John C.M. Riley

Publications (3)6.22 Total impact

• Article: Quantitative and qualitative differences in C6‐volatile production from the lipoxygenase pathway in an alcohol dehydrogenase mutant of Arabidopsis thaliana

  Nicholas J. Bate, John C. M. Riley, John E. Thompson, Steven J. Rothstein
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  ABSTRACT: Six-carbon (C6) volatile products are released from the enzymatic action of hydroperoxide lyase (HPL), a component of the lipoxygenase (LOX) pathway and form the basis of the “green-note” flavour characteristic of many consumed plant products. Arabidopsis leaf tissue contains the C6-aldehydes hexanal, and trans-2-hexenal as well as the C6-alcohols: hexanol, and 3-hexenol. Interconversion between C6-aldehydes and alcohols is thought to proceed through the action of alcohol dehydrogenase (ADH). Using an ADH mutant of Arabidopsis, we have shown that there are large quantitative and qualitative differences in the accumulation of C6-volatiles in the absence of ADH activity. The total quantity of LOX-derived volatiles is greater on a fresh weight basis in the ADH mutant. Qualitatively, hexanol and 3-hexenol levels are approximately 62% and 51% lower in the mutant, respectively, whereas levels of hexenal are approximately 10-fold higher. Hexanal accumulation, however, is unaffected in the mutant. The altered profile of LOX-derived volatiles does not have an effect on the steady-state levels of mRNA for allene oxide synthase (AOS) or LOX. HPL activity and mRNA quantity, however, are higher in the mutant relative to wild type, suggesting that altered product levels in the mutant affect HPL regulation.
  Physiologia Plantarum 01/2002; 104(1):97 - 104. · 3.11 Impact Factor
• Article: Ripening-induced acceleration of volatile aldehyde generation following tissue disruption in tomato fruit

  John C. M. Riley, John E. Thompson
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  ABSTRACT: Hexanal and cis-3-hexenal are principal flavor volatiles in ripe tomato fruit, but whether they accumulate during ripening or are formed upon maceration of the tissue has not been clarified. This has been addressed by measuring levels of these aldehydes in green and ripe fruit with discrimination between intrinsic aldehyde content and aldehyde generation following tissue disruption. Volatile sampling of tomato fruit homogenates was accomplished by purge/trapping, followed by thermal desorption on a gas chromatograph equipped with a mass selective detector. Incubation of some samples with alcohol dehydrogenase to convert the aldehydes to their respective alcohols permitted positive identification of the isomeric form of hexenal as cis-3-hexenal. Red and green tomato fruit homogenized in buffer with saturated CaCl2 contained low (0.1-0.8 µg g−1 fresh weight) levels of hexanal and cis-3-hexenal; thus there is minimal endogenous volatile content in intact fruit. Volatile levels increased rapidly, up to 10-fold, following homogenization of ripe tomato fruit in the absence of CaCl2, and more modestly in corresponding green tomato fruit homogenates. Incubation with the appropriate lipoxygenase/hydroperoxide lyase substrate (linoleic acid for hexanal, linolenic acid for cis-3-hexenal) doubled the amount of volatile compound produced. Hexanal generation was suppressed in the presence of linolenic acid, suggesting that the enzyme complex has greater affinity for this substrate. As well, levels of cis-3-hexenal, but not hexanal, tended to decline within 30 min of homogenization, possibly reflecting a specific degradative process. The results collectively indicate that the contribution of six-carbon aldehydes to tomato fruit flavor is attributable to metabolism invoked following tissue disruption rather than within the intact fruit.
  Physiologia Plantarum 11/1998; 104(4):571-576. · 3.11 Impact Factor
• Article: Lipoxygenase and hydroperoxide lyase activities in ripening tomato fruit

  John C.M. Riley, Claude Willemot, John E. Thompson
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  ABSTRACT: Volatile flavor compounds of lipid origin are produced by the sequential actions of lipoxygenase (LOX) and hydroperoxide lyase (HPL). The activities of these enzymes and their subcellular localization were determined in ripening tomato fruit (Lycopersicon esculentum L. cv. Match W42). Enzyme activity was measured in the microsomal fraction, a post-microsomal pellet (PMP) and a cytosolic fraction obtained from tomatoes at the mature-green, breaker, and red stage. LOX activity was predominantly (~80% of the total) in the PMP fraction for all stages of fruit development, and gel filtration of this fraction indicated that this represents soluble LOX that had been pelleted by centrifugation. For all of the subcellular fractions at all stages of development, LOX activity was higher at pH 6 than at pH 7 or 8. The microsomal form of LOX showed the greatest change in activity with fruit development, increasing by ~69% between the mature-green and breaker stages of development and declining again as the fruit turned red. In contrast, HPL was predominantly (~73% of the total) in the microsomal fraction at all stages of fruit development, proved to be insensitive to pH over the range 6 to 8 and did not show any significant change in activity as the fruit ripened. Moreover, although most of the LOX was found in the PMP fraction, more lipid hydroperoxide is produced by microsomal LOX than is consumed by microsomal HPL. These observations suggest that the microsomal compartment is the site at which volatile flavor formation is initiated in tomato fruit.
  Postharvest Biology and Technology.