Characterization of the expression of an oxygenase involved in chilling-induced damage in citrus fruit

Exposure of chilling-sensitive citrus fruit to low, non-freezing, temperatures may end up with chilling injury (CI), manifested as necrotic areas in the peel. In this study, we have examined changes in the expression of a gene isolated from the flavedo of 'Fortune' mandarin, showing sequence homology with oxygenase genes in other plants, in the response of citrus fruit to chilling. Low temperature storage (1.5 degreesC) caused a transient increase in CIOX (for chilling-induced oxygenase) mRNA accumulation in the flavedo of fruit of the chilling-susceptible 'Fortune' mandarin but this gene was not activated in 'Hernandina' fruit, which belongs to the class of chilling-tolerant mandarins. CIOX transcript accumulated in 'Fortune' mandarins when CI symptoms were initiated, to sharply decrease thereafter although chilling symptoms continuously increased during fruit storage at 1.5 degreesC. Moreover, it has been shown that applying a high concentration (1 mM) of abscisic acid (ABA) and conditioning the fruit for 2 days with 10 muL L-1 ethylene, which sharply increases ABA levels in the flavedo, increased chilling-induced damage in 'Fortune' mandarins and accelerated the transient increase in CIOX mRNA accumulation. By contrast, increasing ABA levels did not cause peel damage in 'Hernandina' mandarins kept at 1.5 degreesC or in 'Fortune' mandarins stored at a non-chilling temperature (12 degreesC) and failed to induce CIOX mRNA accumulation. Thus, this gene appears not to be regulated by ABA but its expression was favored by the combination of low temperature and ABA, conditions that enhanced chilling damage in chilling-susceptible citrus fruit. These results indicate that CIOX expression may serve as a molecular marker for chilling susceptibility and/or damage in citrus fruit. (C) 2004 Elsevier B.V. All rights reserved.