1997, Stress-mduced abicisic acid transients and siimulus-response-coupling. -Physiol. Pianl. !(X): 491^99,Loss of cell turgttr and distortion of the plasma membrane occur a.s a result of dehydration and precede tbe stress-induced bulk increase in concentration of tissue abscisic acid. The latter ha-, been correlated v.iih induction of stress-related gene expression. However, several different stresses may trigger the same coupling mechanism. Thus, al least ibree signalling pathways have been proposed: abscisic acid-requiring, abscisic acid-responsive, and mechanosensors. In this paper, the role and contribution ol slress-induced ahscisic acid transients is examined in an attempi lo explain apparent abscisic acid-dependenl and -independeni stimulus-re.sponse-coupling. Early, intermediate, and late response stages are defined within the stress-induced abscisic acid transient and at leas! four signalling mechanisms are identified. These include, early and laie intracellular modulaiion of gene expression through derepression and/or negative regulation, rapid membrane-initialed calcium release and ion channel activation, and late tskm) ht}rmone-receptor induction of gene expression. An assessment of these proposed .ABA signalling mechanisms in lerms of .AB.A-dependem and -independent stimulus-response-coupling strongly suggests that rapid responses ma\ not be a prerequisite for slow responses and thai the receptor proteins involved have different stehc requiretments. i.e., they are tissue-and/or cell-specific,

Abscisic acid is considered an apocarotenoid formed by cleavage of a C‐40 precursor and subsequent oxidation of xanthoxin and abscisic aldehyde. Confirmation of this reaction sequence is still awaited, and might best be achieved using a cell‐free system capable of both carotenoid and abscisic acid biosynthesis. An abscisic acid biosynthesizing cell‐free system, prepared from flavedo of mature orange fruits, was used to demonstrate conversion of farnesyl pyrophosphate, geranylgeranyl pyrophosphate and all‐trans‐β‐carotene into a range of β,β‐xanthophylls, xanthoxin, xanthoxin acid, 1′,4′‐trans‐abscisic acid diol and abscisic acid. Identification of product carotenoids was achieved by high‐performance liquid chromatography and on‐line spectral analysis of individual components together with co‐chromatography. Putative C‐15 intermediates and product abscisic acid were identified by combined capillary gas chroma‐tography‐mass spectrometry. Kinetic studies revealed that β‐carotene, formed from either famesyl pyrophosphate or geranylgeranyl pyrophosphate, reached a maximum within 30 min of initiation of the reaction. Thereafter, β‐carotene levels declined exponentially. Catabolism of substrate β‐carotene into xanthophylls, putative abscisic acid precursors and product abscisic acid was restricted to the all‐trans‐isomer. However, when a combination of all‐trans‐ and 9‐cis‐β‐carotene in the ratio 1:1 was used as substrate, formation of abscisic acid and related metabolites was enhanced. Biosyn‐thetically prepared [14C]‐all‐trans‐violaxanthin, [14C]‐all‐trans‐neoxanthin and [14C]‐9′‐cis‐neoxanthin were used as substrates to confirm the metabolic interrelationship between carotenoids and abscisic acid. The results are consistent with 9′‐cis‐neoxan‐thin being the immediate carotenoid precursor to ABA, which is oxidatively cleaved to produce xanthoxin. Formation of abscisic aldehyde was not observed. Rather, xanthoxin appeared to be converted to abscisic acid via xanthoxin acid and 1′,4′‐trans‐abscisic acid diol. An alternative pathway for abscisic acid biosynthesis is therefore proposed.

No abstract

1997. Carotenogenic and abscisic acid biosynthesizing activity in a cell-ftee system. -Physiol. Plant. 99: 371-378.Abscisic acid is considered an apocarotenoid formed by cleavage of a C-40 precursor and subsequent oxidation of xanthoxin and abscisic aldehyde. Confirmation of this reaction sequence is still awaited, and might best be achieved using a cell-free system capable of' both carotenoid and abscisic acid biosynthesis. An abscisic acid biosynthesizing cell-free system, prepared from flavedo of mature orange fruits, was used to demonstrate conversion of famesyl pyrophosphate, geranylgerany! pyrophosphate and all-(ranj-^carotene into a range of yS,^-xanthophylls, xanthoxin, xanthoxin acid, 1,4-fran.s-abscisic acid diol and abscisic add. Identification of product carotenoids was achieved by high-performance liquid chromatography and on-line spectral analysis of individual components together with co-chromatography. Putative C-15 intermediates and product abscisic acid were identified by combined capillary gas chromatography-mass spectrometry. Kinetic studies revealed that ^carotene, formed from either famesyi pyrophosphate or geranylgeranyl pyrophosphate, reached a maximum within 30 min of initiation of the reaction. Thereafter, ^-carotene levels declined exponentially. Catabolism of substrate ^carotene into xantfaophylls, putative abscisic acid precursors and product abscisic acid was restricted to the all-frans-isomer. However, when a combination of aii-trans-and 9-cis-^-carotene in the ratio 1:1 was used as substrate, formation of abscisic acid and related metabolites was enhanced. Biosynthetically prepared (''t]-all-;ranj-violaxanthin, ['*C]-all-tranj-neoxanthin and ['*C]-9'-e«-neoxanthin were used as substrates to confirm the metabolic interrelationship between carotenoids and abscisic acid The results are consistent with 9'-cis-neoxanthin being the immediate carotenoid precursor to ABA, which is oxidatively cleaved to produce xanthoxin. Formation of abscisic aldehyde was not observed. Rather, xanthoxin appeared to be converted to abscisic acid via xanthoxin acid and r,4'-rrons-abscisic acid diol. An alternative pathway for abscisic acid biosynthesis is therefore proposed.

No abstract