| Popis: |
Fungi can produce jasmonic acid (JA) and its isoleucine conjugate in large quantities, but little is known about the biosynthesis. Plants form JA from 18:3n-3 by 13S-lipoxygenase (LOX), allene oxide synthase, and allene oxide cyclase. Shaking cultures of Fusarium oxysporum f. sp. tulipae released over 200 mg of jasmonates per liter. Nitrogen powder of the mycelia expressed 10R-dioxygenase-epoxy alcohol synthase activities, which was confirmed by comparison with the recombinant enzyme. The 13S-LOX of F. oxysporum could not be detected in the cell-free preparations. Incubation of mycelia in phosphate buffer with [17,17,18,18,18-2H5]18:3n-3 led to biosynthesis of a [2H5]12-oxo-13-hydroxy-9Z,15Z-octadecadienoic acid (α-ketol), [2H5]12-oxo-10,15Z-phytodienoic acid (12-OPDA), and [2H5]13-keto- and [2H5]13S-hydroxyoctadecatrienoic acids. The α-ketol consisted of 90% of the 13R stereoisomer, suggesting its formation by nonenzymatic hydrolysis of an allene oxide with 13S configuration. Labeled and unlabeled 12-OPDA were observed following incubation with 0.1 mM [2H5]18:3n-3 in a ratio from 0.4:1 up to 47:1 by mycelia of liquid cultures of different ages, whereas 10 times higher concentration of [2H5]13S-hydroperoxyoctadecatrienoic acid was required to detect biosynthesis of [2H5]12-OPDA. The allene oxide is likely formed by a cytochrome P450 or catalase-related hydroperoxidase. We conclude that F. oxysporum, like plants, forms jasmonates with an allene oxide and 12-OPDA as intermediates. |
