22. 糸状菌のアブシシン酸生合成経路-イオニリデンエタンを経由する新しい直接経路

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Two pathways, the carotenoid pathway and the direct pathway, have been proposed for biosynthesis of abscisic acid (ABA) in fungi. This controversy has not come to a conclusion. We have proposed that an ABA-producing fungus, Botrytis cinerea, biosynthesizes ABA by a new direct pathway in which C-1 of FDP is reduced to give 2E-allofarnesene (1), following its conversion to 2Z-α-ionylideneethane (4), and 4 is converted to ABA by oxidation with molecular oxygen. However, conversions of the intermediates to ABA and isomerization step at C-2 have not been proved. Isomerization at C-2 seems to occur at 1 or 2E-α-ionylideneethane (3), suggesting that the pathway has two possible routes via 1→2Z-allofanesene (2)→4 or via 1→3→4 (Figure). We investigated the new direct pathway in B. cinerea by a feeding experiment with [2-^<13>C]-1-4. A feeding experiment was performed in another ABA-producing fungus, Cercospora cruenta also. A feeding experiment with [2-^<13>C]-1-4 in B. cinerea showed that the fungus converted these compounds to ABA with incorporation ratios of 0.7,0.2,2.6 and 17.1%, respectively. The incorporation ratio of 2 was lower than that of 1. However, the lower incorporation of 3 than that of 4,and the ratios of 1 and 2 (4 : 3), and of 3 and 4 (1 : 9) in the mycelia strongly suggested that isomerization at C-2 occurs at 1,not at 3. The low incorporations of 1 and 2 could be due to their volatility and instability. The stereochemistry at C-1' of ABA biosynthesized from (±)-[2-^<13>C]-4 was (S)-configuration (98% ee), showing that (R)-4 was selectively converted to ABA. These findings prefer the route via FDP→1→2→(R)-4→ABA (Figure). 2Z-γ-Ionylideneethane was converted to ABA in C. cruenta, showing that the new direct pathway is working also in this fungus.
植物化学調節学会の論文
2003-10-10

22. 糸状菌のアブシシン酸生合成経路-イオニリデンエタンを経由する新しい直接経路

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