alternative electron acceptors, and chemotaxis to a carbon source. All of these responses share a common signal transduction pathway. An environmental stimulus, such as oxygen concentration or light intensity, modulates the flow of reducing equivalents through the electron transport system. A transducer senses the change in electron transport, or possibly a related parameter such as proton motive force, and initiates a signal that alters the direction of swimming. The Aer and Tsr proteins in E. coli are newly recognized transducers for energy taxis. Aer is homologous to E. coli chemoreceptors but unique in having a PAS domain and a flavin-adenine dinucleotide cofactor that is postulated to interact with a component of the electron transport system. PAS domains are energy-sensing modules that are found in proteins from archaea to humans. Tsr, the serine chemoreceptor, is an independent transducer for energy taxis, but its sensory mechanism is unknown. Energy taxis has a significant ecological role in vertical stratification of microorganisms in microbial mats and water columns. It plays a central role in the behavior of magnetotactic bacteria and also appears to be important in plant-microbe interactions.

Loss of cytochrome c oxidase activity and acquisition of resistance to quinone analogs in a laccase-positive variant of Azospirillum lipoferum

Laccase, a p-diphenol oxidase typical of plants and fungi, has been found recently in a proteobacterium, Azospirillum lipoferum. Laccase activity was detected in both a natural isolate and an in vitro-obtained phase variant that originated from the laccase-negative wild type. In this study, the electron transport systems of the laccase-positive variant and its parental laccase-negative forms were compared. During exponential (but not stationary) growth under fully aerobic (but not under microaerobic) conditions, the laccase-positive variant lost a respiratory branch that is terminated in a cytochrome c oxidase of the aa(3) type; this was most likely due to a defect in the biosynthesis of a heme component essential for the oxidase. The laccase-positive variant was significantly less sensitive to the inhibitory action of quinone analogs and fully resistant to inhibitors of the bc(1) complex, apparently due to the rearrangements of its respiratory system. We propose that the loss of the cytochrome c oxidase-containing branch in the variant is an adaptive strategy to the presence of intracellular oxidized quinones, the products of laccase activity.

A new photosystem II electron transfer inhibitor from Sorghum bicolor

Our study of the mechanism(s) by which sorgoleone (1) acts as a photosystem II (PS II) inhibitor led to the isolation of a new benzoquinone derivative, 2-hydroxy-5-ethoxy-3-[(Z,Z)-8',11', 14'-pentadecatriene]-rho-benzoquinone (2), from the root exudate of sorghum. The structure of 2, which is being given the name 5-ethoxy-sorgoleone, was determined by spectroscopic means. A methoxy derivative (3) of 1 was also prepared. Both 2 and 3 caused a reduction in oxygen evolution by thylakoid membranes and induced variable chlorophyll fluorescence. These compounds, however, were less active inhibitors of PS II than 1.