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item Whitaker, Bruce

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2004
Publication Date: 7/25/2004
Citation: Whitaker, B.D. Oxidative stress and apple scald. Hortscience. 2004. 39(5):924-929

Interpretive Summary: Superficial scald is a costly cold storage disorder of apples that affects fruit of many popular varieties such as Red Delicious, Granny Smith, and Law Rome. Scald is thought to be induced by oxidation products of a volatile compound, alpha-farnesene, which is produced in the peel tissue of apples during storage. To control scald, apples are routinely drenched after harvest with a solution including an antioxidant chemical plus a fungicide. This is expensive and results in unwanted chemical waste and residue on the fruit. Our research is aimed at understanding, at the genetic and biochemical level, why some apples are highly susceptible and others are resistant to the scald disorder. This report summarizes our current knowledge of the genes and enzymes involved in production of alpha-farnesene in apple fruit, including the recent isolation of a gene encoding the enzyme alpha-farnesene synthase, the enzyme that performs the last step in the assembly of alpha-farnesene. The long-range goal of this work is to limit farnesene synthesis in apples, and thereby prevent scald development, by molecular genetic means. This outcome will benefit both the apple industry and consumers by creating new scald-resistant lines that do not require chemical treatment prior to storage.

Technical Abstract: Superficial scald is a costly storage disorder that often develops in fruit of susceptible apple cultivars such as 'Granny Smith' and 'Law Rome' after several months at low temperature. Despite intensive investigation, the biochemical mechanism underlying scald remains unknown. Evidence indicates that the disorder is induced by oxidative stress. The prevailing hypothesis holds that oxidation products of the sesquiterpene alpha-farnesene are directly involved. A dramatic rise in farnesene synthesis often occurs shortly after apples are placed in storage and oxidation of the accumulated farnesene to conjugated trienols proceeds rapidly after about 6 to 8 weeks, particularly in air-stored fruit. Accumulation of conjugated trienols during apple storage is usually correlated with the subsequent incidence and severity of scald development. Further evidence in support of the farnesene oxidation-scald induction hypothesis was the recent finding that prestorage treatment of scald-susceptible apples with 1-methylcyclopropene (1-MCP), a blocker of ethylene action, greatly diminished both farnesene synthesis and scald development. Molecular genetic disruption of genes controlling farnesene synthesis is a strategy that should prove or disprove the direct role of farnesene oxidation in the induction of scald. Logical targets for gene knockouts using antisense or RNAi constructs are genes encoding: 1) a sesquiterpene pathway-specific isozyme of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), and 2) alpha-farnesene synthase. We have recently cloned and characterized an alpha-farnesene synthase gene from the scald-susceptible 'Law Rome' and scald-resistant 'Idared' apple cultivars. Upregulation of the gene in the first 4 to 8 weeks of 0C air storage was markedly suppressed by 1-MCP treatment and expression was about fourfold greater in 'Law Rome' than in 'Idared' peel tissue. We have also cloned complete or partial cDNAs from 'Law Rome' encoding three HMGR isozymes. HMG2 has been identified as the most likely to play a key role in farnesene synthesis and further characterization of this gene is in progress.