title: A new abscisic acid catabolic pathway.
abstract: We report the discovery of a new hydroxylated abscisic acid (ABA) metabolite, found in the course of a mass spectrometric study of ABA metabolism in Brassica napus siliques. This metabolite reveals a previously unknown catabolic pathway for ABA in which the 9'-methyl group of ABA is oxidized. Analogs of (+)-ABA deuterated at the 8'-carbon atom and at both the 8'- and 9'-carbon atoms were fed to green siliques, and extracts containing the deuterated oxidized metabolites were analyzed to determine the position of ABA hydroxylation. The results indicated that hydroxylation of ABA had occurred at the 9'-methyl group, as well as at the 7'- and 8'-methyl groups. The chromatographic characteristics and mass spectral fragmentation patterns of the new ABA metabolite were compared with those of synthetic 9'-hydroxy ABA (9'-OH ABA), in both open and cyclized forms. The new compound isolated from plant extracts was identified as the cyclized form of 9'-OH ABA, which we have named neophaseic acid (neoPA). The proton nuclear magnetic resonance spectrum of pure neoPA isolated from immature seeds of B. napus was identical to that of the authentic synthetic compound. ABA and neoPA levels were high in young seeds and lower in older seeds. The open form (2Z,4E)-5-[(1R,6S)-1-Hydroxy-6-hydroxymethyl-2,6-dimethyl-4-oxo-cyclohex-2-enyl]-3-methyl-penta-2,4-dienoic acid, but not neoPA, exhibited ABA-like bioactivity in inhibiting Arabidopsis seed germination and in inducing gene expression in B. napus microspore-derived embryos. NeoPA was also detected in fruits of orange (Citrus sinensis) and tomato (Lycopersicon esculentum), in Arabidopsis, and in chickpea (Cicer arietinum), as well as in drought-stressed barley (Hordeum vulgare) and B. napus seedlings.
authors.affiliation: Plant Biotechnology Institute, National Research Council of Canada, 110 Gymnasium Place, Saskatoon, Saskatchewan, Canada S7N 0W9.
authors.forename: Rong
authors.initials: R
authors.lastname: Zhou
authors.forename: Adrian J
authors.initials: AJ
authors.lastname: Cutler
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Ambrose
authors.forename: Marek M
authors.initials: MM
authors.lastname: Galka
authors.forename: Ken M
authors.initials: KM
authors.lastname: Nelson
authors.forename: Timothy M
authors.initials: TM
authors.lastname: Squires
authors.forename: Mary K
authors.initials: MK
authors.lastname: Loewen
authors.forename: Ashok S
authors.initials: AS
authors.lastname: Jadhav
authors.forename: Andrew R S
authors.initials: AR
authors.lastname: Ross
authors.forename: David C
authors.initials: DC
authors.lastname: Taylor
authors.forename: Suzanne R
authors.initials: SR
authors.lastname: Abrams
mesh_terms: D000040:Abscisic Acid / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism*; D000123:Acetyltransferases / Q000235:genetics; D029688:Brassica napus / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D003903:Deuterium; D000081031:Fatty Acid Elongases; D015870:Gene Expression / Q000187:drug effects; D017343:Genes, Plant; D006900:Hydroxylation; D013058:Mass Spectrometry; D008954:Models, Biological; D015394:Molecular Structure; D010937:Plant Growth Regulators / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D012639:Seeds / Q000378:metabolism
chemical_list: D010937:Plant Growth Regulators; D000040:Abscisic Acid; D003903:Deuterium; D000123:Acetyltransferases; D000081031:Fatty Acid Elongases