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.
Baerson
Scott R
SR
United States Department of Agriculture, Agricultural Research Service, Natural Products Utilization Research Unit, University, Mississippi 38677. Electronic address: sbaerson@ars.usda.gov.
Dayan
Franck E
FE
United States Department of Agriculture, Agricultural Research Service, Natural Products Utilization Research Unit, University, Mississippi 38677.
Rimando
Agnes M
AM
United States Department of Agriculture, Agricultural Research Service, Natural Products Utilization Research Unit, University, Mississippi 38677.
Nanayakkara
N P Dhammika
NPD
National Center for Natural Products Research, School of Pharmacy, University of Mississippi, University, Mississippi 38677.
Liu
Chang-Jun
CJ
Biology Department, Brookhaven National Laboratory, Upton, New York 11973.
Schröder
Joachim
J
Universität Freiburg, Institut für Biologie II, Schänzlestrasse 1, D-79104 Freiburg, Germany.
Fishbein
Mark
M
Department of Biology, Portland State University, Portland, Oregon 97207.
Pan
Zhiqiang
Z
United States Department of Agriculture, Agricultural Research Service, Natural Products Utilization Research Unit, University, Mississippi 38677.
Kagan
Isabelle A
IA
United States Department of Agriculture, Agricultural Research Service, Natural Products Utilization Research Unit, University, Mississippi 38677.
Pratt
Lee H
LH
Department of Plant Biology, University of Georgia, Athens, Georgia 30602.
Cordonnier-Pratt
Marie-Michèle
MM
Department of Plant Biology, University of Georgia, Athens, Georgia 30602.
Duke
Stephen O
SO
United States Department of Agriculture, Agricultural Research Service, Natural Products Utilization Research Unit, University, Mississippi 38677.
D000483:Alleles; D000595:Amino Acid Sequence; D016227:Benzoquinones / Q000737:chemistry; D004926:Escherichia coli / Q000378:metabolism; D020224:Expressed Sequence Tags; D017343:Genes, Plant; D023281:Genomics; D008055:Lipids / Q000737:chemistry; D013058:Mass Spectrometry / Q000379:methods; D008969:Molecular Sequence Data; D010675:Pheromones / Q000737:chemistry*; D010802:Phylogeny; D018517:Plant Roots / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry; D017386:Sequence Homology, Amino Acid; D045868:Sorghum / Q000235:genetics*
D016227:Benzoquinones; D008055:Lipids; D010675:Pheromones; D011994:Recombinant Proteins; C113738:sorgoleone; C004532:quinone