Baerson, S. R., Dayan, F. E., Rimando, A. M., Nanayakkara, N. P. D., Liu, C. J., Schröder, J., Fishbein, M., Pan, Z., Kagan, I. A., Pratt, L. H., Cordonnier-Pratt, M.-M., and Duke, S. O.: A functional genomics investigation of allelochemical biosynthesis in Sorghum bicolor root hairs. Journal of Biological Chemistry  283: 3231-3247 (2008)
     Sorghum is considered to be one of the Mehr 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 pentadecatrienylresorcinol 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 putativeO-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 benzenederivative substrates tested. SbOMT3 is therefore proposed to be involved in the biosynthesis of the allelochemical sorgoleone.
Reprint request
Zurück zum Text  

Bertin, C., Yang, X., Weston, L. A., 2003. The role of root exudates and allelochemicals in the rhizosphere. Plant and Soil 256, 67-83.
Plant roots serve a multitude of functions in the plant including anchorage, provision of nutrients and water, and production of exudates with growth regulatory properties. The root–soil interface, or rhizosphere, is the site of greatest activity within the soil matrix. Within this matrix, roots affect soil structure, aeration and biological activity as they are the major source of organic inputs into the rhizosphere, and are also responsible for depletion of large supplies of inorganic compounds. Roots are very complicated morphologically and physiologically, and their metabolites are often released in large quantities into the soil rhizosphere from living root hairs or fibrous root systems. Root exudates containing root-specific metabolites have critical ecological impacts on soil macro and microbiota as well as on the whole plant itself. Through the exudation of a wide variety of compounds, roots impact the soil microbial community in their immediate vicinity, influence resistance to pests, support beneficial symbioses, alter the chemical and physical properties of the soil, and inhibit the growth of competing plant species. In this review, we outline recent research on root exudation and the role of allelochemicals in the rhizosphere by studying the case of three plants that have been shown to produce allelopathic root exudates: black walnut, wheat and sorghum.
Zurück zum Text

Duke, S. O., 2007. The emergence of grass root chemical ecology. Proceedings of the National Academy of Sciences of the United States of America 104, 16729-16730.
No Abstract.
Zurück zum Text

Dayan, F. E., Howell, J., Weidenhamer, J. D., 2009. Dynamic root exudation of sorgoleone and its in planta mechanism of action. Journal of Experimental Botany 60, 2107-2117.
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 µg of exudate mg^-1 root dry weight accumulates at the tip of the root hairs. However, Mehr 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.
Zurück zum Text

Duke, S. O., 2003. Weeding with transgenes. Trends in Biotechnology 21, 192-195.
Transgenes promise to reduce insecticide and fungicide use but relatively little has been done to significantly reduce herbicide use through genetic engineering. Recently, three strategies for transgene utilization have been developed that have the potential to change this. These are the improvement of weed-specific biocontrol agents, enhancement of crop competition or allelopathic traits, and production of cover crops that will self-destruct near the time of planting. Failsafe risk mitigation technologies are needed for most of these strategies.
Zurück zum Text

Einhellig, F. A., Souza, I. F., 1992. Phytotoxicity of sorgoleone found in grain Sorghum root exudates. Journal of Chemical Ecology 18, 1-11.
Root exudates of Sorghum bicolor consist primarily of a dihydroquinone that is quickly oxidized to a p-benzoquinone named sorgoleone. The aim of this investigation was to determine the potential activity of sorgoleone as an inhibitor of weed growth. Bioassays showed 125 microM sorgoleone reduced radicle elongation of Eragrostis tef. In liquid culture, 50 microM sorgoleone treatments stunted the growth of Lemna minor. Over a 10-day treatment period, 10 microM sorgoleone in the nutrient medium reduced the growth of all weed seedlings tested: Abutilon theophrasti, Datura stramonium, Amaranthus retroflexus, Setaria viridis, Digitaria sanguinalis , and Echinochloa crusgalli. These data show sorgoleone has biological activity at extremely low concentrations, suggesting a strong contribution to Sorghum allelopathy.
Zurück zum Text

Inderjit, Duke, S. O., 2003. Ecophysiological aspects of allelopathy. Planta 217, 529-539.
Allelochemicals play an important role in explaining plant growth inhibition in interspecies interactions and in structuring the plant community. Five aspects of allelochemicals are discussed from an ecophysiological perspective: (i) biosynthesis, (ii) mode of release, (iii) mode of action, (iv) detoxification and prevention of autotoxicity, and (v) joint action of allelochemicals. A discussion on identifying a compound as an allelochemical is also presented.
Zurück zum Text

Kagan, I. A., Rimando, A. M., Dayan, F. E., 2003. Chromatographic separation and in vitro activity of Sorgoleone congeners from the roots of Sorghum bicolor. Journal of Agricultural and Food Chemistry 51, 7589-7595.
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 C₈ 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.
Zurück zum Text  

Nimbal, C. I., Pedersen, J. F., Yerkes, C. N., Weston, L. A., Weller, S. C., 1996. Phytotoxicity and distribution of sorgoleone in grain Sorghum germplasm. Journal of Agricultural and Food Chemistry 44, 1343-1347.
The relative phytotoxicity of sorgoleone as measured by seed germination and seedling growth of selected crop and weed species and inhibition of photosynthetic oxygen evolution in atrazine-resistant and -susceptible cell cultures of potato (Solanum tuberosum L.) and common groundsel (Senecio vulgaris L.) were investigated. Relatively little or no effect of sorgoleone was observed on radicle elongation at concentrations less than 500 µM in Petri dish bioassays. Sorgoleone was very phytotoxic to large crabgrass (Digitatia sanguinalis), with a GR₅₀ of 10 µM for shoot growth in a hydroponic culture bioassay. Inhibition of shoot and root growth of velvetleaf (Abutilon theophrasti) and barnyardgrass (Echinocloa crus-galli) was also observed at higher concentrations ranging from 10 to 200 µM, but ivyleaf morningglory (Ipomea hederacea) was tolerant. Sorgoleone inhibited photosynthetic oxygen evolution in both susceptible and resistant cell cultures of potato and common groundsel, and the effect was similar to that of diuron, a strong inhibitor of PS II electron transport. Chlorophyll fluorescence response to sorgoleone in both resistant and susceptible cell cultures was nearly the same. Grain sorghum (Sorghum bicolor L. Moench) genotypes varied considerably in the amount of sorgoleone produced. Root exudates generally contained 85-90% pure sorgoleone on the basis of HPLC analysis. These data indicate that sorgoleone is phytotoxic at micromolar concentrations, exhibits marked selectivity, and inhibits photosynthetic electron transport similar to diuron.
Zurück zum Text

Netzly, D. H., Butler, L. G., 1986. Roots of Sorghum exude hydrophobic droplets containing biologically active components. Crop Science 26, 775-778.
Roots of Sorghum are known to exude materials that exhibit allelochemical activity, but the compounds identified do not completely account for the observed species-specific allelochemical activities. The purpose of this investigation was to characterize both water-soluble and water-insoluble exudates from roots of sorghum [Sorghum bicolor (L.) Moench] seedlings grown in petri dishes. Hydrophilic exudates included phenols, protein, and 3-deoxyanthocyanidin derivatives. Hydrophobic droplets, exuded from root hairs, tested positively for phenols and lipids. These hydrophobic exudates strongly inhibit (85%) root elongation in lettuce (Lactuca sativa cv. Great Lakes) seedlings, but do not affect that of corn (Zea mays L. cv. B73Ht), nor the germination of either of these plants. Associated with these hydrophobic droplets are novel quinones, yet unidentified. The results indicate that these hydrophobic exudate droplets contain components that may have species-specific biological activities.
Zurück zum Text

Pan, Z., Rimando, A. M., Baerson, S. R., Fishbein, M., Duke, S. O., 2007. Functional characterization of desaturases involved in the formation of the terminal double bond of an unusual 16:3D^9,12,15 fatty acid isolated from Sorghum bicolor root hairs. Journal of Biological Chemistry 282, 4326-4335.
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.
Zurück zum Text

Netzly, D. H., Riopel, J. L., Ejeta, G., Butler, L. G., 1988. Germination stimulants of witchweed (Striga asiatica) from hydrophobic root exudate of sorghum (Sorghum bicolor). Weed Science 36, 441-446.
Abstract not available.
Zurück zum Text

Rimando, A. M., Dayan, F. E., Czarnota, M. A., Weston, L. A., Duke, S. O., 1998. A new photosystem II electron transfer inhibitor from Sorghum bicolor. Journal of Natural Products 61, 927-930.
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.
Zurück zum Text

Weston, L. A., Duke, S. O., 2003. Weed and crop allelopathy. Critical Reviews in Plant Sciences 22, 367-389.
Allelopathy can be defined as an important mechanism of plant interference mediated by the addition of plant-produced secondary products to the soil rhizosphere. Allelochemicals are present in all types of plants and tissues and are released into the soil rhizosphere by a variety of mechanisms, including decomposition of residues, volatilization, and root exudation. Allelochemical structures and modes of action are diverse and may offer potential for the development of future herbicides. We have focused our review on a variety of weed and crop species that establish some form of potent allelopathic interference, either with other crops or weeds, in agricultural settings, in the managed landscape, or in naturalized settings. Recent research suggests that allelopathic properties can render one species Mehr invasive to native species and thus potentially detrimental to both agricultural and naturalized settings. In contrast, allelopathic crops offer strong potential for the development of cultivars that are Mehr highly weed suppressive in managed settings. Both environmental and genotypic effects impact allelochemical production and release over time. A new challenge that exists for future plant scientists is to generate additional information on allelochemical mechanisms of release, selectivity and persistence, mode of action, and genetic regulation. In this manner, we can further protect plant biodiversity and enhance weed management strategies in a variety of ecosystems.
Zurück zum Text