Endogenous biosynthetic precursors of (+)-abscisic acid. VII. The 1′,4′-trans-diol is formed from ABA, it is not a precursor

Abstract

[¹⁴C]ABA fed to avocado fruit is known to be converted into the 1′,4′-trans-diol and [¹⁴C]1′,4′-trans-diol has been shown to be converted into ABA by several plant tissues. As a ‘cold trap’of trans-diol becomes labelled with ¹⁴C when [¹⁴C]mevalonate is converted into ABA, the trans-diol has been suggested to be the immediate precursor of ABA. This proposal has now been tested by feeding [5-¹⁴C,5-³H₂]mevalonolactone to unripe avocado fruit and measuring the ³H :¹⁴C ratio in the ABA and in the 1′,4′-trans-diol isolated from the fruit after 16 h. Little labelled diol was present unless a ‘cold trap’ of unlabelled 1,4-trans-diol was added with the mevalonate. One ³H atom, derived from those at C-5 of mevalonate, would be expected at C-4′ of the diol, adjacent to the hydroxyl group, and another at C-5 of the side chain of the diol if the diol were a precursor of ABA (³H:¹⁴C ratio of 2:3). However, if the 4′-hydroxyl group had been oxidised to a ketone to form ABA, then the ³H atom at C-4′ of the diol would have been lost and the ³H:¹⁴C ratio would be expected to be 1:3. The normalised ³H : ¹⁴C ratios of ABA and 1′,4′-trans-diol biosynthesised from [¹⁴C,5-³H₂]mevalonate were 0.915:3 and 0.844:3 respectively and after oxidation of the diol to ABA with MnO₂ the ratio was 0.869:3 i.e. there was no ³H at C-4′ of the diol. These ratios are as expected for the trans-diol if it had been formed by reduction of ABA. This, and the absence of labelled diol in the fruit unless a ‘cold trap’was added, establishes that the 1′,4′-trans-diol is formed from ABA and it is not a precursor. The formation of the diols from newly synthesised labelled ABA in cell-free systems can be attributed to the addition to the homogenate of compounds with strong reducing potential. NADPH₂⁺ (8.4 nmol) added to a mung bean seedling homogenate caused the reduction of (±)-[¹⁴C]ABA (0.37 nmol, 22.5 µCi/mol) to trans-diol (1189 dpm) whereas with NADP⁺ only 338 dpm were present in trans-diol. Glutathione (46 nmol) caused the formation of 1214 dpm while oxidised glutathione produced 638 dpm. Less 1′,4′-cis-diol was formed.