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xanthine dehydrogenase (XDH) activity and a model illustrating the proposed metabolic interrelationship is The penultimate step in abscisic acid (ABA) biosyn- presented. thesis involves oxidation of xanthoxal (XAN) catalysed by a molybdenum-cofactor (MoCo)-containing alde- Key words: Abscisic acid, aldehyde oxidase, allopurinol, hyde oxidase (AO) and represents one potential site avocado, cytokinin, molybdenum cofactor, tungstate, of regulation of ABA in plant tissues. In an attempt xanthoxal. to understand the biochemical basis for cytokinin‐ abscisic acid (CK-ABA) antagonism the effect of sev- Introduction eral CKs, molybdate, tungstate and allopurinol (an inhibitor of xanthine oxidase activity and purine meta- Cytokinins appear to antagonize many physiological probolism) on the formation of XAN, ABA and related cesses thought to be mediated, all or in part, by ABA. catabolites in mesocarp of ripening avocado (Persea For example, ABA-induced stomatal closure, leaf senesamericana Mill. cv. Hass) was investigated. Treatment cence, and leaf and fruit abscission are reversed by with either adenine (Ade), isopentenyladenine (2iP) or exogenous application of CK while CK-mediated release zeatin (Z) enhanced conversion of ABA to phaseic acid of seed dormancy contrasts with ABA inhibition of (PA) and caused a reduction in the amount of radio- germination (Salisbury, 1994). This implied antagonism activity incorporated from 3R-[2-14C] mevalonolactone may be the result of metabolic interaction particularly as (MVL) into ABA by stimulating overall ABA metabolism. CKs share, at least in part, a common biosynthetic origin Ancymidol and N-(2-chloro-4-pyridyl)-N-phenylurea with ABA. CK biosynthesis appears to involve the addi(CPPU), while not affecting formation of PA and DPA, tion of dimethylallyl pyrophosphate to the N6 position appeared to retard ABA biosynthesis which resulted of AMP and subsequent conversion of N6D2-isopentenyl in the accumulation of XAN. Tungstate caused accu- AMP to N6D2-isopentenyl adenosine and 2iP, followed mulation of XAN at the expense of ABA and related by hydroxylation of 2iP to yield ‘zeatin-like’ CKs (Binns, acidic metabolites while molybdate and allopurinol 1994). By comparison, ABA is regarded as an apocarotaccelerated ABA metabolism, i.e. formation of XAN, enoid derived from the metabolism of isopentenyl pyroABA, PA, and DPA. These findings are discussed in phosphate via zeaxanthin, 9ae-cis-neoxanthin and XAN, terms of the regulation of the ABA biosynthetic path- which is then oxidized to ABA (Cowan and Richardson, 1997; Duckham et al., 1991; Parry, 1993; Rock and way in avocado fruit by CK-induced suppression of

Regulation of abscisic acid metabolism: towards a metabolic basis for abscisic acid-cytokinin antagonism