title: The pectic disaccharides lepidimoic acid and β-d-xylopyranosyl-(1→3)-d-galacturonic acid occur in cress-seed exudate but lack allelochemical activity.
abstract: OBJECTIVE
  Cress-seed (Lepidium sativum) exudate exerts an allelochemical effect, promoting excessive hypocotyl elongation and inhibiting root growth in neighbouring Amaranthus caudatus seedlings. We investigated acidic disaccharides present in cress-seed exudate, testing the proposal that the allelochemical is an oligosaccharin-lepidimoic acid (LMA; 4-deoxy-β-l-threo-hex-4-enopyranuronosyl-(1→2)-l-rhamnose).
  
  
  METHODS
  Cress-seed exudate was variously treated [heating, ethanolic precipitation, solvent partitioning, high-voltage paper electrophoresis and gel-permeation chromatography (GPC)], and the products were bioassayed for effects on dark-grown Amaranthus seedlings. Two acidic disaccharides, including LMA, were isolated and characterized by electrophoresis, thin-layer chromatography (TLC) and nuclear magnetic resonance (NMR) spectroscopy, and then bioassayed.
  
  
  RESULTS
  Cress-seed exudate contained low-Mr, hydrophilic, heat-stable material that strongly promoted Amaranthus hypocotyl elongation and inhibited root growth, but that separated from LMA on electrophoresis and GPC. Cress-seed exudate contained ∼250 µmLMA, whose TLC and electrophoretic mobilities, susceptibility to mild acid hydrolysis and NMR spectra are reported. A second acidic disaccharide, present at ∼120 µm, was similarly characterized, and shown to be β-d-xylopyranosyl-(1→3)-d-galacturonic acid (Xyl→GalA), a repeat unit of xylogalacturonan. Purified LMA and Xyl→GalA when applied at 360 and 740 µm, respectively, only slightly promoted Amaranthus hypocotyl growth, but equally promoted root growth and thus had no effect on the hypocotyl:root ratio, unlike total cress-seed exudate.
  
  
  CONCLUSIONS
  LMA is present in cress seeds, probably formed by rhamnogalacturonan lyase action on rhamnogalacturonan-I during seed development. Our results contradict the hypothesis that LMA is a cress allelochemical that appreciably perturbs the growth of potentially competing seedlings. Since LMA and Xyl→GalA slightly promoted both hypocotyl and root elongation, their effect could be nutritional. We conclude that rhamnogalacturonan-I and xylogalacturonan (pectin domains) are not sources of oligosaccharins with allelochemical activity, and the biological roles (if any) of the disaccharides derived from them are unknown. The main allelochemical principle in cress-seed exudate remains to be identified.
authors.affiliation: The Edinburgh Cell Wall Group, Institute of Molecular Plant Sciences, The University of Edinburgh, The King's Buildings, Edinburgh EH9 3BF, UK and.
authors.forename: Amjad
authors.initials: A
authors.lastname: Iqbal
authors.affiliation: The Edinburgh Cell Wall Group, Institute of Molecular Plant Sciences, The University of Edinburgh, The King's Buildings, Edinburgh EH9 3BF, UK and.
authors.forename: Janice G
authors.initials: JG
authors.lastname: Miller
authors.affiliation: EastChem School of Chemistry, The University of Edinburgh, The King's Buildings, Edinburgh EH9 3JJ, UK.
authors.forename: Lorna
authors.initials: L
authors.lastname: Murray
authors.affiliation: EastChem School of Chemistry, The University of Edinburgh, The King's Buildings, Edinburgh EH9 3JJ, UK.
authors.forename: Ian H
authors.initials: IH
authors.lastname: Sadler
authors.affiliation: The Edinburgh Cell Wall Group, Institute of Molecular Plant Sciences, The University of Edinburgh, The King's Buildings, Edinburgh EH9 3BF, UK and s.fry@ed.ac.uk.
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Fry
mesh_terms: D001681:Biological Assay; D019607:Brassicaceae / Q000378:metabolism*; D002850:Chromatography, Gel; D002855:Chromatography, Thin Layer; D004187:Disaccharides / Q000378:metabolism*; D004586:Electrophoresis; D006027:Glycosides / Q000378:metabolism*; D006603:Hexuronic Acids / Q000378:metabolism*; D006358:Hot Temperature; D057927:Hydrophobic and Hydrophilic Interactions; D018546:Hypocotyl; D009682:Magnetic Resonance Spectroscopy; D008970:Molecular Weight; D010368:Pectins / Q000378:metabolism*; D010675:Pheromones / Q000378:metabolism*; D053147:Plant Exudates / Q000378:metabolism*; D012210:Rhamnose / Q000378:metabolism; D012639:Seeds / Q000378:metabolism*; D014574:Uronic Acids / Q000378:metabolism*
chemical_list: D004187:Disaccharides; D006027:Glycosides; D006603:Hexuronic Acids; D010675:Pheromones; D053147:Plant Exudates; D014574:Uronic Acids; C000612880:beta-D-xylopyranosyl-(1-3)-D-galacturonic acid; C110298:lepidimoic acid; C007819:galacturonic acid; D010368:Pectins; D012210:Rhamnose
keywords: Allelochemicals; Amaranthus caudatus; Lepidium sativum; high-voltage paper electrophoresis; lepidimoic acid; lepidimoide; nuclear magnetic resonance; oligosaccharin; pectin; rhamnogalacturonan-I; xylogalacturonan