Biosynthetic origin of abscisic acid in ripening avocado fruit.
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Date
2000
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Abstract
Mesocarp of ripening avocado fruit incorporated label from [2-(14)C]mevalonolactone,
[1-(14)C] acetic acid, [1-(14)C] glucose and [1 -(14)C] pyruvate into ABA, although
incorporation from mevalonolactone was significantly higher. Inhibition of the
mevalonate pathway at the HMGR level using mevastatin reduced incorporation from
acetate and MVL, while increasing incorporation from pyruvate and glucose. The
carotenoid biosynthesis inhibitors AMO 1618 (inhibitor of lycopene cyclase) and
fluridone (inhibitor of phytoene desaturase) both decreased incorporation of MVL
into ABA, while the plant growth regulators ancymidol (inhibitor of GA synthesis and
cytochrome P450) and jasmonic acid (senescence stimulator reducing the carotenoid
content of plants) both increased incorporation of MVL into ABA. Tungstate was
found to reduce incorporation from all four substrates into ABA, although more
significantly from MVL and acetate. Further investigation revealed that the tungstate induced
decrease in MVL incorporation into ABA occurred concomitantly with
increased label incorporation into XAN. Cobalt, an inhibitor of ACC oxidase and
therefore of ethylene production, increased incorporation of MVL into ABA. Nickel
had a similar effect. Analysis of the methyl ester of ABA extracted from avocado
mesocarp supplied with either [1-(13)C] acetic acid or [1-(13)C] glucose revealed
incorporation of label from acetate consistent with formation of ABA via the
acetate/mevalonate pathway whereas glucose was incorporated via the triose
phosphate pathway of isopentenyl pyrophosphate formation. Methane, positive ion-chemical
ionisation-mass spectrometry of the cis, trans and all- trans isomers of ABA
indicated more intense labelling of trans, trans-ABA, irrespective of substrate used.
These results indicate that trans, trans- and cis, trans-ABA are derived by different
pathways and that ABA is formed in avocado by both the mevalonate and non-mevalonate
pathways of isopentenyl diphosphate synthesis.
Description
Thesis (M.Sc. Agric.)-University of Natal, Pietermaritzburg, 2000.
Keywords
Avocado., Avocado--Ripening., Abscisic acid., Biosynthesis., Botanical chemistry., Theses--Horticultural science.