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Development and Growth Regulation

Abscisic Alcohol Is an Intermediate in Abscisic Acid Biosynthesis in a Shunt Pathway from Abscisic Aldehyde ¹

Michigan State University-Department of Energy Plant Research Laboratory, East Lansing, Michigan 48824, National Institutes of Health Mass Spectrometry Facility, Michigan State University, East Lansing, Michigan 48824

It has previously been shown that the abscisic acid (ABA)-deficient flacca and sitiens mutants of tomato are impaired in ABA-aldehyde oxidation and accumulate trans-ABA-alcohol as a result of the biosynthetic block (IB Taylor, RST Linforth, RJ Al-Naieb, WR Bowman, BA Marples [1988] Plant Cell Environ 11: 739-745). Here we report that the flacca and sitiens mutants accumulate trans-ABA and trans-ABA glucose ester and that this accumulation is due to trans-ABA biosynthesis. ¹⁸O labeling of water-stressed wild-type and mutant tomato leaves and analysis of [¹⁸O]ABA by tandem mass spectrometry show that the tomato mutants synthesize a significant percentage of their ABA and trans-ABA as [¹⁸O]ABA with two ¹⁸O atoms in the carboxyl group. We further show, by feeding experiments with [²H₆]ABA-alcohol and ¹⁸O₂, that this doubly-carboxyl-labeled ABA is synthesized from [¹⁸O]ABA-alcohol with incorporation of molecular oxygen. In vivo inhibition of [²H₆]ABA-alcohol oxidation by carbon monoxide establishes the involvement of a P-450 monooxygenase. Likewise, carbon monoxide inhibits the synthesis of doubly-carboxyl-labeled ABA in ¹⁸O-labeling experiments. This minor shunt pathway from ABA-aldehyde to ABA-alcohol to ABA operates in all plants examined. For the ABA-deficient mutants impaired in ABA-aldehyde oxidation, this shunt pathway is an important source of ABA and is physiologically significant.

³ Present address: Parke-Davis Pharmaceutical Research Division, 2800 Plymouth Road, Ann Arbor, MI 48105.

¹ Supported by the U.S. Department of Energy under contract DE-AC02-76ERO-1338, by the National Institutes of Health grant DRR00480 to the Michigan State University-National Institutes of Health Mass Spectrometry Facility, by National Science Foundation grant DMB-8703847 to J.A.D.Z., and by a Monsanto Graduate Fellowship and an Michigan State University College of Natural Sciences Doctoral Fellowship to C.D.R.

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